Powdery mildew caused by Blumeria graminis f. sp. hordei is one of the most important diseases of barley in Poland. B. graminis is a genetically diverse pathogen with different special forms and races. The aim of the two-years’ experiment was to assess of B. gramins f. sp. hordei virulence frequency and powdery mildew occurrence on four winter barley cultivars. Virulence frequency of the pathogen depended on place and term of exposition. The occurrence of powdery mildew on four winter barley cultivars depended on virulence frequency of the pathogen and weather conditions.

Spring triticale 'Migo', and spring barley 'Maresi' grown in pure stand and in their mixtures were used in field experiment. Disease occurrence, number of culms and heads as well as yield were assessed. Powdery mildew and scald of barley and Septaria leaf blotch of triticale occurred in lower degree in mixtures than in pure stands, but only in the years with weather conditions favourable for the diseases development. Fusarium stem rot and eyespot of both plant species as well as net blotch of barley occurred in similar degree in all treatments or even in some mixtures in higher degree than in pure stands.