Bacterial, fungal and nematode diseases.

Cassava is affected by a wide range of diseases caused by viruses, bacteria, fungi and nematodes (Table 13.1). With the exception of some of the virus diseases (see Chapter 12) and bacterial blight, most of the other diseases have been regarded as of minor or local importance. This may not always be the case and this perception may be at least partly because cassava in many countries is a subsistence crop, where yields are limited more by other major factors, such as low soil fertility and moisture stress, than by plant diseases. At present, there is often too little information on losses caused by these diseases to draw any conclusions about their importance. There is considerable variation among cassava cultivars in susceptibility to the ‘minor’ diseases, and where susceptible cultivars are grown and conditions favour disease development, losses can be substantial. This is particularly true of the root rot diseases, where cassava is grown in soil with a high water table, or, on land newly cleared from forest or bush. Brown leaf spot is common almost everywhere that cassava is grown, but usually regarded as unimportant, yet the defoliation it causes may have a significant effect on yield, where cassava is grown intensively for commercial production. The diseases considered of economic importance vary to some extent between countries and between continents. Cassava mosaic virus disease occurs wherever the crop is grown in Africa but is absent from South America. Bacterial blight occurs in Africa, South America and Asia. In both Africa and South America the next most important group of diseases are the root rots. In Nigeria, Cameroon and Benin the pathogens causing root diseases of economic importance are Sclerotium rolfsii, Botryodiplodia theobromae, Fomes lignosus, Rosellinia necatrix, Rhizoctonia solani, Phytophthora spp. and Fusarium spp. (Arene et al., 1990; Afouda and Wydra, 1996). In Brazil, Phytophthora is probably the most important root pathogen but in some areas Fusarium spp. are also a problem (Lozano, 1991).