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The Relationship Between Rate-Reducing Resistance to Phytophthora megasperma f. sp. glycinea and Yield of Soybean. P. W. Tooley, Former graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: Department of Plant Pathology, Cornell University, Ithaca, NY 14853; C. R. Grau, associate professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 74:1209-1216. Accepted for publication 12 March 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1209.

Field studies were performed in 1980 and 1981 to evaluate various measures of rate-reducing resistance as predictors of soybean yield loss due to Phytophthora root and stem rot; to determine the relationship of resistance, plant growth, and yield components; and to explore the relationship between resistance and tolerance in reducing yield losses. Yield loss was best predicted by critical-point models with disease incidence assessed between the V5 and R7 growth stages as the predictor variable. The area under the disease progress curve, its square root, the simple interest infection rate, its square root, and disease severity at growth stage R5 were also adequate predictors of yield loss. Based on metalaxyl-treated controls, Phytophthora root and stem rot reduced plant heights, numbers of nodes, internode length, and top and root dry weights. Reduction in the number of yielding plants per row was the most critical component contributing to yield reduction due to Phytophthora root and stem rot; significant reductions in weight per seed and number of pods per plant were also observed. Cultivars differed substantially in the yield components affected by root and stem rot. Cultivars with higher levels of rate-reducing resistance showed less reduction in plant height, dry weight, and yield components than did more susceptible cultivars. Cultivars showed no differences in levels of disease tolerance, estimated by adjusting mean percent yield reduction due to root and stem rot to a common disease incidence. The results strongly suggest that differences in rate-reducing resistance are responsible for observed cultivar differences in yield loss due to Phytophthora root and stem rot.