The Changing Virulence of Stripe Rust in Canada from 1984 to 2017.

Stripe rust, caused by the fungal pathogen f. sp. , is an important wheat disease worldwide. In this study, the f. sp. population in Canada, representing a time period from 1984 to 2017, was analyzed for virulence diversity and geographical distribution. Virulence of 140 f. sp. isolates was evaluated on 17 near-isogenic wheat lines in the 'Avocet S' background, each containing a single resistance gene along with an 18th line 'Tyee'. Seedlings were inoculated with a urediniospore/talc mixture and infection types were evaluated on a scale of 0 to 9. In total, 89 races were identified with various combinations of defeated genes. Clear changes in pathogen virulence have been observed through time that are confirmed by clustering algorithms. The results showed that the tested f. sp. isolates remained avirulent on , , and , and have very low frequency of virulence on , but had high frequencies of virulence on , , , , , , , , and f. sp. virulence spiked on , , and for the first time in 2000, and on and in 2010. Overall, the predominant races in Canada were very similar to those reported in the United States (PSTv-37, PSTv-41, and PSTv-52), which indicates long-distance migration of f. sp. from the United States to Canada. Sixty-four races had unique virulence combinations that had not been previously reported in the United States, which suggested that evolution of virulence/avirulence for host resistance by mutation at local scale, is possible. Analysis of diversity between Canadian isolates and races from the United States since 2010 showed that the f. sp. population in western Canada is similar to that in the western states of the United States, and that the population in eastern Canada is similar to the eastern and/or central regions of the United States, supporting the hypothesis that specific f. sp. populations in North America travel through different wind trajectories.