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Inheritance of Resistance to Peronospora manshurica Races 2 and 33 in Soybean. S. M. Lim, Research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, and also professor, Department of Plant Pathology, University of Illinois, Urbana 61801; Phytopathology 79:877-879. Accepted for publication 26 April 1989. 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, 1989. DOI: 10.1094/Phyto-79-877.

This study determined the inheritance of resistance to races 2 and 33 of Peronospora manshurica in the soybean cultivars Fayette and PI 88.788 and identified the gene (or genes) conferring this resistance. F2 progeny from the susceptible cultivar Williams 82 Union and from Williams 82 Fayette segregated in a ratio of 3 resistant:1 susceptible when inoculated with race 2. F3 families from Williams 82 Fayette segregated in a ratio of 1 resistant:2 segregating for resistance:1 susceptible, confirming that resistance in Fayette is conferred by a single dominant gene. The F2 and F3 progenies from Union Fayette and Union PI 88.788 segregated in a ratio of 3 resistant:1 susceptible when inoculated with race 33 and in a ratio of 15 resistant:1 susceptible when inoculated with race 2, indicating that the gene for resistance in the both Fayette and PI 88.788 segregates independently from the Rpm gene of Union. Fayette was developed from Williams PI 88.788 for resistance to the soybean cyst nematode. Williams is susceptible to both races of P. manshurica. The resistance to these races in Fayette was probably transferred from PI 88.788. The gene symbol Rpm2 was assigned to the gene in PI 88.788. The resistant parents were symptomless, but in crosses of a parent susceptible to race 2 or race 33 with a parent resistant to that race, the F1 progeny developed small flecks. The segregation ratios in the F2 and F3 progenies indicate that homozygous resistant plants are symptomless and heterozygous resistant plants develop small flecks. The F1 progeny from resistant resistant crosses were symptomless when inoculated with race 2, indicating an epistatic effect of the two resistance genes at different loci.