VIEW ARTICLE

Resistance

Microplot Comparison of Rate-Reducing and Race-Specific Resistance to Phytophthora megasperma f. sp. glycinea in Soybean. P. W. Tooley, Former graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: U.S. Department of Agriculture, Agricultural Research Service, Foreign Disease-Weed Science Research Unit, Fort Detrick Building 1301, Frederick, MD 21701; C. R. Grau, associate professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 76:554-557. Accepted for publication 9 December 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-554.

Microplots were artificially infested with races 1, 3, and 4 of Phytophthora megasperma f. sp. glycinea alone and in all possible combinations. Five soybean (Glycine max) cultivars containing different levels of rate-reducing resistance alone or in combination with race-specific resistance to P. m. f. sp. glycinea were grown in microplots for 3 yr in succession. Cultivars with high levels of rate-reducing resistance sustained low disease levels regardless of the race or race combination of P. m. f. sp. glycinea. In microplots infested with incompatible races of the fungus, specific resistance was not significantly more effective than a high level of rate-reducing resistance in all 3 yr. Also, race-specific resistance was ineffective in the presence of race combinations that contained a proportion of compatible races of P. m. f. sp. glycinea. Since diverse races of the pathogen now exist over wide areas, our results indicate that a high level of rate-reducing resistance offers the best means of stable, long-term control of soybean root and stem rot.