The European barley powdery mildew virulence survey and disease nursery 1993-1999

National barley powdery mildew virulence surveys were co-ordinated through COST817 on a European scale from 1993 to 1999 to allow comparison of results actress national borders. The frequencies of virulence matching resistance genes Mla1, Mla3, Mla6, Mla7, Mla9, Mla12, Mla13, Mlk, M1La and Mig were moderate to high in most years and countries. Several additional sources of resistance were matched by virulence frequencies below 5%. Generally, no increase in aggressiveness against Mlo-resistance was detected, but change may be under way as particular isolates of British origin gave higher infection levels on Mlo-resistant varieties than did other groups of isolates. Therefore, it is important in the future to focus on virulence matching the new sources of resistance and Mlo. Multi-location field trials were established in 1998 and 1999 in order to study powdery mildew resistance in barley genotypes in different environments. The trials showed large interactions of location and phenotypic expression of the resistance. A continued exchange of ideas, methodology and plant material between national survey programmes, and a rapid dissemination of results to farmers and plant breeders across Europe is vital.