RACES OF PERONOSPORA MANSHURICA IN THE UNITED STATES

I propose a set of 11 soybean varieties for the differentiation of races of Peronospora manshurica: Pridesoy, Norchief, Mukden, Richland, Roanoke, Illini, S 100, Palmetto, Dorman, Kabott, and Ogden. When I employed these, I found fifteen new races of Peronospora manshurica, the casual agent of downy mildew of soybeans, described them, and assigned numbers 9 through 23. Race 8 was encountered most frequently and occurred at 40% of the locations from which seed samples were obtained. Races 1, 2, 10, 12, 18, and 23 occurred at 5–8% of the locations sampled. Race 10 was common in the southern U. S. Of 247 soybean seed lots sampled, 73% contained some oospore-encrusted seeds. These ranged from 12–26% lighter than healthy seeds. The percentage of oospore-encrusted seeds occurring in seed lots ranged from 0–25%.