Table of Contents 
Introduction vii 
Part I PRODUCTION CONSTRAINTS 
Unit 1 Production constraints 
Part II STRATEGIES FOR OVERCOMING CONSTRAINTS 
Unit 2 Morphology and physiology 
Unit 3 Breeding 
Unit 4 Rapid multiplication 
Unit 5 Tissue culture 
Unit 6 Agronomy 
Unit 7 Crop protection 
Part III POSTHARVEST TECHNOLOGY 
Unit 8 Storage of fresh cassava 
Unit 9 Cassava processing 
Unit 10 Utilization of cassava and its products 
Part IV RESEARCH 
Unit 11 Data collection and organization 
Unit 12 On-farm research 
Glossary 
Recommended reading 
List of Figures and Tables 
Unlt 1 Production constraints Figure 4.4 Hardwood ministem cuttings showing root and 
shoot growth (planted horizontally) 
Figure 1.1 Cassava leaf showing symptoms of Africa Figure 4.5 Semi-matured ministem cuttings showing root 
cassava mosaic virus disease and shoot growth (planted vertically) 
Figure 1.2 Cassava stem showing bacterial gum exu- Figure 4.6 Semi-matured ministem cuttings growing in 
dations polythene bags 
Figure 1.3 Cassava leaf showing an attackof cassava Figure4.7 Hardwood ministem cunings growing in the 
bacterial blight disease nursery 
Figure 1.4 Cassava plant showing defoliated stems, Figure 4.8 cassava stems stored upright 
commonly referred to as 'candelsticks' Figure 4.9 Cassava stems stored on a horizontal support 
Figure 1.5 Cassava leaf showing an attack of brown system 
leaf spot 
Figure 1.6 Infected cassava tuber showing white Table 4.1 Percentage of field establishment for cassava 
mycelial growth ministem cunings pre-sprouted in petforated 
Figure 1.7 Mealybug infestation of cassava leaves polythene bags 
Figure 1.8 Cassava plant showing elegant grass- 
hopper attack 
Figure 1.9 Cassava stem stripped down to the pith Unlt 5 Tlssue culture 
following grasshopper attadc 
Figure 5.1 Process of meristem culture and plantlet devel- 
opment 
Unit 2 Morphology and physiology Figure 5.2 Rapid multiplication of disease-free cassava for 
distribution 
Figure 2.1 General morphology of the cassava plant Figure 5.3 A humidity chamber 
Figure 2.2 Transverse section of young storage root Figure 5.4 Removing the plantlet from the tube 
Figure 2.3 Inflorescence of a cassava plant Figure 5.5 Handling the plantlet 
Figure 2.4 Fruit and seed of a cassava plant 
Figure 2.5 Growth and development in cassava 
Unn 6 Agronomy 
Unit 3 Breeding Figure 6.1 Cassava growing on mounds 
Figure 6.2 Cassava growing on ridges 
Figure 3.1 Pollination by hand Figure 6.3 Cassava growing on the flat 
Figure 3.2 Bagged pollinated flowers 
Figure 3.3 Seedlings growing in a nursery Table 6.1 Nutrients removed by cassavagrown on differ- 
Figure 3.4 IlTA cassava breeding scheme ent types of soil in Madagascar 
Table 6.2 Equivalent amounts of nutrients (kglha) re- 
moved by cassava cultivars and yam species 
Unit 4 RapM multiplication 
through crop harvest in Nigeria, expressed as 
Figure 4.1 Ministem cuttings: tip shoot (left), semi- fertilizers 
matured (centre) and hardwood (right) Table 6.3 Effect of timeof hawest on yield of different 
Figure 4.2 Examples of tools used to prepare mini- varieties (kglplot) 
stem cunings: big secaterus (left), small Table 6.4 Effect of time of harvest on the percentage of 
secateurs (centre) and a hand saw (right) starch 
Figure 4.3 Semi-matured ministem cuttings planted in Table 6.5 Main characteristics of some improved IlTA 
a nursery bed cassava varieties 
Unit 7 Crop protection Unlt 10 Utillzat/on o f  cassava and Its products 
Figure 7.1 Cassava plant damaged by cassavamea- Figure 10.1 Bread with 20% cassava flour made from IlTA 
lybug improved varieties 
Figure 7.2 Cassava plant damaged by cassavagreen 
mite Table 10.1 Compositionof cassavaproductsprepared tra- 
Figure 7.3 Stages in a biological control programs dilionally in Cameroon 
Table 10.2 Composlion of cassava leaves and selected 
other food items in terms of per 1009 edible 
Unlt 8 Storage of fresh cassava portion, fresh weight 
Table 10.3 Animal feed rations using cassava meal 
Figure 8.1 Fully filled trenches under a shed 
Figure 8.2 Cassava roots stored in a trench 
Figu~e8. 3 Three types of containers used for storing Unit 11 Data collection and organization 
cassava roots in sawdust 
Fgure 11.1 Record sheet used in acassava breeding pro- 
gram 
Unit 9 Cassava processing 
Table 11.1 Analysis of variance table for Completely 
Figure 9.1 Steeping cassava roots forthe preparation Randomized Design 
ot lafun Table 11.2 Analysis of variance table for Randomized 
Figure 9.2 Drying manually pulverized cassava on Complete Block Design 
rocks Table 11.3 Two-way table of blocks x treatments 
Figure 9.3 Peeling cassava manually Table 11.4 Complete analysis of variance table 
Figure 9.4 Grating the roots manually Table 11.5 Comparison of treatment means and LSD 
Figure 9.5 Dehydratingandfermentingcassavamash 
Figure 9.6 Sieving cassava mash 
Figure 9.7 Frying gari 
Figure 9.8 Gari being sold 
Figure 9.9 Flow chart of gari manufacture Figure 12.1 Flow chart of OFR activities and their inter- 
Figure 9.10 Washing and grating cassava tubers relationships 
Figure 9.11 Power screw dehydrating press Figure 12.2 Target areas with their representative pilot 
Fgure 9.12 Mechanical sifter tor mash and gari research area 
Figure 9.13 Frying gari (improved) Figure 12.3 Rainfall and evapotranspiration at lbadan. 
Figure 9.14 Flow chart for preparationofcassavachips Nigeria. 1953-19 73 
and flour from low-cyanide varieties Figure 12.4 Cassava-baseds ystems and associated mean 
Figure 9.15 Flow chart for preparation of detoxified rainfall distribution in the Ohosu area 
cassava flour from high-cyanide varieties Figure 12.5 Calendar of farm operations indicating peak 
Fgure 9.16 Flow chart for manufacture of starch periods in the Ohosu area 
Figure 9.17 Layout of cassava-processing industry Fgure 12.6 IlTA cassava-based system survey. Ohosu 
Figure 9.18 A typical cassava grater Figure 12.7 llTA cassava-based system survey. Ohosu 
Figure 9.19 Hydraulic jack press F@ure 12.8 On-station research 
Figure 9.20 RAIDS gari fryer 
Figure 9.21 Manually operated slicing machines Table 12.1 Suggested contents of the report on the pilot 
Figure 9.22 Multipurpose bin dryer research area 
Figure 9.23 Cabinet dryer, showing cross-section Table 12.2 Checklist of information to be collected during 
Figure 9.24 Hammer mill the field survey 
Figure 9.25 Brook dryer, showing cross-section Table 12.3 Treatmentcombinationsin anon-farmtrial with 
improved cassava and soybean 
Table 9.1 Machinery and implements for cassava. Table 12.4 Costs and returnsforcassava/soybean system 
processing industries in the Ohosu area 
CASSAVA IS ONE OF the most important root crops in Africa. It derives its importance from the fact that its 
starchy, thickened, tuberous roots are a valuable source of cheap calories, especially in developing countries 
where calorie deficiency and malnutrition are widespread. In many partsof Africa, the leaves and tender shoots 
of cassava are also used for human consumption. 
Over two-thirds of the total production of cassava is consumed in various forms by humans. Its usage as a 
source of ethanol for fuel, energy in animal feed, and starch for industry is increasing. The crop is amenable 
to agronomic as well as genetic improvement, has a high yield potential under good conditions and performs 
better than other crops under sub-optimal conditions. It is grown widely in several countries in sub-Saharan 
Africa and Madagascar. It was introduced into Africa in the latter half of the1 6th century from South America 
and perhaps also from Central America, where it is believed to have originated. 
This manual is designed to be both a teaching aid for cassava training sessions and a convenient reference 
for those involved in the production and postharvest technology of the crop. It is divided into four parts, and 
together these parts contain 12 units. 
Part I deals with production constraints. It consists of one unit which covers the main factors accounting for 
low yields and the problems associated with postharvest technology: diseases and pests; weeds; soils and 
agronomic factors: and socio-economic factors. 
Part II deals with the strategies for overcoming production constraints encountered in cassava cultivation. 
There are six units. The morphology and physiology unit discusses the biology of the cassava plant and 
suggests possible strategies to increase tuber root yield based on a better understanding of the crop. The unit 
on breeding reviewsthe efforts aimed at incorporating into cassava varieties all the desirable characteristics 
associated with high and stable yields, expressed in terms of both quantity and quality. This is followed by the 
unit on rapid multiplication. Tissue culture appmachesto muniplying and distributing plants which are resistant 
to virus infections are discussed in the unit on tissue culture. The unit on agronomy discusses the production 
aspects in terms of soils and agronomic practices. The final unit in Part II discusses crop protection in terms 
of disease and pest control. 
Part Ill deals with postharvest technology, which encompasses storage, processing and utilization. Cassava 
is a highly perishable crop, and the problems of postharvest losses are well known. The unit on storage 
discusses traditional and improved methods of cassava storage. In the unit on processing, traditional and 
improved methods of cassava processing are described, with emphasis on some of the major products, such 
as gari, flour and starch. The unit on utilizationd eals with the use of cassava and processed cassava products 
in human and animal nutrition and in industry. 
Part IV deals with research and comprises two units. The unit on data collection and organization covers 
commonly used research designs and the collection, analysis and interpretation of data; standard scoring 
systems for the diseases, pests and agronomic characteristics of cassava are also discussed. The final unit 
in this manual deals with on-farm research and on-farm experimentation with cassava. The first part of the unit 
discusses the concept of the farm as a system and the on-farm research process; the second part presents 
an example of on-farm experimentation, including researcher-managed and farmer-managed trials. 
Part I 
PRODUCTION CONSTRAINTS 
UNIT 1 
Production Constraints 
The constraints on cassava production in Africa include diseases, 
pests, weeds, soil and agronomic factors, and socio-economic 
lactors. These constraints have contributed to keeping the aver- 
age cassava yield in Africa at 6.4 tonslha, which is well below the 
world average of 8.8 tonslha. Efforts to increase production must 
be based on an understanding of the constraints in order to 
eliminate or contain them. 
Diseases 
The major diseases of cassava are leaf diseases, stem diseases 
and tuber rot. 
Leaf diseases 
Cassava mosaic. First reported in East Africa in 1894, cassava 
mosaic is the most wides~readd isease of cassava in tro~ical 
Africaand India. Although iiwas postulated in 1906thatthecausal 
organism was a virus, it was not until 1983 that the etiology of 
cassava mosaic was confirmed beyond doubt. 
The causal organism, a geminivirus known as Afr'c an cassava 
mosaic virus (ACMV; paired or bonded virus particles) averaging 
20 x 30 nanometers, is transmitted from one plant to another by 
the whitefly, Bemisia tabaci. It is also spread between plantations 
and from one region to anotherby the use of infected planting 
materials. Symptoms of this disease include characteristic light 
green, yellow or white patches, irregularly intermingled. The chlo- 
rotic areas mav be small flecks or s~otso,  r thev mav cover the 
entire leaf ( s e e ~ i ~ u1r. eI ) .  The motiling 'is sometimes accompa- Figure 3.1 
nied by leaf deformation and a general stunting of the plant. On Cassava leaf showing symptoms of 
plants which are stunted, the diseased leaves are small, with African cassava mosaic virus disease 
1 
asymetrical development of the entire lobes. Yield losses may 
range from 20 to 60%. 
Cassava bacterial blight disease (CBB). This is the most wide- 
spread bacterial disease of cassava and is second in importance 
only to ACMV. Itwasfirst reported in Brazil in 1912 and nowoccurs 
in all cassava-growing areas of the world. In Africa, it was first 
reported in Madagascar in 1946. 
Figure 1.2 
Cassava stem showing bacterial 
gum exudations 
Figure 1.3 
Cassava leaf showing an attack of cassava bacterial blight disease 
The causal organism is a bacterium, Xanthomonas campestris 
pathovarmanihotis. The symptoms include characteristic angular 
water-soaked leaf spot, blight, gum exudation (see Figure 1.2), 
stemdie-back, wilt (seeFigure 1.3) andvascular necrosis. Severe 
attack results in rapid defoliation of the plant, leaving bare stems 
commonly referredto as 'candlesticks'(see Figure 1.4).Y ield loss 
varies from 20 to loo%, depending upon the cultivar, bacterial 
strain and environmental conditions. 
Cassava angular leaf spot. This disease is caused by the 
bacterium Xanthomonas campestrispathovarcassavae. It is not 
as widespread as CBB, being restricted to Uganda, Kenya, Tan- 
zania, Rwanda, eastern Zai're and Malawi. 
The symptoms are similarto those caused by CBB but the disease 
is not systemic. The leaf spots are usually surrounded by a 
Figure 1.4 chlorotic halo. Although infected plants are defoliated, they never 
Cassava plant showing defoliated stems, 'die back'. Yield losses attributable to cassava angular leaf spot 
commonly referred to as 'candlesticks' have not been quantified. 
Cercospora leaf spot. There are three types of cercospora leaf 
spot. The most common one is brown leaf spot, caused by 
Cercosporidiumh enningsii (see Figure 1.5). The other types are 
leaf blight, caused by Cercospora vicosae, and white leaf spot, 
caused by Cercospora caribae. 
Although severe attacks by these micro-organisms have been 
reported in several African countries, they are not known to kill 
plants. The symptoms are restricted to older leaves and set in after 
tuberization hasoccurred. Yieldlossesareminorforwhite leaf spot 
and leaf blight but may reach about 20% for brown leaf spot. 
Figure 1.5 
Cassava leaf showing an attack of brown leaf spot 
Stem diseases 
Cassava anthracnose disease (CAD). Caused by Colleto- 
trichumgloeosporioides f. sp. rnanihotis, CAD is the most impor- 
tant stem disease in Africa; it occurs in all major cassava-growing 
areas. A sap-sucking coreid bug, Pseudotheraptus devastans, is 
reported to be partly responsible for the spread of the disease. 
The fungus attacks mainly the stem, twigs andfruits, causing deep 
wounds ('cankers'), leaf spotting and tip die-back. The first symp- 
toms appearonthe young stems as slightly depressed oval lesions 
which quickly turn dark brown. On the older stems, raised fibrous 
lesionse~entualdl~e velop into deep cankers which make the 
stems brittle. The incidence and severity of the disease have not 
been correlated with yield loss in the field but the infected stems 
produce poor quality planting material; this material does not 
establish well in the following planting season and thus yields are 
reduced. 
Tuber rot 
Some soil-borne pathogens attack cassava roots, which causes 
damping-off disease at the early stages of growth or soft rot or dry 
rot in tubers prior to harvest. 
Sclerofium rot. Caused by afungus. Sclerotium rolfsii, this is the 
most common tuber rot disease and occurs on roots and tubers at 
all stages of development. It can be recognized by the appearance 
of a white mycelialgrowth on infected roots and tubers (see Figure 
1.6). Asthe fungus penetratesthetubers, the plants begin to show 
mild wilting symptoms. 
Figure 1.6 Sofl rot. These diseases are caused by Phytophthora drechsleri, 
Infected cassava tuber showing Pythiumspp., and Fusarium solani, and occur under wet condi- 
white rnycelial growlh tions and cooler temperatures. The causal organisms attack and 
kill small feeder roots and cause necrotic brown lesions on older 
roots. As the roots decay, they infect the tubers which then emit 
pungent odors. Tubers not promptly harvested are more suscept- 
ible to this type of rot. When roots and tubers rot, the entire plant 
wilts, defoliates anddies. Inthe cool, wet conditions that favorthe 
development of these diseases, losses may be as high as 80%. 
Dry rot. Several fungi cause dry rot, including Fomes (Rigidiporus) 
lignosus, Armillariella mellea, Rosellinia necatrix and Botryo- 
diplodia theobromae. The disease usually occurs on land that has 
recently been cleared of trees and shrubs. lnfected tubers are 
typically covered with rhizomorphs (thread-like network of myce- 
lia) of the fungus. The plant wilts, but does not shed its leaves; 
eventually, the entire plant dehydrates, turns brown and appears 
scorched. 
Pests 
Vertebrate pests 
There are two major vertebrate pests of cassava: the African 
bushfowl, Francolinus bicalcaratus bicalcaratus, and the cane rat, 
Thryonomys sweinderianus. 
Bushfowl become pests only after the tubers have been formed 
and after grain crops have been harvested. They peck at the soil 
with their beak until contact is made with the tubers, upon which 
they feed. Tubers damaged in this way are easily invaded by rot- 
causing micro-organisms, leading to their total loss. In highly 
infested areas, tuber loss resulting from bushfowl damage may be 
as high as 30%. 
Cane rats eat cassava stems and tubers. They dig at the tubers, 
and the wounds made on large tubers during feeding become 
sources of infection for the smaller tubers. On unprotectedf arms, 
yield losses can be as high as 40%. 
Nematodes 
At least 45 genera and species of nematodes are known to be 
associated with cassava. They infect the roots and render them 
more susceptibleto rot-causingorganisms.T he root knot nematode, 
Meloidogyne incognita, is a particularly serious problem in Africa's 
cassava-growing areas. Other Meloidogyne species repolled on 
cassava include M. javanica, M. hapla and M. arenaria. Root tips 
of infected plants are devitalized and their growth halted. 
The lesion nematode, Pratylenchus brachyurus, the spiral 
nematode, Helicotylenchus erythrinae, and the reniform nematode, 
Rotylenchulus reniforms, are also found on cassava. An attack by 
these pests causes the plant to lose vigor, and the resulting yield 
losses range between 17 and 50%. 
Mites 
The most important cassava pests in Africa are cassava green 
mite (CGM) and red spider mite (RSM). CGM was first reported 
in Uganda in 1972: it has since spread rapidly over much of Africa. 
Two species of CGM occur in Africa, Mononychellusprogresivus 
and M. tanajoa. The damage caused by CGM appears initially on 
the surface of developing and newly formed leaves. Symptoms 
vary from a few chlorotic spots to complete chlorosis and may be 
mistakenforACMVsymptoms. Heavily attacked leaves are stunted 
and deformed. Mite incidence is high during the dly season and 
may lead to a tuber yield loss of 20 to 80%, depending on the 
severity of the attack. 
There are four species of RSM in Africa: Oligonychus gossypii, 
Tetranychus telarinus, T. neocaledonicus and T. cinnabarinus. 
The pest is visible to the naked eye as a red speck with four pairs 
of legs. Symptoms of attack appear on the upper surface of fully 
mature leavesaschloroticpinpricksalong the mainvein; these pin 
pricks may increase to cover the whole leaf, turning the surface 
reddish-brown. Aprotective webisusually seenonthe leaf. Under 
severe attack, the leaves may die and be shed. infestation starts 
in the dry season, and it is during this season that most damage is 
done. 
Insects 
There are at least six major insect pests of cassava in Africa: the 
cassava mealybug, Phenacoccus manihott the variegated grass- 
hopper, Zonocerus variegatus; the elegant grasshopper, Z. ele- 
gans; the cassava scale insect, Aonidomytilus albus; the coreid 
bug, Pseudotheraptusdevastans;a nd the whitefly, Bemisia tabaci. 
Other pestsinclude thestriped mealybug, Ferrisia virgafa, and the 
green mealybug, Phenacoccus madeirensis. 
Cassava mealybug (CM). This is a very serious pest in Africa. 
It is indigenous to South America but was accidentally introduced 
into Africa in the early 1970sthrough vegetative planting material. 
Its incidence has been repolted in almost all cassava-growing 
areas in Africa. 
Figure 1.7 
Mealybug infestation of cassava leaves 
The mealybug sucks sap from cassava tissue. Initially, it attacks 
the terminal ends of cassava shoots; later, it spreads to the petiole 
and expanded leaves (seeFigure I. 7). The shoot stunting and the 
resultant shorteningo f the internodes are believedt o be caused by 
a toxigenic substance present in the insect's saliva. In cases of 
severe infestation, green shoots die but die-back may not occur. 
A distinct dry season is required for a build-up of the mealybug 
population; drought stress and high temperatures favor pest 
incidence. Tuber loss resulting from mealybug infestation has 
been estimated at 75%. 
Variegated ande legant grasshoppers. The variegated grass- 
hopper occurs in West and East Africa. The elegant grasshopper 
is found mainly in southern Africa, as far north as Angola and 
Mozambique. 
Both species are serious cassava pests. They feed on the leaves 
(see Figure 1.8), petioles and green shoots, and strip the stem 
down tothe pith (seeFigure 1.9). They are particularly devastating 
when the dry season is prolonged. Yield loss resulting from 
defoliation and bark feeding has been estimated at about 60% 
especially if the crop is infested in the first 7 months of growth. 
Cassava scale insect. Found in West and East Africa, these Figure 1.8 
insects cover first the lower stem of cassava plants and then the Cassava plant showing elegant 
leaves and petioles. They occasionally kill the host plant if it has grasshopper attack 
already been weakened by other pests and drought. 
Coreid bug. These sap-sucking bugs are believed to be partly 
responsible for the spread of CAD in Zai're and the Congo. They 
carry enough inoculum, either internally or in a crude mechanical 
way, to cause CAD, and thedisease is knownt odevelopfromtheir 
feeding points on the plant. 
Whitefly. This insect is the vector of ACMV, and is prevalent 
throughout East, Central and West Africa. The reproduction and 
activity of the whitefly are encouraged by high rainfall, a tempera- 
ture range of 2527% and high light intensity. Under field condi- 
tions, the spread of ACMV by whitefly occurs mainly in April, May 
and June when the population is high. 
Weeds 
Cassavacan be seriously affected by early weed infestation. Slow 
initial growth and development make the plant susceptible to weed 
interference during the first 3 to 4 months after planting. 
Flgure 1.9 
Weed competition in cassava crops reduces canopy develop- Cassava stem stripped down to 
ment, tuberization and tuber number. Reduction in tuber yields the plh following grasshopper attack 
varies from 40% in the early-branching cultivars to nearly 70% in 
the late- or non-branchingc ultivars. Depending on previous use 
of the land, soil fertility status and cultivar, yield losses caused by 
uncontrolled weed growth in cassava can reach 100%. 
At least two properly timed hand weedings are needed when the 
plant population exceeds 10 000 standslha; this is particularly 
important in the case of early-branching cultivars that branch at 
heights of less than 1m . However, most farmers grow cassava at 
a lower plant population, which does not provide effective ground 
cover: under these conditions three weedings are necessary for 
good crop yields. 
Failure to plant cassava at the recommended plant population and 
tocar~youtthefirstweedingin  time contributes to low yields, even 
when improved varieties are used. Delaying the first weeding by 
more than 2 months can cause over 20% reduction in tuber yield, 
even if the crop is subsequently weeded three times. Cassava 
production in areas infested with the weed lmperata cylindrica 
requires four or five weedings to minimize weed-related yield 
losses. 
Improved early-branching cassava cultivars are able to develop 
canopy to shade out weeds if: 
early growth is vigorous 
the crop is kept free from weed competition during the first 3 
to 4 months after planting 
thecropisplantedat aplant populationof not less than 10 000 
standslha 
pests are not a major problem 
environmental conditions and soil fertility status are favorable 
to cassava growth and development 
Among the majorweeds associated with cassava production are 
grasses such as Androfmgon spp., lmperata cylindrica, Panicum 
maximumand Pennisetumspp., and broadleavedw eeds such as 
Commelina spp., Chromolaena odorata, Mimosa invisa, Smilax 
kraussiana and Mucunapuriens. The problemw ith I.c ylindrica is 
not limited to direct yield reduction; this weed also causes mech- 
anical damage to cassava storage roots which provides a route of 
entryforfungiandotherpathogensthatcause tuber rot and reduce 
quality of produce. 
Soil and agronomic factors 
The important soil and agronomic factors that affect cassava 
production are soil temperature and moisture, soil erosion and low 
soil fertility, and poor cultural practices. 
Although cassava has a slightly higher optimum range of soil 
temperature regime than maize or soybean, supra-optimal soil 
temperature (above 30%) can cause significant growth reduction. 
There are also significant yield reductions if drought isfrequent and 
if the crop is grown on soils with a low water-holding capacity. 
Some cassava cultivars tend to promote soil erosion because of a 
slow rate of canopy development. Continuous cultivation of cas- 
sava, without adequate erosion control measures, can result in 
severe and irreversible soil degradation. 
In traditional systems, land preparation starts before the onset of 
the rainy season and consists of clearing the vegetation and 
burning it. Moundsor ridgesare madeat thebeginningofthe rains. 
On sandy soils there is little land preparation; farmers merely slash 
weeds and plant cassava cuttings in relatively undisturbed soil. 
Traditional farmers seldomfollow recommendedculturalp ractices 
for cassava, and may be unaware of the existence of improved 
varieties. The use of unimproved varieties, together with inade- 
quate length and age of planting material and incorrect plant 
population,depth and timeof planting,a reamong the reasonswhy 
yields under most traditional systems are low. The selection of 
good planting material is one of the most important aspects of 
cassava production; the material must be fresh and taken from 
healthy and mature stem portions if high yields are to be realized. 
Socio-economic factors 
The main socio-economic factors affecting cassava production 
relate to inadequate resource allocation, infrastructure and exten- 
sion services. 
Resource allocation 
The shortage of labor, land and capital are important resource 
constraints for cassava production. Recent trends indicate a 
decline in the rural farm population, with the result that farm labor 
is scarce and expensive during critical periods, particularly at 
planting and weeding times. Among the reasons for labor short- 
ages are that young adults are migrating to the cities, children are 
at schoolduringperiodsof peaklabordemand, and thereare fewer 
active farmers among the ageing population in the rural areas. 
In several cassava-growing areas, there are no effective land use 
policies and farm holdings are small. Because of population pres- 
sures, fallow periods have been shortened, leading to more inten- 
sive cultivation of marginal lands; also, cassava is seen as a low 
nutrient-requirement crop aand thus is usually the last crop in the 
rotation, resulting in low yields. 
Lack of capital means that farmers cannot afford the cost of hired 
labor. There is no institutionalized farm credit system to assist 
small farmers (the majority of cassava producers in Africa). This 
situation has limited farm sizes and investment in cassava produc- 
tion and processing. The need to develop improved storage and 
processing facilities is particularly important for cassava as it is 
highly perishable and requires processing before consumption. 
Infrastructure 
The necessary infrastructure, such as adequate water supplies 
and transport and marketing systems, is generally lacking in 
cassava-growing areas, giving producers and processors little 
incentive to expand operations. An inefficient, expensive transport 
system adversely affects inputloutput cost andsupply, reducing 
farmers' potential income from marketing their products. Efficient 
marketingis neededt oget the products to theconsumerat theright 
place and time, in the required form, and at affordable prices. 
Extension and input delivery systems 
To diffuse new technology on cassava production, processinga nd 
utilization among rural farmers, it is necessaly to have an efficient 
extension system. Many farmers are not aware of the availability 
of improved planting materials developed by national programs 
and international research centers, such as IITA. This lack of 
information poses a 'demand side' constraint that can be relieved 
with the provision of informal educational programs for farmers. 
There are also situations where there are 'supply side'constraints. 
Forexample,f armers are aware of the existence of inputs, such as 
insecticides or improved cassava planting materials, but have no 
access to these inputs. Efforts must be made to ensure that inputs 
are available at the right time and in the right place. 
Part II 
STRATEGIES FOR 
OVERCOMING CONSTRAINTS 
UNIT 2 
Morphology and Physiology 
Cassava is a perennial crop, although farmers usually hawest it 
during the first or second year. It is propagated mainly from stem 
cuttings; however, under natural conditions, as well as in the plant 
breeding process, propagation by seed is quite common. When 
cuttings are planted in moist soil under favorable conditions, they 
produce sprouts and adventitious roots within a week. When 
propagated by seed, plant establishment is considerably slower 
and the plant itself is smaller and weaker than that producedfrom 
a stem cutting. 
During the few weeks of growth after emergence or sprouting, the 
shoot lengthens and the roots extend downwards and spread. 
Flowering may begin as early as the sixth week after planting, 
although the exact time of flowering depends on the cultivar and 
the environment. Tuber formation begins in aboutthe eighthweek 
after planting. Leaf area approaches its maximum size in 4-5 
months. The average height of a cassava plant is 2m, but some 
cultivars may reach 4m. 
Classification of cassava varieties 
There are many cultivars or varieties under cultivation. They can 
bedistinguishedb y such morphologicalcharacteristicsa s leaf size 
and shape, plant height, wlorof stem and petiole, tuber shape and 
color, time-to-maturity and yield. 
Cassava varieties are often classified according to the levels of 
cyanogenic gluwsides (hydrocyanic acid, HCN) in the tuber and 
leaves. The major groups are: 
cassavawith high HCN level- 1Omg per IOOgmfreshweight 
or more; an example of this group among the IlTA cultivars is 
TMS 50395 
a cassavawith low HCN level -less than 5mg per IO Ogmfresh 
weight; the HCN is often concentrated in the peel; good 
examples of low HCN cassava among the IlTA cultivars are 
TMS 30001 and TMS 4(2)1425 
a intermediate types, in which the levels of HCN range between 
5 and 10mg per 100gm fresh weight; examples among the 
IlTA cultivars include TMS 30572 and TMS 30555 
Ingeneral, low HCN varietiesoften mature early (6-9 months), but 
the tubers tend to spoil quickly when left in the ground for a long 
time after maturity. High HCN varieties take longer to mature: 
tubers may be harvested 12-18 months after planting and can be 
stored longer in the ground after maturity. 
Root and shoot system 
The cassava plant may be divided into two main pacts, as shown 
in Figure 2.1 : 
the shoot system, which consists of stem, leaves and repro- 
ductive structures or flowers 
the root system, which consists of feeder roots and tuberous 
roots 
Root system 
The cassava plant is established from hardwood cuttings. During 
the first 2-3 weeks of growth, adventitious roots develop at the 
base of the cuttings. These roots later develop into fibrous root 
systems which absorb water and nutrients from the soil. Some 
adventitious roots also develop at the base of the axillary buds or 
at the nodes; these are known as 'nodal roots'. 
Depending on variety and age, fibrous roots may be up to IO Ocm 
long. After 30-60 days, the roots begin to swell, marking the 
, beginning of tuber initiation. The process of tuberization involves 
the onset of secondary thickening in fibrous roots; that is, fibrous 
roots swell as a result of cambium activity. The development of the 
tuber consists mainly of an increase in the diameter of a root. 
The actual number of fibrous roots which eventually form tubers 
Fjgure 2.1 depends on several factors, including genotype, assimilate sup- 
General morphology of the cassava plant ply, photoperiod and temperature. 
Genotype. The number of tubers which are produced varies from 
one variety to another. In general. 5-10 tubers per plant may be 
produced. 
Assimilate supply. Generally, the process of cassava tuberization 
isaffected by assimilate supply (that is, the level of photosynthate 
which is available during tuber initiation). The inl i t ionof tuberiza- 
tion requires a critical percentage of assimilate supply. Therefore, 
any factor which affects assimilate supply will also affect the 
number of tubers which are produced. Some examples of these 
tacfors are moisture stress, soil fertility status, radiation and soil 
temperatures. 
