Metalaxyl Sensitivity Selection Within Phytophthora megasperma f. sp. glycinea. Jana S. Lamboy, Department of Plant Pathology, University of Illinois, Urbana, IL 61801. Jack D. Paxton, Department of Plant Pathology, University of Illinois, Urbana, IL 61801. Plant Dis. 76:932-936. Accepted for publication 13 April 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0932.

After six successive transfers on cornmeal agar amended with 0.1 µg/ml metalaxyl, an isolate of Phytophthora megasperma f. sp. glycinea displayed reduced sensitivity to metalaxyl and increased virulence on metalaxyl-treated seedlings of Harosoy 63 soybean (Glycine max). Analysis of 10 single-zoospore progeny from the isolate, derived before the isolate was grown on metalaxyl-amended medium, demonstrated innate variation in virulence and metalaxyl sensitivity. Virulence of several isolates was compared on Harosoy and Harosoy 63 seedlings pretreated with 0, 0.5, 1.0, 2.5, and 5.0 µg/ml metalaxyl in water. Two isolates, which had been grown on cornmeal agar amended with 0.1 µg/ml metalaxyl, were more virulent on seedlings treated with metalaxyl than were their parental isolates. Exposure to metalaxyl in saprophytic growth can select for both altered virulence and decreased sensitivity to the fungicide in metalaxyl-treated plants.