Race typing of the wheat stripe rust pathogen Puccinia striiformis f. sp. tritici in western Canada from 2017 to 2022

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a persistent threat to wheat production in western Canada. Sixty-three Pst isolates, primarily collected between 2017 and 2022 from commercial fields and disease nurseries across the Canadian provinces of British Columbia, Alberta, Saskatchewan and Manitoba, were analyzed for virulence phenotypes. Twenty wheat differentials were used and 63 isolates were characterized into 17 races. Of these, 12 races were unique to Canada, including six novel races not previously reported in either Canada or the United States. Race C-39 was the most prevalent (31.7% of isolates), followed by C-30 (20.6% of the isolates), with both races present across multiple provinces. None of the isolates were virulent on Yr5 , Yr15 , or YrSP and low virulence frequencies (4.2%) were detected on Yr1 , Yr10 , and Yr76 . Resistance conferred by Yr6, Yr7, Yr8, Yr17, Yr44 , and YrTr1 was overcome. Among the isolates tested, 66% of the isolates were virulence on Yr32 , challenging its previously reported effectiveness. None of the Yr genes used in differential wheat genotypes are carried by any modern spring wheat cultivars in western Canada. Overall, the results highlight regional race diversity, a high proportion of races unique to Canada, and stable virulence profiles over time, indicating the need for continued monitoring of this cold-tolerant and regionally evolving pathogen. However, current Pst race typing studies in Canada are not very useful to wheat breeding programs because none of the Yr genes used in differentials is carried by any commercial variety.