Disease Note

Outbreak of Powdery Mildew of Soybean in Minnesota in 1985. B. Kennedy, Department of Plant Pathology, University of Minnesota, St. Paul 55108. J. Orf, Department of Agronomy and Plant Genetics, and W. C. Stienstra, Department of Plant Pathology, University of Minnesota, St. Paul 55108, and J. Wiersma, University of Minnesota, Crookston 56716. Plant Dis. 70:694. Accepted for publication 19 February 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-694c.

Powdery mildew (Microsphaera diffusa Cke. & Pk.) in commercial and experimental soybean (Glycine max (L.) Merr.) plantings was widespread and severe on susceptible genotypes throughout Minnesota in 1985. The disease was also widespread in Minnesota during 1974–1977 but has since been observed in only trace amounts through the 1984 season. In a planting-date study at Crookston, the cultivar Evans planted at eight dates was resistant but cultivars McCall and Maple Amber showed increasing amounts of infection at later dates. Those planted on 8 July (the latest planting date) were 90% infected on 16 September. At St. Paul, where 653 genotypes representing commercial and experimental lines were planted in a disease nursery, 228 remained uninfected by 28 September. In a similar disease nursery at Waseca, 27 of 125 were free from powdery mildew.