Virulence and Molecular Analyses Support Asexual Reproduction of Puccinia striiformis f. sp tritici in the US Pacific Northwest

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, occurs every year and causes significant yield losses in the U.S. Pacific Northwest (PNW). A large number of P. striiformis f. tritici races are identified every year and predominant races have changed rapidly. Barberry and mahonia plants, which have been identified under controlled conditions as alternate hosts for the fungus, are found in the region. However, whether sexual reproduction occurs in the P. striiformis f. sp. tritici population under natural conditions is not clear. To determine the reproduction mode of the P. striiformis f. sp. tritici population using virulence and molecular markers, a systematic collection of leaf samples with a single stripe of uredia was made in 26 fields in the PNW in 2010. In total, 270 isolates obtained from the PNW collection, together with 66 isolates from 20 other states collected in the same year, were characterized by virulence tests and simple sequence repeat (SSR) markers. In total, 21 races and 66 multilocus genotypes (MLGs) were detected, of which 15 races and 32 MLGs were found in the PNW. Cluster analysis with the SSR marker data revealed two genetic groups, which were significantly correlated to the two virulence groups. The analyses of genotype/individual ratio, multilocus linkage disequilibrium, and heterozygosity strongly supported asexual reproduction for the pathogen population in the PNW, as well as other regions of the United States.