Powdery mildew caused by fungi Blumeria graminis f. sp. hordei is one of the most common barley diseases in Polish meteorological conditions. The powdery mildew fungus is made up of different races and forms that are highly specialized. Barley cultivars might be resistant to a certain race of the mildew fungus, but susceptible to another race. Development of the disease is rapid in temperatures from 12 to 20°C, and in humid weather. The aim of the two-year experiment was to assess B. graminis f. sp. hordei virulence frequency and powdery mildew occurrence on five spring barley cultivars. Virulence frequency of the pathogen depended on place and term of exposition. The occurrence of powdery mildew on spring barley cultivars depended on virulence frequency of the pathogen and weather conditions.