Races of Phytophthora megasperma f. sp. glycinea in Wisconsin. P. W. Tooley, Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706. C. R. Grau, Assistant Professor, and M. C. Stough, former Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 66:472-475. Accepted for publication 13 August 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-472.

Studies were undertaken to determine the range of physiologic races of Phytophthora megasperma f. sp. glycinea in Wisconsin in 1978 and 1980 and to investigate the relationship between race frequency and cultivar selection. In 1978, race 3 was isolated at the highest frequency, whereas races 1, 4, 7, 8, and 9 were isolated less frequently. The use of virulence formulae (effective/ineffective host genes) was necessary in 1980 to describe the virulence of the isolates because the variation encountered could not be described under the current numeric classification scheme. Previously undescribed races were found more frequently than were races 3, 9, 1, 5, and 8 and were detected in decreasing frequency on cultivars containing race-specific resistance conferred by the Rps^c₁ allele, the Rps₁ allele, and those with no race-specific resistance, respectively. The results suggest that selection pressure exerted by cultivars containing race-specific resistance has resulted in increased virulence in the pathogen population and cast doubt on the long-term effectiveness of race-specific resistance.

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