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Diversity and Frequency of Races of Phytophthora megasperma f. sp. glycinea in Soybean Fields in Essex County, Ontario, 19801989. T. R. Anderson, Agriculture Canada, Research Station, Harrow, Ontario NOR 1GO. R. I. Buzzell, Agriculture Canada, Research Station, Harrow, Ontario NOR 1GO. Plant Dis. 76:587-589. Accepted for publication 12 December 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0587.

A survey of races of Phytophthora megasperma f. sp. glycinea was conducted in Essex County, Ontario, between 1980 and 1989. Races 1, 3, 4, 5, 6, 7, 8, 9, 13, and 21 comprised 2.7, 24.2, 13.7, 3.5, 0.2, 32.5, 0.2, 16.5, 1.3, and 0.2%, respectively, of 705 isolates. Genes for resistance to root rot in soybean (Glycine max) cultivars recommended in the area between 1976 and 1989 were identified. The diversity of races of P. m. glycinea could be explained on the basis of the genotypes for resistance recommended in the survey area. The frequency of occurrence of races supports the theories of directed and stabilizing selection of plant pathogens based on putative genotypes of P. m. glycinea races in the area.