The wheat stem rust (Puccinia graminis f. sp. tritici) population from Washington contains the most virulent isolates reported on barley.

A diverse sexual population of wheat stem rust, Puccinia graminis f. sp. tritici (Pgt), exist in the Pacific Northwest (PNW) region of the United States due to the natural presence of Mahonia spp. that serve as alternate hosts to complete its sexual life cycle. The region appears to be a center of stem rust diversity in North America where novel virulence gene combinations can emerge that could overcome deployed barley and wheat stem rust resistances. A total of 100 single pustule isolates derived from stem rust samples collected from barley in Eastern Washington during the 2019 growing season were assayed for virulence on the two known effective barley stem rust resistance genes/loci, Rpg1 and the rpg4/5-mediated resistance locus (RMRL) at the seedling stage. Interestingly, 99% of the Pgt isolates assayed were virulent on barley variety Morex carrying the Rpg1 gene, and 62% of the isolates were virulent on the variety Golden Promise transformant (H228.2c) that carries a single copy insertion of the Rpg1 gene from Morex and is more resistant than Morex to many Rpg1 avirulent isolates. Also, 16% of the isolates were virulent on the near isogenic line HQ-1, that carries the RMRL introgression from the barley line Q21861 in the susceptible Harrington background. Alarmingly, 10% of the isolates were virulent on barley line Q21861 that contains both Rpg1 and RMRL. Thus, we report on the first Pgt isolates worldwide with virulence on both Rpg1 and RMRL when stacked together representing the most virulent Pgt isolates reported on barley.