Photoperiod and temperature. Most varieties initiate tubers only 
under short-day conditions. Long days delay tuber initiation and 
thusfewertubersare produced. Long days also tendto encourage B 
I, a 
abundant shoot growth. Photoperiods may affect the hormonal 
balance in the plant; for example, they may influence the level of , , . .  
Gibberellic Acid (GA) and lndole Acetic Acid. Usually, photoperiod 
interacts with temperature, especially night temperature, but var- 
ietal differences in the nature of the interaction are also found. ;: ;;;i~~,, 
3- conical parenchyma 6-m e ~ a r e n c h y ~  
4. phlwm 
The cassava tuber is physiologically inactive andcannot therefore %?&?is B 6bres 
t..l-a, 
be used as planting material. Cassava established from seed first 
develops a tap root system: the radicle grows vertically downward 
and develops into a tap root. Later, adventitious/fibrous roots I 
develop from the upper portion of the tap root. j: 
The cross-sectiono f a young storage root (as illustrated in Figure 
2.2) shows the following dominant features: 
e the periderm, which consists of a few layers of mainly dead tEnlawed ~ i t .hm ail starch prans) 
cells that effectively seal off the surface of the tuber; the peri- 
derm varies in color and may be thick and rough, or thin and 
smooth 
* the cortex, which isthe layer of cells (usually while) just below 
the periderm; the peel of a cassava tuber consists mainly of (I 
the cortex and the outer periderm 
* the flesh, which is the central portion and consists largely of 
storage parenchyma cells; this is the main storage region of 
the plant, where starch grains are deposited; a few xylem 
eh'nents and laticifers occur at random in the starchy flesh 
* Figure 2.2 
the central vascular strands, which consist of xylem bundles Transverse section of ayoung 
and fibers storage root 
13 
Shoot system 
The shoot system develops from axillary buds located on the 
nodeson the cuttings. The numberof shootsthat develop depends 
on several factors, which may include: 
length of cuttings and number of nodes (longer cuttings pro- 
duce more shoots, and cutting orientation affects the number 
and sites of shoots; cuttings planted in a vertical or inclined 
position develop shoots mainly at the basal nodes; those 
planted horizontally may develop shoots at nearly all nodes, 
though often the middle nodes may not develop any shoots) 
size and moisture content of the cutting (large, fresh cuttings 
develop relatively more shoots) 
genotype (some cultivars produce more shoots than others) 
Cassava stems grow up to 4m tall, but dwarf varieties may be only 
l m t all. The stems vary considerably in color (greys, browns or 
silver), and are usually woody with very large pith. The older parts 
of stems consist of prominent knob-like scars which are the nodal 
positions where leaves were originally attached. Each nodal unit 
consists of a node, which subtends a leaf and an internode. The 
rate of node production on each stem is about one node per day 
during early and active growth stages, and about one node per 
week in older plants. 
The internodes vary considerably, depending on varieties and 
environmental conditions. They tend to be long under favorable 
conditions, and short under drought stress; where there is insuffi- 
cient light, they are usually abnormally long. 
Branching. There are two types of branching pattern in most 
varieties growing under normal conditions: 
forking, in which the main stem grows for a while before 
producing (usually) three branches at the apex of the stem; 
after a certain growth period, each branch then produces 
anotherset ofthree branches;forkingoccurs atthe apex of the 
stem when the apical meristem changes to the reproductive 
state, and it is often associated with flowering 
lateral branching, in which branchingoccurson any parlofthe 
main stem at some distance from the apex; branches usually 
arise from one or more leaf axils around the lower portion of 
the stem 
Branching is influencedb y severalfactors, includinggenotypeand 
soil fertility. 
Genotype. The number of nodes which occur before the first 
forking is a function of the variety or genotype. Some clones or 
varieties fork very early, and thus the branches lie close to the 
ground; although this makes weeding diff icun, it does reduce weed 
growth. Some culivars begin to produce branches at a reasonable 
distance above the ground ( l m o r more); the advantage here is 
that the ground beneath the canopy is relatively open and may be 
intercropped with a lowgrowing crop. Where cassava is not 
intercropped, however, weed growth may be a problem. 
Even within the same variety, the branching pattern may vary 
according to environmental conditions. For example, intercrop- 
ping with a more competitive species may alter the branching 
pattern considerably; and where there is competition among crops 
forlight, branching may ocwrat a higher level than inapure stand. 
Time of planting also affects the branching height. 
Soilfertility. The height at which forking occurs may be determined 
by soil fertility. Low soil fertility delays forking, with the result that 
branches usually form at higher stem positions. Some genotypes 
may not produce branches at all where soils are poor. 
Other factors. Water stress and cool temperatures during the 
growth cycle may delay the formation of branches. The level of 
I 
available photosynthate may be a major factor in the formation of young male llowsr 
lateral branches;e xcessphotosynthate,forexample,m ay resunin 
more lateral branches being formed. 
Leaves. Cassava leaves are arranged spirally on the stem (in 
technical terms, the phyllotaxis is a two-fifths spiral). Each leaf is 
subtended by three to five stipules, each about l c m  long. The 
length of the leaf stalk (petiole) varies between 5 and 30cm long. Mature, open 1BmdB IioWBl 
The lamina is simple with a smooth margin but deeply palmate or 
lobed. The number of laminalobes varies between three and nine 
(usually odd numbers). 
Flowering. Cassava is monoecious. Flowering is frequent and 
regular in some cultivars, while in others it is rare or non-existent. 
Mature open malelbrnr 
Cassava flowers are borne on terminal panicles, with the axis of 
the branch being continuous with that of the panicle inflorescence 
(see Figure 2.3). The male flowers occur near the tip, while the 
female flowers occur closer to the base. Each flower, whether Figure 2.3 
female or male, has five united yellowish or reddish sepals but no lnflorescence of a cassava plant 
petals. The male flower has 10 stamens arranged in two whorls of 
five stamens each. The filaments are free and the anthers small. 
The female flower has an ovary mounted on a 10-lobedg landular 
disc. Theovary is3-4cm long and has three locules (each with one 
ovule) and six ridges. The stigma has three lobes which unite to 
form the single style. The female flowers open first, the male 
flowers about a week later. Cross pollination is therefore the rule. 
After pollination and subsequent fertilization, the ovary develops 
into the young fruit, which takes 3-5 months after pollination to 
MATURE CIISSAVA FRUIT mature (see Figure 2.4). The mature fruit is a globular capsule 
(diameter l-1.5cm), with six narrow longitudinalw ings. The woody 
endocarp contains three locules, each with one seed. When the 
fruit is dry, the endocarp splits explosively to release the seeds. 
The cassava seed is ellipsoidal and about 1.5cm long. It has a 
brittle testa which is grey and mottled with dark blotches. There is 
a large caruncle at the micmpylar end of the seed. 
Growth and development 
T R W S Y E R S U  SECTION OF C-VA 
FRUII SHOW ITS nwss 
At high temperatures (24-30°C), the time from appearance to full 
Me-m 
expansion of a given leaf is about 2 weeks. Leaf growth is greatly 
reduced at lowertemperatures. The size of fully expanded leaves 
Endocam increases with the age of the plant; in most varieties, the leaves 
reach their maximum size 4-5 months after planting, after which 
DRY the size decreases. There are great differences in maximum leaf 
CASSAVA FRUIT 
size among varieties; individual leaves in some varieties reach 
800cm2. Leaf size is considerably reduced under adverse environ- 
mental conditions, such as nutrient or water stress. 
The life of individual leaves is usually 60-120 days, but may be as 
long as 200 days, particularly at low temperatures. Drought and 
flooding both cause rapid leaf drop, resulting in shorter leaf life. 
DEHISCENCE OF CASSAVAFRUIT Mutual shading greatly reduces leaf life; in complete darkness, 
leaves may last only 10 days. 
Leaf area index (LAI). This is defined as the leaf area per unit of 
ground area, and is a measure of the leafiness of a crop. In 
general, total leaf area depends on: 
rate of formation of new leaves 
size of individual leaves 
Figure 2.4 
Fruit and seed of a cassava plant longevity of individual leaves 
LA1 in cassava ranges from 3 to 7, depending on variety. Values 
above 7 are very rare; the highest LA1 ever recorded in cassava is 
about 10. In many varieties, LA1 increases as the number and size 
of individual leaves increase, reaching a peak 4- 6 months after 
planting. Thereafter, leaf size and rate of leaf production decrease 
and some leaves die; this marks the beginning of the declining 
phase of LA1 (see Figure 2.5). 
Crop Growth Rate 
(glplanVday) 
12- 
8 - 
4 - 
3 4 5 6 7 8 91011 1213141516171819ZIZl22324 
Age (months) 
3 4 5 6 7 8 910 111213141516171819ZI21222324 
Age (months) 
---- Leaf area 
.-. --- . Total dry matter 
--. .- S
S-upp
tar oltin9 tissue (wood and fibrous mot material) 
ch in storage roots 
Sowca: Modlid homCoun, 1951 
Figure 2.5 
Growth and development in cassava 
In many cases, the decline in LA1 coincides with adry period. After 
the rainy season begins, leaf area increases a second time to a 
maximum which is somewhat less than that of the first season or 
year. Such a pattern has been observed in some improved IlTA 
varieties. The LA1 of TMS 30572, TMS 91934 and TMS 4(2)1425 
planted in June increased to reach apeak in October and declined 
rapidly during the November-March dry season. After the begin- 
ning of the rains late in March, LA1 increased slightly until harvest 
time in June. The increase in LA1 reflects renewed apical activities 
when favorable conditions resume. 
Leaf area duration (LAD). This is defined as the integral of LA1 
overtime and is an important factordetermining storage root yield 
in cassava. Varieties which have longer LAD and relatively high 
LA1 are usually high yielders. Good examples of long LAD varieties 
developed at IlTA are TMS 91934 and TMS 4(2)1425. 
Measurements of photosynthetic rates in cassava leaves show 
values of 15-29mg of CO, dm-2 h-' and 33mg C02 dm-2 h-'. 
However, recent studies at ClAT using improved techniques have 
shown values of up to 40mg dm-2h -' and C0,cornpensation point 
ranging between 50 and 68ppm. This indicates a C3 photo- 
synthetic pathway. 
Dry matter production and partitioning 
The rate of dry matter production follows a similar pattern to that 
shown by LAI, with values increasing to a peak of about 10-12.591 
plantlday after5-6months. Thesevalues represent aCropGrowth 
Rate (CGR) of 70-87.5g/m2/week; this is considerably lower than 
the140-350g/d/week reported for some crops. 
With a good arwunt of solar radiation, CGR may attain a value 
exceeding 120g/m2/week. Values of CGR up to 140glm2/week 
have been achieved in some cassava varieties under high radia- 
tion intensities and long days. 
The optimum LA1 for tuberous root development seems to be 
between 3 and 5. If that value is maintained, tuberous root yield 
can be maximized. At higher LAI, CGR declines mainly because 
of mutual shading. Root growth rate also declines sharply after LA1 
exceeds 4 because, at higher values, less photosynthate is avail- 
able for root growth. 
Tuberous root yield is determined not only by the amount of dry 
matter produced, but also by the pattern of partitioning of the dry 
matter to the different plant parts during growth. In cassava, there 
is simultaneous development of the shoots and storage roots. In 
other words, assimilate supply is partitioned betweengrowthof 
the shoots and tuberous roots, which leads to intensive competi- 
tion between the different parts of the plants. In general. therefore, 
to ensure maximum tuberous root bulking there must be an 
optimum M I .  If the partitioning of assimilate favors shoot growth, 
then there is less dry matter for root bulking, which results in law 
yields. If there istoo little assimilategoing into leaf growth,thenthe 
overall leaf growth will limit photosynthetic production and, again, 
yields will be low. 
This pattern of development differs markedly from that of other 
crops, such as cereals, in which there is phasic development. In 
phasicdevelopment,thephotosyntheticsystem( leaves)develops 
first and the storage system (grains) is filled later. Thus, there is 
little competition for assimilate between the two systems. 
The partitioning of dry matter to the various parts of the plant 
changes considerably during the growth cycle of the crop. For 
example, allocation of dry matter to the tuberous roots varies from 
almost none at the early growth stages of the plant to as much as 
80% of the daily dry matter production at the late growth stages. 
Many experiments which have been conducted to determine the 
pattern of dry matter partitioning to different parts of the plant 
(storage roots, stems or shoots) have shown that the relationship 
between total dry weight and tuberous root weight is linear; this 
suggests that the tuber bulking rate keeps pace with the rate of 
crop growth. 
In describing this linear relationship, two important parameters 
have recently been introduced: 
efficiency ol storage root production (ESRP), which is the 
regression coefficient of the linear equation between storage 
root weight and total weight 
apparent initial start of starch accumulation in tubers (AISS), 
which is the plant weight at which storage root production 
actually starts 
ESRP and AlSS are now becoming extremely useful as selection 
criteria for high yield, replacing Harvest lndex (tuber weight as a 
percentage of total weight). This is because ESRP is constant for 
any length of period (Harvest lndex is time dependent) and both 
ESRP and AlSS give an insight into the growth of cassava before 
and after the beginning of the storage root filling phase. 
Environmental effects on growth and 
development 
Various environmental factors can affect the pattern of growth and 
development in cassava. Long days, for example, may result in a 
marked reduction in storage root yield; low temperatures can 
considerably delay bulking; and drought can hasten the declining 
phase of LAI. In general, however, cassava can be grown in areas 
where the annual rainfall is as low as 750mm and can survive in 
areas with dry seasons as long as 6 months. Because of such 
hardiness, farming families in semi-arid areas rely on cassava as 
a 'famine crop'during the dry season or in times of drought. 
Cassava is able to grow under such extreme conditions because 
it has a very conservative pattern of water use. At the onset of a 
dry season, the production of new leaves is reduced drastically, 
which in turn reducestranspiration. The stomata close as soon as 
they are exposed to dry air, which reduces water loss at the time 
when evapotranspiration is greatest. The reduced leaf area and 
the stomataclosure reduce CGR during periods of drought. 
Other mechanisms ensure that plant growth is not drastically 
affected under drought conditions. These are: 
a heliotropic response mechanism, which allows cassava 
leaves to maximize interceptiono f available sunlight at times 
when transpirational demands are low (for example, in the 
morning and late afternoon the leaves usually turn to face the 
direction of the sun) 
a drooping mechanism, which causes the leaves to droop 
during daily peaks of heat; this reduces the heat load on the 
leaves when the heat is greatest 
an increase in the partitioning of dry matter to the feeder root 
system when plants undergo long periods of drought, which 
enhances the plant's exploitation of soil moisture 
Possible strategy to increase tuberous 
root yield 
Hiah CGR values have been used to Dredict the ceilino vield of 
cGain cassava varieties with differing morphological Zaracter- 
istics. Dry root yield levels of 30ffha have been predicted from 
an ideal plant type. 
In an ideal plant type: 
the largest leaves should have an areaof not less than 500cm2 
first branching should occur not less than 4-6 months after 
planting, so that the leaf canopy develops as high off the 
ground as is practicable 
leaf life should be at least 100days; the higher the capacity of 
the plant to retain leaves, the lower the requirement of dry 
matter in producing and maintaining leaves 
In addition, an attempt could be made to increase root yield by 
increasing CGR. This can be done in two ways: 
by developing and selecting plants in which the leaves have 
a vertical orientation; this would improve light interception 
and help increase yield 
by increasing the photosynthetic rate of individual leaves 
through the incorporation of C4 characteristics into the cas- 
sava plant 
UNIT 3 
Breeding 
-- - -- 
Tiit. ssa. in c j s c x a  breeding is to develop varieties which com- 
3 . i e  i?i;",mi stable yields with good quality characteristics 
:eie;zn!io the ways in which thecrop is utilized in specific regions. 
The objectives of a cassava breeding program should include: 
high yield in terms of dry matter per unit of land area per unit 
time 
resistance to the majordiseases prevalent in target areas (for 
example, ACMV, CBB and CAD) 
resistance lo the major insect pests in target areas (for 
example, CM and CGM) 
improved quality in terms of local consumption requirements 
(for example, low cyanide and mealy varieties in areas where 
the roots are boiled and eaten without further processing) 
adaptability to environmental conditions and cropping sys- 
tems in target areas 
improved plant characteristics in terms of canopy and roots 
Breeding procedures 
Germplasm collection and evaluation 
The most important tasks in any cassava breeding are the acqui- 
sition and selection of superior breeding material. In Africa, there 
is considerable variability among the local germplasm collections. 
There aretwo reasonsforthis. Firstly, someof the materials flower 
and set seed freely, and new cultivars are established from 
volunteer seedlings; because cassava is a cross-pollinated crop, 
continuing recombination and variation occur from outcrosses of 
genetically heterozygous cultivars. Secondly, spontaneous muta- 
tion may give rise to additional genetic variation, although this has 
not been proven. 
Many of the local cultivarsflowerwell. However, some flower only 
to a limited extent ('shy flowering') and others do not flower at all 
under normal growing conditions; this makes their exploitation in 
a breeding program rather limited. 
The systematic introduction of new breeding material from other 
cassava programs (for example, from national or IlTA programs) 
is desirable, especially from areas of similar ecological conditions. 
The African Phytosanitary Council regulations require that intro- 
ductions of new breeding material from outside Africa be confined 
to true seed which has had appropriate chemical and physical 
treatments. However, the movement within the continent of tissue 
culture material which is indexed as being free of pathogens, par- 
ticularly viruses, is permitted by the Council through appropriate 
phytosanitary channels. This is important in order to minimize 
introduction of new diseases and pests in vegetative materials. 
Both the clones developed by a breeding program and those from 
exotic introduction in seed form need to be evaluated in order to 
identify their potential as breeding materialsorasvarieties in terms 
of their agronomiccharacteristics. The agronomic characteristics 
include resistance to diseases and pests, characteristic plant 
architecture, yield, root quality, cyanide content, adaptation to 
agroecology and any additional locally important traits. 
The germplasm may be conserved as clones in field plots, as 
meristem tips in vitro, and/or as seeds in low temperature and 
humidity conditions. 
Source population 
The source population for improvement is made up of genotypes 
which havegenes associatedwithdesirablecharacteristics.These 
may be taken through cyclic recombination and selection proce- 
dures while retaining a high degree of genetic variability. Conven- 
tional methods of creating source population can also be used by 
making crosses between two selected parents. 
Seed production 
As indicated in Unit 2, the stamens and pistils of cassava flowers 
are located in separate flowers on the same inflorescence. The 
female flowers are large, are nearly always located at the base of 
the inflorescence, and open first; the male flowers are small, are 
located at the apical portion of the inflorescence, and usually open 
about a week after the female flowers. Under normal conditions, 
the stigma remains receptive for up to 24 hours after the opening 
of the flowerand dried pollen remainsviablefor about 6daysunder 
controlled conditions. 
Both the stigma and pollen are sticky, and pollination is easily 
carried out by wild bees. Stlucturally and functionally, therefore, 
the cassava flower is well adapted to cross-pollination. 
In the northern hemisphere, cassava usually flowers from July to 
January, with a peak between September and November. In the 
southern hemisphere, it usually flowers from January to July, with 
a peak between March and May. The time of flowering, however, 
depends to a large extent on rainfall distribution, day-length and 
temperature. 
In general, there is a vegetative phase of 1-4 months in most 
cultivars that flower under natural conditions, making it important 
to plant cassava at least 4 months before the peak flowering 
period. In order to synchronize the flowering periods of different 
cultivars or clones, parental genotypes should be planted every 
2-3 months because flowerina-  on an individual ~ l a nuts ually lasts 
for more than 2 months. 
For pollination by hand, pollen is collected early in the morning 
before 1O .OOh and pollination made before 13.00h. Both male and 
female flower sthat areonthe point of opening are used. When the 
anthers are mature, they change from green to yellow, and this 
change in color is a useful indication of when pollen can be 
collected. 
Pollination can be made by hand using the male flower after 
removing the petals or, for mass pollination, by using an applica- 
tor. The applicator can be made from a stick with the tip covered 
with an adhesive piece of velvet-like material to which the pollen 
will readily adhere (see Figure 3.1). Several flowers can be polli- 
nated without recharging the applicator. If the applicator is to be 
usedfor other pollen parents, it should be sterilized; this is done by 
by dipping it into alcohol before using it for new parents. The Figure3.1 
pollinated flowers are bagged with cloth or paper bags (white) to Pollination by hand 
protect them against bees or other insects carrying foreign pollen 
(see Figure 3.2); the bags are rernaved 5 days later. 
Figure 3.2 
Bagged pollinated flowers 
Becausecassava is normally across-pollinated plant, outcrossing 
can occur among selected parents in isolation. There should be 
1-3 selected parent plants per isolation plot, with several replica- 
tions to provide an equal chance of crossing. 
Seeds mature about 70-90 days after pollination. The fruits are 
collected when the coats begin to shrivel and are dried under the 
sunorinanovenat 40-5O0Cuntiltheys hatter. Fruitsfrom isolation 
plots are collected in cloth bags hung on cassava plants for each 
variety or clone and left there until they shatter, releasing hybrid 
seeds which are ready for germination. 
Seed germination and transplanting 
Cassava seeds have a very short dormancy period or, in some 
cases, none at all. Seeds germinate quickly at optimal soil temp- 
eratures (30-35°C) and moisture regimes. Scarification is usually 
unnecessary, but seeds from related wild species can be scarified 
by rubbing them gently on the micropyle with a rough stone or 
sandpaper. 
Seeds may be sown in peat pellets, jiffy pots or plastic bags 
arranged on nursery beds during the dry season. During the first 
3 weeks, the nursery beds are irrigated twice daily, in the rnornings 
and afternoons; thereafter, they are irrigated at regular intervals 
until the transplanting stage. If irrigation is not possible, seeds can 
be planted soon after the first rain. The seeds germinate 10-30 
days after planting and are ready for transplanting when they are 
15-20cm high (see Figure 3.33. Because cassava seedlings are 
weak and grow slowly, weed control is very important at the early 
stages of growth to offset competilion. 
Figure 3.3 
Seedlings growing in nursely 
The field into which the seedlings are to be transplanted is plowed, 
disc harrowed, and divided into 5m-wide beds; if erosion is not a 
problem, the field may be flat with no beds. The seedlings are 
planted at 40cm x 50cm and irrigated until the rains begin. Wider 
spacing can be used if land and labor are not constraints. As many 
as 50 000 seedlings may be produced in any one year. 
Breeding scheme 
To achieve the programobjectives, the IITA breeding scheme 
may be modified lo suit local conditions (see Figure 3.4 overleafj. 
First year. During the growing period, the seedlings are screened 
for resistance to the major diseases and insect pests at 1, 3, and 
6 months after planting in the field. In the case of ACMV, the 
seedlings are exposed to a high population of whitefly (vector of 
ACMV) from spreader varieties planted alongside the nursery. If 
50 000-100 000 seedlings 
Year 1 
Scrccn for res s13ncu to o scac6.s 
an0 nsecls conformal on and roo! 
Source1 Population characteristics. 
500-3000 clones 
year 1 1 I I I ( / 1 Planted single row I m l ong 
Clonal Evaluation 
Further screening for disease and 
insect resistance, conformation 
and root characteristics 
50-100 clones 
Preliminary Meld Trial 
J- Further screening for disease and 
insect resistance, conformation 
and root characteristics I 
20-25 clones 
Year 4 Planted in four rows 10m long 
with four replications in 
three to four locations 
Advanced Meld Trial 
I Further evaluation for yield 
10-15 best clones 
Planted in four rows eachlom 
Year 5 long with four replications. 
Testing locations increased to 10. 
Uniform Meld Trial 
I J- Evaluation for yield and adaptation 
to wide range of environments 
5 elite clones 
Selected for on-farm level 
Year 6 testing and farmer evaluation 
Year Multiplication and release as variety 
(usually by appropriate release committee) 
Figure 3.4 
IITA cassava breeding scheme 
environmental conditions are favorable for disease development, 
Ihe seedlings are also screened for resistance to CBB under 
natural epiphytotic conditions; if not, they are inoculated with CBB 
inoculum using a stem puncture method. Seedlings are also 
screened for pubescence, which is associated with resistance to 
CM and CGM. Towards the end of the rainv season.,  thev, are cut 
back to induce the production of young shdots and are screened 
lor resistance to CM and CGM. Resistant seedlings are selected 
and tagged 
At 3 months after planting, seedlings are also tested for cyanide 
levels using the leaf picrate method, and the low-cyanide seed- 
lings are selected. Seedlings -with a low branching habit (branch- 
ing heights of below 50cm), which is associated with early flower- 
ing, are discarded. At 12 months, all the selected materials are 
harvested and furtherselection is made basedon root shapes, root 
size, number of roots per plant, neck length and poundability. The 
seedlings with a short neck (1-3cm) and uniform short, compact, 
lat roots are selected. 
Second year. The selected seedlings, which may number up to 
3000, are cloned and planted for clonal evaluation in a single row 
plot of 3-5 plants, at lm2s pacing. A standard local variety is 
planted every 10 clones for comparison. At this stage, the obser- 
vations made during the first year on diseases, pests and confor- 
mation are confirmed for each clone; at 1, 3 and 6 months after 
planting, each clone is scored for ACMV and CBB. Later in the 
year, the clones are also assessed for insect damage, particularly 
by CM and GSM. Agronomiccharacters such as branching height 
and angle, canopy spread and the number of stems per plant are 
also scored. 
At harvest (12 months after planting), the individual clones are 
again assessed on the basis of the number of plants which have 
survived, the number of tubers per plant, tuber shapes and size, 
tuber neck-length,t otal tuber yield (kgiplot) and theoverall appear- 
ance of the tubers. The clones which perform poorly in terms of 
establishment,g rowth and resistancet odiseases and insect pests 
are discarded. Only promising clones are further evaluated for dry 
matter, yield potential and other quality characters. Clones se- 
lected for low-cyanide content are further evaluated quantitatively 
forcyanide, using the leaf picrate method or an enzymatic assay 
method. 
Third year.'The best 50-100 clones selected through clonal 
evaluation in the previous year are put through a preliminary yield 
trial in single rows 1O m long with two replications. At this stage, the 
clones are evaluated again for yield, disease and pest resistance, 
root characteristics, conformation, dry matter and consumer ac- 
ceptance qualities. 
Fourth year. The most promising 20-25 clones from the prelimi- 
nary yield trial carried out during the third year are moved to an 
advanced yield trial in four rows, each 1O m long, with four replica- 
tions. Only the lwo central rows are hawested for yield estimation. 
The trials are conducted at three or four locations, representing a 
wide range of environments. The clones are further evaluated for 
tuber yield, disease and pest resistance, dry matter content, 
consumer acceptance qualities and ecological adaptation. 
Fifth year. Based on performance in the advanced yield trial of the 
previous year, the best 10-15 clones are advanced to a uniform 
yield trial. The number of testing sites is increased to 10 and the 
clones are thoroughly evaluated for yield, dry matter content, 
consumer acceptance qualities and ecological adaptation. The 
trials are planted in four-row plots, each 10m long, with four 
replications at each location. Only the two central rows are har- 
vested for yield estimate. 
Sixth and seventh years. Uniform yield trials may be carried out 
for a further year or two in order to confirm the adaptability of the 
clones in the various locations. However, during the sixth year, 
five elite clones from the uniform yield trial are advanced to farm- 
level testing with farmers' participation. 
Clonesfound to be most popularwith farmers are multiplied during 
the seventh year. They are then distributed through established 
national channels. 
Rapid Multiplication 
The phrase 'multiplication ratio' refers to the increase in planting 
material aver what is planted. Cassava is a vegetatively propa- 
gated crop with low multiplication ratios. For example, when a 
cassava stem cutting (25-30cm long) is planted, it gives about 10 
stem cuttings 12 months later; thus the multiplication ratio is I : 10. 
This is low compared with a maize plantwhich may yield acobwith 
about 300 seeds (multiplication ratio 1 : 300). 
Thephrase'rapid multiplication'is used todescribeatechniquefor 
overcoming the handicapof low multiplicationr atios in vegetatively 
propagated crops. It involves using improvedt echniques to rapidly 
increase the quantitiesof planting materialsfrornwhal isavailable. 
In addition to increasing the multiplication ratio of cassava, rapid 
multiplication technique may also be used in other cases. 
1. National programsand internationala griculturalcenters,such 
as IITA, which are involved in cassava breeding can increase 
the few plants of a variety available afier the breeding selec- 
tion process through the rapid multiplication technique. This 
'breeder seed' is high yielding, disease- and pest-resistant 
and of high quality. Institutions, including national seed com- 
panies, and farmers who receive breeder seed can also 
multiply materials supplied to them through this technique. 
2. At certain stages in the breeding program, it is necessary to 
evaluate the materials in multilocational trials or in on-farm 
trials in several locations. The rapid multiplicationt echnique 
may be used to produce enough clean, healthy materials for 
such trials. Resarchers plant speciai multiplication plots to 
produce such materials for the following year's trials in a 
process known as 'back-up multiplication'. The materials 
produced may also be used in other trials (for example, 
agronomic trials). 
3. Healthy, improvedclones received by nationalprogramsfrom 
research centers may be multiplied using the rapid multiplica- 
tion technique to generate enough materials for national 
evaluation. Vegetatively propagated crops such as cassava 
cannot be transferred across country borders unless they are 
certified as being free from diseases and pests by the Plant 
Quarantine Service. IlTA has perfected its tissue culture 
techniques which are used to produce and multiply disease- 
and pest-free cassava plantlets for distribution to collaborat- 
ing national programs. This material first has to be evaluated 
throughout the country by the national program before it can 
be recommended for adoption. The evaluation requires a lot 
of planting materials, which can be obtained through rapid 
multiplication. 
4. The rapid multiplication technique may be used to multiply 
quantities of improved varieties available for distribution to 
farmers in areas where major disease and pest outbreaks, 
such as CBB and CM, havewiped out several hectaresof sus- 
ceptible cassava varieties. 
Principles of rapid multiplication 
The rapid multiplication technique utilizes certain basic morpho- 
logical characteristics of the cassava plant. Examples of these 
characteristics are the dormant axillarv buds which are located at 
the nodes, andthe factthatthe lowest portion of the stem is oldest. 
has agreaterdiameter and more food reserves, and is harderthan 
the ocher portions of the stem; in a typical cassava stem the 
hardwood, semi-mature and soft green portions (the latter is the 
youngest part of the stem) are easily distinguished. 
The basic principles of rapid multiplication of cassava are: 
each axillary bud on the stem can develop into a shoot if apical 
dominance is removed 
the whole stem portion of the plant is utilized 
stem production is the goal (tuber production is of secondary 
interest) 
only clean, healthy, disease- andpest-free stems are usedfor 
multiplication 
clean, healthy planting materials are produced 
Rapid multiplication technique 
Preparation of ministem cuttings 
The stem is cut into several small pieces. Each piece should have 
one or more nodes, depending on the portion of the plant from 
which it is cut. Those pieces cut from the hardwood portion may 
have one or two nodes; those from the semi-mature portion may 
have four to six nodes; and thosefrom the tip portion may have six 
to ten nodes. The number of nodes on a cutting is not rigid and 
depends on such factors as internode length, diameter, age of the 
plant, and weather conditions at and after planting. 
These stem pieces are termed 'ministem cuttings'. Those cut from 
the hardwood stem portion are called 'hardwood rninistem cut- 
tings'; those from the semi-matured portion are called 'semi- 
matured ministem cuttings'; and those from the top green stem 
portion are called 'tip shoots' or 'tip shoot ministem cuttings' (see 
Figure 4.1 .) 
Figure 4.1 
Ministem cuttings: tip shoot (left), semi-matured (centre) 
and hardwood (right) 
The hardwood and semi-matured ministem cuttings are prepared 
using secateurs (small or big), a machete or a hand saw: the tip 
shoots are prepared using secateurs or sharp knives (see Figure 
4.2). Tools must be sharp to ensurecleanlinessof the cut ends.All 
the leaves on the tip shoots, apart from the youngest, unopened 
ones, are removed anddropped intowaterto prevent dehydration. 
