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Field Characterization of Rate-Reducing 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: Department of Plant Pathology, Cornell University, Ithaca, NY 14853; C. R. Grau, associate professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 74:1201-1208. Accepted for publication 12 March 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1201.

Field studies were performed in 1980 and 1981 to compare methods of quantifying rate-reducing resistance to Phytophthora megasperma f. sp. glycinea (Pmg) and to determine how such resistance is expressed under field conditions. Eight variables that provided estimates of resistance were examined in 1980: metalaxyl was applied at four rates so that resistance could be expressed in terms of fungicide equivalency. Quadratic (rather than linear) equations best fit the disease intensity:metalaxyl rate response function. Moreover, cultivars showed significantly different quadratic responses to increasing metalaxyl application rates. Of 21 variables examined in 1981, 14 allowed the detection of significant cultivar differences in rate-reducing resistance. The variables most useful in differentiating cultivars were disease incidence at growth stages R3, R5, and R7; area under the disease progress curve; and disease severity rating at the R5 growth stage. These variables were highly correlated with one another, indicating that the incidence of dead or dying plants may be used to predict the internal disease severity in surviving plants. Most plants of cultivars showing the lowest disease incidence showed no internal discoloration of taproots and lower stems. Thus, resistance was characterized by the apparent ability of some cultivars to restrict the activity of Pmg in the taproot and lower stem tissue. As this phenomenon closely parallels that observed with the same cultivars in response to cotyledon inoculation, we believe that a primary component of rate-reducing resistance to Pmg is the ability to restrict fungal colonization of the plant tissue.