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Soybean Seed Yield Losses Caused by Powdery Mildew¹

Yield losses in soybean (Glycine max (L.) Merr.) caused by natural infection with Microsphaera diffusa Cke. & Pk., the powdery mildew fungus, have not been determined. The objective of this study was to determine if M. diffusa caused loss of seed yield in soybeans. Three susceptible (‘Harosoy 63’, ‘Kanrich’, and ‘Bonus’) and three resistant (‘Lindarin 63’, ‘Wayne’, and ‘Cutler 71’) cultivars were tested. A spray containing benomyl (methyl 1-(butylcarbamoyl)-2-benzimidazole carbamate) was applied to plant rows until runoff, once a week from 1 July until near plant maturity. Nonsprayed rows served as controls. The incidence of powdery mildew increased rapidly during July, and by the end of the month nearly all leaves of all susceptible cultivars were infected. Seed yield of the susceptible cultivars was significantly (p < 0.01) higher in benomyl-sprayed than in nonsprayed plants: 9, 15, and 10% higher for Harosoy 63, Kanrich, and Bonus, respectively. Seed yield of the resistant cultivars was not affected by spraying with benomyl, indicating that the higher yield from spraying susceptible cultivars was only from the control of powdery mildew. The mean loss of yield caused by powdery mildew was 10.1%, which suggests that use of powdery mildew resistant cultivars by soybean producers in areas where powdery mildew occurs, would be beneficial.

Key Words: Disease resistance • Benomyl fungicide • Disease control • Microsphaera diffusa • Glycine max • Disease loss assessment

² Research plant pathologist, ARS, USDA, and professor of plant pathology, Iowa State Univ., Ames, IA 50011.

Received for publication June 17, 1977.