Molecular diversity in the barley pathogen Bipolaris sorokiniana (Cochliobolus sativus)

Spot blotch, caused by Bipolaris sorokiniana, is a prevalent disease of barley in Canada and elsewhere in the world. To evaluate the genetic variation among pathogen isolates, 93 isolates of B. sorokiniana representative of eight different virulence groups (VIGs) identified in a previous study, were subjected to an AFLP assay. AFLP analysis indicated that pathogen isolates collected from different regions of Canada and from other countries had a high level of genetic variability. Isolates possessing low virulence (VIG 0.0.0.0) and differential virulence (VIG 6.0.0.0) on barley genotypes were clearly discernible from other pathogenic isolates. However, molecular analysis did not provide a robust differentiation among the other six VIGs identified by the classical method of pathotype designation. There was a closer correlation between the AFLP patterns and virulence than between AFLP pattern and geographic origin of isolates. To evaluate causes of genetic variation among isolates, the influence of mutation, migration and gene flow, and recombination on the B. sorokiniana population were discussed. The genetic profiles of the three isolates of VIG 6.0.0.0, which were located between low virulence and high virulence isolates, invited speculation that such a virulence group may be the result of genetic recombination between isolates possessing extreme virulences in the population of the pathogen.