Range of the Key Oversummering Regions of Puccinia striiformis f. sp. tritici in Northwestern China Has Expanded Westward.

Wheat yellow rust, caused by Puccinia striiformis f. sp. tritici (PST), is one of the most important wind-borne diseases in all wheat-growing regions, and its occurrence can lead to devastating yield losses in wheat. In China, the wheat fields in Gansu act as an inoculum reservoir during summer periods and provide PST spores for wheat in the fall. The wheat fields in Qinghai provide large amounts of oversummering inocula. The exchange and migration of spores between the two regions are essential to ensure the persistence of PST. To confirm this relationship, we studied the genetic diversity, seasonal population dynamics, role of recombination, and gene flow between PST populations in different oversummering areas of Gansu and Qinghai using molecular markers combined with a spatiotemporal sampling strategy. Shared genotypes provide molecular evidence of migration between the pathogen populations in the two regions. The distribution of genotypic frequencies indicates that the pathogen mainly flows from Qinghai to Gansu in the autumn, whereas the opposite direction of movement occurs in the spring. The inoculum source from spring wheat can be directly transmitted to autumn seedlings, not necessarily through volunteer wheat. Therefore, the bridging effect of spring wheat may play a more important role than the off-season pathogen surviving on volunteer wheat plants. Furthermore, linkage disequilibrium tests indicate that sexual recombination continues throughout the year in the Tianshui and Dingxi regions of Gansu.