Powdery mildew of wheat caused by Blumeria graminis f. sp. tritici is one of the important diseases prevalent in Mazandaran and Golestan provinces in Iran. To control powdery mildew through use of resistance cultivars, it is necessary to determine the virulence level of the isolates. Isolates of B. graminis f. sp. tritici were collected from different fields in the two provinces in 2004. Virulence / avirulence genes of the isolates were verified through differential cultivars carrying resistance genes to wheat powdery mildew. Primary seedling leaves (10-12 day) of differential wheat cultivars were inoculated by single pustule isolates of B. graminis f. sp. tritici. Reaction of inoculated seedlings evaluated by 0-9 scale and virulence frequency of isolates were determined on resistance genes. Eight different pathotypes of wheat powdery mildew were identified through useof a system of designation of pathotypes of the plant pathogen. Pathotype 7.7.7.2.5 carrying virulence on 12 different Pm genes were found to be the most virulent pathotypes, while pathotypes 1.1.6.0.0 with virulence on only four Pm genes were the weakest ones. No virulence was observed on Pm2+4b+8 resistance genes while all isolates were virulent on Pm1 gene. High frequencies of virulence to genes Pm1, Pm3a, Pm3b, Pm4a, Pm8, Pm3c, Pm5 and Pm6 were found in both locations (Provinces). Frequency levels of virulence for Pm2, Pm3d and Pm1+2+9 were intermediate, while frequencies for Pm7, Pm17, Pm2+6 and Pm2+4b+8 were low. It seems that B. graminis f. sp. tritici population in Mazandaran and Golestan provinces is composed of complex and highly varied pathotypes, and this complexity and variation should be taken into account in strategies intended for deployment of resistant genes.
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