Virulence Genes and Virulence Gene Frequencies ofBlumeria graminisf. sp.triticiin Ohio

Field isolates of Blumeria graminis f. sp. tritici were collected from 36 counties in Ohio in 1992 and 1993. Single colony progeny of the 199 isolates were tested against 11 powdery mildew resistance genes (Pm genes) using 8-day-old seedlings of differential host cultivars and lines. Virulence against nine of the resistance genes was found in 1992 while virulence against all 11 resistance genes was found in 1993. All isolates tested were virulent on Pm7 and Pm8 both years and a high percentage (>40%) of the isolates were virulent on Pm2, Pm3a, Pm3b, Pm3c, Pm4a, Pm5, and Pm6 in each year. However, a low percentage (< 5%) of the isolates were found to be virulent on Pm1, Pm3b, and Pm17 both years. Results also suggested that virulence matching resistance gene pairs Pm2+Pm6 and Pm3c+Pm5 always occurred together in the pathogen population. Approximately 92% of the isolates tested gave a compatible reaction with four to eight Pm resistance genes. Therefore, complex races occurred frequently in the pathogen population