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Sources of Immunity and Susceptibility to Downy Mildew of Soybeans¹

The disease reaction of 14 races of Peronospora manshurica (Naum.) Syd. ex Gaum., the downy mildew fungus, was observed on 72 soybean (Glycine max (L.) Merr.) varieties. ‘Mendota,’ ‘Kanrich,’ and a portion of the ‘Kanro’ plants were resistant to all races studied. Only ‘Bansei’ was very susceptible to all races of the fungus to which it was tested. Varieties were rated for response to the fungus on a basis of 1 (immune) to (very susceptible). The distribution with respect to disease rating classes of the responses of the varieties to races of P. manshurica was determined. Nearly one-third of the variety ratings for all race-variety combinations were in class 5. The means for the other four disease rating classes were uniformly distributed in classes 1 through 4. Race 8 was most virulent, with 75% of the disease ratings occurring in classes 4 and 5. In contrast, race 10 was least virulent, with 26% of the ratings occurring in classes 4 and 5.

Key Words: Obligate parasite • Pathogenic specialization • Host-parasite relations • Disease resistance • Glycine max • Peronospora manshurica

² Plant Pathologist, Crops Research Division, ARS, USDA, and Professor of Plant Pathology, Iowa State University.

Received for publication February 5, 1970.