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Pathogenic Variation in Oregon Populations of Sphaerotheca pannosa var. rosae. C. L. Bender, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. D. L. Coyier, Research Plant Pathologist, Horticultural Crops Research Laboratory, USDA, ARS, Corvallis, OR 97330. Plant Dis. 70:383-385. Accepted for publication 4 November 1985. 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, 1986. DOI: 10.1094/PD-70-383.

Pathogenic variation in field and greenhouse populations of Sphaerotheca pannosa var. rosae was monitored on two differential cultivars. Two changes in the virulence of populations of S. pannosa var. rosae were detected by repeated sampling of conidia on greenhouse roses. These changes were associated with the introduction of infected plant material to the greenhouse. Only one pathotype was detected in powdery mildew populations sampled from two commercial greenhouses. Conidial populations on field-grown plants of Rosa rugosa and cultivar Dwarf Crimson Rambler consisted of at least two and four pathotypes, respectively. The presence of cleistothecia on these plants and the ratio in which the pathotypes occurred suggested that the virulence factors may have been recombined meiotically.