Race and virulence of asexual and sexual populations of Puccinia graminis f. sp. tritici in China from 2009 to 2015

Stem rust of wheat caused by Puccinia graminis f. sp. tritici (Pgt) has been under control in China for decades but new races including the Ug99 race group are an ongoing threat. Pgt race surveillance is an essential component for disease prevention strategies. Three hundred fourteen isolates (284 asexual and 30 sexual) obtained from 2009 to 2015 in China were tested on a panel of differential lines and categorized into five race groups (21C3, 34C0, 34C1, 34C3, and 34C6) and 29 races. Race group 21C3 was most frequent (64.0%) and race 21C3CTHTM was the most prevalent (16.6%). Race group 34C6 and 15 races were detected for the first time, accounting for 2.5 and 27.4% of the isolates, respectively. The most widely used stem rust resistance gene (Sr31) is effective against all isolates, however other commonly used resistance genes (Sr5, and Sr17) are only effective to some of the isolates. Novel Pgt races with virulence to both Sr5 and Sr11 were detected. Isolates from the sexual population were strikingly different from those of the asexual population, having higher frequency of new race group, new races and new virulence combination (Sr5 + Sr11). In addition, the higher variation in sexual Pgt population observed in the region growing wild barberries suggests that these sites might be responsible for the occurrence of new races in major wheat growing areas in China.