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Ecology and Epidemiology

Isolation and Identification of Races of Sphaerotheca pannosa var. rosae. C. L. Bender, Graduate research assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; D. L. Coyier, research plant pathologist, U.S. Department of Agriculture, ARS, Corvallis, OR 97330. Phytopathology 74:100-103. Accepted for publication 18 July 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-100.

Nine isolates of Sphaerotheca pannosa var. rosae, the pathogen that causes powdery mildew of roses, were obtained from infections on seven hybrid rose cultivars and two Rosa spp. Isolates were initiated on detached leaves, increased on host plants, and inoculated to rose cultivars to test for differential reactions. Virulence was evaluated by assessing the infection type of individual lesions and the percentage of leaf area covered with sporulating colonies. Five races of S. pannosa var. rosae were identified, and virulence formulae were developed to describe them on four differential rose cultivars. The pathogenicity markers described in this investigation will facilitate future genetic studies of S. pannosa var. rosae.