Leaf spots caused byAlternaria macrosporaandA. alternataare common in cotton crops around the world. Both species are, however, usually considered to be minor pathogens of Upland cotton(Gossypium hirsutum)butA. macrosporacauses premature defoliation with resulting yield loss in cultivars ofG. barbadensein Israel and in the USA.A. macrosporais also able to cause defoliation in Upland cotton if the crop is. predisposed to infection by environmental or physiological stress. In Zimbabwe the main predisposing factor is potassium deficiency. The paper discusses the disease in Zimbabwe against the background of work reported from other countries on its effect on yield and on its epidemiology and control.