Planting ministem cuttings in the nursery 
Ministemcuttings can be planted in nursery beds with well-drained 
soils neara sourceofwater, or instrong blackpolythene bagsfilled 
with good-qualityg arden soil and perforatedon the sides and at the 
bottom to facilitate drainage. Poor-quality bags may break when 
they are filled with soil or moved from one location to another. 
Hardwood ministem cuttings. These cuttings are planted, ei!her 
in nursery beds at a spacing of 10 x 10crn or in black polythene 
bags, at a depth of about 4-5crr1. Adherence to recommended 
planting depths is important; cuttings which are planted too shal- 
low are exposed after water has been applied a few tln-ies and 
become dehydrated. 
The orientation of the cunings is such that two opposite nodes are 
on the right and left sides when buried. This is to avoid the 
placement of one of the nodes at the deepest level, as shoots 
Figure 4.2 developing from such nodes struggle to emerge and are usually 
Examples of tools used to prepare weak and fragile at the base. Such weak seedlings break at trans- 
ministem cunings: big secateurs (left), planting. 
small secateurs (centre) and a hand 
saw (r~ght) Semi-matured ministem cuttings. These cuttings are usually 
7-1Ocm long. They are planted vertically at a spacing of 10 x 1Ocm 
Figure 4.3 
Semi-matured ministem cuttings 
planted in a nursery bed 
in the nursery beds or in polythene bags filled with soil, with two- 
thirds of each cutting buried in the soil. The oldest endof thecutting 
is the buried portion. Figure 4.3 is an illustration of semi-matured 
ministem cuttings planted in a nursery bed. 
Tip shoot minisfem cuttings. The tip shoot ministem cuttings are 
planted in a similar manner to the semi-matured cuttings, at a 
spacing of 10 x 1O cm with two-thirds of each cutting buried in the 
soil. They can be planted in nursery bedsor inpolythene bagsfilled 
with soil. 
Nursery maintenance and care 
The following steps are recommended for proper nursery main- 
tenance and care of planted cuttings: 
1. Apply water to the nursery beds and the potted plants imme- 
diately after planting. Thereafter, limitthe applicationof water Figure 4.4 
to twice a day, once in the morning and once in the evening. Hardwood ministem cuttings showing root 
Soil and atmospheric conditions can affect the frequency of and shoot growth (planted horizontally) 
water application (for example, after agood rain it may not be 
necessaryto apply water as too much water may cause some 
cuttings to rot). 
2. Provide labels stating the variety and date of plantingf or each 
nursery bed or group of potted plants. 
3. Remove by hand any weeds which appear in the beds or the 
bags. 
4. Cover any cuttings which are exposed as a result of water 
application with soil. 
Sprouting and establishment 
The ministem cuttings (especially the hardwood and the semi- 
matured cuttings) sprout 7-10 days after planting. Fibrous roots 
develop at the buried nodes and at the oldest ends of the cuttings. 
The shoots later emerge from the soil and continue to develop 
leaves (see Figures 4.4, 4.5. 4.6 and 4.7) 
The highest plant establishment is obtained from the hardwood 
cuttings, while the tip shoot cuttings usually give the lowest estab- Figure 4.5 
lishment. Tip shoots prepared from field plants usually perform Semi-matured ministem cunings showing 
poorly because they are very young; however, they can be root and shwt g r o w  (planted vertically) 
prepared from shoots which develop from the planted rninistem 
cuttings 8-10 weeks after planting in the nursery. 
Figure 4.6 Figure 4.7 
Semi-matured ministem cuttings Hardwood ministem cuttings growing in the nursery 
growing in polythene bags 
Transplanting 
After they have been in the nursery for 4-6 weeks, the ministem 
cuttings are transplanted into the field. Transplanting is carried out 
in the dry season using irrigation, or in the rainy season when no 
irrigation is necessary. Waterlogged fields are avoided; the per- 
centage of survival or establishment is low in such fields because 
of pooraeration and poor root development. Removalof the plants 
from the nursery beds is done carefully, using hand-trowels or 
hand-forks to avoid damage to the roots. 
The following operations are performed before the sprouted plants 
are removed from the nursery: 
1. The plants are hardened by reducing the amount and fre- 
quency of water application 1-2 weeks before transplanting. 
2. Water is applied heavily on the evening before transplanting. 
3. Water is then appliedagain in the morning on the day of trans- 
planting. 
4. The field is prepared and made ready for transplanting by one 
of the following methods: plowing and harrowing; slashiw the 
top growth and applying herbicides to kill the ;egetation; or 
laying plastic mulch after either of the above preparations 
(irrigation is used before laying the plastic mulch if transplant- 
ing is done in the dry season). 
Thespacing betweenplantsis either 100x 50cmor50x 50cm. For 
transplanting potted plants, holes must be dug after the desired 
spacing is marked because a ball of soil is retained with the plant. 
The soil around each transplanted plant is firmed and the plot is 
then labeledw ith asignboard showing the variety, date of planting 
and number of hectares planted. 
Field maintenance 
Proper field maintenance after transplanting is essential if strong, 
healthy planting materials are to be produced. 
Weed control must be properly carried out during the first 10 
weeks, using such methods as hoeingorapplying herbicides. With 
the use of plastic mulch, weeding is limited, but any weeds that 
develop near the plants must be removed. At transplanting, the 
holes cut through the plastic mulch for planting must be small to 
prevent heavy weed growth. 
Other advantages of the laying plastic mulch are that: 
it allows larger hectarages of land to be put under cassava 
multiplication with greater success because it ensures both 
good plant establishment and vigorous plant growth, parlicu- 
larly in the initial growth stages: the advantage of limited 
weeding which is associated with the useof plastic mulch en- 
courages the planting of large hectarages to cassava for 
multiplication 
there is a higher yield of cassava stems 
soil erosion is reduced and thus there is better soil moisture 
conservation 
Rogueing the off-types (or mixtures) is done during the early 
stages and any vacanciescreated as a result of the death of some 
plants are filled. This promotes bettercanopy cover, which in turn 
helps suppress weed growth. Ferlilizer (NPK) is applied where 
necessary. 
New rapid multiplication technique 
The rapid multiplication method discussed above is a widely used 
and effective method. Latest research, however, has resulted in 
a major improvement: ministem cuttings can be nursed in poly- 
thene bags without soil, thus providing a quicker, less expensive 
and more convenient method. 
Under the method described above, ministem cuttings are nursed 
for4-6 weeks in polythene bags or nursery beds filled with garden 
soil before they are transplanted into the field. Large quantities of 
soil (over 5 tons on an oven-dried basis) are needed to nurse 
cuttings for planting in 1 hectare, and the soil usually has to be 
excavatedfrom another site and transportedt o the nursery. About 
50 man-days are required to fill the bags with soil to nurse cuttings 
for planting in 1 hectare; additional labor is needed to plant one 
cutting per bag (20 000or more plants per hectare) and to care for 
the plants prior to transplanting. 
The planting materials are bulky and heavy totransport to the field, 
and the soil used could carry disease-causing organisms such as 
nematodes,fungi and bacteria. Sterilizingthe soilto overcomethis 
problem is expensive and facilities to do this are not easily 
available. 
With the new rapid multiplication technique, the ministem cuttings 
are dipped into a fungicidehater suspension. They are then put 
directly intoperforatedpolytheneb agsandstored inashadedarea 
or under a roof. The bags are tied with pieces of string, leaving 
about one-third of the top space empty to allow for aeration. 
Various sizes of bags can be used, as long as they are not 
completely filled. 
Depending on the cassava variety, 95.100% of sprouting occurs 
in 3-5days. In an experiment carried out inTogo, 100% sprouting 
was achieved with the variety 'Nakoko' in 2-3 days, but some 
varieties may require afew more days to give a large percentage 
of sprouting. High humidity and temperature inside the polythene 
bag promote a rapid and uniform sprouting. In recent experiments, 
the sprouted ministem cuttings established well in the field at 8 
weeks after transplanting, as shown in Table 4.1. 
.- 
I he new technique has other advantages: the ministem cuttings 
can be stored for a few days, fairly large numbers can be carried 
by hand or transported over long distances with a limited space 
requirement, and they can be used for mechanical planting. 
Table 4.1. 
Percentage of field establishment for cassava ministem cuttings 
pre-sprouted in perforated polythene bags 
Cassava Condition of Number of Percent 
variety materials before ministems establishment 
transplanting planted (8 WAT)' 
TMS 4(2)1425 Sprouted with 950 
shoots and roots 
TMS 4(2)1245 Sprouted with 1260 89.4 
shoots only 
TMS 50395 Sprouted with 420 86.9 
shoots only 
Note: WAT - wee& anw fran~planiing 
Source: IlTA Annud Repon and Rwearch Highligma. 1987188 
Harvesting the stems 
If the field is maintained properly, stems can be cut and supplied 
to farmers or institutions 6-7 months after transplanting. As the 
objective of rapid multiplicationo f cassava is to produce stems, the 
plants are not uprooted at harvest. They are cut at a height of 20- 
25cm from the ground after it has been ascertained that they are 
physiologically mature and pest- and disease-free. 
This practice of leaving the stumps standing in the field after 
harvest is known as 'ratooning'. Several shoots sprout from a 
ratoon left in the field but these are reduced to two or three. 
Herbicide and fertilizer are applied to the ratooned plots. Another 
set of stems is cut again about 6 months later. At IITA, as many 
as three ratoons have been taken from rapid multiplicationp lants. 
The number of ratoons is influenced by several factors, including 
variety, soil type and fertility, weed control and field maintenance. 
After harvest, the stems are tied together in bundles; in Nigeria, 
these bundles consist of 50 stems, each l m l ong, and it is in this 
form that the stems are sold. Stems must be handled with care 
throughout the harvesting, loading, transporting and unloading 
procedures, toavoidtoo much bruising. If axillary budsare bruised, 
they may never develop into plants if the nodes are used in rapid 
multiplication. 
Distribution 
Multiplication of planting materials per se is not enough unless 
steps are taken to ensure their effective distribution to the farmers 
or institutions for whom the materials were multiplied. Cassava 
stems are bulky and do not store well for a long time. Their 
transportation and distribution, therefore, deserve special effort by 
those people who are responsible for making the materials avail- 
able to farmers. Some farmers who need the improved varieties 
will go to the sources of supply and collect them. Many farmers, 
however, lack the means to go to the sources or may not be aware 
of the existence of superior varieties. 
Planting materials can be effectively distributed using one or more 
of the following channels: 
special government and/or donor-assisted agricultural or 
multiplication projects 
strategically located National Seed Service multiplication 
centers of Ministries of Agriculture 
private and mission agricultural projects 
school farming projects 
agricultural meetings (such as incountry training courses, 
farmers' field days and agricultural shows) 
transporting planting materials in trucks and vehicles to vil- 
lages and farms 
demonstration plots 
multilocational on-farmtrials where the varieties are supplied 
to farmers for testing, with the farmers retaining the good var- 
ieties 
farmer-to-farmer movement of planting materials 
Storage 
As planting materials, it is important for cassava stems to be 
properly stored. Long-terms torage is not possible because stems 
dehydrate during storage. They are also attacked by insects and 
diseases, which results in a lower sprouting percentage. 
Storage of cassava stems is necessary when: 
the plants are harvested for tubers off-season and the stems 
need to be preserved for planting some weeks later 
a farmer acquires stems for planting before hislher field is 
ready for planting 
the stems, especially of improved varieties, are sold by farm- 
ers on the roadside and thus must be stored properly during 
the period of sale 
Storage methods 
Cassava stems can be stored effectively in one of three ways. 
1. The stems are tied into bundles and stored upright under a 
roof, in a well-ventilated shed or under a well-developed tree 
providinggood shade (seeFigure4.8). Theoldest endsof the 
stems are inserted in soil, and water may be applied to the 
base. Stemscan be stored in thisway for up to about 8 weeks. 
Flgure 4.8 
2. The oldest ends of im-long cassava stems are inserted Cassava stems stored upright 
upright into the soil in a cool, well-shaded area. The basal 
portions of the stems should touch each another. The stems 
are inserted so that they lean on astrong support (atree stem 
or bamboo stick) which has been tied horizontally between 
two trees afew meters apart (see Figure 4.9). 
3. Stems are stored horizontally under well-developed tree 
shade for up to about 8 weeks. 
Flgure 4.9 
Cassava stems stored on a horizontal 
support system 
Precautions 
When storing cassava stems, there are a number of important 
points to be borne in mind: 
avoid direct sunlight and hot or cold winds 
let the buds face upwards when stems are stored vertically 
long stems store better than short ones 
use mature stems from healthy cassava plants or plantations 
the viability of stems under storage depends on a number of 
factors, including the plant variety, the storage methods and 
conditions, the length of storage and the quality of planting 
material 
UNIT 5 
Tissue Culture 
Tissue culture is a means ofgrowinga plant'scells ortissues under 
controlled conditions. It may be defined as the culture of single 
plant cells, a group of cells, tissues or organs in an artificial 
environment under aseptic conditions. Under such a manipulated 
environment, the cells, tissues and organs multiply and continue 
to grown in an unorganized way or regenerate into a whole plant. 
The phrase in vitro, which means 'growingoutsidet he living body, 
in an artificial environment' is often used in association with tissue 
culture. 
The traditional method of propagating cassava is by using stem 
cuttings. However, the risk attached to this rnethod is that many 
diseases and insects can persist in the stem cuttings and be 
carried over from one vegetative generation to the next; examples 
of such d~seasesa re ACMV and CBB. This has imporlant implica- 
tions in the collection and maintenance of healthy germplasm for 
breeding purposes and the movement of cassava clones across 
national and international borders. 
In the t rad t~naalp proach, germplasm collections of vegetatively 
propagated crops are grown inthefield each season. This requires 
many hectares of land, involves considerable labor costs and 
leads to a significant loss of germplasm materials as a resun of 
insect damage, disease attack and other unpredictable environ- 
mental factors. In comparison, tissue culture offers an approach 
for the safe storage and maintenance of germplasm in an in vitro 
environment; in vifrorapidmultiplicationcanproducel argeamounts 
of planting material and is not restricted by seasonal changes. 
Meristem andlor shoot-tip culture is the most effective method for 
virus elimination in a wide range of crop species. When placed on 
a suitable culture medium and incubated under favorable condi- 
lions, the isolated meristems regenerate into plantlets. Using 
various vilus indexing methods, the regenerated plants are then 
indexedf or freedom fromvirus infections. Plants which are regen- 
erated in this way usually retain thecharacteristicsof their mother 
plant, thus making this a very useful method for cleaning up 
disease-infested material for distribution localyl as well as inter- 
nationally. 
Culture media composition 
The success of in vitro culture is largely the result of a better 
understanding of the nutritional requirements of cultured cells and 
tissues. Culture mediumcomposition is one of the most important 
factors that determines the success of the culture. 
The components of plant tissue culture media include inorganic 
salts, plant growth regulators, vitamins, amino acids, complex 
organic supplements, carbohydrate, water and the medium ma- 
trix. 
There are a number of formulated media which are used in either 
basic or in modified form by tissue culure workers. Some of these 
formulated media, including Heller's, Nitsch's. White's and Mu- 
rashige and Skoog's media (MS), are commercially manufactured. 
Others may be prepared by using stock solutions, examples of 
which are   resented below. 
Composition of stock solutions for the 
MS medium 
The MSmediumconsistsof morethan 15different chemicals. The 
quantity of eachchemical required forthe preparation varies; in the 
case of some chemicals, the requirement is minute. 
Stock solutions which are prepared at a higher concentration (10  
or 100 times) are used to increase accuracy and convenience 
when preparing media. 
Stock solution I 
mgll 
NH,NO, 33000 = 339 
KNO, 38000 = 389 
CaC1,.2H20 8800 = 8.8g 
Mg S0,.7H20 7400 = 7.49 
K H, PO, 3400 = 3.49 
Stock solution II 
mgll 
Stock solutlon Ill 
mgll 
Fe SO,. 7H,O 5560 = 5.569 
Na,. EDTA. 2HO,  7460 = 7.469 
Vitamin mixture stock solutl~n 
Thiamine hydrochloride 10 = 0.01g 
Pyridoxine 50 = 0.059 
Nicotinic acid amide 50 = 0.059 
Glycine 100 = 0.lg 
Stock solution for growth regulators 
Most of the growth regulatorsdissolve in dilute NaOH or HCI, 95% 
ethanol or distilled water with heating. 
NAA stock solution. Using an analytical balance, weigh 1Omg of 
NAA and dissolve it in a few drops of 0.5N NaOH. Add distilled 
water to make the solution up to 100ml. Mix the solution well and 
store the mixture in a refrigerator. The solution gives 0.1 mgof NAA 
per ml of solution used. 
BAP and GA, stock solutions. Measure 1O rng of the respective 
chemicals and dissolve them separately with 95% ethanol. Add 
distilled water to make the solution up to 100ml. Mix the solution 
well and store it in a ref rigerator. The solution gives O.lmg of BAP 
or GA, per ml of solution used. 
Dilution of stock solution of growth regulators. If the quantity 
required is less than O.lmg, the solutions are diluted by 10 or 100 
tihes, to give more accurate measurements 
Modified forms of the MS media are commonly used for tissue 
culture. The following are used for cassava meristem culture. 
Cassava meristem culture medla 
(for 1 #Her media) 
Stock solution I 
Stock solution II 
Stock solution Ill 
Vitamin stock solution 
Sucrose 
lnositol 
Adenine sulfate 
Naphthalene acetic acid (NAA) 
Benzyl amino purine (BAP) 
Gibberellic acid (GA,) 
Agar 
Multiplication media for cassava are simpler than the media used 
for meristem culture because the size of the plant material used in 
multiplication is much larger. 
Multiplicatlon medium for cassava 
(for 1 liter medium) 
Stock solution I 
Stock solution II 
Stock solution Ill 
Sucrose 
Vitamin stock solution 
lnositol 
NAA 
BAP 
Agar 
Culture media preparation 
If acommercially produced medium is not used, stock solutions of 
macro-elements, micro-elements, vitamins and growth regulators 
are prepared and stored in the refrigerator,w hile vitamins must be 
kept in the freezer. The chemicals used for such preparations are 
analytical grade, and double distilled water is used to ensure that 
the purity of the media is improved. However, for routine tissue 
culture work, refined grocery sugar is generally sufficiently pure 
andcan be used as acarbon source insteadof sucrose. Examples 
of culture media preparation are presented below. 
A. Procedure for media preparation using a ready-made me- 
dium package (1 pack for 1 liter culture media) 
1. Dissolve the powder in 500ml of distilledw ater in a 1-liter 
beaker 
2. Add 309 sucrose 
3. Add additives (e.g. growth regulators NAA O.lmg, BAP 
0.05mg, GA, 0.02mg) 
4. Add distilled water to over 900ml mark 
5. Adjust pH to 5.7k0.1 with dropwise of 0.5N NaOH or 0.5N 
HCI 
6. Make up final volume to 1 liter 
7. Put solution in erlymeyerflask(s) and add 0.6-1% agar if 
solid media is preferred 
8. Meil the agar 
9. Distribute to culture tubes 
10. Autoclave at 121°C for 15 minutes (ift o be poured into 
pre-sterilized culture containers, media are autoclaved 
first for 15-20 minutes) 
11. Let cool and solidify 
B. Procedure for the preparation of culture media using a plant 
salt mixture package (1 pack for 1 liter culture media) 
1. Dissolve powder in 500ml of distilled water in a 1-liter 
beaker 
2. Add 5ml vitamin stock solution 
3. Add 100mg inositol 
4. Follow the rest of the procedure in A from step 2 
C. Procedure for the preparation of culture media using stock 
solutions (to prepare 1 liter of culture medium) 
1. Fill a 1-liter beaker with 200-300ml distilled water 
2. Pipette in 50ml of stock I 
3. Pipette in 5ml of stock II 
4. Pipette in lOml of stock Ill 
5. Follow the rest of the procedure in B from step 2 
After autoclaving, the culture media are stored in a transfer room 
or ina refrigerator inaplastic bag. Someof the additiveswhich are 
heat liable and not suitable for autoclavecan be sterilized using an 
autoclaved millipore finer. 
Procedure for cassava meristem-tip culture 
technique 
The procedure for cassava meristem-tip culture technique is 
described here (see Figure 5.1). 
Mother plant 
1. Obtain woody cuttings from vigorously growing plants in the 
Exdsed field. Wash cuttinas thorouahlv and disinfect with dilute chlo- 
meristem rox solution by immersing ihe  cuttings in the solution for 5 
minutes. 
2. Plant cuttingsinsterilesoil (chemically treatedsoil) in pots and 
place them in an isolated place, such asa greenhouse. Ap- 
a ply water to the soil; avoid watering the stem and leaf parts. It 
is recommended that insecticide be sprayed once a week to 
prevent infestation. 
Meristem in 
culture medium 3. Transfer sprouted plants to a growth chamber, with a regu- 
lated day and night temperature of 37°C and with 12 hours 
U photop&iod, for 1 month. 
4. Remove apical buds from the mother plant and transfer them 
Leaf developed tothe laboratory inacontainerwith asmall quantity of distilled 
water. While both apical and lateral buds may be used for 
meristem culture, the successful rate of plantlets regenerated 
from a lateral bud is low compared with that of the apical bud. 
U 5. Discard the distilled water and take the materials to the 
transfercabinet. Disinfect budswith 70% ethanol for 3-5 rnin- 
Shoot developed utes,followed by 1O %sodium hypochlorite solution with afew 
drops of detergent for 20 minutes. The buds always float on 
the surface of the disinfectant so it is advisable to agitate the 
container once every few minutes to promote contact and 
penetration. 
Plantlet 6. Discard the sodium hypochlorite solution and rinse the buds 
(Ready for transplanting) with three changes of sterile distilled water at 5- minute inter- 
vals to remove the disinfectant. 
Figure 5.1 
Process of meristem cunure andplantlet 7. Remove the buds from the container using a sterile forceps 
development and transferthem to a sterile petri dish with sterile filter paper 
or to the stage of a dissecting microscope which has been 
disinfected with 70% ethanol. The forceps is sterilized by Plantlet grown in an 
dipping it in 70% ethanol and flaming with a spirit lamp. insect-free isolation room 
where they are indexed 
for freedom from viruses 
8. Place the petridishunderthedissecting microscope and, with 
the aid of a sterilizeddissecting needle and scalpel, gradually 
remove the leaf primordia until the meristem is excised. Use 
the needle totransferthe meristemtothe cuiture media. Only 
a small proportiono f the meristem is embedded in the media, 
leaving a greater proportion above the surface of the media. 
9. Label the culture tube and/or container with the appropriate 
variety number and the date of culturing. 
10. Incubate the cultures in a culture room with a temperature 
range of 25-28°C and 12 hours photoperiod. Plantlets can be 
obtained afler 8-10 weeks. 
Procedure for multiplication 
The procedure for multiplication is as follows (see Figure 5.2): 
1. Obtain lcm-long single node cuttings consisting of the bud 
and part of the petiole and stem from the green stem of a 
cassava plant. 
2. Place the nodes in a container and disinfect with 70% ethanol 
for 5 minutes and 10% sodium hypochlorite with a few drops 
of detergent for 20 minutes; then rinse with three changes of 
sterile distilled water. 
3. Remove the nodes from the container with a sterile forceps 
and place them in a sterile petri dish with sterile filter paperto 
remove the excess water. 
4. Place the nodes in the culture media and incubate in the 
cuiture room. 
After 5 weeks, plantlets of four or five nodes are obtained and can +d istribution 
be transplanted for hardening and eventually planted in the field, 
or packed for international distribution, or used for further multi- 
plication. If the material isforfurthermultiplication, it issubcultured 
by removing the plantlets from the culture tube, cutting them into 
several one- or two-node cuttings under the laminar+flowc abinet, Figure 5.2 
and transferring to fresh culture media. It is estimated that the Rapid multiplication of disease-free 
multiplication ratio of cassava is 5 per 6 weeks. cassava for distribution 
Distribution and handling of tissue 
culture material 
Distribution 
Tissue culture materials are distributed as plantlets in test tubes 
after has been ascertained that they are free from the diseases 
and insect pests of the original parent material. This is important 
because of the threat of many diseases and insects being spread 
by the use of vegetative propagating material. The test tubes are 
packed in a cardboard box together with a phytosanitarty certifi- 
cate, the import permit, a shipment form and a booklet explaining 
recommendations for handling tissue culture material. Materials 
intended for transplanting are packed separately; in the case of 
cassava, these materials include jiffy peat pellets, vermiculite and 
jiffy pots. 
Handling during transportation 
Transportation time should be as short as possible. A prolonged 
dark period (in the box) results in a low survival rate. If the journey 
takes longer than 4 days, exposure of the tube to light (not direct 
sunlight) during transit is required. Temperatures below 1O °C and 
above 40°C are to be avoided, and the package should be kept in 
an upright position and protected from rain and direct sunlight. 
Receipt of material 
The tissue culture materials should be transplanted as soon as 
possibleaflerreceipt. Ifthisisnot possible, it is advisable tounpack 
the box and place the cultures under sufficient light (not direct 
sunlight), at temperatures between 20 and 30°C. 
Plants in tissue culture are adapted to high relative humidity, 
almost 100% RH, and thus materials must be transplanted in an 
environment with high humidity. Transplanted materials probably 
have less epicuticular wax and their vascular development be- 
tween root and shoot may not be complete. These two factors 
increase water loss and restrict water transport respectively. 
A simple humidity chamber can be constructed using plywood, 
nails and covers made of transparent plastic sheets (see Figure 
5.3).T he humidity chamber is placed in shade and, if possible, 
Figure 5.3 
A humidity chamber 
the temperature is maintained at 25.35%. The humidity inside the 
chamber can be maintained by spraying water to saturate the air. 
Factors affecting the survival rate 
Selection d cultureand pre-treatment. Select wlturesthat are 
in good condition and use plantlets that are at least 3cm tall and 
have weli-developed root systems. In cassava, certain practices 
have been used to strenghthen the root system. These include 
exposing the cultures to higher light intensify, and loosening the 
culture caps to gradually decrease the humidity in the tube before 
transplanting. 
Handling from tube lo substratum. Transplanting the plants 
fromtube to substratum requiresgreat care. It is advisable to use 
a blunt-end forceps (a pointed one might damage the plant) to 
bring out the plant from the tube. Avoid breakage of the stem and 
especially of the root system. 
Humidity control. Before transplanting the materials from the 
tube to the soil, a humidity chamber is made ready. The humidity 
in the chamber can be maintained by spraying water twice a day 
tosaturate the air. The humidity is maintained at almost 100% RH 
for at least 3 days and then may be decreased gradually. 
Temperature. The humidity chamber is placed in shade, 
preferably in a glasshouse.The temperature range ismaintained 
at 25-35%. If a glasshouse is not available, it is advisable to put 
the chamber under a tree or any other shade to avoid direct 
sunlight. The humidity chamber may also be placed on a labora- 
tory bench but some artificial lighting would be required. 
Watering. This is a very impoltant factor in the survival of the 
Dlantlets. It is recommendedt hat sufficient water is supplied each 
day. Avoid over-watering and flooding of the humiditychamber. 
After transplanting to the field, irrigation is necessary. 
Transplanting cassava plantlets 
The procedure for transplanting cassava plantlets is described 
below. 
1. Prepare the humidity chamber and place under shade 
2. Soak the jiffy peat pellets in water; the peat pellet attains its 
final volume after soaking for about 3 hours. 
Fi.a ure 5.4 
Removing the plantlet from the tube 3. Remove the net from the pellet and break the peat moss into 
fine pieces. 
4. Mix two parts of the peat moss with one part of vermiculite. 
5. Write a label to indicate variety number and dateof transplant- 
ing. 
6. Hali fill the jiffy pot with vermiculite and peat moss mixture. 
7. Remove the screw cap from the culture tube. 
8. Hold the tube in the right hand and gently tap it with the left 
hand until the plantlet is hail way out of the tube; if necessary, 
use a blunt-end forceps to assist in the operation (see Figure 
5.4). 
9. When the plantlet is out of the tube, do not hold its stem; 
holding the stem increases the possibility of breaking the 
whole root system from the stem (see Figure 5.5). Allow the 
plantlet to rest on the palm. If the culture medium remains 
attached to the root, place the palm with the plantlet in water 
and shake gently to remove the medium. 
Figure 5.5 10. Hold the plantlet over the half-filled jiffy pot with the roots 
Handling the plantlet hanging inside the pot. Add the vermiculite and peat moss 
mixture until the roots and the base of stem are covered. 
Press the mixture very gently to allow slight compaction. 
Insert the label in the jiffy pot. 
11. Immediately after transplanting, place the jiffy pot in the 
humidity chamber. 
12. Spray the humidity chamber with distilled water (if available), 
or cooled boiled tap water, to saturate the air. 
13. Make sure that the chamber is closed properly. 
14. During the first week of transplanting, spray the humidity 
chamber twice a day and water the plants once a day. 
15. Water the plants with fungicide mixture (Benlate 5g1500ml) 
every other day to prevent fungal growth. 
16. One week after transplanting, spray the humidity chamber 
only once a day and water the plants once a day. 
17. Two weeks after transplanting, remove the plants from the 
humidity chamber, break the jiffy pots and transplant the 
plants intopotsorplasticbagsfilledwithordinaryorsterilesoil. 
18. Keep the plants in the pots or plastic bags under shade and 
water them once a day. 
19. Two weeks after planting the plants in the pots or plastic bags, 
they can be planted in the field. At this stage, irrigation is very 
important. Any drought occurring at this stage will destroy the 
plants. 
UNIT 6 
Agronomy 
The agronomic practices associated with cassava are discussed 
in this unit under the headings land preparation, planting, inter- 
cropping and harvesting. 
Land preparation 
Cassava production requires good soil preparation. Land prepa- 
ration practices vary considerably, depending mainly on climate, 
soil type, vegetation, topography and degree of mechanization. 
Where no mechanization is available and cassava is grown as the 
first crop afterclearingf orest, no land preparation is required other 
than the removal of the forest growth by cutting down small trees, 
shrubs and vines, and cutting off the branches of large trees to 
admit sunlight. Trees and bushes are piled and burned at the end 
of the dry season. When the first rains have softened the ground, 
the soil is loosened with a hoe, planting stickor sharp instrument 
SO that the cassava stem cuttings can be planted easily. 
The field may be prepared as mounds, ridges, flat-tilled or zero- 
tilled, depending upon soil type and drainage (see Figures 6.1,6.2 
and 6.3 overleaf). The size of the ridges or mounds and the place- 
ment of crops on them are influenced by drainage. Thus, water- 
loving crops such as rice may be placed between mounds or ridges 
in areas prone to water-logging, while cassava, maize and leg- 
umes may be planted on the sides and/or tops of the mounds or 
ridges. 
Cassavacultivation on mounds is common in West Africa. The top 
soil is gathered into more or less conical heaps at various points 
in the field. Mounds that are specifically made for cassava range 
from 30 to 60cm high; on average, they are lower than those 
prepared for yam but have much broader bases. This is perhaps 
a recognition of the fact that cassava tubers tend to spread more 
widely and penetrate less deeply than yam tubers. The space 
between mounds varies from 0.6 to 2m. 
Figure 6.1 
Cassava growing on mounds 
Figure 6.2 
Cassava growing on ridges 
Where mechanization is available, many cassava growers plow 
and harrow the land to prepare a good seedbed. Plowing may be 
to a depth of 25cm. The cassava is planted on the flat, on ridges 
or in furrows. For planting on the flat, the cuttings are inserted 
directly into the land after i t  has been harrowed. For planting on 
ridges or furrows, the land is ridged or furrowed after harrowing. 
