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Identification and Quantitative Characterization of Rate-Reducing Resistance to Phytophthora megasperma f. sp. glycinea in Soybean Seedlings. Paul W. Tooley, Graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; Craig R. Grau, assistant professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 72:727-733. Accepted for publication 21 September 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-727.

Cotyledons of 9- to 11-day-old seedlings were wound-inoculated with Phytophthora megasperma f. sp. glycinea (Pmg). A 10-μl drop containing encysted zoospores was delivered to cotyledons by using a micropipette with a disposable tip modified to allow simultaneous wounding and delivery of spores. After incubation for 5 days, plants were scored as dead or living. Resistance was characterized by the inability of the fungus to move into the hypocotyl and kill the plant. Controlled-environment studies showed that the expression of this resistance was affected by inoculum density, temperature, photoperiod, and plant age, and was active against four physiologic races of Pmg. Based on inoculum density studies using this method with Pmg race 7, estimated log LD50 values were obtained for eight soybean cultivars that differed in susceptibility to Pmg in the field. LD50 values correlated well with yield, disease severity, and (simple interest) infection rate (R) for each cultivar when grown in naturally infested field plots. Thus, this method may prove useful for evaluating germ plasm lines in breeding programs, in evaluating cultivar resistance in cases where field evaluations are not possible, and as a technique to study the nature of rate-reducing resistance to Pmg.