Prevalence of Soybean Downy Mildew in Iowa. J. M. Dunleavy, Research Plant Pathologist, USDA, ARS, Department of Plant Pathology, Seed, and Weed Sciences, Iowa State University, Ames 50011. J. W. Keck, Research Associate, K. S. Gobelman-Werner, Biological Laboratory Technician, USDA, ARS, and M. M. Thompson, Research Assistant, Department of Plant Pathology, Seed, and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 68:778-779. Accepted for publication 2 March 1984. 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, 1984. DOI: 10.1094/PD-68-778.

Downy mildew of soybean, caused by Peronospora manshurica, occurred in 50% of 825 Iowa soybean fields examined in 1982, and the disease was found in all sections of the state. Mean disease severity, based on a rating ranging from 1 (slight disease, trace to 25% of leaves showing symptoms) to 4 (very severe disease, 75–100% of leaves showing symptoms), was 2 (moderate disease). Greatest prevalence of downy mildew occurred in an 11-county area in the north central section of the state in which 88.5% of the fields were affected by the disease. Only 5.1% of the fields were affected in the 12-county northwestern section. Downy mildew was observed in 81 of 95 counties (85%) sampled and was present in all counties in the north central, central, and east central sections of the state. There was no correlation between the percentage of fields affected by downy mildew and the mean disease severity in affected fields. Soybean acreage of the nine sections of the state sampled was not correlated with either the percentage of fields affected by downy mildew or disease severity.

Keyword(s): Glycine max.