Figure 6.3 
Cassava growing on the flat 
Planting material 
Cassava propagation material is vulnerable to adverse climatic 
conditions, as well asto pestsand diseases. When exposed to the 
sun after cutting, i t  can lose viability quickly through dehydration; 
on the other hand, excessive moisture may cause bud sprouting. 
Pathogens and pests are also common causes for poor sprouting 
after planting. Sprouting is better if stemcuttings harvested sholtly 
before planting are used, rather than stored stem cuttings. Also. 
there are varietal differences in the sprouting vigor of stem cut- 
tings, which are emphasized if the storage period is extended. (In 
this publication, 'stem cutting' is used instead of 'stake'todescribe 
cassava planting material.) 
For the best results in any cassava production enterprise, fresh 
stemcuttingsfrom mature plantsare ideal. However, ifthey arenot 
available because of cold, prolonged drought or excess moisture, 
producers have to depend on reliable methods to preserve them. 
Those stored in a dry, well-ventilated, shaded area where direct 
sunlight and dampness are avoided maintain their viability longer. 
Quality of planting material 
The quality of cassava stem cuttings depends on stem age, thick- 
ness, number of nodes per stem cutting (size) and health. Control 
of these factors is essential for the sprouting of vigorous plants 
capable of producing a good number of roots. 
Age of the stem. In general, cuttings taken from the older, more 
mature parts of the stem give a better yield than those taken from 
the younger portions. Although cuttings from green stems will 
sprout, they are extremely susceptible to attack by soil-borne 
pathogens and sucking insects. Also, the immature herbaceous 
green stems cannot be stored fora long period because they have 
a high water content and tend to dehydrate rapidly. 
When stem cuttings are taken from plants more than 18 months 
old, the stem is highly lignified and contains only small amounts of 
food reserves for the shoots. This adversely affects storage 
quality, root and shoot formation, and development, and the 
sprouting buds will have reduced viability. This is manifested in 
delayed sprouting and/or the production of shoots with little vigor. 
It is recommended that planting material be taken from plants 
which are between 8 and 18 months old. 
Thicknessof cultings. Although any part of thecassava stem can 
be used for propagating material in a commercial operation, thin 
stems which have poor nutrient reserves should not be used. This 
is because the shoots which develop from such stems tend to be 
weak and only a few, small tuberous roots are produced. 
As a general rule, it is recommended that the thickness of the 
stems used for cuttings is not less than 1.5 times the diameter of 
the thickest part of the stem of the particular variety being used. 
Number of nodesper cutting.The nodes on cuttings are import- 
ant asoriginsof shootsand, if buried,of roots. Acassavaplant may 
be obtained from a very small cutting with only one bud, but the 
possibilities of sprouting under field conditions are low, especially 
when soil moisture is limited. Cuttings with one to three nodes 
have low percentages of sprouting under field conditions because 
they are short and thus have lower food reserves and are more 
susceptible to pathogen attack and rapid dehydration. Cuttings 
with few buds are more likely to lose the viability of all their buds 
during propagation, transportation and planting. Long cuttings 
with more than 10 nodestheoretically have a betterchanceof con- 
serving their viability because of the greater number of buds. 
Long cuttings have been reported to give higher yields than short 
ones, presumably becausethe former had more buried nodes than 
the latter and thus produced more stems and leaves, which 
resulted in higher yields. However, when long cuttings are used, 
much more propagating material per unit of surface area is 
required. 
The recommendation is that cuttings should have five to seven 
nodes and a minimum length of 20cm. 
Health of cutting. Propagation material should be selected from 
disease-free plants. Generally, cassava stands from which plant- 
ing material isto beobtained should becut asclose to plantingtime 
as possible. It is very important to avoid rough handling when 
cutting and transporting the selected stems or branches. The 
epidermis and buds of cuttings may be bruised or damaged by 
friction and machete wounds during preparation, transportation, 
storage and planting; each wound is a potential site of entry for 
micro-organisms that cause rot during storage or after planting. 
The cut is made with a well-sharpened machete or circular saw. 
Fungicide treatment may be appliedas a protectant whenapplying 
insecticide to control the insect pestsfound on thecuttings. This is 
not common practice among cassava farmers. 
Planting 
Cassava cuttings may be planted upright or at an angle in the soil. 
or horizontally beneath the soil, as follows: 
for planting in the vertical position, the cutting is usually 
inserted so that about iwo-thirds of its length isbeneath the 
soil 
for planting at an angle, about two-thirds of the length of the 
cutting is beneath the soil, and the angle of the cutting to the 
soil surface varies from just slightly above horizontal to about 
60' 
forplanting horizontally, the cutting is inserted horizontally so 
that the entire cutting lies beneath the soil; depth of planting 
varies from 5 to 20cm but is usually about 1O crn 
The orientation of the cutting influences several growth character- 
istics of the plant. Cuttings planted vertically sprout and develop 
appreciable foliage slightly more rapidly than do angled and 
horizontal plantings. Vertical plantings produce deeper storage 
roots than angled plantings; horizontal plantings produce the 
shallowest storage roots. Storage roots produced by vertical or 
angled plantings are more compactly arranged, and more difficult 
to haWest by pulling, than those resuiting from horizontal plant- 
ings. 
Most mechanical planters in use today are designed to plant 
horizontally. The machine opens a furrow, the cutting is dropped 
horizontally, and soil is placed over the cutting. 
Experience in many cassava-growing areas of different countries 
has shown that: 
in areasof rnediumto heavy soilswithadequate rainfall (1000- 
2000mmlyear) it does not matter whether cuttings are planted 
horizontally,vertically or at an angle because the moisture will 
be adequate for sprouting 
in areas of sandy soils or erratic rainfall, vertical planting is 
safest; cuttings which are 20-30cm long will have at least 
15-20cmi nthesoilandthus have bettercontactwith available 
moisture 
Time of planting 
The aim in deciding time of planting is to ensure maximum 
utilization of the growing season. Cassava is planted as early as 
possible after the beginning of the rains or just before the main 
rains begin. Delayed planting leads to considerable reduction in 
yield. When planted early, the cutting sprouts, establishes well, 
and receives sufficient moisture for growth during the growing 
season; this enables the plant to withstand attack by diseases and 
pests later in the season. In Nigeria, the ideal time for planting 
cassava in most years is AprilIMay. 
Depth of planting 
Thedepthof planting mustbe regulatedaccordingt othe prevailing 
environmental conditions. Too much exposure of the cuttings in 
areas where soil moisture is below optimum can result in poor 
stands and, hence, low yields. 
A good practical rule is that where there are dry sandy soils, 
cassava cuttings should be plantedfairly deep, and where the soil 
is moist and heavy, the planting depth should be fairly shallow. In 
the latter case, it should be remembered that deep planting will 
make harvesting difficult and increase productionc osts; however, 
deep planting is advisable in areas prone to termite attacks. 
Plant population 
Optimum plant density of cassava is largely dependent on edaphic 
and climatic factors, cassava varieties, soil feltility, cultural prac- 
tices and the final utilization of the roots. 
In traditional systems, cassava is often grown as an intercrop 
among yams, maize, bananas and melons. The distance between 
cassava plants depends on how much space is taken up by the 
other crops, but in general the distances range from 1 to 4m. 
Where cassava is grown as a monocrop, the rows and the spacing 
within the rows are both 80-100cm apart. Although there is no 
universal spacing recommendation which can be applied to all 
cassava-growing areas in Africa, a population of between 10 000 
and 15 000 plantslha generally gives a good crop of cassava. 
Weed control 
Like many othercrops grown in the tropics, cassava is susceptible 
toearlyweedcompetition. Slow initialdevelopmentof sproutsfrom 
cassava cuttings makes all cassava cultivars susceptible to weed 
interference during the first 3-4 months after planting. 
Improved cuttings from cultivars with early branching habits are 
able to develop canopies which will reduce weed growth i f :  
sprouts from cuttings are vigorous 
m the cmp is kept free fmmweed competitionduring the first 
3-4 months after planting 
m the plant population is not less than 10 000 plantslha 
there is low pressure from diseases and pests 
m environmental conditions and soil fertility status are favorable 
to cassava growth and development; when conditions are 
less than adequate or canopy fails to provide sufficient cover, 
weed problems could be as severe as in other arable crops. 


Table 6.1. 
Nutr ients removed b y  cassava grown on different t ypes  o f  so i l  in 
Madagascar 
Soll Portion Nutrient removal in kglha Starch Mean 
type of plant content of  yield 
N P K Ca Mg roots (%) (Vha) 
Young, 
fenile, root 153 17 185 25 6 29 42 
alluvial wood 100 11 65 17 23 
soils 
Total 253 28 250 42 29 
Lateritic 
clay root 178 20 91 26 3 23.5 26 
soils wood 107 16 31 30 9 
Total 285 36 122 56 12 
Laterites, 
high in 
phosphate root 138 28 24 47 6 16 8 
lowin wood 108 23 12 42 30 
potash 
Total 246 52 36 89 36 
Souros; Jacob. A and H. vion Uexkuil 1983'NulriSan and Manuting of Tropicalcrw' 
Table 6.2 
Equivalent amounts  o f  nutr ients (kglha) removed b y  cassava culti. 
vars  and y a m  species th rough c rop  harvest in Nigeria expressed as 
fertil izers 
Cassava cultivars Yam species 
53101 60506 D. slafa D.rofundata 
(var. Efuru) 
Tuber dry matter yield (kgiha) 7370 9350 9034 12133 
Ammonium sulphate (21%N) 129 176 609 738 
Single superphosphate (18% P,O,) 89 115 215 232 
Muriate of potash (60% K,O) 142 228 323 352 
Source: Oblgbesan. G.O. 1977 'Nuirkional Problems in Root Crops Produnion' in Procssdings of the 
Fin1 Nationaiseminaron R o n  andTuber Cmpr. Umudike. March, 1987 
The figures show that cassava requires large quantities of nutri- 
ents and will respond to fertilizer treatment when grown on low- 
fertility soils. Like all starch or sugar-producing plants, cassava 
requires nitrogen, phosphate and large quantities of potash. 
Nitrogen. Cassava requires a considerable amount of nitrogen. 
Nitrogen occurs in the soil in various forms. It is readily available 
in the form of NO,-N and can be leached into lower layers of the 
soil, particularly by rain. Nitrogen deficiency can be easily recog- 
nized by stunted growth of the plant; the leaves are narrow and 
pale green, with the discoloration starting at the leaf tips and 
margins, and they are shed prematurely. 
Sufficient nitrogen is needed to develop a large bulkof foliage and 
lhus an extensive assimilating area is a pre-requisite for good 
development of the tubers. However, excessive application of 
nitrogen, without the simultaneous application of phosphate and 
especially potash, may promote leaf and stem growth without a 
corresponding increase in tuber yield, or may result in lowertuber 
yield. 
Phosphorus. This is important for the development of the root 
system. Although cassava has modest requirement for phos- 
phorus, its response to phosphorus application under field condi- 
lions is low and varies greatly on different soils. Phosphorus 
deficiency can be recognized by stunted growth and a violet 
discoloration of the leaves. 
Potassium. Although cassava removes large quantities of potas- 
sium from soils, an adequate supply of nitrogen and phosphorus 
seems to be more important in producing good tuber yield than a 
large supply of potassium. 
The symptoms of potash deficiency begin withstunted growth; the 
leaf color is often dark and then gradually becomes paler. Dry, 
brown spots develop from the tips and margins of the leaves. In 
Ihefinalstage, necrosisoccuronthemarginsoftheleaves. Potash 
deliciency results not only in reduced yields and a lower starch 
content but also has an unfavorable effect on root quality. 
Application. A satisfactory balance between nitrogen and potas- 
sium in the fertilizer mixture is important in fertilizing cassava. The 
interaction of the various nutrients applied needs tobe considered. 
Inliming the applicationof nitrrogen, it must be borne in mindthat 
nitrogen fertilizers are easily leached out by rain, and thus it may 
be more expedient to postpone the application until the plants are 
well developed. 
lntercropping 
Multiple cropping (growing two or more crop species on the same 
field in the same year) is almost the rule in tropical agricultural 
systems; cassava is rarely grown as a sole crop except on a few 
large-scale mechanized farms. 
Muliplecropping includes intercropping( growingt woor more crop 
SDecies simultaneouslv on the same ~ i e coef  land) and seauential 
intercropping two or more'crop species, one aiter the 
other, on the same piece of land in one year). 
lntercropping is the most dominant multiple cropping system in 
most parts of the humid tropics, especially under rainfed condi. 
tions. It is associated with shifting cultivation or rotational bush 
fallow in which farm land is abandoned after 2-3 years of cropping 
so as to allow it to revert to natural fallow, a method used to 
maintain soil fertility. 
lntercropping may be practised as: 
mixed intercropping (growing two or more crop species in an 
irregular arrangement) 
row intercropping( growing two or morecropspecies in awell. 
defined row arrangement) 
stripintercropping (growingt wo or more crop species in strips 
wide enough to allow independent cultivation and yet narrow 
enough to induce crop interactions) 
relay intercropping (planting one or more crop species within 
an established crop so that the final stage of the first crop 
coincides with the initial development of the other crop or 
crops) 
Of these various types of intercropping, the most common one 
practised in the cassava-growing areas of the humid tropics is 
mixed intercropping. 
Because the humid tropics are characterized by high rainfall (that 
is, where rainfall exceeds potential evapotranspiration for 5 or 
more months in the year) and thick vegetation cover, soil manage- 
ment in traditional aclriculture is such that the topsoil is covered by 
the canopies of a multispecific crop mixture. in  such a system'. 
opening up new farm land is done with simple tools, usually a hoe, 
which disturbonly the topsoil. Some large trees and palm trees are 
left, but the rest of the cleared land is burnt, leaving ash mulch on 
the soil. Soil erosion and pest and disease incidence is reduced 
by growing a mixture of crops with varying canopy configurations. 
Yields are maintained at a fairly stable but low level, while the soil 
fertility status is maintained by fallowing. Farmerstend to adapt to 
changes in soil fertility by planting those crops which require most 
nutrientsfirst (suchcrops include maize, yamandplantains); tuber 
and legume crops, which have a lower nutrient requirement, are 
planted later. 
The advantages of intercropping are: 
higher gross returns per unit area of land 
yield stability 
satisfaction of family dietary requirements 
* control Of pests, diseases, weeds and erosion 
r more even distribution of labor 
The disadvantages of intercropping include: 
difficulty in mechanizing planting and harvesting operations 
r difficultyi napplyingferlilizersandpesticides in mixedcultures 
difficulty in managing experiments (these are usually more 
complex in intercropping than in sole cropping) 
Cassava is usually intercropped with vegetables, plantation crops, 
yam, sweet potato, melon, maize, rice and legumes. The inter- 
cropping patterndepends on the environmentalconditionsa nd the 
food preferences of the region. 
Cassava-based intercropping systems can be divided into simple 
mixtures (which consist of only two crop species) and complex 
mixtures (which consist of three, four or more crop species). As a 
long-duration crop (9-18 months), cassava is well suited to inter- 
croppingwith short-duration crops such as maize, cowpea, ground- 
nut, melon, okro, rice, cocoyam and several leafy vegetables. 
In simple mixtures, arable crops are usually selected on the basis 
of differences in growth habit and time of maturity. For example, 
cassava (slow initial growth, 9-18 months to maturity) is often 
grown with maize (rapid growth, about 100-120days to maturity), 
cowpea, melon (rapid growth, 70-80 days to maturity), groundnut 
(rapid growth, 120 days to maturity) or okro (harvested over a 
period of 50-100 days). 
Higher returns and a greater number of calories have been ob- 
tainedfromthefollowingcomplex mixtures: cassava/maizelmelon; 
cassava/maize/okro/melon; cassava/maize/okro/cowpea; and 
yam/rnaize/cowpea. 
These complex mixtures are also knownto suppress infestationb y 
weeds, reduce soil temperature, retain higher soil moisture up to 
a depth of about 20cm and produce higher organic matter than in 
the case of sole cropping or simple mixture intercropping. Nutrient 
loss resulting from erosion under complex mixtures is less than in 
sole cropping. 
Harvesting and yields 
The exact time for harvesting a cassava crop depends on several 
factors- the cultivar,t he rainfall,soilconditions andthe tempera- 
ture regime. 
It is best to harvest cassava at a time when the tubers are old 
enough to have accumulated a sufficient amount of starch but not 
so old as to have become excessively woody or fibrous. Late- 
maturing cassava cultivars are ready for harvesting 12 months 
after planting, while some early-maturing cultivars are ready at 7 
months. 
Table 6.3 
Effect of time of harvest on yield of different varieties (kglplot) 
Variety Time of harvest (months) Mean 
(Uha) 
12 15 18 21 24 
Mean (Vha) 11.5 15.5 16.7 14.8 13.2 14.4 
LSDP = (0.15)13.4kg/plot 
Sour-: Hahn e! d. 1979'Cassava Improvement in Afnca' Feid Cmps Rssearch' 2:193~226 
Studies have shown that several cassavacultivars attain optimum 
fresh weight at about 18 months after planting. This corresponds 
to the time of highest starch accumulation. The effect of the time 
of harvesting on yield and on the percentage of starch for four 
cassava varieties is shown in Tables 6.3 and 6.4 respectively. 
In practice, cassava plots are seldom harvested all at once or all 
at the recommendedt ime of harvesting. The main reason forthis 
is that the cassavatuber is highly perishable and, once it has been 
harvested, cannot be kept in good condition for more than 2 days 
after harvesting. Therefore, farmers harvest the amount of tubers 
that they require for immediate use, leaving the remaining tubers 
unharvested until needed. 
Table 6.4 
Effect of time of harvest on percentage of starch 
Variety Time of harvest (months) Mean 
12  15 18  21 24 
Mean 17.2 20.6 22.6 15.6 17.1 18.6 
LSD (P = 0.05) = 5.2 (variety) 
Source: Hahn eld. 1979'Cassava lmomvemant in Africa' Fsld Crma Research 2:193~226 
In traditional agriculture, hand-harvesting, an extremely laborious 
process, isthe rule. Limited mechanical harvestingo f cassava has 
been reported but no satisfactory cassava harvester has yet been 
develo~ed. 
In handharvesting, a machete is used to cut off the stem a few 
centimeters above the ground. The soil around the tuber is then 
loosened, using the machete, and the stub of the stem is pulled to 
lift out the tuber. 
Whatever harvesting method is used, the task is easier when the 
soil is wet. It also tends to be easier if planting is done on ridges or 
beds and in loose or sandy soils, rather than on flat ground and in 
clay or heavy soils. 
Yield, resistance to major pests and diseases, HCN content and 
other characteristics of some improved llTA cassava varieties are 
presented in Table 6.5. 
Table 6.5 
Main characteristics of some improved IlTA cassava varieties' 
Variety Av. Percent Dry HCN Resistance to' Garifi- Gari3 
yield dry yield rngil00g cation quality 
tiha matter Vha CMV CBB CGM CM 
TMS 50207 23.2 28.0 6.5 6.6 2.0 1.7 3.3 3.5 17.0 G 
TMS 4(2)1425 21.4 34.4 7.4 3.3 1.8 1.8 2.3 2.8 22.5 VG 
TMS 50395 20.4 27.5 5.6 10.7 1.8 1.7 3.0 3.5 18.5 G 
TMS 30337 20.4 28.0 5.7 6.5 2.3 1.7 3.8 3.0 15.0 M 
TMS 30572 20.2 31.5 6.3 4.7 1.9 1.7 3.3 3.5 20.0 VG 
TMS 63397 19.5 33.6 6.5 6.2 1.6 1.4 3.0 2.8 18.0 G 
TMS 3021 1 18.1 29.5 5.3 3.5 1.8 1.6 3.8 3.2 16.5 M 
TMS 4W81 17.9 31.0 5.5 4.4 1.7 1.4 4.0 3.0 16.5 G 
TMS 30555 15.4 31.5 4.8 3.7 1.9 1.8 3.3 3.8 21.0 G 
TMS 4(2)0267 15.0 34.7 5.2 5.0 2.2 1.6 2.5 2.3 20.0 M 
TMS 3000 1 14.1 31.2 4.4 4.8 1.4 1.8 4.0 3.5 18.5 VG 
TMS 42025 13.2 35.8 4.7 7.0 2.0 1.6 2.3 2.3 21.5 VG 
60506 12.4 27.4 3.4 5.7 2.7 2.6 4.0 3.0 17.0 G 
TMS 60142 11.1 35.4 3.9 3.7 2.2 2.2 2.4 2.0 21.5 G 
LSD (5%) 2.6 0.14 
SE (f) 0.95 0.39 
Not-: 
1 Average of four locationr n Nigeria, 1983~1985 
2 Seeunit 11 lor sxpianatono! the scoing sysiem 
3 VG = very gocd. G = gWd.M  - modsrafe. and P - poor 
UNIT 7 
Crop Protection 
Crop protection against pests and diseases is acrucial element of 
cassava production. More than 50 cassava diseases induced Dy 
fungal, bacterial, mycoplasmal, phytomonal and viral agents have 
been reported.T hese diseases can affect plant establishment and 
vigor, inhibit photosynthetic efficiency and cause preha~esot r 
p0SthaNeSt deterioration. Severe infestation often leads to a 
considerable yield loss and thus it is important to undertake control 
efforts as early as practicable. 
Cassava pests represent a wide range of arthropods; most of them 
are minor pests but a few, including mites and whiteflies, may be 
classified as major pests. Insects can cause damage to cassava 
by reducing photosynthetic area, which results in yield reductions; 
by attacking stems, which weakens the plant and inhibits nutrient 
transport; and by attacking planting material,w hich reduces germ- 
ination. Those mites and insects that attack the stem also lessen 
the quality and quantity of planting material taken from these 
plants, thus affecting production. Soil-borne insects attack cut- 
tings, causing wounds or boring holes through which soil-borne 
pathogens can enter. 
Diseases 
African cassava mosaic virus and 
cassava bacterial blight 
The use of disease-resistant, improved varieties is recommended 
as a method for controlling ACMV and CBB. A considerable 
amount of work was put into cassava breeding before varieties 
which were resistant to both disease were obtained. Resistance to 
ACMV and CBB is highly correlated, and thus when a cultivar is 
bred for resistance to ACMV, resistance to CBB is automatically 
included. 
llTA uses recurrent selection to improve resistance to ACMV and 
CBB, together with other agronomic characteristics, while main- 
taining a large genetic variation. Resistance alone was improved 
in one cycle of 2 years; to combine resistance with high yield took 
4-5 years. 
Cercospora leaf spot 
No control measures are required with Cercospora leaf spot 
because the disease sets in after the plant has matured and 
tuberized. It is essentially a disease of older plants and no yield 
loss has yet been traced to it. 
Cassava anthracnose disease 
Little work has been undertaken on controlling CAD because the 
incidence and severity of this disease have not been correlated to 
yield loss. However, screening studies are being carried out to 
identify CAD-resistant varieties which can be recommended to 
farmers. 
Tuber rot 
It has beenfoundthattuberrot is prevalent in heavy, poorly drained 
soils. Forthis reason,friable, well-drained soils are recommended 
for cultivating cassava. 
Pests 
Vertebrate pests 
Ina reas where African bushfowl and cane rat populations are high. 
various methods of reducing these populations are practised, 
including hunting, trapping, snaring and poisoning. Because most 
people in such areas eat bushfowl and cane rats, poisoning is not 
recommended. 
Nematodes 
Nematodes are usually controlled by the application of nemati- 
cides, such as Nemagon and ethylene dibromide. In developing 
countries, these chemicals are not usually available; when they 
are available, they are expensive. An effective alternative is crop 
rotation. Good weedcontrol preventsthegrowthof alternate hosts 
when cassava has been harvested from the field. The use of 
nematode-trapping crops such as Crotolaria spp. during the fallow 
year is advocated as acontrol method. Soilorganic matter amend- 
ment using cocoa pod husks and cassava peels has been found 
to be successful in reducing the parasitic nematode population in 
the soil. 
Mites 
CGM needs to be controlled because it damages cassava leaves 
and reduces tuber yield. Biological control using natural enemies, 
as well as the use of pubescence in young leaves and shoots, is 
being investigated. 
Insects 
Usually, there is no need to control insect pests of cassava, apart 
from CM and the variegated grasshopper. 
CM represents a classical example of an insect that can be 
biologically controlled by using other insects and natural enemies 
(see'Biological Control' below). The parasitic wasp, Epidinocarsis 
lopezi, is an exotic natural enemy at present being released in 
Africa and has been found to be effective in controlling CM. An in- 
digenous natural memy, Hyperaspis pumila, plays a secondary 
role in biological control. 
Pubescent plants prevent the establishment of the mealybug and 
are being investigated in cassava breeding work. 
Early planting is recommendedt o allow the cassava plant a good 
growth before the dry season when plants are invaded. Before 
planting, cuttings may be treated with dimethoate solution to kill all 
insects and mitesto prevent their establishment in a newly planted 
field. 
In the case of the variegated grashopper, the easiest and most 
econom'ical protection method is to control the freshly hatched 
nymphs; however, the success of this method depends upon the 
extentof cooperationb y neighboringfarmers.Oncefreshlyh atched 
nymphs are detected, they should be treated with insecticides 
(such as Rogor and Gammalin 20) or poisonous bait should be 
laid. 
Biological control 
Biologicalc ontrol is a pest management procedure which relies on 
and augments the activity of natural enemies of a pest organism. 
It has been usedfor hundreds of years. The first modern example, 
which was later repeated in Africa, was the spectacular control in 
the USA of the cottony cushion scale, lcelya purchasi, by the 
ladybeetle, Rodolia cardinalis, introducedfrom Australia in 1888. 
Modern biological control relies mostly on specific insect natural 
enemies, predacious mites and microbial agents. Among the 
beneficial arlhropods are the predators (which feed on many host 
individuals) and the parasitoids (which need only one host individ- 
ual for their development). 
Biological control strategies establish a new ecological balance by 
using biological agents. Typically, a pest problem arises when the 
natural balance is disrupted, as is the case when a pest invades a 
new geographical area which is devoid of its natural enemies. An 
undesirable pest level is attained because the natural balance that 
usually exists in the pest's area of origin has been upset. 
Three types of biological control can be distinguished: 
1. The action of indigenous predators and parasitoids (foriu- 
nately, in the African environment, beneficials are often still 
relatively undisturbed) 
2. Periodic manipulation, including inundative releases of natu- 
ral enemies (except for the application of microbial patho- 
gens, this approach is generally too sophisticated and costly 
for African conditions) 
3. Classical biologicalcontrol, which isthe introduction of natural 
enemies for permanent establishment (often, the pest con- 
cerned has been accidentally introduced from abroad but 
some deliberate introductions are known to have been suc- 
cessful against indigenous pests) 
Classical biological control is the attempt to restore an ecological 
balance by introducing the natural enemies that keep the pest in 
check in its native habitat. Such a control strategy is particularly 
appropriate in developing countries because farmers benefit from 
a relatively quick, permanent and ecologically sound technology 
without the extra capital expenditures or special~zedt raining 
required for most of the other control practices. 
Major cassava pests for biological control 
Thecassavamealybug, Phenacoccusmanihoti,M at.-Ferr. (Horn.: 
Pseudococcidae), and the cassava green mite, Mononychellus 
tanajoa, Bondar (Acari: Tetranychidae), were introduced into 
Africa in the early 1970s and have successfully spread through 
most of the cassava belt. Yield losses from the activities of these 
pests are as high as 80%. 
The Africa-wide Biological Control Program of Cassava Pests . 
(ABCP) was established by llTA with two major objectives: to 
reduce yield losses by re-establishingt he natural control found in 
the pests' area of origin: and to create an African biological control 
capacity by training specialists and assisting in the establishment 
of National Biological Control Programs. 
Cassava mealybug. First discovered in Africa in CongolZaire in 
1973, CM is a parthenogenetic species which, under tropical 
conditions, develops from egg to adult in 27 days. The adults live 
for about 20 days, during which time they produce up to 400 eggs, 
most of them in the first 10 days. 
".. ' % 
CM attacks the growing points of the plant first, producing a Figure ,., 
stunted, bunched effect in the terminal shoots. A toxin present in Cassava damagedby cassava 
its salivary juice contributes to this leaf distortion. Further symp- 
toms are short internodes, little new leaf growth, and curling of 
leaves. Very young plants may be killed, and any attacked plant 
is significantly weakened (see Figure 7.1). 
Cassava green mite. CGM was first observed in Africa in 1971, 
attacking cassava fields in Uganda. It is believed to have been 
introduced from South America. 
CGM is principally a dry-season pest but it may be found through- 
out the year on new shoots and on the undersurface of young 
leaves. During their lifetime of 3-4w eeks, adult females produce 
between 20 and 90 eggs each, depending on the quality of the 
available foliage; the development time from egg to adult is about 
14 days. 
The mite feeds by inserting a pair of needle-like mouthparts into 
individual plant cellsand suckingoutthefluidcontent. Thedamage 
which is caused to cassava plants by CGM is similar in appear- 
ance to the symptoms of ACMV attack. Leaf damage varies from 
a few chlorotic spots to complete chlorosis, depending on the 
extent of CGM feeding activity. Leaves which have been heavily 
attacked are stunted in growth and become deformed as they 
mature (see Figure 7.2). The leaves may become mottled and 
eventually dry out, die and abscise. 
CGMfeeding leadstoa reductionof dry matter inthe leaves, stems 
and storage rootsof heavily infested plants. Depending on theage 
of the plant and the time of the season, dry matter reductions of up 
lo  45% have been reportedf or improved varieties. Losses in local 
varieties are generally higher, especially those susceptible to plant 
pathogens. CGM damage also exacerbates weed probelms and 
affects the quality and quantity of cutting material available for 
replanting. 
The principal way of dispersing CGM is to collect and move the 
planting material from one area to another. It may also be dis- 
persed aerially. 
Figure 7.2 
Cassava plant damaged by cassava green mite 
Control measures 
Aclassical biologicalcontrol programconsists of fourwell-defined 
stages (see Figure 7.3). 
1. Foreign exploration for natural enemies. The first step in 
planned biological control against a pest organism, which is 
supposed to have been introduceda ccidentally, is to locate its 
area of origin, where parasitoids and predators have co- 
evolved with it over a long period. Exploration in this area 
usually provides numerous species of natural enemies but 
often in very limited numbers. 
2. Quarantine processing and rearing of identified natural ene- 
mies. The collected species are identified in a quarantine 
station; because there are no fully equipped stations in 
tropical Africa, arrangements are usually made by govern- 
ments to use the quarantine facilities at the Commonwealth 
Institute of Biological Control in London. The species are 
reared as safely as possible to prevent escape, their biology 
is studied and they are checked to ensure that they are 
disease-free. Their host specificity is evaluated and hyper- 
parasitoids (parasitoids of parasitoids) are excluded. If these 
processes are carried out properly, biological control is acom- 
pletely safe procedure. Nocases are known, or are expected 
to be found, of parasitoids and predators switching to plant 
food once their insect host has been reduced; they simply 
survive at the very low population levels at which their host 
survives. 
I 
Figure 7.3 
Stages in a biological control program 
3. Mass-rearingandintroduction.F rom quarantine, the selected 
beneficial organisms are mass-reared in a laboratory, then 
released into the field under the best possible conditions. 
Establishment often occurs with releases of less than 200 in- 
dividuals; in general, however, larger releases are favored to 
allow ample genetic recombination in the new conditions. 
4. Impact evaluation. The released populations are then moni- 
tored. Ideally,t heimpact is measuredand relatedt ocroploss, 
but, in practice, proving the impact of the released organisms 
is often difficult because nearby control fields cannot be kept 
uninvaded by theestablished beneficial. Experimental exclu- 
sion of parasitoids and predators by using insecticides,s leeves 
or other measures sometimes proves the efficiency of the 
controlagent, and studiesondensity-dependentb ehavior, life 
tables and mathematical models are conducted to support 
this proof (examples of this procedure are given below). 
