An Evaluation of the Independence of Certain Virulence Genes ofErysiphe graminisf. sp.tritici

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Abstract
Single-colony isolates of E. graminis f. sp. tritici were collected in Pennsylvania [USA] during 1976-1978 and 1980. Races were distinguished by using 9 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 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.