Virulence dynamics of the western Canadian Cochliobolus sativus population infecting barley

Spot blotch, caused by Cochliobolus sativus, is a major disease of barley in western Canada. Understanding the virulence diversity of C. sativus is essential for deployment of host resistance. In this study, 49 C. sativus isolates were evaluated on 12 barley differential host genotypes and classified into nine virulence groups (VGs) based on host responses. The three most prevalent VGs (0.0.0.0, 5.7.0.0 and 5.7.0.4) accounted for 67% of the isolates, and five novel VGs were identified. Classification using the North Dakota differential system revealed a high prevalence of pathotype 1 (50%) and the emergence of pathotype 3 (28%), consistent with increased cultivation of two-row, ‘Bowman’-derived barley cultivars that are more susceptible to this pathotype. No correlation was observed between isolate virulence and geographic origin. Hierarchical clustering supported the VG classifications, revealing four major clusters aligned with infection response (IR) patterns and host genotype specificity. Virulence groups causing severe spot blotch exhibited high regression coefficients, reflecting a greater increase in IR on susceptible hosts, whereas VG 0.0.0.0 consistently showed low coefficients and caused only mild symptoms across all hosts. Regression analyses indicated that several six-row (e.g. ND B112, ‘Robust’, ‘Stander’) and some two-row genotypes (TR 251) maintained relatively durable resistance, whereas other two-row genotypes, including ‘Bowman’ and its derivatives, exhibited greater variability in susceptibility. These findings suggest that host selection pressure may be contributing to shifts in the distribution of virulence phenotypes within the C. sativus population, underscoring the importance of ongoing pathogen monitoring and strategic resistance gene deployment.