Procedure for cassava green mite 
Identification and importation of natural enemies. Mites of 
the family Phyloseiidae are used by the ABCP as the primary 
agents against CGM. Their ability to control spider mites in many 
agro-ecosystems in temperate climates is well established, and 
their potential as biological agents is well documented. 
Exploration for natural enemies of CGM in the neo-tropics is 
undertaken by the Centro lnternacional de Agricultura Tropicale 
(CIAT) and the Empresa Brasileira de Pesquisa Agropecuaria 
(EMBRAPA). These institutions explore areas that correspond 
ecologically to areas in Africa where the mite is a severe problem. 
As species are identified during exploration, their biological char- 
acteristics are noted and added to a database for selection 
purposes. 
Other work which is being carried out in the field of the biological 
control of cassava includes: the international quarantine services 
at the University of Amsterdam, Netherlands; taxonomy of 
tetranychids at the University of Sao Paulo, Brazil; taxonomy of 
phytoseiids by EMBRAPA in Petrolina, Brazil; regional liaison 
assistancefromtheCAB lnternationall nstituteof BiologicalC ontrol 
in Nairobi, Kenya; simulation modeling of the cassava ecosystem, 
including CGM, atthe University of California, Berkeley, USA, and 
the Federal Institute of Technology (ETH) in Zurich, Switzerland; 
and artificial diets for transporting natural enemies of CGM, a 
survey of entomopathogens of CGM, and the biotaxonomy of 
CGM atthe InternationalC enter for Insect Physiology and Ecology 
(ICIPE). 
Selection of release sites. Potential release fields are identified 
and Surveyed for CGM and associated natural enemies prior to 
making any releases. In the fields which are chosen for releases, 
the cassava plants are vigorous enough to support increasing 
CGM populationsduringt he dry season but also young enough not 
to be hawested during the following wet season. 
Packing and transport. Most phytoseiids packaged for shipment 
are a mixture of age classes. The egg stage travels without 
problems as long as the humidity is above 50% RH. However, in 
Ihe active stages special attention is required. If badly packed, 
actives are susceptible to rapid dehydration. Starvation causes 
cannibalism. 
Phytoseiid shipments are packed in containers or vials with agar 
as a source of confined moisture and some type of inert, non- 
hygroscopic material which is folded to increase the surface area. 
No plant material or live host material is included. These contain- 
ers are placed inside a coolbox where the temperature is main- 
tained at about 15°C. The package is kept closed while being 
transported. Exposure to X-rays or other forms of radiation (for 
example, security screening devices at airports) is avoided. 
Handling. Upon the arrival of a shipment in the country where the 
release is to be made, about 300 phytoseiids are added to 10 well- 
infested leaves and placed in individual paper bags. The bags are 
closed by folding over the top a couple of times and securing the 
fold with a staple, paper clip or straight pin. Care is taken not to 
crush the paper bags or expose them to direct sunlight while the 
predators are being transported. The predators feed and repro- 
duce on the leaves for about 5 days. The bags are stored in a cool 
place. 
Releases. Once appropriate release fields have been identified, 
individual plants are selected to receive the released material and 
are marked with some type of flag. Each phytoseiid species is 
released in a separate field. Predators are released by placing 
individual infested leaves, complete with predators and their eggs, 
on the young, fully developed leaves of the cassava plant. 
Taking into account the mortality suffered by the phytoseiids in 
transit and the number of CGM-infested leaves provided as food, 
infested leavesare distributedto provide a density of 10-50 actives 
per cassava plant in a defined part of the release plot. The best 
times for releasing natural enemies are at the beginning of the dry 
season and just after the first rains of the long wet season, when 
CGM populations rapidly increase to high levels. 
Monitoring. After exotic natural enemies have been released, 
routine follow-up monitoring is carried out in order to determine 
whetherthe species has become established. Itsdispersal and its 
impact on the target species is measured. CGM populations in the 
release and control fields are monitored, using approved census 
procedures. Natural enemies in the release fields are sampled, 
and the leaves and green stems from three or four plants of the 
most abundant weed species are examined for tetranychids and 
associated natural enemies. Any specimens found are collected 
in the field. 
This follow-up activity is done twice a month during peak periods 
of CGM activity, once a month during the transition periods 
between seasons, and bi-monthly during the wet season. Speci- 
mensfromweeds arecollectedon every alternate trip to these field 
sites. 
Procedure for cassava mealybug 
Identification and imporlation d natural enemies. The wasp 
Epidinocarsis lopezi (De Santis) (Hymenoptera. Encyrtidae), 
which parasitizes CM, has been established successfully in many 
cassava-growing areas in Africa where it has been released by the 
ABCP. A natural enemy is considered established when it has 
survived afull rainy season -t he period of low CM population - 
and has been located again 12 monthsafter release. E. lopezi has 
spread rapidly to other cassava-growing areas and is now estab- 
lished in 13 countries, over a total area of 650 000 kmz. It has 
caused considerable reductions in CM populations, making it a 
successful biological control agent against this pest. 
The search for exotic natural enemies of CM began in central and 
northern Swth America becausecassava, the only natural host of 
CM in Africa, was introduced from South America. In 1981, CM 
was finally discovered in Paraguay by CIAT. Several predators 
and parasitoids were collected and quarantined by the Common- 
wealth Institute of Biological Control in London. After approval by 
the lnter-African Phytosanitary Council and the Nigerian plant 
quarantine authority, they were then sent to IITA. 
Further explorations in Paraguay, Brazil and Bolivia led to the 
collection of E. lopezi. After collection and importation to IITA, 
E. lopeziwas reared on CM on potted cassava plants. 
Selection of release sites. Before the release of E. lopeziat any 
site, an extensive survey is carried out in order to: 
investigate the distribution and abundance of the pest and 
select release sites based on infestation levels 
estimate the population of the pest, thereby providing a basis 
for impact assessment of the natural enemies 
determine the species wmpositionof infestedcassavafields, 
a procedure which is also relevant to the impact assessment 
This survey is usually conducted during the dry season when CM 
populations are at their highest. Aparticular area is sampledonce 
(or, at most, two or three times) during this period. The location of 
sampling points is determined to a large extent by accessibility. 
Where there are suitable roads or trails, the selection of sampling 
points is more often systematic (at regular intewals of lOkm, for 
example) than random. However, in order to obtain population 
estimates for future impact studies, a combination of systematic 
and random sampling is involved. Collection and conservation of 
infested shoots provide data on species composition. The sam- 
pling unit is the terminal shoot because this is where CM is usually 
concentrated. Infestation levels are determined as a basis for site 
selection. 
Releases. Most releases are made in the second half of the dry 
season into fields with high CM populations. All releases are 
conducted in collaboration with local agronomists or entomolo- 
gists. Ground releases are made by pouring the insects directly 
onto infested plants. In some areas, aerial releases are also 
undertaken. 
Monitoring. The efficiency of a released natural enemy depends 
on its searching capacity and rate of dispersal; these factors 
indicate to a large extent whether the natural enemy will be 
established or not. The first stage in the impact assessment 
therefore involves determining the spread and establishment of 
the released natural enemy. Starting from the release site, 
systematic samples are taken from various points over a large 
area. This may be done two orthree timesduring the dry season. 
Toobtain additional informationonthepopulationsofb oth the pest 
and the natural enemy, a field which is selected systematically is 
randomly sampled. 
Sampling consists of randomly selecting 50 plants and carefully 
breaking off the upper 1Ocmof the tipsof the shoots. These are put 
inside paper bags to prevent any escape by natural enemies, and 
the bags are then sealed andtaken to the laboratory. Here, the tips 
are dissected, and living CM and dead, hardened and parasitized 
CM ('mummies'),aswellas natural enemies, arecounted. Mummies 
are kept in gelatine capsules in the laboratory for parasitoid 
emergence. 
The living CM are reared on cassava leaves or, preferably, on 
detached fleshy leaves of water leaf, Talinum tflangulare (Jacq.). 
an alternate host of CM which remains fresh for a longer period 
than cassava in petridishes. The rearing continuesfor3-4 weeks. 
Daily obsewations are made on emergence of parasitoids, which 
are immediately removed to keep them from stinging the remain- 
ing CM. Coccinellid and other larvae are also reared to adults for 
proper indentification. Monitoring is done every 2 weeks. 
Parasitization rates are calculated by relating the number of 
emerged parasitoids from mummies and living CM (second to 
fourth instars) to the total number of second to fourth stage CM 
(E. lopezidoes not reproduce in first instar CM). 
Integrated disease and pest 
management 
A sound integrated control program for pests and diseases is 
essential in any program aimed at yield improvement and stability. 
The progam should involve not only biological control practices, 
but also good cultural practices and ecologically adapted resistant 
varieties. The use of chemical control should be considered only 
when other control measures are ineffective. If an outbreak does 
require pesticide applications, it should be done selectively, bear- 
ing in mind the possible lethal effects on beneficial agents. 
Cultural practices 
There are many cultural practices that contribute to pest and 
disease control. Uniform practices cannot be recommended for all 
cassava-growing areas; they must be adapted to the specific 
characteristics of each ecosystem. In general terms, however, the 
following practices are likely to reduce pest and disease stress: 
proper soil preparation 
the use of clean, high-quality planting material 
good weed control 
removal and destruction of infected plant materialldebris 
crop rotation 
intercropping cassava with other crops 
well-planned spacing of plants 
proper fertilization 
strict quarantine regulations 
Varietal resistance 
Yield stability is related to climatic, edaphic, pathological and 
entomological stresses, and to the genetic capacity of clones to 
tolerate these stresses; these stresses are known as negative 
productive factors (NPFs). The cassavalecosystem interaction is 
considerable because, for a long time, cassava clones have been 
selected in localized areas and perpetuated vegetatively. 
A well-adapted clone with tolerance to a given ecosytem could be 
severely affected by the NPFsofanotherecosystem.Thus, ineach 
ecosystem regional clones or clones from similar ecosystems 
should take preference over those introduced from ecosystems 
with different sets of NPFs. Introductions are made specifically to 
improve the gene pool existing in an ecosystem (regionalc lones). 
Clonal evaluation should be based on the following criteria: 
a satisfactory yield of fresh roots, starch and foliage, accord- 
ing to the utilization of the plant 
a good production of high-quality planting material 
highly acceptable root quality, according to regional socio- 
economic requirements 
Clones selected according to these criteria would be the most 
acceptable to farmers and therefore be the most stable over time. 
Clonal evaluation in each ecosystem should be directed at identi- 
fying genotypes with the widest type of resistance to the NPFs 
existing in it. This evaluation should be performed in areas of a 
particular ecosystem where NPFs are most severe and most 
frequent. This should not eliminate or underrate evaluations di- 
rected at identifying tolerance to specific important biotic prob- 
lems; such evaluations could be neededto improve clones which 
have wide resistance but are deficient in certain required charac- 
teristics. 
Varietal resistance obviously enhances the impact of biological 
control because economic damage occurs only at higher popula- 
tion levels, facilitating the increase of beneficial biotics and reduc- 
ing or eliminating the need for pesticides. 
PART Ill 
POSTHARVEST TECHNOLOGY 
UNIT 8 
Sforage of Fresh Cassava 
Cassava tubers are extremely perishable. They can be kept in the 
ground prior to harvesting for up to about 2 years, but once they 
have been harvested they begin to deteriorate within 40-48 hours. 
The deterioration is caused by physiological changes and, subse- 
quently, by rot anddecay. Mechanicaldamageduringthe harvest- 
ing and handling stages also renders the crop unsuited to long- 
term storage. 
Deterioration of cassava has an adverse effect on the processed 
product, andthusthecrop must be stored properly. Traditionaland 
modern methods of storage have been devised to combat post- 
harvest losses. 
Traditional storage methods 
In most areas where cassava is grown under subsistence farming 
conditions, the problem of storage is overcome by leaving the 
mature cassava crop in the ground until needed. The main dis- 
advantages of this method are that: 
large areas of land are used as a storehouse for the already 
mature crop and therefore cannot be used for further crop- 
ping; this decreases the economic output of the land and 
increases pressure on the land (there is already a consider- 
able amount of pressure on the land in many countries in 
Africa because of high population growth rates) 
susceptibility to loss is increased because the tubers are 
vulnerable to attack by rodents, insects and nematodes 
roots become more fibrous, lignification occurs, and conse- 
quently the crop's starch content and its suitability for many 
food preparations decline 
Other traditional methods, based on the principle of preventing 
moisture loss from the tubers, include: 
storing harvested tubers in pits (this involves burying them in 
pits lined with straw or some other vegetative material) 
piling them into heaps and watering them daily to keep them 
fresh 
coating them with a paste of mud 
storing them under water 
These methods prolong the shelf life of cassava by only afew days 
and are not hide~yu sed. 
Improved storage methods 
Among the improved storage methods forfreshcassava are those 
based on techniques involving freezing, gamma irradiation, con- 
trol of storage environment (relative humidity and temperature) 
and waxing. However, none of these techniques has been suffi- 
ciently tested. Three improved storage methods which have 
undergone sufficient testing, including field testing, involve: 
dipping fresh roots in fungicide and packing them in polythene 
bags 
storing them in specially prepared trenches 
storing them in moist sawdust 
Although these three methods are not yet widely used, they are 
useful for small- and medium-scale cassava production. 
Storage in polythene bags 
This method appears to be the simplest way of storing cassava 
roots. If properly conducted, it ensures a shelf-life of 2 weeks or 
more. 
The method is based on the principle of 'curing', which is the 
capacity of the root to form a new layer of cells over damaged 
tissues. Freshly harvested roots are treated with 0.4% solution of 
Mertect, a thiabendasole-basedf ungicide. They are then packed 
in polythene bags and sealed. Inside the bags, the rootscreate the 
necessary temperaturelhumidity environment (temperature should 
be 30-40°C and RH should exceed 80%). The fungicide treatment 
preventsthe growthof micro-organismsi nthe humid environment. 
Storage in trenches 
This low-cost method, developed by the Nigerian Stored Products 
Research Institute, keepscassavafreshfor at least 6-8weeks and 
can be implemented easily by farmers and processors. 
A trench is dug in the ground at a site which has a low water table, 
thus preventing flooding of the roots by seepage of underground 
water. The trench should be 2m long, 1.5m wide and l m d eep. 
Depending on the size of the cassava roots, a trench of this size 
can store 0.5-0.7 tons of cassava. 
A shed made of wood and iron, or bamboo, with a thatched roof, 
is constructed over the trench. It is economical to make several 
trenches under the same shed (see Figure 8.1). 
Figure 8.1 
Fuiiy filled trenches under a protective shed 
Two layers of palm branches or raffia leaves are laidon the boHom 
of the trench. One or two layers of freshly harvested, undamaged 
cassava roots,w ithstems attached. arearrana- ed onthe branches1 
leaves. This process is repeated until the trench is almost full. The Figure 8.2 
final layer of brancheslleaves is covered with soil, 7-10cm deep; Cassava roots storedrn a trench. 
thesoil is moistenedonceaweekwithcleanwater(seeFigure8.2). covered with soil 
Storage in sawdust 
Cassava roots stored in sawdust must be freshly harvested with 
15-20cmo f the stern attached. Thethree typesof containerswhich 
can be usedforthis method arewoven baskets, papercartons and 
wooden boxes with covers (see Figure 8.3). Roots can be stored 
by this method for 6-8w eeks. 
I I 
Figure 8.3 
Three types of mntainers used for storing cassava roots in sawdust 
A layer of sawdust is spread at the bottom of the container. A layer 
of fresh cassava roots, carefully arranged so that the roots do not 
touch eachother, is then placedon the sawdust. Another layer of 
moist sawdust is putonthe roots, followed by second layerof roots. 
Sawdust is packed between the roots and also at the top of the 
container, and is then moistened. The containers can be trans- 
ported or stored in this way. 
It is essential in this type of storage to inspect cartons every three 
days to ensure that the sawdust is moist. It is also important to 
ensure that the harvested cassava roots have no mechanical 
damage, as this method is suitable only for storing undamaged 
roots. 
UNIT 9 
Cassava Processing 
Cassava consists of 60-70% water. Processing it into a dry form 
reduces the moisture content and converts it into a more durable 
and stable product with less volume, which makes it more trans- 
portable. 
Processing is also necessary to eliminate or reduce the level of 
cyanide in cassava and to improve the palatability of the food 
products. Processed cassava products are also used as raw 
materials for a number of small- or medium-scale industries in 
Africa. 
The tubers and leaves of cassava contain cyanide which can be 
poisonous, depending on the levels in aparticularvariety. Thus, to 
ensure they are safe for human consumption, thecyanide must be 
removed or considerably reduced. According to the processing 
procedure used, the percentage of cyanide reduction varies from 
69.85 to 100%. 
The tubers are detoxified by hydrolysis of linamarin and lotaus- 
tralin into HCN (hydrogen cyanide) which is volatile and evapo- 
rates rapidly at temperatures above 28°C. Some measure of 
detoxification can also be achieved by mechanical disintegration 
(pounding, grating or chipping the tubers). 
The objectives of cassava processing are to: 
reduce postha~eslto sses of fresh tubers 
eliminate or reduce the cyanide content 
improve the taste of cassava products 
provide raw materials for small-scale, cassava-based rural 
industries 
Traditional methods of cassava 
processing 
Traditional cassava processing technologies can be divided into 
three main groups: 
preparation of cassava chips and flour (unfermented or fer- 
mented) 
technologies based on fermented cassava dough 
minor technologies 
Cassava chips and flour 
Preparation of unfermented cassava flour. This process is 
suitable for low-cyanide cassava varieties only. The cyanide 
content in thesevarietiesis50mgorlessper1OOOgoffresh weight, 
whereas in high-cyanide cassava varieties the cyanide content is 
1OOmg or more per 1OOOg of fresh cassava. Flour prepared from 
high-cyanide cassava and used for food preparations may result 
in acute cyanide poisoning. An example of unfermented cassava 
flour is 'kokonte', in Ghana. 
The traditional processforpreparing unfermentedcassavaflour is 
as follows: 
1. The cassava tubers are peeled manually. 
2. The peeled tubers are washed. 
3. They are then cut into chunks (in some countries, including 
Rwanda and ZaTre, the peeled and washed tubers are dried 
as whole tubers). 
4. The cassava chunks are dried on the ground (or, rarely, on 
elevated platforms);d rying takes 2-5d ays, depending on the 
weather. 
5. Thedriedcassavais normally stored in the lormof chips in jute 
sacks and then sold, or it is milled for family use when 
necessary. 
Preparation of fermented cassava flour. In Nigeria, fermented 
cassava flour is known as 'lafun'. It is particularly popular in the 
south-westem states of Lagos, Ogun, Ondo and Oyo. There are 
slight variations in the preparation of lafun, depending on locality, 
but basically the process is as follows: 
1. The cassava roots are washed (in areas with water supply 
problems, this step is often omitted). 
2. The roots are steeped in water, usually in drums, pots or 
natural ponds inareasclose to cassavafarms. It isduring this 
stage that the fermentation occurs. The minimum time for 
fermentation is 3 days; the process is slower in the rainy sea- 
son than in the dry season. 
3. The fermented cassava roots are peeled. After fermentation, 
the peel comes off easily, as a result of partial disintegration 
of the cassava roots. 
4. The roots are then dehydrated by putting them into bags and 
placing stones on top of the bags. Figure 9,1 
Steeping cassava roots for the 
preparatbn of lafun 
5. Thedehydrated, pulverized mashissun-dried inthinlayerson 
mats, concrete surfaces or, very often, on rocks. Drying the 
mash on rocks has the advantage of allowing drying to 
continue overnight because the rocks absorb heat during the 
day and give it out at night. Drying takes 1-3days, depending 
on the weather. 
6. The dried cassava is milled and stored for household con- 
sumption and sale. 
To produce better-qualityf lour, the roots are peeled before steep- 
ing in water, and disintegration is carried out using a cassava 
grater set for larger clearance. 
Deficiencies of traditional flour preparation methods. The 
deficiencies of preparing unfermented and fermented cassava Figure 9.2. 
using the traditional methods outlined above are as follows: Drying manually pulverized cassava 
on rocks 
althoughdryingthechunks orthe whole tubersusually resuits 
in their outer surfaces being sufficiently dry, the moisture 
level inside the chunks or tubers is still considerably higher 
than its safe value 
the process is quite unhygienic; spreading the product on the 
ground makes it vulnerable to contaminationb y, for example, 
foreign bodies or dust 
drying causes a major bottleneck in flour production, particu- 
larly during the rainy season when the product can become 
moldy and lose quality 
Fermented cassava dough 
The most typical and popular product which is prepared from fer- 
mentedcassavadough in West Africa isgari. Gari is afree-flowing 
product,consistingof cassava particles which have beengelatinized 
and dried. The size of these particles varies from one locality to 
another according to consumer preferences; a finer gari is pro- 
duced by sieving the product after roasting. Gari is creamy-white 
or yellow, depending on the type of cassava used orwhetherpalm 
oil has been added. 
For good storability, the moisture content of gari should be below 
12%, preferably 8-10% Good-quality gari swells to about three 
times its initial volumewhen placed in water. The popularity of gari 
is probably based on the fact that the granules are precooked and 
a very short time is needed to prepare them as main dishes or 
snacks. An additional advantage is that well-prepared gari stores 
well for at least 12 months. 
Traditional gari preparcttion. The traditional process for prepar- 
ing gari is as follows: 
1. The roots are peeled manually. Usually, this is a family or 
group activity, with women helping each other or being hired 
by processors (see Figure 9.3) 
Figure 9.3 
Peeling cassava manually 
The peeled roots are washed (this step is sometime omitted 
in areas with water shortages). 
The peeled roots are grated. This is usually done with hand 
graters (perforated tin sheets, nailed to a bench or set in a 
frame), but mechanical graters are available and are being 
used in some areas (see Figure 9.4.) 
The grated cassava mash is fermented and dehydrated. This 
is done by putting it in sacks. Logs or stones are placed on top 
of the sacks or, alternatively, the sacks are pressed between 
two boards attached by ropes; as the ropes are tightened, 
the water is squeezed out from the cassava mash. Fermen- 
tation usually takes 3-5 days, but in localities where a bland 
gari taste is preferred (for example, Bendel State in Nigeria) 
the mash is fermentedforonlyl day (seeFigure9.5). Fermen- 
tation is very important because it givesgari its preferred sour 
flavor, and detoxifies the cyanide. The safe level of cyanide in 
gari as specified by the Nigerian Food and Drug Administra- 
tion is 10ppm (irng HCN per 1009 of gari): the cyanide level 
for low- cyanide cassava is 50ppm (5mg HCN per lOOg of 
fresh tubers). 
Flgure 9.4 
 rating the roots manually 
Figure 9.5 
Dehydrating and fermenting cassava mash 
5. Sieves made of plant material are used to separate the gari 
particles and to remove fiber and poorly grated material (see 
Figure 9.6). 
6. Theparticlesarethen ready forfrying. Garifrying can be seen Figure 9.6 
as two processes: starch gelatinization of the particles, and Sieving cassava mash 
93 
drying. The particles are fried in shallow earthenware, alumin- 
ium or iron cast fryers (see Figure 9.7). In certain parts ol 
Nigeria, an oil d ~ mcu, t longitudinally and set into a specially 
prepared fireplace, is used. Palm oil is added to the frying 
surface to prevent burning or to give the gari a yellow color. 
During the frying process, a calabash or a little broom is used 
to toss the particles. 
Figure 9.7 
Fiying gari 
7. Thefriedparticlesarecooled by spreading themonafloor;the 
floor is usually covered with some sort of sheeting. 
Figure 9.8 
Gari being sold 
8. The cooled gari is sieved with locally made sieves to ensure 
uniformity of grain size. Large particles are normally milled 
and added10 the sieved gari. This is then packed inpolythene 
bags, jute sacks, propylenesacks or paper bags, and mar- 
keted. 
Deficiencies in traditional gari production. The deficiencies in 
the traditional gari production process are as follows: 
0 Manual peeling results in lowproductivity. Attemptsto mecha- 
nize peeling have not been successful because of the irregu- 
larshape and size of cassava tubers. However, for small- and 
medium-scale processing, manual peeling by hand has the 
advantage of providingpart-time workforwomenandchildren 
in rural areas. 
Grating normally results in low output (not more than 20kg of 
cassava can be grated per day), and may cause injuries to 
fingers. 
. Gari fryers otten have afuel efficiency of less than 10% and 
frying exposes those cooking the gari to heat, smoke and 
cyanide fumes. 
0 There isoften little or noqualitycontrol of thetinishedproduct, 
which may result in the product having a higher moisture con- 
tent than recommended, making it unsuitable for long-term 
storage. Sometimes, to save on fuel, gari is deliberately 
removedfrom the fryers before its moisture content has been 
sufficiently reduced, and then sundried. This practice gives 
satisfactory results during the dry season, but in the rainy 
season gari reabsorbs the moisture and becomes unsuitable 
for storing for more than 1 week. 
Minor processing technologies 
In all cassava-growing areas in Africa, starch is produced in small 
quantities. The process involved in starch production is sumrna- 
rized here: 
1. The cassava tubers are peeled and washed. 
2 The tubers are grated or pulverized 
3. The cassava mash is mixed with large quantities of water and 
sieved to extract the starch. 
4. The starch granules are allowed to settle overnight 
5. The water is decanted, and the starch cake which settles at 
the bottom of the container is broken into pieces for drying; 
often it is sold as wet chunks. 
6. The wet starch is sun-dried for 1-2 days 
Deficiences in traditional starch production. The problems 
associated with traditional methods used for starch production are 
as follows: 
Manualgrating,especially whenpoor-quality gratersareused, 
has low productivity and does not allow starch to be released 
from the cells efficiently. 
Sun-drying isdifficultduringthe rainy seasonandoften results 
in contamination of the finished product. 
The quality of starch depends on the quality of the water 
There is no quality control of the finished product 
Improved cassava processing 
Compared to the traditional methods, the improved method for 
processingcassavai ncreasespmductivitya nd improves the 
and storabilitv of cassava Droducts. It also enhances the ~otential 
for cassava growers in ~ f i i c atno develop non-traditionalcassava 
products (suchascassavastarch, an important raw material in the 
food, textile, paper and other industries; cassava flour, for use in 
various bakery preparations, alone or as composite flour; and 
cassavachips and pellets, which are incorporated in animal feed 
rations by EECcountries because of the low price and high energy 
content of cassava compared to cereals). 
The objectives of improved cassava processing are: 
to reduce the drudgery and labor intensiveness of traditional 
cassava processing methods, and thus increase productivity 
to produce an end product of better and more uniform quality 
to ensure the reduction ortotal eliminationof undesirabletoxic 
constituents in cassava so that it is suitable for human con- 
sumption 
0 to promote the establishment of economically viable small- 
and medium-scalec assava-based industries and create new 
opportunities for employment in rural areas 
to reduce the amount of fuel used for drying cassava by 
introducing fuel-efficient devices and techniques 
to promote the export potential of cassava products such as 
starch and cassava chips and pellets Manual peeling 
Improved cassava processing for three cassava products -g ari, 
cassava chips and flour, and cassava starch - is presented in Washing <Water 
detail below. These specificcassava products have beenselected 
because: 
Grating 
they are relatively easy to manufacture within the existing 
infrastructures of the rural areas in Africa 
they could play an important role in the food security of rural aFermenting 
communities  
the machinery requiredto processthem isorcan be manufac- juice ('MC 47.50%) 
tured by the informal engineering sectors of many African 
states 
Fibrouz 
matter 
they provide the raw material for a number of important 
industries 
Gari 
A flow chart depicting the improved gari processing method is Cooling 
presented in Figure 9.9: the moisture content of the cassava 
product at each stage of the process is indicated. Illustrations of 
various stages of the process are provided in Figures 9.10 to 9.13 Final sieving 
(see overleaf). 
Raw material requirements. The ratio of the fresh unpeeled Packaging 
cassava input to gari output is approximately 5 : 1 (that is, 2 tons 
of fresh cassava are required to produce approximately 400kg of 
gari). However, these figures are not absolute because many 'MC= Moisture Content 
factors affect the ratio of output to input, such as type of cassava 
used, thickness of peel, age of cassava and moisture content. It 
is important to weigh the cassava before and after peeling. 
Peeling and washing. Attempts to mechanize the peeling of Figure 9.9 
cassava have not been successful because of the irregular shape How chart of gari manufacture 
and size of the tubers. Manual peeling with stainless steel knives 
is recommended. 
The peeled tubers are washed and packed in woven baskets to 
allow the waterto drain. The tanks in which the tubers are washed 
should be made of stainless steel, plastic or ceramic material; if 
these are not available, galvanized steel tanks may be used. After 
washing, the tanks are cleaned and dried. 
Figure 9.10 
Washing and grating cassava tubers 
Figure 9.11 
Power screw dehydrating press 
Grating. The washed tubers are conveyed with wheelbarrows or 
other means to the cassava grater. These graters vary in size and 
shape but basically they ail have a rotating drum covered with a 
perforated metallic sheet. By attaching a discharge chute to the 
grater, the grated cassava particles can be delivered straight into 
bags (made of polypropylene or other materials) for fermentation. 
Fermentation. Fermentation racks are built from wood and have 
drainage lanes directly beneath them to allow the juice from 
cassava to flow out. Fermentation takes 1-5 days, depending on 
the preferred gari flavor in a given locality. 
Dehydration. Some water drains through the holes in polypro- 
pylene bags during the fermentation stage. However, mod of the 
moisture is removed by using power screw presses; two people 
operate the extension arms of thescrew shaft. The moistureof the 
dehydrated cassava mash is 47-50%. 
Sieving. Dehydration produces cassava cake which has to be 
sieved before frying. Sieving machines or sieves made of plant 
material are used. The cassava cake is pressed or rubbed against 
the surface of the sieve. 
Figure 9.12 
Mechanical s#ter for cassava mash 
and gari 
Figure 9.13 
Fiyfng gari (improved) 
Frying. The cassava particles are fried on metal trays measuring 
1.2 x 2.4m and fixed into a fireplace built of bricks or mud blocks. 
The fireplace has a chimney to allow smoke to escape and to 
improve heat efficiency. The particles are fried until crisp and dry. 
The recommended moisture content of the finished gari is about 
10%; this can be tested with a moisture meter. 
Cooling. The gari is cooled by spreading it out on polythene 
sheets on the floor. It can be cooled overnight and packed in the 
morning. 
Final sieving and packing. The cooled gari is sieved again to 
ensure uniformity of the final product. It is then packed. 
Cassava chips and flour 
Fresh cassava roots 
*I  Using low-cyanide varieties. The process to be followed in the 
preparation of dried cassava chips and flour from low-cyanide 
cassava varieties is shown in Figure 9.14. 
Peeling 
Notes on preparation method 
The ratio of the cassavachips output to freshcassava input is 
Washing approximately 1 :4 . Depending on the type of cassava used, 
*I  1 ton of cassava tubers gives 250kg of cassava chips. 
It is recommended that peeling is done with stainless steel 
Chipping knives. 
4 Peeled cassava should be washed in cemented tanks or 
plastic drums, to avoid adverse effects of corrosion. 
Artificial sun or solar drying 
.1 Washed cassava may be sliced using manual or mechanical 
slicing machines. 
Packing and storing in chip form During the dry season it is convenient to sun-dry chips on 
elevated platforms built in an open area, about 900m2. How- 
ever, during the rainy season such an arrangement is not sat- 
isfactory, and some sort of enclosed drying chamber using 
Grinding and sieving solar energy or fuel is recommended. Drying is done until the 
(milling into flour) chips have a moisture content of 8-10% afterwhich the chips 
are cooled and packed. 
