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, exists in the Pacific Northwest region of the United States because of the natural presence of Mahonia spp. that serves 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 ipg4/5-mediated resistance locus (RMRL) at the seedling stage. Interestingly, 99% of the P. graminis f. sp. tritici 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, which 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, which contains both Rpg1 and RMRL. Thus, we report on the first P. graminis f. sp. tritici isolates worldwide with virulence on both Rpg1 and RMRL when stacked together, representing the most virulent P. graminis f. sp. tritici isolates reported on barley.

Automated summary

A diverse population of wheat stem rust exists in the Pacific Northwest region of the United States, and novel virulence gene combinations that could overcome existing resistances have been found. This study reports the first worldwide isolates of Puccinia graminis f. sp. tritici that are virulent on both Rpg1 and RMRL, representing the most virulent strains of this pathogen reported on barley.