Barley pathogens in Iceland: Identification, virulence and genetic structure of major barley pathogens in Iceland
Leaf samples were collected from 27 barley fields in Iceland in August 2007 in order to analyze the diversity of fungal species growing on barley. Twelve fungal species were found on 277 successfully analyzed leaves, including two widespread barley pathogens, Rhynchosporium secalis and Pyrenophora teres f. teres, the causal agents of scald and net blotch, respectively. The genetic structure of the Icelandic R. secalis population was studied using thirteen microsatellite markers and results compared to six European R. secalis populations. The genetic structure of P. teres was examined with four AFLP markers and compared to populations from Finland and Russia. Both studies revealed a significant differentiation between the Icelandic and European populations. The mean Fst value between the Icelandic R. secalis population and five Scandinavian populations was 0.165 and pairwise differentiation between the P. teres populations from Iceland and Finland was 0.640. Of the P. teres populations analyzed the gene diversity was highest in Iceland, with a value of 0.26, compared to 0.17 and 0.04 in the Russian and Finnish populations, respectively. The gene diversity in the seven R. secalis populations ranged from 0.43-0.73 and was 0.55 in Iceland. The distribution of genetic diversity was notably different for the R. secalis and P. teres populations as 84% of the total diversity was distributed within populations and only 16% between populations for R. secalis where as more than half of the total genetic diversity, or 51%, was distributed between the P. teres populations and only 49% distributed within the three populations. The existence of mating types 1 and 2 at the same locations was confirmed in both R. secalis and P .teres populations from Iceland, providing evidence of sexual reproduction. The virulence of Icelandic R. secalis and P. teres populations was assessed on a number of standard barley genotypes. Both populations had high pathogenic variation. Fourteen R. secalis isolates were grouped into eleven pathotypes and twenty P. teres isolates were grouped into nineteen pathotypes. None of the barley cultivars currently used in Iceland were resistant to a mixture of Icelandic P. teres isolates. These findings suggest that the Icelandic R. secalis and P. teres populations possess high variability both in neutral molecular markers and virulence, supporting the classification of both R. secalis and P. teres as high risk pathogens with considerable evolutionary potential. It is therefore recommended to use quantitative resistance along with major gene resistance in cultivar mixtures and multilines in breeding projects for durable resistance to R. secalis and P. teres in Iceland.
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