Virulence and molecular characterization of Blumeria graminis isolates from wheat fields across Ontario

In Ontario, Canada, the prevalence and severity of powdery mildew (PM) disease caused by Blumeria graminis on wheat has been escalating. Continual monitoring of the effectiveness of PM-resistance genes (Pm) and the pathogen virulence spectrum in the field is essential for breeding durable resistance to control the disease. In 2021 and 2022, a total of 74 single-colony isolates of B. graminis were purified from field samples from 40 randomly selected wheat fields and greenhouses in Ontario, then tested for virulence on 24 single-Pm gene wheat lines. Compared to a previous survey in 2018–2019, the virulence frequencies to most Pm genes have increased over the years, including Pm1a, Pm1b, Pm1c, Pm2, Pm3a, Pm3f, Pm5a, Pm5d, Pm17, Pm30, Pm34, Pm35, Pm43, MlAG12 and NCAG13. Nevertheless, Pm12, Pm16, Pm21 and Pm37 each remained effective against all isolates. Nine genes (Pm1a, Pm1b, Pm1c, Pm3d, Pm4b, Pm25, Pm29, MlAG12 and NCAG13) were generally effective, with resistance to more than 80% of the isolates. The remaining 11 genes were ineffective, having susceptible reactions to 37–97% of the isolates. DNA sequences of 116 isolates (2018–2022) for the fragments of alternative oxidase (AOX), protein kinase A (PKA), protein phosphatase type 2A (PPA) and β-tubulin (bTUB) genes resulted in only four informative segregating single nucleotide polymorphisms (SNPs). The linkage disequilibrium analyses of these four SNPs suggested frequent genetic recombination. Additionally, network analyses of virulence phenotype data showed no genetic subdivision, suggesting B. graminis in the sampled area is likely a panmictic population.