Figure 9.14. Cassava may be stored as chips and milled into flour when 
Fbw chart for preparation of cassava chips needed; chips store better than flour. There are many types 
and flour from lowcyanide varieties of grinding machines in use in urban and rural areas in Africa. 
100 
Using high-cyanide varieties. This process for preparing cas- 
savachips and flour from high-cyanide cassavawas developed by 
the Ceylon Institute of Scientific and Industrial Research in Sri 
Lanka (see Figure 9.15). It removes 95% of the cyanoglucosides 
in cassava. 
The first drying stage disrupts the cell membranes of the cassava 
tissue, causing increased permeability. Soaking inwater results in 
a loss of a large fraction of soluble material, including free HCN, 
glucosides and cyanohydrins. By the end of the second drying 
stage. 90% of the HCN has been removed. Drying at 100°C 
causes quick decompositiono f cyanohydrins and thus almost total 
removal of cyanide. 
Flour prepared in this way can be used for various purposes (for 
example, in the paper industry, and in the textile industryforcotton 
warp sizing and textile finishing). The peels may be dried, soaked, 
redried, ground and used as poultry or cattle feed. 
Cassava tuber 
Traditionally, grinding chips into flour is done manually by pound- 
ing them in a wooden mortar. However, grinders are now common vI 
in rural and urban Africa, particularly the serrated plate type.  
Peeling and washing 
Notes on preparation method 
1. Freshly uprooted cassava is preferred because it gives a 
better-quality product. However, tubers which are 1-3 days Chipping 
old may be used if there is no obvious spoilage. I 
2. Chipping is done manually or with mechanically operated 
chippers; the thickness of slices is 4mm. 
3. In the first drying stage, the slicedchips aredried in thesun for 
about 3 days to a moisture content of 14%. Drying is done on 
trays made of wood or other plant material. During the rainy sun-drying 2 
season, protected solar driers are recommended. (90% cyanide removed) 
4. The chips are soaked in water for 8 hours in tanks; a drain 
leads from the tank. The ratio of water to chips is 1 gallon of 
water to 1 pound of chips. 100°C overi dryirg 
(95% cyanide removed; 
5. The second sun-drying stage takes 1-2 days. 
6. Dried chips are placed in an oven at 100°C for 2 hours and Flgure 9.15. 
dried to a moisutre content of 6-8%. Ovens with a capacity to Flow chari for preparation of 
dry 200kg of chips per batch and handle about four batches a detoxified cassava flour fmrr high 
day may be used. cassava variet~es 
 1 ( Cassava starch 
Cassava starch is an important industrial raw material which is 
Iused in the manufacture of a number of products, including food, 
Fresh tubers adhesives, thickening agents and pharmaceuticals. The process 
for improved starch production, shown in Figure 9.16, is very 
similar to the traditional process. It is based on a maximum 
(18 % loss) productiono f 200kg/day, which is the capacity for an average rural 
starch factory. 
Stream water Notes on preparation method 
a 1. When selecting tubers for starch production, age and tuber 
Grate  variety are the criiical factors. Tubers contain 20% starch by 
weight, but as a result of losses during processing this is 
reduced to 10%. Thus, 1 ton of cassava produces 100kg of 
Filtered cassava starch. 
clean water )( Wash out starch I I 
1 2. Manual peeling is still the cheapest way of peeling cassava; 
18% loss of weight is assumed. 
(0.5% loss) 
1 3. The Deeled tubers are washed in cemented tanks. As water 
/ Cil+,..-A .
c ~ L X Z i e r  p*I ! quality is not critical at this stage, stream water can be used. 
Scrape; ,r esettle ] 1 
4. Grating is important because it affects the quantity of the 
starch released from cassava. The percentage of starch set 
free is called 'rasping effect'. Its value after one rasping varies 
between 70 and 90%. Secondary grating using a hammer mill 
with a fine screen is recommended. 
5 .  The starch is washed out using clean water. If the water 
contains ferrous compounds, a simple filtering system may 
be used. If piped treated wateris available, filtering is not nec- 
Pulverise essary. 
Fora 200kglday starch production, the cheapest way to wash 
out the starch is to use a woven basket with a piece of clean 
calicoclothtiedaroundtheoutside. Thisforms adouble sieve. 
The grated pulp is put into the basket and handwashed with 
IStore water until no more mi lky starch comes out. The remaining 
pulp is discarded. The milky starch solution is collected into 
plastic drums (30-gallon drums are a convenient size for 
starch removal) and left to settle overnight. 
Figure 9.16. The discarded pulp can be fried into gariordried and incorpo- 
Flow chart for manufacture of rated in animal feed. Gari from discarded pulp is of inferior 
cassava starch quality. 
617. After it hassettled overnight, the clearwater isdrainedoff and 
the topsurface of the starchcake is scraped clean; the bottom 
part of the cake may also need scraping. The starch is then 
dug out in lumps, which are again mixed with water and 
allowed to settle overnight. This process may be repeated the 
following day to get good-quality starch free of any dilt. 
8. After the final settling, a clean starch cake results which is 
broken into small bits by hand in preparation for drying. The 
crumblingc an bedonewith sieves, similartothoseused in the 
production of gari. 
9. Drying is done in ovens if the quantity of starch produced is 
above IOOOkg per day. For small quantities, sun-drying is 
used. Starch is deposited on trays which are placed on racks, 
about 1m above the ground at an angle of about 30". These 
simple measures increase the drying speed by a factor of 3. 
If the starch is not dry by the evening, the trays are stacked 
inside and returned to the racks in the morning. Solar dryers 
may be useful in speeding up the drying process. 
An important advantage of sun-drying is the bleaching action 
of the ultraviolet raysof the sunlight. The starch isdried to not 
more than 12% moisture content. During the rainy season, 
when it is difficult to sun-dry starch, it is recommended that 
drum dryers are used to facilitate the drying process. 
Establishing a cassava-processing 
cottage industry 
This section outlines the measures to be taken in establishing a 
rural cottage industry producing gari, cassava flour and cassava 
starch. 
When deciding on the location of the industry, two important 
factors must be taken into account: fresh cassava tubers have a 
high moisture content and are therefore bulky and difficult to 
transport; and cassava is a highly perishable commodity. For 
these reasons, the industry should be located within the cassava- 
producing area or not more than 20km away from it; and links 
should be established with farms or plantations capable of supply- 
ing not less than 50% of the industry's annual raw material 
requirements. 
A separate economic viability analysis should be undertaken for 
each locality before a cassava-processing industry is established. 
Overhead 
. . water tank 
. ..  ..... . . .: ... 
j j  **.**. ..*. a* 
: : Fermentation/sedimentation rack 
a 0 
. : :  peeling 
,W.-aste 
I I -.-.--.., 
I__________ Wet milling 
-D-rain-ag-e -----a-rea- -- 
area 
r& Paadreoan g Bin dryers 
Store 
[ofice 1 storeroom I ~ti~izationu  1 
Figure 9.17 
Layout of rural cassavaprocessing industry 
The profitability of such an industrydependsonmany factorsquite 
independent of its technical viability, such as government policies. 
supply and price of cassava, and the comparative price of the 
imported commodity which may be substituted. 
Where a market for non-traditional cassava products has not been 
developed and is therefore unpredictable, it is more practical for 
the cottage industry to produce afew cassava products instead of 
concentrating on only one. This allows the flexibility to switch 
quickly to the product for which a market demand exists, and will 
also make maximum use of those implements and machines that 
can be used for processingdifferentt ypes of cassava products(for 
example, graters are usedfor bothgari and starch production). The 
recommended output from the industry is: 400kglday of gari; 
250kglday of cassava chips and flour; and 200kglday of starch. 
Structure and design 
Figure 9.17 (on page 104)presents the recommended layout of a 
rural cassava-processing industry. To reduce the cost of construc- 
tion, it is an open structure, apart from the section where the 
finished products are stored; this areashould be walled to the roof. 
The dry milling area should have a concrete floor which is about 
18cm thick; the floor of the wet milling area should also be 
concreted but about 8cm lower than the dry area..Alternatively, a 
cement wall about 15cm high can be built to separate the two 
areas. 
The sink for washing tubers is 60cm deep and fitted with a tapand 
drainage pipe; it stands on a 60cm-high plinth. Some space is 
provided within the structure for stacking the starchlchips drying 
trays during the night. Wooden battens laid between each layerof 
trays allows air to circulate. The drying racks outside are made of 
bamboo poles. The center pole is about IOcm higher than the 
others to provide the necessary tilt. 
Machinery and implements 
The type and quantity of implements and machinery required for 
a rural cassava processing industry are given in Table 9.1 (see engine 
overleaf). 
Machinery for processing gari. The gari-processing machinery 
itemized in Table 9.1 is described here, apart from the petrol and 
diesel engines. 
Cassava grater 
The cassava grater (stationery or mobile) has become a perma- 
nent feature of cassava processing in rural communities. These chute 
graters can grate at least 4 tons of fresh tubers per day, and thus 
only one is needed to handle all the garilstarch processing opera- Figure 9.18 
tions of a rural industry. A typical cassava grater 
Table 9.1. 
Machinery and Implementsfor ruralcassava-processing 
industries 
Item Quantity Description 
Machinery for gar1 
Cassava grater. 1 Powered by 5hp diesel engine 
Cassava press 1 Ouvut 500kglhr: manually operated 
Diesel engine 1 Screw type; 5hp 
Sieving machine 1 Poweredb y 3hppetrolengine 
Gari fryer 2 200kglday capacity 
Petrol engine 1 3hp 
Machinery for 
cassava chips and flour 
Slicing machine 
Dryer 1 Bin Gpe; 200kglday 
Milling machine 1 5hp engine 500kgIhr 
Machinery for cassava starch 
Drum dryer 2 50kgIhr 
Implements and accessories 
Knives (for peeling cassava) 20 Stainless steel 
Plastic drums 30 For fermentation 
Fermentation racks Made of wood 
Drying trays 100 Made of localwooda nd plastic net; 0 . 7 ~1O m 
Basket for sieving starch 10 Localtype 
hishl re meter 1 Multipurpose, suitable for grains and flours 
Weighing scale 1 For fresh cassava up to m k g  
Weighing scale 1 Forfinishedproductupt o 1Okg 
Atypicalcassavagrater incorporatesacylindrical,r otating,w ooden 
drum which is covered with a nail-punched metal sheet (galva- 
nized or tin), as shown in Figure 9.18. The rotary drum is set into 
a casing, with the critical dimension being the clearance between 
the lower part of the drum and the casing; this clearance deter- 
mines the size of the grated particle. 

ated, have beendeveloped in West Africa. The most cost-effective 
type in industries producing less than 500kg of gari perday seems 
to be the RAIDS gari fryer (see Figure 9.20). It consists of a 
rectangular tray set into a brick fireplace w lh  a chimney. The tray 
is made of 3mm-thick mild steel sheets and has a side openina for 
tray discharging the finished product. To produce 400kg per day,lwo 
gari fryers are required. 
outlet cover 
outlet Machinery for processing cassava chips and flour. The three 
blmk wl l items required for processing cassava chips and flour are a slicing 
machine, drying equipment (for natural or artificial drying) and a 
fired milling machine. 
Slicing machine 
A manually operated or mechanized slicing machine is an impor- 
tant investment for producing cassava slices of uniform thickness 
Figure 9.20. to ensure more uniform drying. Such an investment will save time 
RAIDS gari fryer and energy, improve productivity, increase the surface area avail- 
able for drying and produce better-quality chips and flour. 
Slicing machines are popularin Asia but uncommoni n West Africa. 
The tvDe used in Asia consists of a steel framework su.~ . ~or t ina o  
feed hopper, a casing containing the rotor disc and a petrol/di&el 
engine(seeFigure9.21). Thecuningd~misfittedwithlourblades 
which rotate at about 500rpm. The size of the cassavachipis 1Ocm 
x lOcm x 50cm; the optimal thickness of the chip is 6mm for 
through-flowdryingand 1Ommforcmss-flowdrying. The machine 
produces 1 tonof chips per hour, and a single machine is adequate 
for a rural cassava-processing industry. 
Figure 9.21. 
Manually operated slicing machines 
Dryers 
Drying is carried out to reduce moisture content and is essentially 
a process of simultaneous heat and moisture transfer. Heat is 
required to evaporate the moisture from the inside and the surface 
of a product by an externaldrying medium, usually air. Ina  number 
of agricultural crops, including cassava, the drying of single par- 
ticles under constant external conditions exhibits a constant-rate 
moisture loss during the initial drying period, followed by a falling- 
rate moisture loss. This implies that the drying rate decreases 
continuously during the course of drying. 
Drying methods can be classified as natural or artificial. 
(a) In natural drying, the material is subjected to the combined 
action of sun rays and atmospheric air. Naturaldrying can be 
divided into three categories: sun drying, solar drying and 
natural ventilation. 
Sundrying. This is the most common methodofdrying 
in Africa. The material is spread on the ground, roof top, 
damped soil, concrete floor or, more rarely, on an ele- 
vated platform. The material is occasionally turned. 
There are numerous disadvantagesto this method, such 
as reabsorption of moisture from the ground, uneven 
drying, insect and animal invasion, and exposure to dirt 
and dust. 
Solar drying. Sun drying can be speeded up throug the 
introductiono f solar dryers to enhance the effect of solar 
radiation. Solar dryers can be simple box structures cov- 
ered with polyihene sheets, plastic sheets or complex 
structures which incorporate blowers and transparent 
plastic sheets. 
Natural ventilation. Agricultural crops are sometimes put 
onto platforms where they are allowed to dry by natural 
air. The downward flow of air is increased by placing a 
windbreak in front of the platform. 
(b) Artificial drying methods are those which use blowers, heaters 
and other external energy sources. In areas where solar 
radiation and relative humidity are conducive to sun-drying, 
sun-drying should be encouraged. Artificial methods should 
be used only as a supplement to sun-drying during the rainy 
season or during the night or early morning. The three meth- 
ods of artificial drying described here involve the use of drying Figure 9.22 
trays, artificial forced circulation dryers and solar dryers. Multipurpose bin dryel 
Drying trays, measuring 0.7 x lm ,  can be made of plant 
material, but investing in more durable plastic mesh/ 
netting wooden is more cost effective in the long run. 
Cassava chips are laid on the trays (about lOkg per 
square meter), and the trays are placed on specially built 
racks inclined at an angle of 25-30". The number of trays 
needed depends on the rate of production of the chips. 
During the rainy season, when it becomes impossible to 
dry cassava chips outside, artificial forced circulation 
dryers can be used. A suitable design is shown in Figure 
9.22 (onp age 109). The drying chamber is made of ply- 
wood, and the product to be dried is arranged on the 
shelves. The air is blown by a centrifugal-bloweri nto a 
heat exchangerwhich consists of a series of pipes set in 
a fireplace with a chimney; the blower is driven by an 
electric motor ora small petrol engine with a powerof 0.7- 
1 .Okw. The air is heated by the pipes and passes into the 
drying chamber; here it picks up moisture from the 
product and escapes through the opening at the top of 
the drying chamber. About 500kg of cassava chips with 
a moisture content of 12% can be dried in 40-48 hours. 
The blower is driven by an electric motor or a small petrol 
engine with a power of 0.7-1.0kw. 
Solar dryers, such as the cabinet dryer shown in Figure 
9.23, can be constructedfrom locally available materials. 
They enhance the insulation effect and contribute to- 
wards the generation of higher air temperatures and 
lower relative humidities, both of which are conducive to 
Figure 9.23. improved drying rates and lower final moisture content ol 
Cabinet dryer, showing cross-section the dried crop. The highertemperaturesa lso deter insect 
(below) and microbial infestation. 
Milling machine 
The most common type of mill used in Africafor grinding chips into 
flour is a plate mill. This has stationary and rotatings errated plates. 
The clearance between these plates regulates the degree of 
fineness of the milled product. The output of the milled material 
depends on the size of the plate and the power of the motor or 
engine. 
Another type of milling machine, the hammer mill, has a series of 
reversible, flexible hammers fixed radially inside the casing (see 
Figure 9.24). The material is fed through the hopper, and moved 
over the wire mesh screen by the hammers; the size of the milled 
particle is regulated by the screen. 
hHousing 
Screen 15 HP motor 
Figure 9.24 
Hammer mill 
Although the hammer mill uses less energy to produce the same 
output as that produced by the plate mill, it not as suitable for 
small-scale rural cassava-processing industries as the plate mill 
because: 
the availability of spare parts for the hammer mill in most 
cassava-growing areas in Africa is limited (for example, 1 is 
necessary, to replace the screen regularly, and obtaining a 
new screen often poses problems) 
it can be used only for dry material, whereas the plate millcan 
be used for grinding both wet and dry material 
Machinery for processing starch. The Brook dryer is a simple 
device which consists of three 200-liter drums and a screen. The 
drums are laid end to end and are joined together, as illustrated 
in Figure 9.25. Above the drums is the screen. A fire is built in 
the first drum, and the warm air from the fire passes through the 
starch. 
After being dried in the Brookdryer, the starch is ground in a plate 
mill and is then sieved. It is necessary to sieve the starch because 
the standard particle size for starch used for most applications is 
very small. However, if the demand for cassava starch increases, Figure 9.25. 
it may be necessary for the rural industry to invest in pulverizing Brook dryer, showing cross-section 
equipment. (below) 
Packing and storage 
A number of African research institutions, including the Nigeria 
Stored Products Research Institute (NSPRI), have analyzed the 
suitability of locally available packaging methods for long-term 
storage of processed cassava products. 
The most cost-effective storage measures are outlined below. 
Ensure that the product to be packaged is dried to a safe 
moisture content. The amount of moisture in an agricultural 
crop or product is the most important factor determining its 
storability. When determining the moisture content, it is im- 
portant to ensure that: 
(a) the sample is representative of the batch which is being 
examined and the samplings are sufficiently large 
(b) the sample is kept in a sealed container before determin- 
ing the moisture content 
The amount of moisture in a sample of produce which 
does not decompose when the produce is heated can be 
determined by weighing some of the ground produce and 
thendrying it in aforced draft airoven at agiven tempera- 
ture for a predetermined length of time. The drop in the 
weight of the produce is measured according to its initial 
weight (wet basis). 
Allow the product to cool sufficiently before packing it. Latent 
heat which is not released will later condense inside the 
sealedcontainer,r esulting in moldgrowth and insect develop- 
ment. 
The material which has been prepared according to the 
procedure described above can be stored in polythene-lined 
sacksor brown paper bags inquantitiesof about 25kgor more 
for at least 12 months. If the material is to be packed into 
smaller packages (in quantities of 2, 5 or lOkg), thick poly- 
thene bags with a gauge of at least 0.15mm should be used. 
The largerbagsare tiedwithapieceof string, whilethesmaller 
bags are sealed. 
It is important to note that if the moisture content of the flour 
or gari is not sufficiently low and the product is not intended 
for long-term storage, polythene or hessian sacks provide 
better conditions for short-term storage. 
Cassava chips store better than cassava flour. If flour is re- 
quired, the cassava should be stored aschips andthen milled 
into flour just prior to sale or immediate use. Cassava flour, 
like gari, can be stored in polythene or paper bags, as 
described above. 
A fumigant, in the form of a tablet, should be put inside the 
bags. Phostoxin is safe as long as the user follows directions 
for its use provided by qualified agricultural extension person- 
nel. 
Measures to control rodents by mechanical or chemical 
means must be applied. 
UNIT 10 
Utilization of Cassava and 
its Products 
Cassava is an important food in the tropical areas of Africa, Asia 
and Latin America. It isestimatedthatthecropprovidesabout40% 
of all calories consumed in Africa. 
Cassava has often been considered an inferior food because it is 
low in protein, essential minerals and vitamins (see Tables 10.1 
and 10.2 overlea?. However, in many cassava-growing areas its 
use as food helps to alleviate problems of hunger and carbo- 
hydrate intake deficiency and thus its importance in terms of food 
security in these areas cannot be over emphasized. 
Utilization for human consumption 
The cassava tuber is utilized ih many food preparations in Africa. 
It provides most of the calories in a meal, while the vegetables, 
legumes and meatlfish provide the necessary protein, minerals 
and vitamins. Various types of cassava flour are cooked into thick 
pastes by adding water to the flour and stirring the mixture rapidly 
over the fire. Cassava leaves are used as a vegetable in many 
African countries. It is estimated that the crude protein content of 
the leaves is about 25% (dry weight basis). The leaves contain 
more cyanogenicglucosides but the traditional methods of detoxi- 
fication by grinding and prolonged boiling of the leaves seem to be 
satisfactory. 
There are many traditional cassava-based food preparations in 
Asia and Latin America which may be used alongside traditional 
African preparations. In addition, in many African countries home 
economists and nutritionists have developed a number of non- 
traditional food uses by substituting an imported ingredient in a 
recipe with locally grown cassava. 
Compositlon of cassava products prepared traditionally in Cameroon* 
- - -  
cooked n o ~ ~  
Raw Root Bhton Gari Cooked Steeped Steeped Root, cooked Peel, cooked 
pealed cooked gar1 without with and washed and washed 
root in water peel peel (medus-me. 
mbong) 
Calories 
Proteins (g) 
Lipids (g) 
Total carbohydrate (g) 
Indig. carb. (g) 
Ash (g) 
Calcium (mg) 
Phosphorus (mg) 
CaiP 
Fe (mg) 
Thiamine (pg) 
Riboflavin (kg) 
Niacin (pg) 
Ascorbic acid (mg) 
Note: ' Per iWg dry maw, 
SOUICB: Favier, J.C. el A. 1971 'Lafechnologiet rad'Rionel1 du manioc au Cameroun: Influence sur lavaleur nutritive' 
Only low-cyanide cassava varieties are recommended for meals 
prepared from fresh cassava without fermentation because nei- 
ther boiling nor frying efliciently eliminates cyanide from cassava. 
A list of well-testedc assava recipes has been compiled in various 
countries of Asia and Africa. 
Cassava flour is used in many bakery products, especially bread. 
Research into the use of cassava flour in bread has shown the 
Composit ion of cassava leaves and selected other foods in terms o f  per 1009 edible portion, fresh weight 
Calories Moisture Protein Fat Total Fibre Ash Ca 
% B g Carbo- g II mg 
hydrate g 
Cassava leaf, raw 91 71.7 7.0 1. O 18.3 4.0 2.0 303 
60 81.0 6.9 1.3 9.2 2.1 1.6 144 
Chinese cabbage, raw 17 94.2 1.7 0.2 3.1 0.7 0.8 102 
Spinach, raw 19 93.0 2.4 0.4 2.8 0.7 1.4 62 
Soybean whole seeds 
salted, bladc 330 20.1 18.1 9.4 46.3 8.5 6.1 29 
Wheat whole grain, hard 332 12.5 11.6 2.2 72.1 2.1 1.6 48 
Maize, yellow 349 13.6 9.1 4.2 71.7 2.3 1.4 14 
Rice, unhuiled, rough 341 13.7 5.8 2.3 73.4 10.4 4.8 24 
P Fe Vitamin A Thiamine Rlboflavln Niacin Ascorbic Reference 
mg mg O Carotene mg mg mg acid 
equivalent pg mg 
Cassava leaf, raw 119 7.6 11,775 0.25 0.60 2.4 8 a 
68 2.8 8,280 0.16 0.32 1.8 82 b 
Chinese cabbage, raw 46 2.6 2,305 0.07 0.13 0.8 53 b 
Spinach, raw 39 3.9 3,640 0.06 0.22 0.7 56 b 
Soybean whole seeds 
salted, black 163 1.1 520 0.07 0.27 18.6 
Wheat whole grain, hard 382 3.3 0 0.37 0.12 4.6 0 b 
Maize, yellow 245 2.8 270 0.29 0.1 1 2.1 0 b 
Rice, unhulled, rough 236 1.4 0.33 0.06 5.6 b 
Sour-: a: Food C o w h i o n  T a b l or Use in Africa Food and Agric Org, and US Depl. Healh. Educ. and Werare. 1968 
b: Food Corrposhion T a b f or Use in East Asis. Food and Agi i .  Org. and US Depl. Health. Educ, and Wenars. 1972 
process to be a viable technical proposition. Good quality breads 
have been produced with 10-100% of the wheat flour being 
substituted by cassava flour (see Figure 10.1). With regard to 
cakes and biscuits, 100% cassava flour can be used with good 
results. There is little information on the storability of these prod- 
ucts, but it is known that they can be stored for up to 2 days. 
Figure 10.1. 
Bread with 20% cassava flour made from llTA improved varieties 
Some of the most common cassava-based foods in Africa are 
listed below. 
Abacha Boiled, shredded and dried cassava slices (simi- 
lar to noodles); eaten in salads. (Nigeria) 
Ampesi Boiled cassava roots; normally eaten with vege- 
tableimeat soups or stews. (Ghana) 
Agbele k d l o  Deep-fried snack in the shape of croquettes or 
balls preparedfromgratedcassava mash; eaten 
as snacks. (Ghana) 
Akple Thick porridge preparedf rom a mixture of maize 
and cassava dough; eaten with okro soup or 
stew. (Ghana) 
Attieke Steeped, pounded, fermented cassava roots 
which arepressed, cntmbled and steamed; eaten 
with milk or meat and vegetables. (CBte d'lvoire) 
Baton du manioc Wet cassava paste wrapped in leaves, shaped 
as a long stick (30-60cm) and cooked. (Cam- 
eroon) 
Chickwangue Like baton du manioc, but shaped into a ball. 
(Cameroon) 
Elubo lafun Thick paste prepared from traditional cassava 
flour (lafun): eaten with vegetablelmeat soup. 
(Nigeria) 
Fufu Boiled cassava pounded with plantain or with 
cocoyam; eaten with various soups. (Ghana) 
Foofoo Soaked, pounded and fermented mash which 
is then mixed with water, sieved, and cooked 
into a thick paste; eaten with stew. (Sierra 
Leone) 
Gari (garri) Grated, fermented, sieved and fried cassava 
mash; in its final form, it is afree-flowing granu- 
lar meal; used inavariety of ways in main meals 
and as a snack. (West Africa) 
Garifoto Combination of gari and fish or egg sauce to 
make a one-dish meal. (Ghana) 
Kokonte Dried, unfermented cassava chips, milled into 
flour and made into a thick paste; eaten with 
soups. (Ghana) 
Kowou-kowou Thin gruel made by adding some fermented 
cassava flour to boiling water. (Cameroon) 
Kumkum Cassava flour prepared from fermented tubers 
by grating, forming into balls, and drying over 
the fireplace; the dried balls are stored until 
required and marketed as balls or flour. 
(Cameroon) 
Kpokpo gari Peeled and soaked tuberswhicharethen grated, 
washed, dried, roasted into large hard grains 
and then soaked in water again; eaten with side 
dishes. (Nigeria) 
Njambo Dried, fermented cassava chips, milled into 
flour and made into a thick paste. (Gambia) 
Wet c; panlally dried sieved starch particles 
heatsd with continuous stirring, forming 
gelatinised, cb:ic;d grani~lese; aten as breakfast 
porridge. (Wcst Africa) 
Dried cassava chips pr~ducedby  sun-drying, or 
stnepad and fermented priorto drying, and then 
made into a thick paste; eaten with soup. (Tan- 
zania) 
Yakeyake Steamed cassava dough. (Ghana) 
Utilization for livestock feed 
There is great potential for using cassava feed rations in local 
livestock industries (see Table 10.3). The production of cereals, 
especially maize, is not high enough to meet the energy require- 
ments of both human beings and livestock. Since the 1960s, some 
EEC countries have used cassava chips in the preparation of 
compound animal feed because of cassava's high energy content 
and low price. 
Research carried out by the Institute of Agricultural Research and 
Training in Nigeria has shown that substituting up to 44% of the 
maize in pig feed withcassavadoes not causeany reduction in the 
performance of pigs; in fact, with the addition of 0.1-0.2% DL 
methionine, the performance of pigs fed on diets containing more 
than 50%cassava meal is improved. It has also been reportedt hat 
the use of cassava in the diet of white fulani herds in Nigeria has 
increased milk production by 22%; this has been accompanied by 
an increase in percentagesof butterfat, protein and non-fat solids. 
Utilization in industry 
Cassava starch is an important industrial raw material. More than 
100 starch derivatives (chemically modified starch) have been 
developed to provide products which have the physical andlor 
chemical properties required for speclic applications. However, 
the capital investment for the production of starch derivatives is 
fairly high, and a careful economic analysis must be made before 
establishing an industly to produce these derivatives. 
Raw, unmodified cassava starch can be used successfully,a nd in 
many instances advantageously,f or industrial applications where, 
formerly, cheaply produced maize (USA and Canada) or potato 
Animal feed rations using cassava meal 
Cassava meal lncluslon rates7 
Percentage cassava meal (dry) 
Type of feed Cautious maximum 
Broiler starter 5 10 
Broiler finisher (4 weeks) 10 20 
Chick starter 5 10 
Pullet grower 10 25 
Layer 25 40 
Sowlboars 10 30 
Piglets (to 8 weeks) 5 10 
Pigs (8-16 weeks) 10 25 
Pigs (16 wweks-maturity) 15 30 
Lsyers mash' 
Cassava tuber meal 40.8 
Cowpea* 20.0 
Blwd meal 16.8 
Bone meal 2.5 
Oyster shell 7.8 
Salt 0.24 
Minlvit mix (Pfizer) 0.24 
Nnes: ' C~wpeam ust be r m s d  before milling to remove aminutrlional radon 
Dia is nn balanced for aulphoamim acas. Mvbable to chec* accept&illty to ch'ck~ns 
sow-: -Dr T-. Unlvenny n lbadan Ngena; ' 'Fed international MayiJune, lPBD 
(Europe) starches were used. This is because starches are highly 
interchangeable. 
Starch has wide applications in industry. It is used in the food 
industry in many preparations, including sauces, gravy, mustard 
powders, baby foods, tapioca products, glucose production, con- 
fectionery and bakery products; it is also used as a jelly or 
thickening agent. It is used extensively in the manufacture of 
adhesives, dextrins and pastes and as a filler in the manufacture 
of paints. In the textile industry, it is usedforwarp sizing, cloth and 
felt finishing. 
Good quality cassava starch can be produced by cassava growers 
to meet standard specificationsfort he local market or export. This 
has been done in some developing countries, including India, 
Thailand and Malaysia. The important factors affecting the quality 
of starch are its color, uniformity of size, moisture content, purity 
and pH. The most desirable end-product should be a clean, white 
starch, free from specks, dirt and insect infestation, with a mois- 
ture content of about 12%. 
Part IV 
RESEARCH 
UNIT 11 
Data Collection and 
Organization 
In most field trials a number of treatments are involved. These 
treatments are assessed on the basis of some formof experimen- 
tal design which allows 'analysis of variance' to be carried out; in 
other words, the experimental design provides the means for 
comparing the variation attributable to differences between the 
treatments with the unavoidable variation between plots (the 
error). 
The most commonly used and simple experimental designs are 
Completely Randomized Design and RandomizedC omplete Block 
Design. 
Completely Randomized Design 
A simple field trial carried out to test the differences between four 
varieties of cassava -which will be called varieties A, B, C and 
D - is used here to illustrate the use of Completely Randomized 
Design. Some degree of replicationof each treatment is essential 
to obtain an estimate of error, so assume that there are four 
replicates. 
How should these 16plots (4varietiesx4replications) bearranged 
in the field and how should the data collected from the plots be 
analyzed? 
Unless the experimental site is absolutely uniform (which is never 
the case in practice), some form of random allocation of varieties 
or treatments to individual plots is necessary in order to avoid any 
systematic treatment bias. The simplest way to do this is to 
allocate all 16 plots completely at random to form a Completely 
Randomized Design. An example of a Completely Randomized 
Design is shown below: 
The form of the analysis of this design is to apportion the total 
'variation' between the yields of the various plots into that part 
which can be attributed to treatment effects and a remaining 
residual part which gives an estimate of the 'variation due to 
sampling error'. Variation is measured in terms of variances (that 
is, Sums of Squares of Deviations, SS, divided by Degrees of 
Freedom). 
