Phytophthora root rot of soybean [Glycine max (L.)Merr.], caused by Phytophthoru megasperma Drechs, f. sp. glycineu Kaun and Erwin, is a major disease problem in the Upper Midwest. This study was conducted to determine soybean performance with metalaxyl IN- (2,6-dimethyl phenyl)-N-(methylacety1)alanine methyl ester] fungicide on cultivars with different resistance and tolerance to this disease under low to moderate Phytophthora root rot potentials. Yield, plant density, and a Phytophthora root rot index were determined for metalaxyl treatments applied at labeled rates in furrow (IF) and as a seed treatment (ST) on 10 cultivars in two Wisconsin locations in 1985 and 1986, and IF at 0, 85, 170, and 340 g ai. ha−1 on five cultivars at two 1987 locations. Yields of two, four, and five cultivars were increased by metalaxyl ST, IF, or IF+ ST treatments, respectively, in two moderate Phytophthora root rot diseased environments. Soybean yields with ST did not differ from IF for any cultivar. The cultivar Am00 with IF+ST treatment yielded more than IF alone. The AP200 plant stand at V2 stage was increased by IFfST compared with the control. In furrow metalaxyl treatment decreased plant stand compared with ST for ‘S18-84’. In two 1985 and 1986 environments, without detected Phytophthora root rot, soybean yields were decreased by 280 g ha−1 metalaxyl applied IF at 0.42, 0.25, 0.26, and 0.20 Mg ha−1 for S18-84, AP200, ‘Century 84’, and ‘L1771’, respectively. However, in additional studies conducted in 1987 at two locations in the absence of Phytophthora root rot, metalaxyl applied at 85 to 340 g ha−1 IF did not influence the yield of five cultivars. Metalaxyl can be an effective fungicide for soybean Phytophthora root rot control; however, yield for certain cultivars may be adversely affected where there is no disease control benefit.
Submitted as partial fulfillment of requirements for the Ph.D. degree in Agronomy by senior author. This research was supported in part by grants from Ciba-Geigy Co., Graduate School, and College of Agric. and Life Sci. as Federal Hatch project 1890.