RACE COMPOSITION AND VIRULENCE OF WHEAT POWDERY MILDEW (ERYSIPHE-GRAMINIS TRITICI) AND THE RESISTANCE OF WHEAT-VARIETIES IN HUNGARY

The race composition and virulence of wheat powdery mildew as well as the effect of known resistance genes on the pathogen have been studied in Martonvasar since 1970. The powdery mildew population is very heterogeneous; a total of 72 physiological races were isolated, with between 15 (1993) and 35 (1986) races being identified each year. Within the pathogen population, the number of virulence genes per race rose steadily from 2.03 (1973) to 5.63 (1993). Significant changes have occurred in the race composition and virulence of the pathogen. Correlation coefficient (r) calculations and the results of principal component analysis (PCA) have confirmed that the wheat powdery mildew population during the period in question can be divided into at least four distinct groups on the basis of race composition and degree of virulence. The vast majority of powdery mildew isolates is virulent to genotypes with known resistance genes. Complete resistance is only provided by gene Pm 4a. The cultivation of varieties with vertical resistance due to the resistance genes Pm 8 and, probably, Pm 2+6 has led to a change in the race composition and virulence of the pathogen. With the spread of races virulent to resistant genotypes, previously resistant varieties have become susceptible, so the search for new resistance sources providing efficient biological protection is a constant necessity, requiring the breeding and cultivation of new, resistant varieties. Resistance breeding is thus a permanent process. Among the wheat varieties developed in Martonvasar, Mv 22, Mv 23, Mv 24, Mv 25, Mv Magma and Fatima 2 have good powdery mildew resistance.