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T-Toxin Production by Near-Isogenic Isolates of Cochliobolus heterostrophus Races T and O. K. J. Tegtmeier, Research associate, Department of Agricultural Biochemistry, University of Nebraska, Lincoln 68583; J. M. Daly(2), and O. C. Yoder(3). (2)Professor, Department of Agricultural Biochemistry, University of Nebraska, Lincoln 68583; (3)Associate professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:1492-1495. Accepted for publication 28 April 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1492.

Mycelia and culture filtrates from four near-isogenic isolates of Cochliobolus heterostrophus (Helminthosporium maydis), segregating monogenically for race differences, were extracted for host-specific toxin known to be produced by C. heterostrophus race T. Extracts from the race T isolates produced large amounts of an acetone- and chloroform-insoluble white precipitate that was chemically and biologically identical to previous preparations of T-toxin. Although a slight precipitate sometimes formed in extracts from cultures of the race O isolates, this precipitate was nontoxic. Unprecipitated material remaining in extracts of race T cultures also exhibited specific toxicity, but similar fractions of race O cultures did not. The mycelium of race T isolates accounted for 95% of the toxin extracted, which conservatively can be estimated to amount to 2% of mycelial dry weight. Thus, the single gene shown to determine virulence of these isolates to corn with Texas male-sterile cytoplasm also appears to control the abundant production of a chemically defined product that is essential for high virulence, but not for fungal growth or survival.

Additional keywords: Bipolaris maydis, Drechslera maydis, HmT-toxin.