An example of an 'analysis of variance' table for Completely 
Randomized Design (with hypothetical SSs and variances) is 
shown below in Table 11 .l. 
Table 11.1 
Analysis of variance table for Completely Randomized Design 
Sources of Degrees of Sums of Variance 
Variation Freedom Squares of or Mean 
(DF) Deviations Lquares) 
(SV) (SS) (MS) 
Total 15 100 
Treatments 3 40 13.33 
Residual (or error) 12 60 5.00 
Nmm: 
1 The DF foilhetotal is 1 lsu than the lotd number d plots: that fortreatrents i8 1 brs than the 
number ot 1reaPments. 
2 For calculation d T a d  and Treatram SS. m page 127. 
3 R s i d ~ aSiS  and OF are -1 e*iiy sakulated by dineren-: 8s. total rncnus lrealmnl. 
The residual or errorvariance gives a measure of normal variation; 
the square rootof this measure is termed the 'Standard Ermr (SE) 
of asingle plot', which is an estimateof the standarddeviation( SD) 
of individuals. Thus SEs of treatment means and SEs of differ- 
ences between treatment means can be calculated, allowing a t- 
testto be carriedout between a given pairof treatments. However, 
a prior step in testing significance is normally an overall test of all 
treatments by comparing the treatment variance with the error 
variance; this is termed the F test. (Details of these tests are 
discussed later in this unit.) 
Randomized Complete Block Design 
In practice, the Completely Randomized Design is used only when 
the experimental site is extremely uniform (for example, in a 
laboratory or greenhouse). Randomized Complete Block Design 
usually offers a considerable advantage over the Completely 
Randomized Design. 
In RandomizedCompleteB lock Design, thetreatments are grouped 
into blocks (or replicates) containing 1 plot of each treatment 
arranged at random. For example: 
Block I B A C D  
Block II C A D 6  
Block Ill D C B A  
Block IV B D C A  
This Randomized Complete Block Design allows the effects of 
environmental differences between the blocks to be measured. 
Whereas in the Completely Randomized Design these effects 
remain part of the residual variation, in the Randomized Block 
Design the residual variation is decreased by the removalof these 
block effects. This usually leads to a lower error variance, lower 
SEs and a better chance of achieving significant differences 
between treatments. 
Using the example which formed the basis of Table 11.1, the 
analysis of variance table takes the following form (with hypo- 
thetical SSs and variances): 
Table 11.2 
Analysis of variance table for Randomized Complete Block 
Design 
Total 15 100 
Treatments 3 
Blocks 3 
Error 9 
N a a :  
1 Block DF is 1 less than the nurrfsr d bl& 
2 Em! DF and SS again dculael by d'merence 
w 
Example of analysis of variance for 
Randomized Complete Block Design 
Three cassava varieties - N, E and F - are compared in a 
Randomized Complete Block Design with four replications. The 
layout of the plots and yield in kglplot are as shown below. 
Block I Block II 
Treatment F N E  E N F  
Tuberous 
root yield 359 330 372 340 288 337 
Block Ill Block IV 
Treatment F N E  E F N  
Tuberous 
root yield 373 295 343 341 302 313 
The objective is to carry out an analysis of variance to determine 
if these varieties are significantly different in yield. 
Step 1 
Form a two-way table of blocksx treatments andformthe blockand 
treatment totals and the 'grand total'. 
Table 11.3 
Two-way table of blocks x treatments 
Block l Block ll Block Ill Block lV Treatment totals 
Treatment N 330 288 295 313 1226 
Treatment E 372 340 343 341 1396 
Treatment F 359 337 373 302 1371 
Block totals 1061 965 1011 956 3993' 
Ndm: 
mb ward total b termed the'gmnd t ~ d w. h en drawing up any ray tabla ahays dleckthr~th e 
sum d the totals on each ol t b m o  rides is qua1 tothli grand mid. 
Step 2 
Calculate the Sum of Squares of Deviations. The Sum of Square? 
of Deviations for N individual x values is best calculated using the 
following formula: 
In this experiment, the basic x values are the weight of tuberous 
rootslplot and the 'Total' Sum of Squares of Deviations for treat- 
ments or blocks is calculated thus: 
Total SS = 330' + 372'+ 359' + 288' ..... 302' - (3993)' 
= 1337535 - 12 
1328671 = 8864 
Whenthesexvalues aregrouped intotreatmentorblocktotals,the 
Sumof Squares of Deviationsfortreatmentsorblocks iscalculated 
from these tatals using a similar formula: 
where T represents each treatment or block total and N, repre- 
sents the number of x values which are added to form each 
appropriate total. 
For this experiment the calculations are: 
Block SS = 0612 + 9652 +  10112+9562 - (3993)2 
3 12 
Treatment SS = 12762 + 1390 + 13712 - G?B%v 
4 12 
Note that ..  occurs in all the calculations of Sum of Squares 
of Deviations, so in practice it is convenient to calculate this first. 
This term is usually called the Correction Factor (CF). Note also 
that although the second formula above uses totals, it does in fact 
give the Sum of Squaresof Deviations of meansfromt heir general 
mean. 
Finally, the 'Error' Sum of Squares of Deviations is calculatedt hus: 
Error SS = Total SS - Block SS - Treatment SS 
= 8864 - 2330 - 421 2 = 2322 
Step 3 
Calculate the Variances (or Mean Squares) by dividing the Sumof 
Squares of Deviations by the appropriate Degrees of Freedom: 
Block Variance 2330 = 776.66 
3 
Treatment Variance -42-12  - 2106.00 
2 
Error Variance 2322 
- = 387.00 
6 
Step 4 
Test for significant differences by means of an 'F test'. An 'F'v alue 
(or Variance Ratio) for treatments is formed by dividing the 
Treatment Variance by the Error Variance: 
F for Treatments = 2106.00 = 5.44 
387 
If this F value is large, it means that the treatment differences are 
much larger than those attributable to normal variation (remember 
that the Errorvariance is takenasanestimateof normalvariation). 
The level of significance of an Fvalue is determined by comparison 
with an F value from tablesobtainable from any standard statistical 
textbook. These table values show that an F value derived from 
2 Degrees of Freedom for treatments and 6 Degrees of Freedom 
for error must be at least equal to 5.14 to achieve 5O/0 significance 
and 10.12 to achieve 1% significance. In this experiment, the 
calculated F value is 5.44, indicating that the treatment effects are 
significant at 5%. 
It may also be of interest to test for significant differences between 
blocks. Int hiscase. thecalculated Fvalue is 2.00 (seeTable 11.4) 
and the tabulated value is 4.76 at 5% therefore, there was no 
significant difference between blocks. 
The complete analysis of variance table may now be drawn up: 
Table 11.4 
Complete analysla of varlance table 
SV DF SS MS F 
Total 11 8864 
Block 3 2330 776.66 2.00 
Treatments 2 4212 2106.00 5.44' 
Error 6 2322 387.00 
Step 5 
Calculate a'Least Significant Difference' (LSD) todetermine which 
treatments are significantly different. 
This patiof the analysis is essentially aformof the simple t-test. In 
a t-test, a t value for two treatment means is cakulated by 
expressing the actual difference between the means as a number 
of SEs. This is then compared with the tabulated twhich indicates 
the least numberof SEs necessaryforsignificance. Whencompar- 
ing a number of treatment means in an analysis of variance, 
however, it is usually more convenient to calculate the absolute 
difference which is necessary for significance; this is termed the 
Least Significant Difference. It is calculated by multiplying the SE 
of the difference behnreen two means by the appropriate tabulated 
tvalue obtainable from any standard statistical textbook. The SE 
is based on the Errorvariance because it is against some estimate 
of normal variation thattreatment differences are being compared. 
Thus this SE can be written as: 
Error Variance (for mean A) + Error Variance (for mean B) 
N N 
where N isthe numberof individual itemsofdatacontributingto the 
mean. Since, in a simple analysis of variance, N is the same for all 
treatment means, this SE can also be written as: 
/ - - 
~ r GVra riance x 2 
and the LSD (at 5%): 
4 eOr x t (at Error DF and at 5%) 
N 
For this experiment: 
LSD = x 4 = 34.038kg 
This is therefore the least difference which must occur between 
two treatments for them to be declared significant at 5%. (LSDs 
can also be calculated at 1% or 0.1%) 
Table 11.5 
Comparison of treatment means and LSD 
Treatments LSD 
at 5% 
N E F 
Comparing treatment differences with the LSD, we conclude that 
the means of treatments E and Fare significantly greater than for 
N, but treatments E and F are not significantly different. In other 
words, varieties E and F significantly outyieided N (which may be 
a local check) but varieties E and F are not significantly different. 
(E - N = 349.0 - 306.5 = 42.5 sig.) 
(F - N = 342.7 - 306.5 = 36.2 sig.) 
(E - F = 349.0 - 342.7 = 6.3 NS) 
Step 6 
Calculate the Coefficient of Variation (CV). The CV simply ex- 
presses the SE of a single plot 
q ( ~ r r oVra riance) 
as a percentage of the general mean, thus giving a relative 
measure of the degree of variability within the data. A high CV 
represents a high degree of variability. Such variability is usually 
only partly the result of the inherent variability of the parameter 
being measured, because considerable variability can arise as a 
result of the way experiments are handled. Thus the CV helps to 
indicate the degree of accuracy with which an experiment has 
been conducted. For this experiment: 
CV = 4E rror variance x 100% ',!zzE x 100% = 5.91% 
General mean 332.75 
For yield trials of this sort, a CV of up to 15% or even 20% is 
acceptable, so this CV is good and indicates a high level of 
accuracy. It is important to note, however, that in this particular 
example the analysis of variance was carried out using the yield 
per plot. For better comparison, yield per plot is convelted to yield 
per hectare before carrying out the analysis. 
Some practical considerations in 
designing experiments 
The following practical points should be borne in mind when 
designing an experiment. 
Need to avoid bias 
It is very important to avoid designing an experiment to test a 
preconceived opinion. For exanlple, when carrying out a variety 
trial to test three varieties A, Band C, the experiment is biased in 
favorof variety A if the person carrying out the experiment is 
convinced that variety A is superior. 
Treatments 
Include a large enough number of treatments to provide as much 
informationa spossible. Forexample, whencarrying out afertilizer 
experiment it is important to include a wide range of levels from 
minimum to maximum so as to reveal all possible effects. It is 
important to include a control treatment or check as the basis of 
comparisons (for example, a 'no fertilizertreatment' in the case of 
afertilizer experiment, ora known variety or local check in the case 
of a variety trial experiment). 
Record taking 
It is essential to take records regularly throughout the duration of 
the experiment. This helps provide an account of the relative 
significance of each treatment. For example, one or two plots may 
be severely damaged by pests ordiseases during the course of the 
experiment and, consequently, yield forthe treatmentsconcerned 
may be extremely low. If regular records are taken, it is possible 
to account for the low yields. 
Plot size 
Both the accuracy and the value of the results which are obtained 
from an experiment are influenced to a great extent by plot size. It 
is important to avoid using extremely small plots for field experi- 
ments because small plots tend to exaggerate the results and 
increase the error. 
For example, an error of 5kg on a .05ha plot could easily convert 
to an errorof 1OOkg per hectare. For this reason, the results which 
are obtainedfrom a plot experiment are neverexpressedo n a hec- 
tare basis. Experimental error diminishes as plot size increases, 
although this reduction is smallerwhenthe plot size is greaterthan 
,025ha. 
However, small plots are usually necessary when a large number 
of varieties are being tested (for example, in a breeding program 
when a large number clones are being evaluated). At IITA, a 
standard plot for Uniform Yield Trial for Cassava consists of four 
rows of 10m each. The central two rows are harvested for yield 
estimates and converted to yield per hectare. In general, the 
following factors influence the size of the plots used: 
type of crop (this influences the spacing and, therefore, the 
size of the plot; it is common practice to use relatively larger 
plots for cassava) 
number of treatments (the larger the number of treatments 
involved, the smaller the plot size) 
type of machinery used in planting, weeding and harvesting 
land area available for experiment 
labor and funds available for the field trial 
Plot shapes 
It is important to select plot shapes that minimize both the effects 
of soil fertility differences and bordereffects. Where there are large 
differences in soil fertility, long narrow plots are probably the best 
compromise when plots are laid along the contours. Fertility drift 
is usually at right angles to the contours. If the land and soil fertility 
are reasonably uniform, square plots are usually preferred be- 
cause these types of plots have the shortest perimeter and 
therefore the smallest discards for a given area. For example: 
30m 90m 
A B 
Area = 900m2 Area = 900m2 
Perimeter = 120m Perimeter = 200m 
Square plotstend to minimize bordereffects. In practice, however, 
thereare alwaysdiscard rows to reduce errorresultingfrom border 
effects. 
Interplot competition 
Competition often occurs between adjacent rows of different 
treatments and this may lead to serious errors invariety trials. This 
is particularly likely to occur where the varieties involved differ 
markedly intheirgrowth habits. Forexample, interplotcompetition 
may raisethe yieldof avigorousvariety and lowerthe yieldof a less 
vigorous one when they are planted in neighboring plots. Where 
interplot competition is expected, the effect can be minimized by 
using wider alleys. 
In spraying and dusting experiments, it is also vital to consider the 
effects of drifting chemicals. This can be minimized by the use of 
so-called 'drift rows' between treatment plots. 
Intra-plot competition 
Intra-plot competition arises as a result of uneven distribution of 
plants within each plot (rectangularity effects). In other words, for 
the same number of plants per plot, it is possible to have different 
distribution patterns which lead to differing levels of competition, 
resulting in serious error in yield estimates. 
Similarly, differences in the stands or number of plants per plot 
could lead to serious errors. Thus it is usually important to try to 
achieve a uniform stand in any variety trial experiment. 
Randomization 
Experimental conditions are rarely uniform except in laboratories 
or greenhouses. Random allocation of treatments is essential to 
minimize the effects of unknown factors. This reduces the bias 
which may be introduced in the results. It is important to have a 
separate randomization for every experiment. Avoid using the 
same randomization for several seasons or experiments because 
this will destroy the value and applicability of the results. 
Replication 
Differences in soil fertility in the experimental field are a major 
source of error. The most practical way to reduce error resulting 
from differences in soil fertility is to replicate the treatments. 
Replications provide an estimate of the magnitude of the error and 
reduce the error, thereby increasingt he applicability of the results. 
The number of replications required in any given experiment 
depends to a large extent on the variability of the test material and 
to some extent on the cost of labor involved in maintaining the ex- 
periment. 
Generally, it is betterto use more than four or five replications in a 
field trial. The rule of thumb is to provide for not less than 10 DF 
for the error term. 
Local control 
This refers to the grouping of treatments into blocks. It allows for 
the elimination of a certain proportion of the totalvariation which is 
irrelevant in making comparisons. 
In simple experiments, each block or replicate contains the same 
numberof treatments distributed at random. Thus the experimen- 
tal error is reduced because variation between plot yields can be 
partly attributed to a measurable amount caused by block differ- 
ences. This leads to a lower Error Variance, IowerSEs anda better 
chance of achieving significance. 
Standard scoring system for major 
diseases, pests and agronomic 
characteristics 
The Root, Tuber and Plantain Improvement Program at IlTA con- 
ducts research on cassava, yam and plantain. The overall objec- 
tive of the program is to develop varieties which have high stable 
yieldsand resistancetothe majordiseasesandpests, andaresuit- 
able for the main types of utilization. 
The elite materials from IITA are tested by the national agricultural 
research systems (NARS) underconditions likethose in wtiich the 
materials will eventually be cultivated by farmers. In order to 
ensure that information received from NARS is useful for further 
improvement, it is important that a standard procedure for record 
taking and scoring is developed and adopted by the collaborating 
scientists. 
This section provides some information on record taking and 
scoring for the major diseases, pests and agronomic characteris.. 
tics used in a cassava breeding program. During the growing 
season, data should be recorded on data sheet similar to the one 
shown below in Figure 11 . l .  
Figure 11.1 
Record sheet used in a cassava breeding 
program Diseases 
The major diseases of cassava are ACMV, CBB and CAD. 
Cassava mosaic virus. Leaves are reduced in size, misshapen 
andtwisted, with chloroticareas Separated by green areas. Leaflets 
may show a nearly uniform mosaic pattern. 
ScoringforACMV is done at 1,3a nd 6 months afterplanting, using 
the following scoring system: 
1 = no symptoms observed 
2 = mild chlorotic pattern on entire leaflets or mild distortion at 
base of leaflets, rest of leaflets appearing green and healthy 
3 = strong msaic pattern on entire leaf, and narrowing and dis- 
tortion of lower one-third of leaflets 
4 = severe mosaic, distortionof two-thirdsof leaflets andgeneral 
reduction of leaf size 
5 = severe mosaic, distortion of four-fifths or more of leaflets, 
twisted and misshapen leaves 
Cassava bacterial blight. The manifestation of the severity of 
CBB depends on plant age and time of evaluation. Thus, scoring 
is done once during the peak of the rainy season and once at the 
end of the rainy season, using the following scoring system: 
1 = no symptoms 
2 = only angular leaf spotting 
3 = exclusive leaf blight, leaf wilt and defoliation, and gum 
exudation on stems and petioles 
4 = extensive leaf blight, wilt, defoliation and stem die-back 
5 = complete defoliation and stem die-back; stunting and die- 
back of lateral shoots 
Cassava anthracnose disease. CAD is characterized by light to 
dark brown oval lesions on the soft green stems and at leaf axils; 
the lesions on the axils lead to petiole epinasty, petiole necrosis, 
wilting, and defoliation. On older plants, pale brown, shallow de- 
pressions appear on the stems and, as the stem becomes woody, 
develop into deep cankers. The following scoring system is used: 
1 = no symptoms 
2 = few shallow cankers on woody stems, late in the growing 
season 
3 = many deep cankers on woody stems followed by distortion 
4 = many oval lesions on green stems 
5 = many lesions on green stems and severe necrosis at leaf 
axils, followed by wilting and severe defoliation 
Pests 
The major pests affecting cassava are CGM and CM 
Cassava green mite. This dry-season pest attacks the young 
portion of the shoot. Initially, yellowish (chlorotic) 'pinpricks' ap- 
pear on the surlace of newly formed leaves. Symptoms vary from 
a few chlorotic spots to complete chlorosis. Heavily attacked 
leaves are stunted and deformed, and severe attacks cause 
terminal leaves to die and drop, producing a 'candlestick' appear- 
ance. Scoring is best done at the two transition periods (the rainy1 
dry season transition and drylrainy season transition), using the 
following scoring system: 
1 = no obvious symptoms 
2 = moderate damage, no reduction in leaf size, scattered chlo- 
rotic spots on young leaves 
3 = severe chlorotic symptoms, slight reduction in leaf size 
4 = severe chlorotic symptoms and leaf size of young shoot 
severely reduced 
5 = very severe chlorosis and significant reduction in leaf size 
and young shoot portion; extensive defoliation; candlestick 
appearance of young shoots 
Cassava mealybug. CM is also adry-season pest. The terminal 
shoots of damaged plants become stunted and deformed. Inter- 
node length is reduced, causing twisted stems. Severe attack 
leads todeath, starting at the plant tip. Scoring is done atthe peak 
of the dry season, using the following scoring system: 
1 = no obvious symptoms 
2 = slight bunch topappearance, and slight reduction in leaf size 
and internode length 
3 = moderate bunchtop symptoms, and serious reduction in leal 
size and internode length 
4 = severe bunch top symptoms; obvious reduction of internode 
length and severe reduction in leaf size and leaf area 
5 = candlestick appearance; internode length reduced, young 
potion of shoot curved and completely defoliated 
Agronomic characteristics 
The main agronomic characteristics to be scored are root size, root 
shape, neck length, skin color, lodging and flowering. 
Root size. The scoring system used for root size is: 
1 = very small (less than 0.5kg) 
2 = small (0.5-1k g) 
3 = medium (1-2kg) 
4 = large (2-5kg) 
5 = extra large (more than 5kg) 
Root shape. The scoring system used for root shape is: 
1 = round 
2 = oval 
3 = medium, long 
4 = fat, long 
5 = thin, very long 
Neck length. The scoring system used for neck length is: 
1 = short or no neck 
2 = about 5-7cm long 
3 = about 7-10 cm long 
4 = about 10-15cm long 
5 = over 15cm long 
Skin color. The scoring system for skin color is: 
1 = white 
2 = light brown 
3 = dark brown 
Lodging. Record the number of plants lodged at an angle of more 
than 45". 
Flowering. Record the number of days (not dates) from planting 
to the time when 50% of the seedlings flowered. 
UNIT 12 
On-Farm Research 
The objective of On-Farm Research (OFR) is to identify, in co- 
operation with farmers, improved farming practices which are 
adaptable to farmer conditions and will raise productivity in a 
sustained way. With the farmer as a research partner, new 
technologies are tested under farmers' conditions; their accepta- 
bility and profitability is closely monitored; what is not appropriate 
is rejected;what ismodifiable is modified; and, in the light of results 
obtained, newtechnologiesare incorporated. Inotherwords, OFR 
is a continuous process, with each phase built upon the experi- 
ences of previous phases. 
For new technology to be adopted by farmers, it must solve some 
of their constraints without creating new ones of the same magni- 
tude or it must tap some of their unused, readily available re- 
sources. An adequate choice of technologies therefore requires 
good knowledge of farmers' conditions and of the farming system 
they practise. 
The farm as a system 
A system is an orderly arrangement of parts which are performing 
various functions to achieve an overallobjective.A  farming system 
emerges as the result of the decisions made to devote a set of 
resources to a set of activities in order to meet the requirementsof 
the farm family. 
There are two dimensions to the farming system environment - 
material and human. The material environment consists of physi- 
cal elements (such as precipitation, temperature, topography. 
solar radiation and soil) and biological elements (such as natural 
vegetation, and plant and animal pests and diseases). These 
physical and biological elements determine what type of crops 
can be grown in aparticular area, given a suitable human envimn- 
ment. 
The human environment consists of economic, institutional and 
social elements. Economic elements include the economic policy 
of the country or region. This policy determines quantities, abso- 
lute and relative prices of inputs and outputs, and the physical 
infrastructure (such as transportation, water supply, health serv- 
ices, and marketing, processing and storage facilities). Institu- 
tional elements include the laws of the area, credit and marketing 
conditions, contractual arrangements, extension services, prop- 
erty rights to land, water, trees and pasture, seed distribution, 
quality control of inputs and outputs, grading and measuring 
systems, educational institutions and taxation. The social eie- 
ments include the culture and customs of a community. 
On-farm research process 
Research under farmers' conditions starts with the collection of 
data on the farming system and its environment. This includes a 
study of existing sources of information, the gathering of second- 
ary data and an informal exploratory survey. 
The purpose is to analyze the system's major material and human 
elements, to understand the goals of the farmer, to determine the 
major factors that influence hislher decisions, and to describe the 
resource flows and how they relate to eachother (see Figure 12.1) 
Monitoring the degree of adoption (the only valid proof of success) 
is partot the OFRpmcess. Educationf or massadoption, however, 
isan extension activity beyond the scope of OFR, although pre- 
extension tests which validate a technical package and guarantee 
its readiness for mass adoption are a component of OFR. 
In essence, OFR embraces: 
1. Choice of research area 
2. Initial collection of data through exploratory surveys and the 
study of existing secondary information 
3. Choice or design of new technologies for testing 
4. On-farm testing and evaluation, including monitoring of adop- 
tion 
5. Special studies 
+On-farm research 
Setting up pilot area  
1 
Broad environmental 
I-+ information characterisation 
1 1 Choice of innovations 14 
+ Prototype innovations 
I udemonstration  
1 
Extension takes over 
Figure 12.1. 
Flow chart of OFR activities and their inter-relationships 
The on-farm research team 
The OFR teamcomprises scientists, field assistants and extension 
agents. It is recommended that the core includes at least two 
experienced research officers - an agronomist and an agricul- 
tural economist - and that the field assistants should have 
received training in implementing trials and collecting agronomic 
and economic data. The field team is headed by a junior re- 
searcher, pethaps a first-degree holder. Researchers in other 
disciplines (fmm the team's institution or from other institutions, 
such as universities) are invited to participate when needed; for 
example, soil scientists and sociologists can provide cnrcial input 
for the exploratory survey and design of trials. 
The team, by the nature of its work, enters an area that has 
traditionally been Sewed by the extension service. Tasks overlap; 
the extension agents have much to offer the team as a result of 
their experience in the area, and will ultimately be responsible for 
disseminating successful technologies deriving from the team's 
work. It is therefore important to have one or two local extension 
agents associated with the field team to participate in such activi- 
ties as the exploratory survey, trial design, supervision, monitoring 
and farmers' field days. 
The target area and the pilot research area 
Because of the intensive nature of OFR, the area chosen by the 
field team must be manageable and it must be representative of a 
larger area. This is sometimes called the 'extrapolation' or the 
'target area'forwhichthe research results are relevant (seeFigure 
12.2). 
Flguro 12.2. 
Target areas with their representative pibt research area (PRA). 
Arrows indicate assumed applicability of the resuns 
Choice of the target area. The choice of a target area for OFR 
should reflect: 
the research institute's mandate (the crop, region or ecologi- 
cal area that is the focus of the institute's activities) 
the government's development priorities (for example, 'prob- 
lem areas' or 'high-potential areas' 
the need for a single homogeneous ecological zone (that is, 
with minor differences in climate, soil associations and vege- 
tation) where differences in population density, ethnic groups 
or farming systems are minor 
In  choosing the target area, soil maps, climatic charts and other 
geographical documents are studied and a reconnaissancet our is 
conducted. 
Choice of a representative pilot area. In conventional multi- 
location and demonstration trials, the experimental sites are dis- 
tributed across the target area. ~ecauseotfh e intensive nature of 
OFR, one or a few compact pilot areas that can be considered as 
a model for the whole target area are chosen. A pilot area should: 
incorporate all the microvariability of the target area, such as 
differences in access and distance to roads and markets, 
small-scale soil variations and population density 
be manageable during the testing phase (not more than 10- 
15km can be traveled daily by field staff on bicycles or 
mobylettes and acceptable living quarters must be available 
for field assistants) 
be close enough to the research station to enable the scien- 
tists to visit it frequently to monitor the on-farm tests 
Collecting initial information on the research 
area 
Initial information is collected to provide a basis for defining re- 
search priorities. Data collection is carried out in two phases: 
1. an analysis of the existing base data 
2. an exploratory survey 
The findings are then analyzed and a report is written. 
Base data. The following sources may provide valuable base 
data: 
meteorology bulletins or records 
soil and relief maps 
m publications of the national statistical bureau 
e regional project reports 
e local government offices 
e university students' village studies 
written or verbal information from extension services, agro- 
services centers and special interest groups such as mission- 
aries 
The base data are analyzed and a preliminary report is prepared, 
using the formula recommended for the final version (see Table 
12.1). This serves as the framework to which the results of the 
exploratoly survey are added. 
Table 12.1 
Suggested contents o f  the repori on the pi lot research area 
General features of ihe area Maps, administrative divisions, area, population, settlement pattern, 
ethnic groups, traditional hierarchy, religions 
Physical znd biaioglcs: axvirnitr?lent Climate 
Evapotranspiration, rainfall regime, median and quartiles of rainfall, 
critical periods, temperature, humidity 
Vegetation 
Land, soil and water 
Land form, land types and associated soils with frequency of occurrence, 
texture and color of top soil, soil depth, hardpans, water table heights, 
water storage capacity, chemical fertility 
Human environment Economic environment 
and physical infrastrirct~ri Imports of capital goods, foodstuffs, agricultural exports, exchange rate 
policy, employment opportunities, urban migration 
lnstitutbnal environment 
Credit facilities, input supply services, extension services, marketing 
facilities, farmers' organizations 
Social environment 
Land tenure, labor distribution by gender, community help, festivities 
Physical infrastructure 
Road conditions, availability of transport, markets, large-scale storage. 
schools, water supply, electricity, medical services 
Farming systems Cropping patterns and land use 
Crops, cropping patterns and crop associations, cropping patterns and 
fertility, utilization of land types, fallow, products collected from the bush 
Crop varieties 
Characteristics of varieties 
Cropping operations and crop calendar 
Land preparation, planting, crop densities, weeding, manuring, harvest- 
ing and cropping 
Inputs and yields 
Sourceof seed and planting material, use of fertilizer and agro-chemicals, 
tools, crop yields 
Crop disorders 
Pests, diseases, weeds and their control, nutrient deficiencies 
Postharvest activities and consumption 
Storage, processing, marketing, prices of farm products, nutritional hab- 
its, wnsumption 
Livestock 
Factors of production Land 
Ownership and access to land, farm sizes 
Capital, capifalgoods and capital formation 
Cash sources and use, input purchases and cost, cash flow, investment 
Labor 
Labor profile, division of labor, sources and cost of labor 
Management and information 
Educational level, farm management systems 
Decision-making and production choices 
Gender roles in decision-making, production choices (food, cash crops, 
livestock, non-farming activities) 
Analysis of farmers' conditions Typology of farms and fields 
Constraints and opportunities 
~ 
Exploratory survey. Through the exploratory survey, the team 
aims to establish an understanding of the system and its con- 
straints and potential in an intensive, informalw ay, combining field 
observations and farmer interviews. Most of the time is spent 
visiting farmers' fields, but these visits are preceded and followed 
by a group meeting with farmers in the village. 
Formal questionnaires are avoided during field visits but a check- 
list is used to keep track of the topics that have not been covered 
(seeTable 12.2). For recording physical information on individual 
fields, a simple data sheet is completed in the field. Field note- 
books, soil augers, magnifying glasses, and sample bags for 
plants and soil are carried (see Mutsaers et al., 1986) 
Table 12.2. 
- 
Checklist of information to be collected during the field survey 
Field Group 
vislt discussion 
General features of the area 
Ethnic groups, traditional hierarchy, religions x 
Physical and biological environment 
Ctmafe 
Farmers' perception of rainfall and consequences for cropping 
Vegetation 
Vegetation type (data sheet) 
Land, SOTI and water 
Land form, land types, soils (data sheet) 
Soil fertility 
Seasonal availability of water 
Human envlronment 
Economic environment 
Availability and origin of items not produced locally (market visits) 
Urban migration 
Institutionale nvironment and services 
Availability and prices of capital goods, inputs (ask traders, 
distribution centers, etc.) 
