Race Characterization of the Stripe Rust Pathogen in Canada from 2020 to 2024 and Long-Term CYP51 Mutation Analysis

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici (Pst) remains a major threat to global wheat production. In Canada, long-term collection of Pst isolates since 1984 provides a unique perspective on pathogen evolution over four decades. This study extends previous race structure analyses by characterizing 73 isolates collected since 2020 from wheat, barley, and wild barley. Among 56 wheat-infecting isolates, 23 races were identified, including 16 new races (C90 to C105). The predominant races C17 and C43 and the newly emerged C96 accounted for 50% of isolates, with C96 likely evolving stepwise from C43 through acquisition of virulence to cultivar Tyee (YrTye); this cultivar was resistant in previous years. Alberta remains a hotspot of Pst diversity (14 races), whereas Saskatchewan showed lower diversity (5 races). Most isolates were virulent to Yr6, Yr7, Yr8, Yr9, Yr17, Yr27, Yr43, Yr44, YrTr1, and YrExp2, whereas Yr5 and Yr15 remained undefeated; Yr1, Yr10, Yr24, Yr32, and YrSP conferred resistance to most of the isolates. Cultivated barley-derived isolates infected both wheat and barley and displayed distinct virulence profiles, suggesting interspecific adaptation and possible hybridization with P. striiformis f. sp. hordei (Psh). Additionally, analysis of the Cyp51 gene revealed the presence of a nonsynonymous mutation Y134F in one Psh isolate but its absence across tested Pst populations over the past 40 years in Canada, signifying potential evolution of fungicide tolerance or fitness adaptation in barley-infecting form.