EVOLUTION OF WHEAT POWDERY MILDEW POPULATIONS DURING 1971-1997 IN HUNGARY LASZLO SZUNICS AND LUDMILLA SZUNICS Agricultural Research Institute of the Hungarian Academy of Sciences, Martonvasar, Hungary Background and objektives The wheat powdery mildew pathogen (Erysiphe graminis f. sp. tritici) has been known in Hungary for a long time, though the first nation-wide epidemic was not recorded until 1961. Since then the pathogen has appeared and caused damage every year. The success of resistance breeding (biological plant protection) is promoted by a knowledge of the pathogen race composition, virulence and by the identification of effective resistance genes and their use in breeding. A knowledge of the resistance possessed by registered varieties is a useful guideline for growers. Materials and methods Studies have been underway in Marionvasar since 1971 to trace the physiological specialisation [1] and virulence of wheat powdery mildew [2]', the role of the host genotype in changes in the race composition and degree of virulence and the resistance of wheat varieties. Experiments carried out in the greenhouse and in the field. Correlation coefficient (r) calculations, principal component analysis (PCA) and cluster analysis (CA) were used to process the data. Results and conclusions 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. During the period under examination, significant microevolutionary processes took place in the pathogen population, leading to a substantial change in the powdery mildew race composition, including changes in the prevalent races. While in 1971, for instance, races 3, 9, 0, 46 and 35 were dominant, in 1976 the prevalent races were 52,26,4,2 and 9, in 198535,46, 75,85 and 44, in 199051,75,46,72 and 85 and in 1997 90, 51, 72, 70 and 46. Within the pathogen population, the number of virulence genes per race rose steadily from 2.03 (1973) to 5.63 (1993). In 1997 as in previous years, the majority of powdery mildew isolates were virulent to genotypes with known resistance genes. Complete resistance was only provided by gene Pm 4a (Khapli). Some 30% of the races were virulent to genes Pm 4b+ P 31512 and Pm 1+2+9 (Normandie). In recent years there has been an increase in the proportion of races (51, 67, 72, 75, 90) virulent to genes Pm 4a+ crP 309/A) and Pm 4b (Weihenstephan Ml), so the latter now have reduced breeding value. Using the statistical methods listed above, the powdery mildew population of the years 1971 to 1997 could be divided into 4 distinct groups on the basis of race composition and virulence. It would appear that smaller changes in the pathogen population occur within an interval of 2-5 years and greater changes after 5-7 years, especially if varieties with a different type of resistance are put into cultivation. In the Martonva's~r institute, breeders have succeeded in developing a number of varieties resistant to powdery mildew. These varieties (Mv 22, Mv 23, Mv 24, Mv 25, Mv Magma, Fatima 2, Mv Magdale'na and Mv Mezofo Id) could thus be effective tools in biological plant protection. References I Nover I, 1957. Phytopathologische Zeitschrift 31, 85-107. 2.Wolfe MS, Schwarzbach E, 1978. Annual Review Phytopathology, 16, 159-180.