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Temperature-Induced Suppression of Alternaria Leaf Spot of Cotton in Arizona. P. J. Cotty, Department of Plant Pathology, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 71:1138-1140. Accepted for publication 24 August 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-1138.

Lesion formation by Alternaria macrospora, the causal agent of Alternaria leaf spot of cotton (Gossypium barbadense), is influenced by the temperature at which plants are held after infection. Forty to 100% reductions in lesion number occurred on cotton plants incubated at elevated temperatures compared with plants maintained at 30 C. Lesion formation was reduced more than 70% after exposure to 43.5 C for 2 hr. Results indicate that daily temperature maxima may be important in limiting Alternaria leaf spot in Arizona and may partially explain increased disease severity in central Arizona during cotton seasons in which daily temperature maxima are relatively low. The influence of high temperature on the pathogen in vitro was also studied. Germ tubes of A. macrospora explosively lysed on potato-dextrose agar (PDA) incubated at 4246 C. After 6 hr at 42, 43.5, and 46 C, respectively, 7, 52, and 82% of the germ tubes lysed. Spore viability on PDA declined 65% after 4 hr at 46 C but remained stable for 24 hr at 42 C. Results provide indirect evidence that germ tube lysis and reduced spore viability may be mechanisms of temperature-mediated reduction in lesion number. However, other temperature-induced alterations in the host-pathogen interaction may be equally important.