Alternaria leaf spot
Alternaria leaf spot is caused by Alternaria macrospora, a fungus that infects the leaves, bracts and bolls. Pima cotton (Gossypium barbadense) is very susceptible, while Upland cotton (G. hirsutum) is fairly tolerant under normally dry weather conditions. A. macrospora survives in cotton debris and on weeds. Under high humidity or rainfall, spores are produced that are windblown or splashed on cotton plants. Red lesions appear where spores have germinated and grown into the host tissue (photo 1). Infections late in the season are not considered a problem, but early infections during summer rains in July and August can cause severe defoliation. Disease can be controlled with preventative applications of mancozeb (Penncozeb 2(ee) available) fungicide.
