Race structure of Pyrenophora triciti-repentis (tan spot of wheat) in Alberta, Canada

Tan spot is an important foliar disease of wheat caused by Pyrenophora tritici-repentis. Eight races of this fungus have been identified based on their virulence on a wheat differential set. Previous surveys revealed that most isolates collected from the Canadian Prairies belong to races 1 and 2 of P. tritici-repentis and carry the ToxA gene. However, pathogen populations from Alberta, a major wheat-producing province, have not been characterized. In this study, the virulence of 45 isolates of P. tritici-repentis from Alberta was evaluated on six differential wheat genotypes. Race 1 was found to be most common, representing 62% of the isolates tested, while race 2 was the second most common (36% of isolates). A single isolate of race 3 was also found (2% of total), but no other races were identified. Polymerase chain reaction and Southern blot analyses with ToxA and ToxB-specific primers and probes were conducted to confirm the race designations of the tested isolates. The ToxA gene was identified in each isolate classified as race 1 or race 2, but not in the race 3 isolate. Sequencing of ToxA from 16 isolates collected in different parts of the province revealed that the sequences were identical. The ToxB gene was not found in isolates classified as races 1 or 2, but a ToxB homologue was identified in the race 3 isolate. These results confirm that the P. tritici-repentis populations in Alberta follow the toxin or inverse gene-for-gene model, and serve to validate the race-based classification system for this pathogen. © 2013 The Canadian Phytopathological Society.