Periodic surveys of virulence frequencies of the causal agent of wheat (Triticum aestivum) powdery mildew (Blumeria graminis f. sp. tritici) help to select resistance sources to use in plant breeding programs. This work reports the results of five years of analysis on effective resistance genes to pathogenic populations of Blumeria graminis f. sp. tritici from Brazil and Chile. Powdery mildew samples were received at Embrapa Trigo, Passo Fundo, RS, in 1995, 1996, 1997, 1999, and 2000. Each monopustular isolate was inoculated in plants of a differential series. Ninety different combinations of effective and ineffective resistance genes were identified. In 1995 and 1997, most isolates were virulent to Pm3a, Pm3c, Pm8, Pm1+..., and PmD1 genes; in 1999, to Pm3a, Pm3c, Pm4b, Pm8, PmD1,and PmD2; and, in 2000, to Pm3a, Pm3c, Pm4b, Pm6, Pm8, PmD1,and PmD2. During the period, Pm2 and Pm4a+... genes remained effective to all isolates. The number of ineffective genes to each isolate varied from one to nine in a total of 13 genes or combinations of genes tested. Isolates with virulence to five genes alone or in combinations were more frequent in 1995 and 1997, to six in 1999, and to seven in 2000, probably indicating an increase in racial pathogen composition each year.
Triticum aestivum; especialização fisiológica; Erisyphe graminis f. sp. tritici
