African Mosaic Disease

AMD affects almost all cassava plants For a growing number of Africans, cassava is a vital crop. It is a staple food in forested areas and a life-saver during famines in savanna areas. 50 million tonnes of fresh tubers are produced each year in Africa and most of them are consumed directly by the families who grow them. About 15 years ago, however, cassava cultivation began to be seriously affected by both traditional and newly introduced diseases which have considerably reduced yield (see SPORE N° 4). As its names implies, African Mosaic Disease (AMD) is native to Africa. It is endemic in all cassava producing countries and is estimated to be responsible for losses of 10-25 million tonnes per year (the subsistence nature of cassava production means that such data are not well recorded). Because of the impact of this disease a meeting was held from May 4-8, 1987 in Yamoussouko, Cote d'lvoire that brought together researchers and officials of producing countries to discuss methods of controlling it. They summarized the state of scientific knowledge on AMD and gave an update on the current situation in the field. This international seminar on controlling AMD was initiated by the FAO, funded primarily by CTA and organized by ORSTOM with the assistance of IITA and CPI. The geminivirus which is responsible for AMD was isolated in Kenya about ten years ago and has since become well known among virologists. It now affects almost all cassava plants and spreads in two ways. A tiny, 1 mm whitefly Bemisia tabaci is responsible for the initial dispersion of the virus through crops by feeding on young leaves. Whitefly populations fluctuate according to weather conditions, notably wind. Favoured by the rainy season in East Africa, they also benefit from the high temperatures found in West Africa It now appears, however, that this fly is not the major means of contamination by or persistence of AMD. It is the farmers themselves who keep re-introducing the disease by planting infected cuttings. Research in Kenya has shown that the whitefly actually plays only a minor role in maintaining the epidemic. As it turns out, it is the cassava itself that harbours the virus and is the main carrier. Infected cassava plants are characterized by leaves covered with yellow blotches. When severely infected, the leaves shrivel up, stunting growth. The degree of such symptoms, which are the only way to recognize AMD, varies according to the means and date of contamination. Plants infected at an early stage (especially as cuttings) are more likely to have their foliage affected and thus produce less. On the contrary, whitefly attacks on wellestablished plants (2-3 months old) have hardly any effect on yield. For this and other reasons, whitefly control is somewhat utopian especially in view of the high cost of insecticides and their contamination of edible cassava leaves. Recent epidemiological studies have pointed towards two control strategies. The simplest, sanitation, is ensuring that only healthy cuttings taken from uninfected plants are used in conjunction with the systematic eradication of plants showing symptoms of the disease. Even if this method does not entirely remove the risks of recontamination, it does result in a remarkable increase in production because the disease is restricted to less sensitive, adult plants. Kenya adopted this approach three years ago and other East African countries, notably Malawi, Uganda and Tanzania, are following in its footsteps. However, sanitation alone is not always enough to control AMD, especially when it is strongly entrenched and the whitefly plays a stronger role in its spread, which seems to be the case in many areas of West Africa. This means that sanitary measures must be complemented by the use of resistent varieties. Work in this direction has been undertaken by the IITA which has developed such varieties. By combining thermotherapy with cloning, a virus-free strain has been produced and is now being propagated. Unfortunately, resistent varieties have only been developed for the bitter strains of cassava, which are not only less favoured in many areas but also more difficult to prepare. These strains are also poorly accepted by farmers who do not appreciate their sprawling growth amidst traditional crop associations. A further complication is that infected leaves are more tender and apparently taste better than healthy ones! It thus seems preferable to concentrate on local strains, crossing them with resistant ones, in order to produce plants that are both well adapted to the local environment and please local consumers. In Zaire, a collection of local varieties is being used to establish a planting stock of healthy cuttings thanks to in vitro techniques. Given the rapidity of such a method, plants will soon be able to be distributed to farmers. The development and distribution of healthy cuttings is, in fact, the biggest obstacle to reducing the incidence of AMD in many countries. Most of the resistent strains and healthy cuttings are still found on experimental farms, not farmers' plots. International research institutes have developed appropriate techniques but it is now up to national organizations to see that they are adapted to local conditions and then implemented. Many African officials have noted, however, that their countries cannot afford to invest heavily in an AMD control programme alone given the need to develop a comprehensive package to deal with all cassava pests, including bacteria, scale and mites. Finally, if the goal is to encourage farmers to control cassava diseases in order to increase production, attention must also be given to processing techniques and marketing methods. There is no doubt that cassava can and must be saved. The problem, as usual, is transforming such goodwill into action For a copy of the proceedings of this seminar, write to: ORSTOM BP V 51 Abidjan Cote d lvoire or CTA Postbus 380 6700 AJ Wageningen The Netherlandsinternational seminar on controlling AMD May 4-8, 1987 in Yamoussouko, Cote d'lvoire For a copy of the proceedings of this seminar, write to: ORSTOM BP V 51 Abidjan Cote d lvoire or CTA Postbus 380 6700 AJ Wageningen The Netherla