High levels of genetic and genotypic diversity in field populations of the barley pathogen Ramularia collo-cygni

The ascomycete pathogen Ramularia collocygni causes Ramularia leaf spot (RLS) on barley. Although R. collo-cygni is considerd an emerging disease of barley, little is known about genetic diversity or population genetic structure of this pathogen. We applied a set of polymorphic AFLP (Amplified Fragment Length Polymorphism) markers to investigate population genetic structure in two Northern European populations of R. collo-cygni. The distribution of AFLP alleles revealed low levels of population subdivision and high levels of genetic diversity at both locations. Our analyses included 87 isolates and of these 84 showed a unique genotype pattern. The genetic structure of populations in Scotland and Denmark is highly similar and we find no evidence of population sub-division. An analysis of molecular variance was used to show that 86 % of the variance is attributable to within field genetic variance. In spite of the high levels of genetic and genotypic diversity in the R. collo-cygni populations, we find significant evidence of linkage disequilibrium among the AFLP alleles using a multilocus analysis. We propose that the high levels of genotypic diversity and the lack of population differentiation result from considerable levels of gene flow between populations most likely mediated by seed borne dispersal of inoculum.