Diversity and complexity of Erysiphe graminis f.sp. hordei collected from barley cultivar mixtures or barley plots treated with a resistance elicitor

Powdery mildew populations were analysed to determine the effects of a resi stance elicitor and cultivar mixtures on genetic complexity and diversity. Isolations were made from a range of spring barley monocultures and mixture s in a field trial, and characterised for virulence and RAPD profile. In a second trial, isolates were taken from a single mixture from untreated and resistance elicitor-treated areas and from the components of the mixture in monoculture. The mildew population was not only highly heterogeneous for v irulence characteristics, but also proved heterogeneous within pathotypes f or molecular markers, indicating the major impact of sexual recombination o n population structure and the lack of clonal dominance. Various diversity measurements were compared and the value of dissimilarity measurement for r evealing genetic distance within a population was highlighted. There was a trend towards increasing complexity as the season progressed, but there was no consistent relationship between cultivar or mixture, disease control tr eatment, fertiliser treatment, replicate or position in trial, and pathogen genotype. Whilst the resistance elicitor did reduce mildew by 78% in the f irst trial, and there was no interaction with fertiliser level in its expre ssion, control was substantially less in the second trial. There were no di fferences between mildew isolates from elicitor and control treatments. It was felt that more effective and consistent resistance elicitors need to be developed before it can be stated that they are unlikely to be eroded by s electing resistant or adapted mildew genotypes.