Africa is rich in both natural and human resources, yet nearly 200 million of its people are undernourished because of inadequate food supplies. Comprehensive strategies are needed across the continent to harness the power of science and technology (S&T) in ways that boost agricultural productivity, profitability, and sustainability -- ultimately ensuring that all Africans have access to enough safe and nutritious food to meet their dietary needs. This report addresses the question of how science and technology can be mobilized to make that promise a reality.
In all farming systems there are major factors that reduce crop growth. This also holds for animal production systems. Pests, diseases and weeds are a problem in nearly all farming systems of importance.
In Africa, many pests and diseases are known to occur and seriously threaten the productivity of major crops in some areas. Yield losses of up to 50 percent are mentioned for cassava: Cassava Mosaic Disease (CMD) can completely destroy a crop in heavily infected areas. Major pests of maize include stem and ear borers; armyworms; cutworms; grain moths; beetles (weevils, grain borers, rootworms, and whitegrubs) and virus vectors (aphids and leafhoppers). Major fungal diseases also affect maize. Ear rot, caused by Fusarium verticillioides, decreases yield but - more importantly - can produce mycotoxins that threaten human and animal health. Combined attacks by pests and weeds can severely damage cowpea plants and cause losses as high as 90 percent. Bananas are also vulnerable to diseases, especially panama disease (Fusarium oxysporum f.sp. cubense) and black Sigatoka leaf spot disease. The latter may reduce yield in banana and plantain by up to 40 percent. Even higher losses are reported for plants infected with banana streak virus (IITA, 2003).
A major pest in maize in Sub-Saharan Africa is witchweed (Striga). In the Nigerian savannah, for example, weed-related yield losses ranging from 65 to 92 percent have been recorded. Also crops like sorghum, millet and cowpeas are infested. Depending upon the extent of infestation, reductions in per hectare grain yield of 30-60 percent are common. A good method of estimating grain loss in an infested field is 3-4 kilograms per100 striga plants per hectare for sorghum and 5-6 kilograms per 100 striga plants per hectare for maize, the lower number being used for fields or areas with less productive potential (Shank, 2003).
In Africa, the possibilities for chemical control of pests and diseases are restricted, due to the limited availability and high cost of pesticides. As a consequence, farmers in most farming systems have to find alternative solutions. The choice of resistant varieties is one of the most powerful tools, whenever appropriate varieties are available. Genetic modification offers a new tool for developing resistant varieties. To date, genes conferring resistance to pests and diseases have been transferred to certain target crops from a wide range of sources, far exceeding the limits set by the fertility constraints of conventional breeding. Although this is a powerful technique, it has not yet been applied to its full potential in many parts of the world, including Africa. Chapter 4 will discuss this topic in more detail.
Intrinsic properties of the farming systems themselves may limit damage caused by pests and diseases. In many Western countries, interest in intercropped farming systems is increasing because they demonstrate a higher buffering capacity against diseases, as demonstrated by Zhuand colleagues (2001) for rice in China. Therefore the complex intercropping systems used in Africa may be appropriate to limit the effects of diseases. This may reflect the use of indigenous knowledge by African farmers and needs further research.