Isolates of the barley mildew pathogen from the air spora over a large part of Europe and from fields of variety mixtures, were tested for virulence against 12 host resistance alleles . Subsamples were tested for their response to triadimenol fungicide and analyzed for 10 DNA loci using RAPD markers and PCR . There was a large range of haplotypes spread over Europe ; irregularity in the distribution was probably due mainly to non-uniform use of the corresponding host resistances and fungicides . A large range of variation was also detectable within individual fields . Positive gametic disequilibria distorted the distribution of virulence alleles among haplotypes and reduced the number of haplotypes detectable in the sample . Analysis of the spread of the newly selected Val3 allele into different European sub-populations indicated that gene flow throughout the population may be rapid for alleles that have a selective advantage .Fungicide resistance was widespread in areas known for intensive use of fungicides for mildew control . Four classes of fungicide response were detectable and particular virulence haplotypes were found to be characteristic for each class .Variety mixtures used in the former German Democratic Republic (GDR) reduced mildew infection, and thus fungicide use, during the years 1984-1991 despite the limited variation in host resistance among the mixtures . A tendency for complex pathogen races to increase in mixture crops was reversed by the large-scale re-introduction of fungicides for mildew control in 1991 . The mixture strategy appeared to be more successful than using the same resistance alleles in pure monoculture or combining them in a single host genotype .

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