Molecular evidence of the west-to-east dispersal of Puccinia striiformis f. sp. tritici in central Shaanxi and the migration of the inoculum from Gansu

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is an airborne disease. In China, it frequently develops initially in central Shaanxi and southwestern Gansu, and from there, inoculum spreads to the eastern wheat production regions. Field investigations have suggested that Pst could spread from the west to the east within central Shaanxi and that Gansu could serve as the inoculum source for central Shaanxi, but there is no direct evidence for this hypothetical dispersal route. In the current study, 321 Pst isolates collected from central Shaanxi and Gansu in the 2019–2020 and 2020–2021 winter wheat cropping seasons were genotyped using 23 pairs of KASP-SNP markers. The dispersion among subpopulations was analyzed using several approaches, and overall, the populations were found to exhibit high levels of genetic diversity. There was little genetic divergence (0.05>F_ST>0) within central Shaanxi. However, significant gene flow (Nm>4) driven by wind-oriented dispersal from west (Baoji) to east (Weinan) occurred. There was also gene flow among the 4 Gansu subpopulations of Tianshui, Longnan, Pingliang, and Qingyang. Migration of the pathogen occurred between central Shaanxi and Gansu. Migration from Gansu to central Shaanxi was major compared with that from central Shaanxi to Gansu that was minor. Genetic variation occurred among isolates, instead of among subpopulations and within isolates. Linkage disequilibrium revealed that there was strong genetic recombination in the subpopulations from Gansu and central Shaanxi. Therefore, the present study provides molecular evidence that Pst spread from west to east in central Shaanxi and showed that Gansu (especially Longnan and Tianshui) was one of the major origins of the pathogen inoculum of wheat stripe rust in central Shaanxi. The results revealed the west-to-east transmission route of wheat stripe rust in central Shaanxi, being used to guide integrated management of the disease.