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Microsphaera diffusa, the Perfect Stage of the Soybean Powdery Mildew Pathogen. M. R. McLaughlin, Graduate Research Assistant, Department of Plant Pathology, University of Illinois, Urbana, IL 61801; Julia S. Mignucci(2), and G. M. Milbrath(3). (2)(3)Graduate Student, and Assistant Professor, respectively, Department of Plant Pathology, University of Illinois, Urbana, IL 61801. Phytopathology 67:726-729. Accepted for publication 3 December 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-726.

Profuse development of cleistothecia of a powdery mildew pathogen was observed on soybeans grown in the greenhouse. The pathogen was identified as Microsphaera diffusa. Cleistothecia were white at first, then during maturation changed sequentially to yellow, tan, brown, and finally black. Mature cleistothecia were 92 to 125 μm in diameter and possessed about twenty appendages 124 to 212-μm long that were dichotomously branched four to five times at the tips. Multiple pyriform asci contained up to six light-yellow ovoid ascospores which measured 9 18 μm. A relatively simple and rapid procedure is described for handling leaf material with powdery mildew for scanning electron microscopy (SEM). The cleistothecia of M. diffusa were hemispherical with deeply convoluted outer walls and appendages originating in a ring from the base. A comparative SEM study of Erysiphe polygoni is presented.