Structure and migration in U.S. Blumeria graminis f. sp. tritici populations

Authors: Cowger, Christina; Parks, Wesley; KOSMAN, EVSEY - Tel Aviv University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2015
Publication Date: 1/4/2016
Citation: Cowger, C., Parks, W.R., Kosman, E. 2016. Structure and migration in U.S. Blumeria graminis f. sp. tritici populations. Phytopathology. 106:295-304.

Interpretive Summary: Wheat powdery mildew occurs throughout the south-central and eastern U.S.A, but epidemics are especially damaging in the Mid-Atlantic states. The structure of the U.S. Blumeria graminis f. sp. tritici population was assessed. A set of 238 strains of the fungus was collected from 16 locations in 12 states. DNA was analyzed using two different molecular markers. Of the 238 strains, 210 had unique genetic profiles. Genetic distance was weakly related to geographic distance; i.e., more widely separated isolates were less genetically related to each other. However, locations in the Southeast, Mid-Atlantic, and Great Lakes regions were relatively highly similar to each other genetically, and in turn were genetically separated from two of three Southern Plains locations and an intermediate location in Kentucky. Evidence was found for migration of some strains from west of the Appalachian Mountains into populations to the north and east, despite the fact that the Atlantic states experience more frequent and damaging wheat mildew epidemics. This is consistent with prevailing wind directions. Overall, there seems to be a large-scale mosaic of overlapping populations that re-establish themselves from local sources, rather than the kind of annual continental-scale extinction and re-establishment of populations that is seen with rusts and downy mildews.

Technical Abstract: While wheat powdery mildew occurs throughout the south-central and eastern U.S.A, 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 polymorphisms (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 (Kosman diversity = 0.18). There was a weakly positive relationship between genetic distance and geographic distance (R2 = 0.19, P = 0.003), indicating modest isolation by distance. However, locations in the Southeast, Mid-Atlantic, and Great Lakes regions were strongly genetically clustered, and were genetically separated from two of three Southern Plains locations and an intermediate location in Kentucky. One-way migration was detected of approximately six individuals per generation from west of the Appalachian Mountains into populations to the north and 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.