Virulence Characterization of the <i>Puccinia striiformis</i> f. sp. <i>hordei</i> Populations of 2010-2020 in the United States

Puccinia striiformis f. sp. hordei (Psh), causing stripe rust epidemics on barley, has been continually changing in virulence since its introduction in the United States in the early 1990s. To monitor virulent changes, 344 Psh isolates recovered from stripe rust samples collected from barley, grasses, and wheat in 2010-2020 were tested on the set of 12 barley differentials. Among the 12 virulence factors, the frequency of virulence to Topper was the highest (99.4%), followed by those to Abed Binder 12 (77.9%), Trumpf (35.2%), and Bancroft (27.6%). The virulence frequencies of all remaining differentials were below 20.0% with 16.6% on Hiproly; 15.7% on Bigo; 14.5% on Varanda; 10.8% on Emir; 9.9% on Astrix and Heils Franken, 8.1% on Muzurka, and 2.6% on I 5. Except the consistently high frequencies of the virulence to Topper from different years, epidemiological regions, and hosts, virulence frequencies of other differentials varied from year to year, region to region, and host to host. A total of 41 Psh races were identified from the isolates, with the numbers of virulence factors ranging from 0 to 12. Among these races, 12 were first identified in the United States in the present study while the other 29 races were first detected before 2010. Race PSH-33 (virulent on differentials Topper and Abed Binder 12) had the highest frequency (36.0%), followed by PSH-48 (virulent only on differential Topper) with a frequency of 20.6%. Cluster analysis and discriminant analysis of principal components revealed differentiations among years with major changes occurring between 2016 and 2017, and to a less extent to regions and hosts. The results should be useful for developing resistant barley cultivars for control of stripe rust.