Polymorphisms for lesion type, fungicide tolerance, and mating capacity in Cochliobolus carbonum isolates pathogenic to corn

Of 189 isolates of Cochliobolus carbonum from the southeastern and midwestern United States, 3 were race 1, 133 were race 2, and 53 were race 3. The rarity of race 1 which produces a host-specific toxin suggests that, in the absence of host plants sensitive to the toxin, stabilizing selection favors genotypes lacking the gene for toxin production. Race 3 which causes long, linear lesions on corn leaves was predominant among isolates from the Mountains of North Carolina and adjacent states, but was rare in the Piedmont and Coastal Plain where race 2 predominated. Race 2 which causes small round or oval lesions was the only race found in the midwestern United States.Polymorphisms for tolerance of cycloheximide, carboxin, and cadmium were found in race 2 but not in race 3. Only 17% of race 3 isolates had the ability to form perithecia, whereas 67% of race 2 isolates formed perithecia in compatible matings. These differences in polymorphism in addition to the quantitatively inherited difference in lesion type between race 2 and race 3 indicate that there has been little or no gene exchange between races 2 and 3 in nature even though fertile crosses can be obtained readily in the laboratory.