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Title: Virulence structure of the eastern U.S. wheat powdery mildew population

Author
item PARKS, W. RYAN - NORTH CAROLINA STATE UNIV
item CARBONE, IGNAZIO - NORTH CAROLINA STATE UNIV
item MURPHY, J. PAUL - NORTH CAROLINA STATE UNIV
item Marshall, David
item Cowger, Christina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2008
Publication Date: 7/1/2008
Citation: Parks, W., Carbone, I., Murphy, J., Marshall, D.S., Cowger, C. 2008. Virulence structure of the eastern U.S. wheat powdery mildew population. Plant Disease. 92:1074-1082.

Interpretive Summary: Little is known about the population structure of wheat powdery mildew in the eastern 2 U.S., and the most recent report on virulence in this pathogen population involved isolates 3 collected in 1993-94. In the present study, wheat leaves naturally infected with powdery mildew 4 were collected from 10 locations in the southeastern U.S. in 2003 and 2005 and a collection of 5 207 single-ascospored powdery mildew isolates was derived from the leaves. Frequencies of 6 virulence to 16 mildew resistance (Pm) genes were determined by inoculating the isolates 7 individually on replicated plates of detached leaves of differential wheat lines. These virulence 8 frequencies were used to infer local effectiveness of Pm genes, estimate virulence complexity, 9 detect significant associations between pairs of pathogen avirulence loci, and assess whether 10 phenotypic differences between pathogen subpopulations increased with geographic distance. In 11 both years, virulence to Pm3a, Pm3c, Pm5a, and Pm7 was present in more than 90% of sampled 12 isolates and virulence to Pm1a, Pm16, Pm17, and Pm25 virulence was present in fewer than 10% 13 of isolates. In each year, 71%-84% of all sampled isolates possessed the same multilocus 14 virulence haplotypes, although there were significant differences among locations in frequencies 15 of virulence to individual Pm genes. Several potentially useful significant positive associations 16 were detected between alleles for avirulence to pairs of Pm genes. Genetic (phenotypic) distance 17 between isolate subpopulations increased significantly (R2= 0.40, P < 0.001) with increasing 18 geographic separation; possible explanations include differential commercial deployment of Pm 19 genes and restricted gene flow in the pathogen population.

Technical Abstract: Little is known about the population structure of wheat powdery mildew in the eastern 2 U.S., and the most recent report on virulence in this pathogen population involved isolates 3 collected in 1993-94. In the present study, wheat leaves naturally infected with powdery mildew 4 were collected from 10 locations in the southeastern U.S. in 2003 and 2005 and a collection of 5 207 single-ascospored powdery mildew isolates was derived from the leaves. Frequencies of 6 virulence to 16 mildew resistance (Pm) genes were determined by inoculating the isolates 7 individually on replicated plates of detached leaves of differential wheat lines. These virulence 8 frequencies were used to infer local effectiveness of Pm genes, estimate virulence complexity, 9 detect significant associations between pairs of pathogen avirulence loci, and assess whether 10 phenotypic differences between pathogen subpopulations increased with geographic distance. In 11 both years, virulence to Pm3a, Pm3c, Pm5a, and Pm7 was present in more than 90% of sampled 12 isolates and virulence to Pm1a, Pm16, Pm17, and Pm25 virulence was present in fewer than 10% 13 of isolates. In each year, 71%-84% of all sampled isolates possessed the same multilocus 14 virulence haplotypes, although there were significant differences among locations in frequencies 15 of virulence to individual Pm genes. Several potentially useful significant positive associations 16 were detected between alleles for avirulence to pairs of Pm genes. Genetic (phenotypic) distance 17 between isolate subpopulations increased significantly (R2= 0.40, P < 0.001) with increasing 18 geographic separation; possible explanations include differential commercial deployment of Pm 19 genes and restricted gene flow in the pathogen population.