Virulence, Frequency, and Distribution of Races of Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei Identified in the United States in 2008 and 2009

Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei, the causal agents of stripe rust on wheat and barley, respectively, can change rapidly in virulence, and such changes may overcome resistance in cultivars and result in severe epidemics. To monitor virulence changes in the pathogen populations, isolates obtained from stripe rust samples collected by the authors and collaborators from 17 U.S. states in 2008 and 13 states in 2009 were tested on 20 wheat and 12 barley differential lines to identify races of P. striiformis f. tritici and P. striiformis f. sp. hordei, respectively. In 2008, 33 P. striiformis f. tritici (PST) races were detected, including a new race, PST-138, which was similar to previously identified PST-127 (virulent on wheat differentials 1, 2, 3, 4, 6, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, and 20) but not virulent on differential 8. The five most frequent races were PST-114 (virulent on differentials 1, 3, 4, 8, 9, 10, 11, 12, 14, 16, 17, 18, 19, and 20), PST-100 (virulent on differentials 1, 3, 8, 9, 10, 11, 12, 16, 17, 18, 19, and 20), PST-116 (similar to PST-114 plus virulent on differential 5), PST-101 (similar to PST-100 plus virulent on differential 2), and PST-98 (similar to PST-100 but not virulent on differential 9). In 2009, 26 P. striiformis f. tritici races were identified, including two new races, PST-139 and PST-140. PST-139 was similar to PST-127 but not virulent on differentials 16 and 20. PST-140 was similar to PST-114 but not virulent on differential 9. The five most frequent races were PST-139 (19%), PST-140 (14%), PST-114 (11%), PST-116 (10%), and PST-127 (9%). However, the most widely distributed races were PST-98 (in 10 of the 14 states) and PST-102 (in 7 of the 14 states). Differential genotype AvSYr5NIL (Yr5) was the only one among the 20 differentials that remained resistant to all of the identified races. Virulence diversity of the P. striiformis f. tritici populations was higher west of the Rocky Mountains. For barley stripe rust, P. striiformis f. sp. hordei (PSH)-33 (virulent on barley differentials 1 and 7) was the most common (46%) of the 11 races detected in 2008, including a new race, PSH-82 (virulent only on barley differentials 1 and 11). In 2009, six previously identified races were detected, of which five (PSH-16, PSH-38, PSH-46, PSH-54, and PSH-71) were detected in Washington and two (PSH-54 and PSH-70) in Oregon. The information on P. striiformis f. sp. tritici and P. striiformis