DIVERSITY AND FREQUENCY OF RACES OF PHYTOPHTHORA-MEGASPERMA F SP GLYCINEA IN SOYBEAN FIELDS IN ESSEX COUNTY, ONTARIO, 1980-1989

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.