Striga: small seeds, big losses
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CTA. 1990. Striga: small seeds, big losses. Spore 29. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/45353
Striga, a parasitic weed of most savannah crops, is gaining ground in Africa and farmers have only limited weapons against it. Its control should therefore be added to the list of priorities for research Over the last 15 years, striga has become a...
Striga, a parasitic weed of most savannah crops, is gaining ground in Africa and farmers have only limited weapons against it. Its control should therefore be added to the list of priorities for research Over the last 15 years, striga has become a pest of crops in the Sudano - Sahelian region and Eastern Africa. This parasitic plant sustains itself while damaging cultivated plants. It produces considerable yield losses that can reach 100% in some fields and affects major crops such as sorghum, millet, maize, cowpea, tobacco and sweet potato. Lack of rain, the reduction of fallow and the cultivation of poor soils promote its spread. The minute seeds of striga germinate in a few days following the first rains. As soon as the young rootlet is within half a centimetre of the roots of another plant, it attaches itself. Once attached to the root system of the host plant, striga develops a haustorium that siphons off the moisture and nutrients needed for it to grow. The weed grows underground for several weeks and exhausts the parasitized plants before its own presence is evident. As soon as the first leaves appear, striga develops its own root system but remains attached to the host plant. Within a few weeks it produces its attractive pink flowers while the parasitized plant withers and often dies. At the end of the cycle, each striga plant produces between 40,000 and 90,000 seeds that lie dormant in the ground until favourable conditions permit germination. Seeds can survive in the soil for 15 to 20 years. The complete stock of seeds in the soil must therefore be destroyed in order to eradicate this parasite. Inadequate solutions There are two radical solutions. The first one is aimed at destroying the seeds in the soil with methyl bromide during the period of dormancy. The second involves provoking an artificial germination, which can be done with ethylene, but without the support of a nearby host plant the seeds die very quickly after germination. However, as with herbicides, such costly and difficult remedies are beyond the means of resource-poor farmers A third technique of control that is based on cultural practices has been developed. It consists of growing 'false hosts' that induce germination but are not parasitized. Cotton, groundnut and soyabean can be used. If the striga infestation is light, crop rotation, even of only one year, leads to a remarkable improvement. But in cases of serious infestation, getting rid of the stock of striga seeds would take several years of such cropping. But inevitably, the greatest problem is for those farmers whose land is so infested that they do not have sufficient clean fields in which to sow their cereals. Another method being explored by the International Centre for Research in the Semi-Arid Tropics (ICRISAT), is the selection of resistant varieties. Some resistant sorghum and cowpea varieties have already been found. The work on millet is less advanced. However, this resistance is not final, as in a few years striga can adapt itself to new varieties. A further approach being investigated by research workers is biological control. Some suitable insect parasites have been identified, but much has still to be done before they can be used against striga on a wide scale in the field. Farmer awareness At the present fume, farmers have only one option; to pull out the weeds by hand and burn them before they produce seeds. But this method is difficult to carry out except on very small plots and it will not be readily accepted by farmers who do not understand the relationship between the pretty pink flowers in their fields and the withering of their crops. In some areas, farmers think that they can win by changing crops. When sorghum is too badly infested they plant millet. But after a few years, millet becomes infested as well and farmers have to abandon their fields. It is therefore essential to improve their knowledge so that they can at least limit damage due to striga, while research workers identify the weaknesses of this parasite and suggest more efficient control methods. Striga exposed A bilingual (English and French) poster has just been published to familiarize extension workers with the threat striga represents to crops. Using drawings and pictures, it shows the stages of underground and above ground development of the parasite as well as the different striga species. A short strip cartoon gives practical advice for its control. To receive this poster free of charge, write to: G Salle, Universite Pierre et Marie Curie Laboratoire de Cytologie experimentale et Morphogenese vegetale 2, place Jussieu, Bat. 2 75252 Paris cedex 05 France