Yield Reduction in Soybeans Caused by Downy Mildew. J. M. Dunleavy, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 71:1112-1114. Accepted for publication 24 June 1987. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1987. DOI: 10.1094/PD-71-1112.

Seed yield reductions in soybeans (Glycine max) caused by downy mildew (Peronospora manshurica) were determined. In a 2-yr field test, there was good disease spread early in the season from the Lincoln A117 border to leaves of the unsprayed susceptible cultivars in the plots. Infected leaves on these cultivars produced abundant conidia, and the mean percentage of diseased plants was 98.5. No conidia were found on metalaxyl-sprayed or unsprayed leaves of resistant cultivars. The mean seed yield of sprayed susceptible cultivars was 11.8% more than that of the same unsprayed cultivars. Metalaxyl effectively controlled downy mildew on three susceptible cultivars. A mean of 10.6% oospore-encrusted seed occurred in unsprayed, susceptible cultivars.