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Race-Specific Resistance in Soybean cv. Davis to Phytophthora megasperma f. sp. glycinea. Malcolm J. Ryley, Plant Pathologist, Queensland Department of Primary Industries, P.O. Box 102, Toowoomba, Queensland, Australia 4350. Neale R. Obst, District Experimentalist, Queensland Department of Primary Industries, P.O. Box 102, Toowoomba, Queensland, Australia 4350. Plant Dis. 76:665-668. Accepted for publication 23 January 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0665.

In Queensland, Australia, the soybean (Glycine max) cultivar Davis has been used extensively in the public breeding program as a source of resistance to Phytophthora megasperma f. sp. glycinea. This type of resistance, associated with roots and conditioned by the Rps2 gene, provided a high level of protection against many races until recently. Davis and other cultivars derived from it are grown throughout the state and have rarely been affected by Phytophthora root and stem rot. Since 1988, P. m. glycinea has been isolated from affected plants of these cultivars at several localities. Every isolate was classified as race 15, which comprised less than 10% of all isolates collected in Australia between 1979 and 1988. Race 1 was dominant during this period. Two root inoculation experiments showed that plants of Davis, Manark, and Centaur were resistant to races 1 and 4 but highly susceptible to race 15. Bragg and Dragon were moderately susceptible to all three races, whereas Semstar was highly susceptible to races 1 and 15 and moderately resistant to race 4. There is evidence that the long-term cultivation of cultivars with high levels of resistance to race 1 at Hermitage Research Station via Warwick, Queensland, has resulted in an increase in the proportion of race 15 isolates relative to race 1 isolates. It is apparent that field resistance in Davis is race-specific and that the continued use of genotypes with resistance derived from Davis will result in the selection of races virulent to such genotypes.