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Heterothallism in the Apple Powdery Mildew Fungus, Podosphaera leucotricha. Duane L. Coyier, Research Plant Pathologist, Western Region, Agricultural Research Service, United States Department of Agriculture, Mid-Columbia Experiment Station of Oregon State University, Rt. 5, Box 240, Hood River 97031, Present address: 3420 S.W. Orchard St., Corvallis, Oregon 97330; Phytopathology 64:246-248. Accepted for publication 9 October 1973. DOI: 10.1094/Phyto-64-246.

Fourteen cultures of Podosphaera leucotricha, each derived from a single conidium, were maintained on Malus sylvestris seedlings in special isolation chambers. Cleistothecia developed on leaves, stems, and petioles of plants inoculated with certain randomly paired cultures, but did not form on plants inoculated with single cultures. Therefore, P. leucotricha is heterothallic, and two mating types are suggested.