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Genetic Diversity in Field Populations of Cochliobolus carbonum on Corn in North Carolina. K. J. Leonard, Research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Cereal Rust Laboratory, University of Minnesota, St. Paul 55108; S. Leath, research plant pathologist, USDA-ARS, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 80:1154-1159. Accepted for publication 11 May 1990. 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, 1990. DOI: 10.1094/Phyto-80-1154.

Mean frequencies of race 2 (round and oval lesions) and race 3 (long, linear lesions) among 314 isolates of Cochliobolus carbonum from corn leaves in 10 North Carolina fields were 0.80 and 0.20 in the western piedmont and 0.82 and 0.18 in eastern North Carolina. Even though races 2 and 3 occurred in the same fields, they were genetically distinct. There were clear distinctions in lesion types, which are polygenically inherited, and the frequencies of cycloheximide tolerance and ability to form pseudothecia differed significantly in the two races. Frequencies of race, mating type, ability to form pseudothecia and asci with ascospores, and tolerance of cycloheximide and carboxin varied considerably from field to field, even within short geographical distances. This suggests that inoculum dispersal and gene flow among populations is restricted in C. carbonum. Calculations of Nei?s genetic distances between field populations based on frequencies of these polymorphic traits were not correlated with geographical distance between the fields, indicating that the traits are not good indicators of microevolutionary divergence between populations, probably because the traits are not selectively neutral.

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