T-Toxin Production by Near-Isogenic Isolates ofCochliobolus heterostrophusRaces T and O

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Abstract
Mycelia and culture filtrates from 4 near-isogenic isolates of C. heterostrophus (Helminthosporium maydis), segregating monogenically for race differences, were extracted for host-specific toxin 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 that determines virulence of these isolates to corn with Texas male-sterile cytoplasm also apparently controls the abundant production of a chemically defined product that is essential for high virulence, but not for fungal growth or survival.