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Research Project: Genetic Improvement of Small Grains for Biotic and Abiotic Stress Tolerance and Characterization of Pathogen Populations

Location: Plant Science Research

Title: Virulence differences in blumeria graminis f. sp. tritici from the central and eastern United States

Author
item Cowger, Christina
item MEHRA, LUCKY - North Carolina State University
item ARELLANO, CONSUELO - North Carolina State University
item MEYERS, EMILY - North Carolina State University
item MURPHY, J - North Carolina State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2017
Publication Date: 1/23/2018
Citation: Cowger, C., Mehra, L., Arellano, C., Meyers, E., Murphy, J.P. 2018. Virulence differences in blumeria graminis f. sp. tritici from the central and eastern United States. Phytopathology. 108:402-411.

Interpretive Summary: Wheat powdery mildew is a disease of international importance that occurs across a wide geographic area in the USA. The disease is caused by Blumeria graminis f. sp. tritici. A survey of virulence, or which resistance genes can be overcome in wheat, was conducted by sampling 36 wheat fields in 15 U.S. states in the years 2013 and 2014. Mildew isolates were obtained from three separated plants at each of five separated sites within each field in order to assess the spatial distribution of virulence profiles. A total of 1,017 isolates from those fields were tested individually on single-gene differential cultivars containing a total of 21 powdery mildew resistance (Pm) genes. Several mildew resistance genes recently bred into bread wheat from wild wheat relatives (Pm37, Pm53, MlAG12, NCAG13, and MlUM15) exhibited full resistance to all local B. graminis f. sp. tritici populations from across the sampled area. One older gene, Pm4b, also retained at least some efficacy across the sampled area. The B. graminis f. sp. tritici population sampled from Arkansas and Missouri, on the western edge of the eastern soft red winter wheat region, had virulence profiles more similar to other soft wheat mildew populations than to the geographically closer population from hard wheat fields in the Plains states of Oklahoma, Nebraska, and Kansas. The Plains population differed in that it was avirulent to several Pm genes long defeated in the soft wheat-growing areas. On average, the pathogen could overcome more Pm genes in the eastern states, and fewer toward the west. Several recently introduced Pm genes (Pm25, Pm34, Pm35, and NCA6) that are highly effective against mildew in the field in North Carolina were unexpectedly susceptible to eastern-U.S. B. graminis f. sp. tritici populations in detached-leaf tests. The research confirmed that the U.S. B. graminis f. sp. tritici population has different virulence profiles in the hard and soft wheat regions, probably due to which host genes are used and limited pathogen migration.

Technical Abstract: Wheat powdery mildew is a disease of international importance that occurs across a wide geographic area in the USA. A virulence survey of Blumeria graminis f. sp. tritici, the causal agent, was conducted by sampling 36 wheat fields in 15 U.S. states in the years 2013 and 2014. Using a hierarchical sampling protocol, isolates were derived from three separated plants at each of five separated sites within each field in order to assess the spatial distribution of pathotypes. A total of 1,017 isolates from those fields were tested individually on single-gene differential cultivars containing a total of 21 powdery mildew resistance (Pm) genes. Several recently introgressed mildew resistance genes from wild wheat relatives (Pm37, Pm53, MlAG12, NCAG13, and MlUM15) exhibited full resistance to all local B. graminis f. sp. tritici populations from across the sampled area. One older gene, Pm4b, also retained at least some efficacy across the sampled area. The B. graminis f. sp. tritici population sampled from Arkansas and Missouri, on the western edge of the eastern soft red winter wheat region, had virulence profiles more similar to other soft wheat mildew populations than to the geographically closer population from hard wheat fields in the Plains states of Oklahoma, Nebraska, and Kansas. The Plains population differed in that it was avirulent to several Pm genes long defeated in the soft wheat-growing areas. Virulence complexity was greatest east of the Mississippi River, and diminished toward the west. Several recently introgressed Pm genes (Pm25, Pm34, Pm35, and NCA6) that are highly effective against mildew in the field in North Carolina were unexpectedly susceptible to eastern-U.S. B. graminis f. sp. tritici populations in detached-leaf tests. Sampled fields displayed a wide range of pathotype diversity and spatial distribution, suggesting epidemics are caused by varying numbers of pathotypes in all regions. The research confirmed that the U.S. B. graminis f. sp. tritici population has different virulence profiles in the hard and soft wheat regions, which are likely maintained by host selection, isolation by distance, and directional gene flow.