Virulence races of Puccinia striiformis f. sp tritici in 2006 and 2007 and development of wheat stripe rust and distributions, dynamics, and evolutionary relationships of races from 2000 to 2007 in the United States

Stripe rust, caused by Puccinia striiformis f. sp. tritici (PST), has historically been the most frequently destructive disease of wheat (Triticum aestivum) in the western United States and has become a more frequent problem in the central and southeastern states since 2000. The race composition of PST has been determined every year from rust-infected leaf samples of wheat and grasses collected in the United States on a set of 20 differential wheat genotypes. In 2006, a total of 18 races were detected, of which five were detected for the first time. In 2007, a total of 30 races were detected, of which 11 were newly detected. Among the 16 new races detected in 2006 and 2007, PST-127 was the most important as it has the broadest virulence spectrum identified so far (virulent to all 20 differential genotypes except for 'Moro', AVS/6*Yr5 (Yr5), and 'Tres') and combined virulence factors to 'Tyee' (YrTye) and 'Hyak' (Yr17 and YrTye) and those common in the race group detected since 2000. The distribution, frequency changes, and evolutionary relationships for races detected from 2000 to 2007 were analyzed. Three major waves of race changes were identified during the eight-year period. From 2000 to 2002, the predominant races were PST-78 and PST-80, which were virulent on wheat genotypes 'Lemhi', 'Heines VII', 'Lee', 'Fielder', 'Express', AVS/6*Yr8, AVS/6*Yr9, 'Clement' and 'Compair'. Race PST-80 is also virulent on 'Produra'. From 2003 to 2006, the predominant race was PST-100, with the same virulence formula as PST-80 plus virulence on 'Yamhill' and 'Stephens'. Starting in 2006, races with the same virulence formula of PST-100 plus virulence to Yr1 became predominant in California and races with the virulence of PST-100 plus virulence on Yr10 became predominant in the Pacific Northwest. During 2000 to 2007, races with more virulence factors became more predominant in the United States, indicating that races with increased virulence factors are at an advantage in the pathogen population over those with fewer virulence factors because they are able to infect more wheat cultivars.