Effect of Temperature and Soybean Cultivar on Metalaxyl Efficacy Against Phytophthora megasperma f. sp. glycinea. S. O. Guy, Graduate Research Assistant, Department of Agronomy, University of Wisconsin, Madison 53706. C. R. Grau, E. S. Oplinger. Professor, Department of Plant Pathology, and Professor, Department of Agronomy, University of Wisconsin, Madison 53706. Plant Dis. 73:236-239. Accepted for publication 31 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0236.

Four soybean cultivars, treated or not treated with metalaxyl, were evaluated at 24, 28, and 32 C for reaction to two isolates of race 3 of Phytophthora megasperma f. sp. glycinea. In addition, metalaxyl was evaluated at rates of 0, 0.05, 0.25, 0.60, and 1.20 ?g/ml at 24, 28, and 32 C. Internal stem discoloration was measured acropetally from the point of inoculation and was positively correlated (r = 0.90) with plant mortality. Lesions were longest at 32 C and shortest at 24 C for all cultivars and rates of metalaxyl. Regression analysis of metalaxyl concentration vs. lesion length showed a difference in intercepts, but not slopes, with the highest intercept at 32 C and the lowest at 24 C. Metalaxyl was effective in reducing lesion length at all temperatures, in all cultivars, and with both pathogen isolates. However, greater amounts of metalaxyl were needed to reduce disease severity at 32 C than at 24 and 28 C. The Rps₆ resistance gene expressed an incompatible reaction at 24 C but a compatible reaction at 32 C.

Keyword(s): disease evaluation, disease resistance, Glycine max, Phytophthora root and stem rot.