Hierarchical structure of pathogenic variation among Rhynchosporium secalis populations in Idaho and Oregon

Abstract

Ninety-four isolates of Rhynchosporium secalis, the causal organism of the barley scald disease, from four populations (infected fields) in Idaho and eight in Oregon, were tested for pathogenicity to 14 barley differential cultivars; 60 distinct pathotypes, 33 in Idaho and 28 in Oregon, were found among the 94 isolates. Pathogenicity to most of the differentials was found in each local population sampled. One differential cultivar, CI 5831, was resistant to all isolates tested. Isolates from the same cultivar in different locations usually had different pathogenicities, and most pathotypes were genetically heterogeneous mixtures of many different genotypes based on previous electrophoretic data. The sampled cultivars generally had few or no resistance genes, suggesting that the R. secalis populations in Idaho and Oregon are characterized by large amounts of seemingly unnecessary pathogenicity. When the total pathogenic diversity was partitioned into hierarchical components using the Shannon information statistic, 58% was accounted for by the within-population component, 33% among populations within regions, and only 9% by the between-region component. Because most of the pathogenic variation is within local populations, these populations should adapt rapidly to introduced resistance genes; therefore traditional methods of breeding for resistance to R. secalis using single major resistance genes are not likely to be effective in the Pacific Northwest. Key words: pathogenicity index.