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An Evaluation of the Independence of Certain Virulence Genes of Erysiphe graminis f. sp. tritici. M. H. Royer, Graduate student, Department of Plant Pathology, Pennsylvania State University, University Park 16802, Current address: United States Department of Agriculture, Agricultural Research Service, Plant Disease Research Laboratory, Building 1301, Ft. Detrick, Frederick, MD 21701; R. R. Nelson(2), and D. R. MacKenzie(3). (2)(3)Evan Pugh professor, and professor, respectively, Department of Plant Pathology, Pennsylvania State University, University Park 16802, (3)Current address: Department of Plant Pathology and Crop Physiology, Louisiana State University, Baton Rouge 70893. Phytopathology 74:1007-1010. Accepted for publication 8 April 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1007.

Single-colony isolates of Erysiphe graminis f. sp. tritici were collected in Pennsylvania during 1976- 1978 and 1980. Races were distinguished by using nine near-isogenic lines of the winter wheat cultivar Chancellor with known powdery mildew resistance (Pmx) genes. The occurrence of virulence on lines carrying Pm1, Pm3a, and Pm3b was rare relative to virulence on lines carrying Pm2, Pm3c, Pm4, Pm5, and a line carrying a gene from cultivar Michigan Amber (MA). Nonindependent occurrences of the following virulence gene (p) pairs were found: (p2,p5), (p3c,p4), and (p3c, pMA). Genotype p2,p5 appeared to be more prevalent in the 1976- 1978 collection, but the apparently greater frequency of genes p3c and pMA in combination with genotype p2,p5 may have produced an increase in racial complexity in 1980. These hypotheses emphasize areas of future research that may be of interest to plant pathologists who plan and interpret plant pathogen racial surveys.