Tanzania is one of the largest producers of Upland cotton (Gossypium hirsutumL.) in Africa. The crop is raingrown by smallholders over a large ecologically diverse area with a variety of disease problems. The main diseases areFusariumwilt (Fusarium oxysporumf. sp.vasinfectum(Atk.) Sny. & Hans.) and bacterial blight (Xanthomonas malvacearum(E. F. Smith) Dowson).Fusariumwilt is established over much of the area close to Lake Victoria where the soils are sandy and acidic. The disease is seed borne but spread is reduced by issuing seed produced inFusariumfree villages. Bacterial blight has become less important due to success in breeding resistant varieties.Verticilliumwilt (Verticillium dahliaeKleb.) and root-knot nematode are potentially serious in some areas. Many other diseases occur at the seedling stage or on leaves and bolls but cause crop loss only under unusual environmental conditions or after insect damage.Alternaria macrosporais the most common cause of leaf spot andColletotricum gossypiithe main boll rot pathogen. Screening and selection for resistance toFusariumwilt is conducted in the glasshouse and on land artificially infested with high levels ofFusarium. Field selection for blight resistance involves assessment of naturally occurring leaf infection together with stem and boll inoculation.