Maize diseases in Africa and their role in the varietal improvement process

Diseases are greater in number and intensity in the tropics than in the nontropical regions of the world. In Africa, annual yield losses to diseases are commonly in the range of 15 to 50%, compared to the world average of 9%. Some years, epidemics of such Africa-specific diseases as maize streak virus can cause yield losses of 100%. The yield-depressing effect of maize diseases is among the principal causes of the instability of maize production in Africa. This paper discusses the groups of diseases that are most widespread and important in Africa, their symptoms, control measures and the availability of sources of resistance in maize. The groups discussed are seedling rots and other seedling diseases, local-spot foliar diseases (including leaf blights, rusts, leaf spot, and brown spot), the systemic foliar diseases (including the economically important maize streak, maize mottle/chlorotic stunt, maize dwarf mosaic, and downy mildew) and stalk and ear rots. Maize is also prone to attack by the parasitic seed plant Striga (witchweed), which feeds on the plant, depriving it of essential nutrients, metabolites, and water. IITA has developed techniques for disease-resistance screening by exploring host-plant resistance (among adapted varieties first in order to hasten farmer adoption). Three parameters are considered in disease assessment: incidence, intensity/severity, and crop yield. The role of maize diseases in catalyzing the variety development process in Africa has been tremendous; the birth of many national programs of maize research was a direct result of the spread of disease on the continent. IITA and CIMMYT have collaborated to develop varieties resistant to the systemic foliar diseases, especially maize streak. Future efforts of IITA will continue to be concentrated on the development of maize varieties with combined resistance to the major economically important diseases, maize mottle/chlorotic stunt, Striga, and stalk and ear rots