Availability and organization of credit 
Field Group 
visit discussion 
Access to extension and input delivery systems 
Farmers' organizations 
Social environment 
Access to land and tenurial arrangements 
Division of labor by age and gender 
Health conditions 
Festivities 
Physical infrastructure 
Accessibility, availability of transport 
Location, frequency, role of markets 
Large-scale storage facilities 
Schools, water supply, electricity, medical services 
The farming system 
Cropping patterns and land use 
Crops, cropping patterns, crop associations 
Differences in cropping pattern among fieldslland types; reasons 
Ownership of crops within same field 
Criteria for choosinglabandoning field 
Duration and utilization of fallow 
Products collected from the bush 
Obsolete, new crops; reasons 
Other changes in farming practices over past 40 years (ask old people) 
Crop varieties 
Crop varieties and their characteristics 
Cropping operations and cmp calendar 
Plant spacing and arrangement 
Time and method of land preparation, planting, weeding, harvesting 
Inputs and yield 
Sources and maintenance of seedslplanting material 
Use of organic, inorganic fertilizers, household refuse, agro-chemicals 
Farm implements 
Estimates of yields 
Crop disorders 
Weeds, time and method of control 
Pests and diseases and their control 
Nutrient deficiencies 
Postharvest activities and consumption 
Storage facilities (household and community) 
Field Group 
VlSlt dlscussion 
,. y; 2:q. izationo f crops, proportions marketed and consumed 
Processing of crops and food by the farm household or community 
Prices of farm products 
Consumption patterns and f w d  preferences, sorts of purchased food 
Water and fuel requirements and sources 
Utilization of crop residues and by-products 
Livestock 
Livestock systems, species, husbandry, feeding pattern, 
interaction with cropping 
Factors of production 
Land 
Availability of land 
Number, size and location of fields per household 
Accessibility of fields 
Capita!, capitalgoods and capital formation 
Sources and principal usages of cash 
Labor 
Sources and cost of labor, family and hired 
Distribution of labor, peaks, slack periods and bottlenecks 
Management and information 
Educational level of farmers 
Decision-making and production choices 
Gender roles in these processes 
Analysis o f  farmers' conditions. Immediately after the survey, 
the findings are analyzed in a few round-up meetings. The core 
features of this analysis are: 
w a typology of farms and field: classification of the farms and 
fields according to criteria appropriate to the aim of the OFR 
(for example, size, or degree of market orientation) 
an identification of constraints and opportunities: listing ele- 
ments in the farming system and the environment that limit 
productivity and for which solutions may be sought, and 
describing features of the system which may be better ex- 
ploited to increase productivity 
Writing the report. After an analysis has been made of the 
farmers' conditions, a draft report is completed before the on-farm 
trials aredesigned .The report beginswith a brief descriptionofthe 
location of the area and its size; administrative divisions; patterns 
of population settlement: and ethnic groups, traditional hierarchy 
and religions. 
The report should also include a description of: 
the physical and biological environment (climate, vegetation, 
land, soil and water) 
the human environment and physical infrastructure (the eco- 
nomic, institutional and social environment, and physical in- 
frastructure) 
farming systems (cropping patterns and land use, crop varie- 
ties, cropping operations andcropcalender, inputsand yields, 
crop disorders, postharvest activities and consumption, and 
livestock) 
On-farm experimentation 
Choice of technologies. From analysis of the existing base data 
and exploratory survey data, theconstraints (those elements in the 
farming systems and their environment that limit the systems' 
productivity) are carefully examined. Opportunities (features of 
the system that may be better exploited to increase productivity 
and farmers' welfare) are also examined. 
The next step is consider whether a particular technology is 
available or can be developed to alleviate a constraint or exploit an 
opportunity. This technology could be a new crop or cropping 
pattern, a fertilizer, a new variety, a labor-saving machine (for 
example, in field preparationo r crop processing) or a crop protec- 
tion chemical. 
Inevitably, there are some constraints and opportunities which, in 
terms of the OFR team's mandate and time schedule, cannot be 
addressed. The addressableconstraints oropportunitiesa re those 
for which solutions can be sought in on-farm trials in the season 
following the exploratory survey. They are arranged in order of 
priority, and the technologies which are appropriate to each one 
are considered. The criteria determining which technologies are 
considered may be divided into necessaly criteria and desirable 
criteria. 
Necessary criteria 
the technology should address constraints or exploit opportu- 
nities that actually exist in the localities in which it is to be 
tested 
the technology should be simple enough10 be demonstrated 
by trained extension personnel and operated by the target 
farmers 
the technology should be economically viable in terms of the 
yield levels farmers may be expected to achieve and prices 
and costs prevailing in the villages 
Desirable criteria 
8 the seasonal labor requirements of the new technology 
should complement rather than compete with other farm op- 
erations 
the technology should require no resources or inputs (capital 
outlay, expert maintenance or service facilities) that are not 
available to the target farmers 
the technology should not be more vulnerable to weather, 
pests, diseases or other risk factors than is the case with the 
existing production practices, and should be compatible with 
the prevailing livestock-herdingc onventions 
Design of on-farm trials 
The word 'design' in the context of on-farm trials is used broadly to 
mean: 
the choice of representative villages and farms for siting of 
trials 
the selection of treatments to be compared in the trials 
8 the choice of the number of replicates and of the distribution 
of these replicates within and between farms 
8 the choice of the most appropriate experimental design 
the size of plot (in the statistical sense) to be used 
Types of on-farm trials 
On-farm trials can be grouped according to: 
1. The type of innovations being tested: 
improvement of crops and cropping techniques in existing 
cropping patterns 
improved or new cropping patterns and new crops 
soil, vegetation, and water-management practices 
2. The state of knowledge: 
exploratory trials 
verification trials 
pre-extension trials 
3. The degree of farmer involvement; 
researche-managed, researcher-executed 
researcher-managed, farmer-executed 
farrner-managed, farmer-executed 
On-farm experimentation with cassava 
This section outlines the methodology for conducting on-farm 
experiments once the preparations described above have been 
completed. Examples from a case study of the Ohosu area in 
Nigeria are used to illustrate the on-farm experimentation concept. 
Diagnostic survey 
It is important to provide clear objectives to guide the team of sci- 
entists (biological and social) in drawing up questionnaires de- 
signed to provide answers to the objectives of the study. Usually, 
these objectives are: 
to understand the resource base of cassava farmers 
to ascertain the importance of cassava in the welfare of the 
farmers 
to determine the cassava varieties grown and their sources 
to understand the cropping systems in the area 
to ascertain the labor available and its distribution between 
sexes, and between adults and children 
to identify the constraints to cassava production 
In the Ohosu study, the objectives were: 
to determine the extent of adoption of improved cassava 
varieties in the specific location 
to measure the relative yields of traditional and improved 
varieties obtained by small-scale farmers 
to identify the factors that might be impeding the realization of 
the yield potential of improved cassava varieties adopted by 
the small-scale farmers 
to identifya ny potentially undesirable or adverse effects of the 
expansion of cassava production in the area 
to design field trials to address the constraints 
Methodology 
In the Ohosu study, the methodology used involved: 
site selection 
description of the physical characteristics and cropping pat- 
terns of the selected area 
study of the history of the spread of improved varieties into the 
area 
determination of the relative importance of improved and local 
cassava varieties in the farming systems 
measurement of the yields of local and improved varieties 
calculation of economic returns from the adoption of the 
improved cassava varieties by the farmers 
lnformation which was not expected to vary significantly from 
farmertofarmerwasobtained by usingagroup inte~iewquestion- 
naire. lnformation expected to vary significantly from farmer to 
farmer was obtained by using individualq uestionnaires. (See Fig- 
ures 12.6 and 12.7, at end of unit). 
Three villages in the Ohosu area were selected for study. In each 
village, 15 farmers were chosen by a random method from a list ol 
farmers compiled by the community head. Questionnaires were 
given to the selected farmers. The fields belonging to these 
farmers which were planted to cassava were measured. In addi- 
tion, 40rn2 plots were harvested to determine farmers' yield and 
plantdensity: andsoil samples inthe0-15cm layerweretakenfrom 
four locations within the 40m2, composited, and analyzed for 
physical and chemical characteristics using standard methods. 
Analysis and interpretation of diagnostic data 
The data collected was analyzed to provide information on: 
1. Biophysical features: 
vegetation 
soil (physical and chemical properties) 
topography 
climate (rainfall, light and temperature) 
cassava yield 
diseases and pests 
cropping systems 
Examples of rainfall and evapotranspiration for lbadan, Nigeria, 
are given in Figure 12.3. Examples of major cassava-based 
systems and associated mean rainfall distribution for the Ohosu 
area are given in Figure 12.4 (see overleaf). 
M A M J J A S O N  
Figure 12.3. 
Rainfall and evapotranspiration at lbadan, Nigeria, 1953-1973 
I I I 
Late maize I Late cassava I Fallow for 2 years I 
Early maize I I 
I Cassava I Fallow for 2 years 
1 I 
Maize / / I I 
I I 
1 Cassava Cassava 
I 
I I 
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l l l l  l l l l l l l l l l l l l l l l l  
J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J  
Figure 12.4. 
The major cassava-based systems 2. Socioeconomic features: 
and associated mean rainfall distribution 
in Ohosu area, Bendel State, Nigeria farmfactors: farm and non-farm enterprises, farm size, yield, 
farm labor, farm inputs. Figure 12.5 illustrates labor for farm 
operations. The peak labor demand period is from April to 
June. Any technology requiring additional labor may not be 
easily adopted during this period. 
household factors: size of household, composition, religion, 
income, tribal origin, ages, sexes, social standing, second- 
ary occupation 
infrastructural factors: absence or presence of such facilities 
as adequate transportation, health services, water, indus- 
tries, credit and extension services 
market factors: volume of malket, condition of end product 
and packaging, effect of substitutes 
policy factors: pricing policy, input prices compared to cost of 
production, extension policy and research policy 
Activity Months 
O N D J F M A M J J A S O N D J F M A  
I I l l  I I I I I I I I I I I I I I I I 
1 Land clearing 
2 Planting 
cassava 
maize (early) 
maize (late) 
melon 
yam 
3 Weeding 
4 Harvesting 
cassava 
maize (early) 
maize (late) 
melon 
yam 
oil palm 
5 Processing 
cassava 
melon 
oil palm 
1 1 I1  11 I  Peak activity 
Figure 12.5. 
Calendar of farm operations indicating peak periods in the Ohosu area, 
Bendel State, Nigeria 
Constraints identified from diagnostic data 
The main constraints identified from a village in the Ohosu study 
were: 
rapiddecline of soil fertility after first yearof cassavafollowing 
a 3-year fallow 
a acute weed infestation and decline in yields 
80% of the farmers still planted an unimproved variety 
95% of the populationa te cassava in at least two meals aday, 
based upon 24-hour recall period;malnourishment and pro- 
tein deficiencies were acute problems 
a major pests were CM and CGM 
intercropping cassava with green maize and melon was 
common, with maize spaced at suboptimum populations 
most farmers had never been visited by an extension agent, 
did not use fertilizers and, because of processing problems, 
stored 70% of their cassava in the ground 
Identifying researchable issues 
The constraints are then listed in order of priority and those for 
which technologies are available for immediate on-farm research 
are identiiied. 
If, for example, the constraint relating to malnourishment and 
proteindeficienciesisselectedasapriorityforattention, it might be 
decided that the most effective way to address this problem is to 
introduce soybean into the farming system. The OFR approach 
would be divided into three stages: 
1. introducing many improved soybean varieties into the cas- 
sava system by first testing them under researchers' control 
or on-station (see Figure 12.8, at end of unit); this includes 
exposing farmers to production problems through field days 
and obtaining their reactions 
2. identifyingo ne or two acceptable varieties and working with a 
team of nutritionists and food technologists to introduce soy- 
bean processing and utilization methods to farmers 
3. conducting researcher-managed trials in the fields of a few 
farmers using acceptable soybean varieties; and popularizing 
soybean utilization 
Designing on-farm research 
In researcher-managedt rials, farmer involvement is minimal. The 
researcher simply uses the farmer's field to conduct a trial. The 
location of the trial may enable the farmer to observe the diifer- 
ences betweent reatments and perhapsfacilitatesfarmer adoption 
of new technologies. 
An example of a typical researcher-managed trial is given here. 
Researcher-managed trial 
Title: Cassavalsoybean intercropping at farm level 
Objectives: To evaluate two varieties of soybean in 
cassava-based systems at farm level, and to 
assess farmer reaction to soybean as food 
Number of farmers = 6 
Re-o, l~ica tion-s /f-a rrn = 2 
(Treatment description): Two soybean varie- 
ties intercropped with TMS 30572cassava 
vs TMS 30572 cassava as sole crop 
Thus: 
Cropping systems = 2 (sole vs intercropped) 
Soybean varieties = 2 
Total number of plots = 8 per farm 
Exercise: 
I.I dentify relevant data to collect in this trial 
2. Prepare a data collection schedule 
3. Note especially farmers' reactions to these 
trials during field days 
Having identified the appropriate soybean variety, ascertained 
its compatibility with cassava, assessed the reactions of the 
farmers and introduced utilizationm ethods to the farmers,t he next 
stage involves the introduction of soybean into the farmers' cas- 
sava-based system. 
This trial is made as simple as possible and managed by the 
farmers, as shown in the example provided here. 
Farmer-managed on-farm trial 
Treatment factors Level 
Cassava variety 2 
Soybean variety 2 
Management: 
Population 
Pattern of planting 
Weeding, etc 1 Farmers 
Costs 
I nt his trial, the risk is minimal. Soybean varieties are simply being 
introduced into the cassava system and other trials have demon- 
strated that soybean in association with cassava does not reduce 
cassava yield and that soybean is compatible with cassava as an 
intercrop. The farmer is most likely to benefit. 
Setting up the on-farm trial 
Identify up to 30 project farmers whose fields are within the study 
area. With other OFR team members (such as subject-matter 
specialists) hold group meetings with the farmers to: 
discuss the treatments with them, including rationale for 
choice (see Table 12.3) 
make modifications based on farmers' inputs 
arrive at conclusions on management issues including as- 
sumptions of risks (for example, crop failures, plot area 
needed for trial, help required to set up trials accurately and 
timing of provision of planting materials) 
Other considerations 
Plot size. Use plots big enough for farmers to notice differences 
arisingfrom different treatments. If the researcher plans to sample 
Treatment combinations in an on-farm trial with improved 
cassavaandsoybean 
(i) Farmer's cassava (ii) Farmer's cassava 
Improved soybean var. 1 Improved soybean var. 2 
(iii) Improved cassava (iv) Improved cassava 
Improved soybean var. 1 Improved soybean var. 2 
for cassava yield at different ages, plot size is adjusted to accom- 
modate this. The minimum plot size should be 10m x 15m but 
some farmers may not have as much. The entire trial area should 
not be more than 20-30% of the farmer's field. 
Replications. Usually, farmer-managed trials are not replicated 
within farms. This helps to minimize complications and to reduce 
total plot area committed to the trial. 
Non-treatmentv ariables. Non-treatment variables are routinely 
recorded. These variables relate to management (for example, 
weed control, planting patterns and populations, and cropping 
history of the land as related to inherent soil fertility) and to site- 
specific features (such as shade, land types, unusual incidence of 
pests and diseases, and seasonal or annual variations in rainfall). 
Labor data and farmer participation in the trials is also recorded. 
Yielddata. Determine yielddata accurately because they provide 
the most important information. For root crops, an OFR team 
usually determines plot size, number of plants and weight of roots 
and shoots at harvest. 
Data analysis 
Compile data and examine the mean effects of treatments (see 
Unit 11). Observe farmer-to-farmer variations, and group the 
project farmers into categories on the basis of yield. Compare 
yields and interpret trends. 
Statisticalanalysis. Having interpreted trendsof yield results, use 
statistical analysis to establish confidence levels. Indicate the SE 
or LSD. If interpretation were limitedt o statisticaldifferences, non- 
significant LSD or t level would indicate that no further discussion 
of result was necessary. However, in on-farm trials, statistical 
analysis is a tool to determine confidence level. A non-significant 
statistical difference may have economic importance. 
Economic analysis. Economic analysis of the data is now 
conducted. It is important to record all inputs so that output can be 
accurately interpreted. An example of economic analysis for a 
cassavalsoybean system is presented in Table 12.4. 
Table 12.4. 
Costs and returns for cassava/soybean system in the Ohosu area (In Naira) 
Item Wlth TGx536-02D Wlth Malayan 
Average yield (Wha cassava)+ 
Average yield (Vha soybean) 
Net yield (Vha cassava) 
Net yield (Vha soybean) 
Gross lield benefit N22O/ton (cassava) 
Gross field benefit soybean (N1500lt) 
Gross grain (cassava + soybean) 
Variable costs 
Land preparation (5OMDIha) at N9/MD1 
Planting (13MDlha cassava at N91MD 
Planting (15MDIha soybean at N9IMD) 
Weeding 60 MDIha at N9 (2 weedings) 
Seeds 5Okg at N3lkg soybean 
Cassava cutting 50 bundles at N3 
Harvesting 70 MD cassava 
Harvesting 35MD soybean 
Threshing (32kglMD) 
Total variable costs 
Net benefit 
BIC 
Return to laborza t N9ID per man-day 
ReNrn to labor = (N91MD) 
The data in Table 12.4 shows that intercropping cassavaand soy- 
bean is highly profitable, resulting in a realization of about 2 times 
the value of labor invested or more than 2.5 times the amount of 
money invested. However, this calculated benefit may not be 
realized because of the socioeconomic factors, such as volume of 
gari and soybean markets to absorb increased production,g overn- 
ment policy and infrastwctural limitations. 
Verification trial 
The next phase in OFR is the pre-extension verification of the 
result. This serves as a demonstration as well as a production 
scale operation through which confidence in the trial package 
(cassava + soybean) is established. Plots are selected on a few 
strategically located farms, and plot size may be as large as 0.5- 
Ih a. Analysis is mainly economic and much of the interaction is 
between the farmers and extension officers. If the trial is success- 
ful (that is, meets the farmers' expectations), the package is 
popularized among target farmers. 
Afinal phase is tostudy mass adoptionof the package at aspecific 
time in the future. Among the issues addressed during this phase 
are the number of farmers who have adopted the package, the 
amount of land that has been brought under cultivation using the 
package and the effect of the package on the farmers' welfare. 
Figure 12.6 IlTA cassava-based system survey, Ohosu area 
Group Interview 
1. Location name: 2. Location no. 
3. Type of community: Camplsenlement 
- Traditional village 
4. Distance from camplvillage to: 
Water supply km (which) 
Gari market km 
Tarred road km 
Sec. school km 
Medicare' km (which maternity) 
Agric. people2 km (which extension) 
Notes: 
1. Hospital, clinic, maternity, doctor, nurse, midwife, etc. -indicate which 
2. Agric. officer, extension worker, agric. project, etc. - indicate which 
5. What is the major problem of cassava production in this camplvillage? 
6. Cassava variety grown, year of introduction, initial source of cutting, estimated yield 
Variety name Year inimducsd Initial source estimated yield 
Figure 12.7 IiTA cassava-based system survey, Ohosu area 
9. No. of years spent in school: 
10. Secondary occupation: 
1 1. No. of children: 
12. No. of children in school: 
14. Have you ever given cassava cuttings to anybody outside your camphillage? 
15. If yes, where is (are) the person@)? 
16. What crops were in each of your fields in the last planting season, and did you apply fertilizer in each of the fields 
8. If yes, whiah one? 
9. How many times has an extension worker visited you in the past 12 months? 
22. How much did you spend last year on the following farm inputs? 
Hire labor W 
Planting materials W (materials bought) 
Feltilizer W 
Herbicide W 
Insecticide W 
Yam stake W 
Farm tools W 
Tractor hire W (for what purpose) 
23. Which and how many of the members of your household workon your farm and during what periods (months) 
Type and number of livestock owned 
26. Tree crops grown (plantation) 
27. Items of food eaten in the past 24 hours (contd. overleaf) 
'Name them: Vegetables 
29. Transport vehicle owned 
30. O~nershipo f cassava mill 
31. Ownership of TV 
Spraying equipment Yes: 
Figure 12.8 On-stat ion research . 
Title: Soybean Cassava lntercropping Trial 
Objectives: To determine if a reasonable yield of both soybean and cassava 
can be produced by intercropping the two crops 
Experiment: Split plot design 
Main plot - intercropping 
Monocropping 
Sub plot - 11 different varieties of soybean: Replication-3 
Land preparation -slash and burn - no tillage 
Date planted cassava and soybean - 17 July 1987 
Spacing: Cassava - 1.33m x 0.75m (10,000 plantslha) 
Soybean - 0.75177 x 0.05m (266,667 plantslha) 
Fertilization - NONE 
Insecticides - NONE 
Chemicals for disease control - NONE 
Weed control - hand weeding as needed 
Annexes 
GLOSSARY 
Agronomic trials Research experiments aimed at investigating field-crop production andlor soil 
management practices. 
A ~ i c adlo minance The condition whereby the shoot apex regulates the growth and development of the 
lateral buds and branches. Auxin, a growth hormone, has been indicated as involved 
in the process. 
Apomixis Reproduction involving specialized generative tissues but not dependent on 
fertilization (for example, seed development in the ovary of sexually reproducing 
plants where the embryo is formed without union of sperm and egg). 
Althropods Animals with jointed legs, including Crustacea, Myriopoda, lnsecta and Arachnoidea. 
Axillary bud A bud formed in the axil of a leaf 
Bacteria Unicellular micro-organisms belonging to the Kingdom Protista , producing no 
chlorophyll. They reproduce by binary fission and are related to the fungi. Most are 
saprophytic, some are autotrophic, and some are parasitic to plants and animals. 
Biotics Pertaining to life. 
Breeder seed Purest form of planting material (for example, cassava stem cuttings) of an improved 
variety produced by the breeder or hislher agents. The quantity of such material is 
usually small, and further multiplication at experimental or other sites controlled by a 
station is necessary to increase the quantity. The resulting material from this 
multiplication is referred to as foundation seed. 
C , cycle Photosynthetic carbon reduction cycle in which the first stable product of 
photosynthesis is a 3 carbon compound. 
C , cycle The photosynthetic carbon reduction cycle in which the first stable product of 
photosynthesis is a 4 carbon compound. 
C:N ratio The carbon to nitrogen ratio in any chemical or foods, especially proteins. 
CO, compensation point CO, concentration at which the amount absorbed is equal to that given off 
Cambium A layer, usually 1-2 cells thick, of persistently meristematic tissue that divides to give 
rise to secondary tissues, resulting in growth in diameter. 
Cercospora A common parasitic fungus in the subdivison Senteromycotina. It causes leaf spots 
on different monocotyledonous and dicotyledonous plants. The genus Cermspora is 
responsible for three leaf spot diseases of cassava. 
The loss of chlorophyll content by a tissue. Usually, tissue turns yellow or pale or 
even white. The mndition is termed chlorosis and may be brought about by disease, 
genetic factors, lack of light and deficiency in magnesium or iron. 
171 
A group of plants originating from a single individual and reproduced by vegetative 
means. 
Cultivar/variety A unform group of cultivated plants obtained by breeding or selection. 
Cyanogenic glucosides Substance called Linamarin found in cassava. Linamarin hydrolyzes in the presence 
of enzymes (linamarase), giving rise to hydrocyanic acid (HCN). 
Die-back A condition in which the plant shoot dies from the top downwards. Leaves may or may 
not be shed. 
Dormancy State of suspended biological activity (for example, dormant seeds do not germinate 
despite provision of normal environmental requirements for the process). 
Epiphytotic Of, relating to, or being a plant disease that tends to recur sporadically and to affect 
large numbers of susceptible plants. 
Etiology The origin of causes of a disease; the study of causes of a disease 
Fungus A large group of filamentous and non-filamentous micro-organisms belonging to the 
Kingdom Protista. They lack photosynthetic pigments (chlorophyll). They are either 
saprophytic or parasitic. The saprophytic fungi cause f w d  spoilage and wood and 
debris decay; parasitic fungi cause diseases of plants and animals. 
Genotype The assemblage of genes in an organism (cf. phenotype). 
Genus (pl. genera) A group of closely related species of plants, animals or micro-organisms 
Halo A portion of plant tissue devoid of chlorophyll (chlorotic) which surrounds some leaf 
spots (for example, the chlorotic halo surrounding the bacterial angular leaf spot of 
cassava caused by Xanthomonas campestris pv cassavae). 
Hardwood (of cassava) The lower matured portion of a cassava stem 
Heterozygous Having two genes at corresponding loci on homologous chromosomes different for one 
or more loci. 
Host specificity Usually found in pathogens. Host-specific pathogens cannot attack any plants other 
than their host (for example, Xanthomonas campestris pv manihotis has not been 
shown to attack any plant apart from cassava, hence it is host specific). 
Hydrocyanic acid (HCN) A very toxic compound found in many plants, including cassava. In cassava, HCN is 
concentrated in the peel of the tuberous root and in the leaves. 
Interspecific crosses Interbreeding or hybridization involving representatives of different species 
In vitro Literally means 'in glass'. It is now applied to any process carried out in sterile 
cultures. 
Laticifers Latex tubes found in the cassava tuber flesh 
172 
Leaf picrate method A rapid means of testing the amount of HCN released by discs cut from cassava 
leaves where the quantity of HCN corresponds to the intensity of color (dark red for 
high HCN) developed when afiker paper soaked in sodium picrate is suspended in a 
vial containing the leaf disc and a few drops of toluene. 
Lesion A wound; a well-marked but limited diseased area. 
Meristem culture Apical meristem cukure; cultivation of the apical dome tissue distal to the youngest 
leal primordia in prepared nutrient media. 
A minute opening in the integument of an ovule of a seed plant through which the 
pollen tube penetrates to the embryo sac. 
The study of form and its development; the form and structure of an organism or any 
of its parts. 
Mottling The presence of a mixture of many colors on the leaf surface caused usually by 
viruses and occasionally by other factors. 
Mutation A relatively permanent change in hereditary material involving either a physical 
change in chromosome relations or a biochemical change in the make-up of the 
genes; the process of producing a mutation; an individual or strain resulting from 
mutation. 
Mycelium (pl: mycelia) A collection of fungal filaments (hyphae). 
Necrosis The death of part of orthe whole of a plant. 
Outcross A progeny resulting from interbreeding involving relatively unrelated individuals 
Parthenogenetic Plants developed from seed or ovum without fertilization by pollen. 
Perennial A plant that lives an indefinite number of years (perennis: lasting for the whole year) 
Plantlet A small rooted shoot or germinated embryo. 
Polypbid Having or being a chromosome number that is a mukiple greater than 2 of the genetic 
number. 
Propagation Methods of raising or establishing crop plants 
Pubescence Quality or state of being covered with soft short hairs (pubescent: covered with soft 
hair, as is the case with young leaves and shoots of some cassava varieties. 
Recombination The formation through the processes of crossing-over and independent assortment of 
new combinations of genes in progeny that did not occur in the parents. 
Recurrent selection A plant-breeding practice whereby progenies from intermating of a group ofparents 
selected for their breeding values are tested for good performance. Selected plants 
are constituted as parents of the next generation through intermating. 
Replication Systematic or random repetition of experimental units like agricultural test rows or 
plots to reduce error. 
Rogueing The removal of unwanted varieties or plants of undesired characteristics to prevent 
them from mixing with or contaminating the desired variety. 
Secondary thickening Formation of additional, secondary vascular tissue by activity of cambium, with 
accompanying increase in diameter of stems and roots of plants; providing additional 
conducting and supporting tissue for the growing plant. 
Species A group of interbreeding individuals not interbreeding with another such group; a 
systematic unit which includes geographic races and varieties and is included in a 
genus. 
Stem puncture method Procedure whereby cassava stems are punctured with a needle for inoculation 
purposes. Usually, plant pathogens are inoculated into such punctures. 
Subculture Subdivision of a culture for transfer to fresh medium. 
Supra-optimal temperature The temperature most ideal for a biological process such as disease development or 
the growih of afungus or bacterium. 
Systemic Generally distributed throughout the system (that is, the plant system) 
Tetranychids Having to do with mites. 
Tuber A thickened, fleshy underground root or stem. 
Variation The divergence in the structural or physiological characters of an organism or 
biotype from those typical or usual to its group; the extent or range of such 
divergence. 
Vascular necrosis Death of the plant vascular system. 
Vector A carrier of pathogenic organisms; any agent transferring a parasite to a host. 
Viral Consisting of or resulting from a virus. 
Virus One of the nucleoprotein-like entities able to pass through bacteria-retaining 
filters, having many characteristics of living organisms and recognized by its toxic or 
pathogenic effects in plants and animals, including man. 
Volunteer seedlings Seedlings resulting from seeds other than those intentionally sown (for example, 
through germination of seeds from an earlier planting). 
Xanthomonas One of the five bacteria genera pathogenic to plants. The genus is usually 
characterized by rod shape with a single polar flagellum and yellow colonies (with 
a few exceptions), and causes angular necrotic leaf spots, gum exudation, wilt, 
and die-back of young shoots (for example, Xanthomonas campestris or 
manihotis). 
RECOMMENDED READING 
Production constraints 
Hahn, S.K., E.R. Terry, K. Leuschner. 1.0. Akobundu, C. Okali, and R. Lal. 1979. 'Cassava improvement in Africa' 
Field Crops Research 2 : 193-226 
Onwueme, I.C. 1978. The Tropical Tuber Crops John Wiley, Chichester 
Storey, H.H., and R.F.W. Nichols. 1938. 'Studies on the mosaicdiseases of cassava' Annals of Applied Biology 
25 : 790-806 
Morphology andp hysiology 
Cock, J.H. 1973. 'Some physiological aspects of yield in cassava (Manihot esculenta Crantz)' in Proceedings of 3rd 
International Symposium on Tropical Root Crops, Ibadan. Nigeria 
Cock, J.H., and J.A. Reyes (eds). 1985. Cassava Research, Production and Utilization UNDPICIAT 
Enyi, B.A.C. 1972. 'Growth rates of three cassava varieties under varying population densities' Journal of 
Agricultural Science. U.K. 81 : 15-28 
Hunt, L.A, D.W. Wholey, and J.H. Cock. 1977. Growth Physiology of Cassava Field Crops Abstracts (vol 30, no 2) 
Onwueme, I.C. 1978. The Tropical Tuber Crops John Wiley, Chichester 
Breeding 
Hahn. S.K., A.K. Howland, and E.R. Terry. 1980. 'Correlated resistance of cassava to mosaic and bacterial blight 
diseases' Euphytica 29 : 305-31 1 
Jennings, D.L., and C.H. Hershey. 1985. 'Cassava breeding: A decade of progress from international programmes' 
in Russell, G.E. (ed) Progress in Plant Breeding - 1 Butlerworth 
Jones, W.O. 1959. Manioc in Africa Stanford University Press, Stanford 
Kawano, K. 1980. 'Cassava' in Fehr, W.R. and Hadley, H.H. (eds) Hybridization of Crop Plants American Society 
of Agronomy. Madison, Wisconsin, USA 
Nichols. R.F.W. 1947. 'Breeding cassava for virus resistance' East African Agricultural and Forestry Journal 
12:  184-194 
Multiplication and distribution 
Cock. J.H., and J.A. Reyes (eds). 1985. Cassava: Research, Production and Utilization UNDPICIAl 
Dahniya, M.T., and S.N. Kallon. 1984. 'Rapid multiplication of cassava by direct planting' Proceedings of 2nd 
Triennial Symposium of the International Society for Tropical Root Crops - Africa Branch. Douala, Cameroon, 
14-19  August 1983 
Heys, G.E. 'Large-scale multiplication of cassava at IITA, Ibadan' in (ed) H.C. Ezumah Cassava Production and 
Extension in Central Africa: Proceedings of a Wokshop held in Mbaza-Ngungu, Zaiie, 19-22 May 1980 Proceedings 
Series No. 4, IlTA 
Lozano, J.C., and D.W. Wholey. 1974. 'The production of bacteria-free planting stock of cassava.' World Crops 
Journal 26: 114-117 
Lozano, J.C.. J. Toro, A. Castro, and A.C. Belloni. 1977. Production of Cassava Planting Material ClAT Series GE-17 
Terry, E.R. 'The production of pathogen-free cassava planting material for distribution' in (ed) H.C. Ezumah Cassava 
Production and Extension in Central Africa: Proceedings of Workshop held in Mbaza-Ngungu. Zaire. 19-22 May 1980 
Proceedings Series No. 4, IITA 
Tissue culture 
Bhojwani, S.S., and N.K. Razdan. 1983. Plant Tissue Culture: Theory and Practice Elsevier. Amsterdam 
Evans. D.A., W.R. Sharp, P.V. Ammirato, and Y. Yamada. 1983. Handbookof Plant CeN Culture Macmillan. New 
YorklLondon 
Fogh, J. 1973. Contamination in Tissue Culture Academic Press, New York 
George, E.F., and P.D. Sherrington. 1984. Plant Propagation by Tissue Culture Exegetics Ltd, UK 
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Agronomy 
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