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Isozyme Variation Within and Among Pathogenic Races of Cochliobolus carbonum on Corn in North Carolina. H. G. Welz, Wissenschaftlicher Mitarbeiter, Institute of Plant Breeding, Seed Science, and Population Genetics, University of Hohenheim (762), 70593 Stuttgart, Germany; former postdoctoral research associate, Tropeninstitut der Justus-Liebig-Universität, 35390 Giessen, Germany; W. Köhler(2), and K. J. Leonard(3). (2)professor, Biometrie und Populationsgenetik, Justus-Liebig-Universität, 35390 Giessen, Germany; (3)research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Cereal Rust Laboratory, University of Minnesota, St. Paul 55108; former research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7616. Phytopathology 84:31-38. Accepted for publication 26 August 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, 1994. DOI: 10.1094/Phyto-84-31.

We used starch gel electrophoresis to investigate isozyme variation among 194 isolates of races 0, 2, and 3 of Cochliobolus carbonum collected in 1987 in two corn fields in North Carolina as well as six isolates of race 1 from previous collections in different states of the United States. Of eight enzyme systems studied in detail, only one, phosphogluconate dehydrogenase, was monomorphic. Diaphorase, malate dehydrogenase, glutamate oxaloacetate transaminase, and esterase were highly polymorphic in race 0, whereas isozyme variation in races 2 and 3 was very limited. Race 0 accordingly had a significantly higher average gene diversity than did race 2 or race 3. Genetic similarity between races 2 and 3 was very high. Their allele frequencies differed clearly at only two putative esterase loci. Races 2 and 3 were only distantly related to race 0, as demonstrated by cluster analysis and principal coordinates analysis. There was no apparent genetic variation among five race 1 isolates from North Carolina and South Carolina. Their common electrophoretic type occurred also in race 2 but not in race 0 or race 3, suggesting a common ancestry with race 2. Only a small proportion of race 2 and race 3 isolates or race 2 and race 0 isolates shared common electrophoretic types, which suggests that there is little, if any, genetic exchange between these races.

Additional keywords: Bipolaris zeicola, Helminthosporium carbonum, maize, Zea mays.