Structure and Migration in U.S. Blumeria graminis f. sp. tritici Populations

While wheat powdery mildew occurs throughout the south-central and eastern United States, epidemics are especially damaging in the Mid-Atlantic states. The structure of the U.S. Blumeria graminis f. sp. tritici population was assessed based on a sample of 238 single-spored isolates. The isolates were collected from 16 locations in 12 states (18 site-years) as chasmothecial samples in 2003 or 2005, or as conidial samples in 2007 or 2010. DNA was evaluated using nine single nucleotide polymorphism (SNP) markers in four housekeeping genes, and 10 simple sequence repeat (SSR) markers. The SSR markers were variably polymorphic, with allele numbers ranging from 3 to 39 per locus. Genotypic diversity was high (210 haplotypes) and in eight of the site-years, every isolate had a different SSR genotype. SNP haplotypic diversity was lower; although 15 haplotypes were identified, the majority of isolates possessed one of two haplotypes. The chasmothecial samples showed no evidence of linkage disequilibrium (P = 0.36), while the conidial samples did (P = 0.001), but the two groups had nearly identical mean levels of genetic diversity, which was moderate. There was a weakly positive relationship between genetic distance and geographic distance (R(2) = 0.25, P = 0.001), indicating modest isolation by distance. Most locations in the Mid-Atlantic and Great Lakes regions clustered together genetically, while Southeast locations formed a distinct but adjacent cluster; all of these were genetically separated from Southern Plains locations and an intermediate location in Kentucky. One-way migration was detected at a rate of approximately five individuals per generation from populations west of the Appalachian Mountains to those to the east, despite the fact that the Atlantic states experience more frequent and damaging wheat mildew epidemics. Overall, the evidence argues for a large-scale mosaic of overlapping populations that re-establish themselves from local sources, rather than continental-scale extinction and re-establishment, and a low rate of long-distance dispersal roughly from west to east, consistent with prevailing wind directions.