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Reduced Fitness Associated with TOX1 of Cochliobolus heterostrophus. C. J. R. Klittich, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011, Current address: Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506; C. R. Bronson, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Phytopathology 76:1294-1298. Accepted for publication 20 May 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1294.

Cochliobolus heterostrophus strains near-isogenic except for a gene for T-toxin production were examined for differences in fitness on normal cytoplasm maize in the field and greenhouse. A mixture of two strains (one race O, nontoxin-producing, and one race T, toxin-producing) was used to inoculate a field plot of the susceptible hybrid Cornell 281 in Ames, IA, in 1983 and 1984. Different pairs of strains were used each year; lesions were harvested periodically during the season, and the number of lesions yielding race T and race O strains was determined. The frequency of the race T strain decreased significantly compared with the race O strain both years, indicating that the race T strains were less fit than the race O strains. Differences in fitness of near-isogenic strains were also expressed as differences in lesion length. Lesions produced by race T strains were significantly shorter than those produced by race O strains from four backcross generations of the fungus and on three genotypes of maize. We concluded that the toxin gene (TOX1) or a gene closely linked to it reduces pathogen fitness on N-cytoplasm maize. Reduced fitness associated with the toxin locus could explain the rapid decrease in frequency of race T after susceptible Texas male sterile cytoplasm maize was replaced with normal cytoplasm maize following the 1970 southern leaf blight epidemic.

Additional keywords: Bipolaris maydis, stabilizing selection.