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Phenotypic Variation and Parasitic Fitness of Races of Cochliobolus carbonum on Corn in North Carolina. H. G. Welz, Formerly, postdoctoral research associate, Tropeninstitut der Justus-Liebig-Universität, 6300 Giessen, Germany, Current address: Wissenschaftlicher Mitarbeiter, FSP Biotechologie und Pflanzenzüchtung, Universität, Hohenheim (762), 7000 Stuttgart 70, Germany; K. J. Leonard, research plant pathologist, U. S. Department of Agriculture, Agricultural Research Service, Department of Plant Pathology, P. O. Box 7616, North Carolina State University, Raleigh 27695-7616, Current address: supervisory research plant pathologist, U. S. Department of Agriculture, Agricultural Research Service, Cereal Rust Laboratory, University of Minnesota, St. Paul 55108. Phytopathology 83:593-601. Accepted for publication 28 January 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/Phyto-83-593.

Two field populations of Cochliobolus carbonum on corn (Zea mays) in the piedmont of North Carolina, where races 2 and 3 coexist, were sampled repeatedly (four times during 51 days in the Wilkes County field and three times during 32 days in the Yadkin County field) during 1987. Isolates were tested for lesion type and several polymorphic traits, including mating type, fertility, and fungicide tolerance. The relative fitness of race 3, as determined from frequency changes, was significantly lower than the fitness of race 2 in each field. The fitness values of race 3, with an estimated generation time of 7 days, were 0.82 and 0.84 versus 1.0 for race 2. A third race, race 0, was also common in the Wilkes County field and had a relative fitness of 0.42. The mean frequency of race 3 was 29% in the Wilkes County field, 30% in the Yadkin County field, and 92% in a third field in the Appalachian Mountains in Tennessee that was sampled only once in 1987. These data show that the frequency of race 3 in the piedmont and mountains changed little from 1977 to 1987, and parasitic fitness alone cannot account for the distribution of race 3. The frequencies of seven polymorphic traits and the genetic diversity within races 2 and 3 remained stable over the 1987 sampling period, and race frequencies changed, suggesting the phenotypic traits are selectively neutral. As in previous years, trait frequencies differed significantly between races, indicating their genetic isolation. Within races, however, there was evidence that sexual reproduction may have occurred. In races 0 and 3, there were no indications of gametic phase disequilibrium typical of asexual populations. In race 2, there were no significant associations of pairs of traits, but some phenotypes occurred significantly more frequently than was expected based on the frequencies of their component traits. Also, there were significant differences in phenotype frequencies between mating types MAT-1 and MAT-2 in race 2. The two mating types differed more from a 1:1 ratio in race 2 than in races 0 and 3, suggesting that sex may be less important in race 2 than in races 0 and 3.

Additional keywords: Bipolaris zeicola, Helminthosporium carbonum, maize.