|WOLFENBARGER, SIERRA - Oregon State University|
|MASSIE, STEPHEN - Oregon State University|
|OCAMB, CYNTHIA - Oregon State University|
|ECK, EMILY - Oregon State University|
|GROVE, GARY - Washington State University|
|NELSON, MARK - Washington State University|
|PROBST, CLAUDIA - Washington State University|
|TWOMEY, MEGAN - Oregon State University|
|Gent, David - Dave|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2016
Publication Date: 6/1/2016
Citation: Wolfenbarger, S.N., Massie, S.T., Ocamb, C., Eck, E.B., Grove, G.G., Nelson, M.E., Probst, C., Twomey, M.C., Gent, D.H. 2016. Distribution and characterization of Podosphaera macularis virulent on hop cultivars possessing R6-based resistance to powdery mildew. Plant Disease. 100(6):1212-1221. doi: 10.1094/PDIS-12-15-1449-RE.
Interpretive Summary: Host resistance is often the preferred long-term approach for disease management because it is inexpensive, simple, and avoids use of purchased inputs such as pesticides. In 2012, an outbreak of powdery mildew occurred in Washington and Idaho on previously resistant cultivars whose resistance was putatively based on a resistance gene designated R6. In this research, we describe the spread of disease on cultivars with similar resistance, characterize which other resistance genes might be effective against the pathogen, and provide some evidence that R6 may have some residual value in breeding. Surveys of commercial hop yards during 2012 to 2014 documented that powdery mildew is now widespread on cultivars possessing R6 resistance in Washington and Oregon, and the incidence of disease is progressively increasing. However, such strains have not yet been confirmed in the Upper Midwest or eastern U.S. It is possible that there is a reduction in fitness of R6-viruelence that may reduce disease severity in field, even when R6-virulent strains of the pathogen are present. Resistance genes R1 and R2 appear to remain effective against R6-virulent isolates of the pathogen in the Pacific Northwestern U.S.
Technical Abstract: In 2012, an epidemic of powdery mildew occurred in Washington and Idaho on previously resistant cultivars whose resistance was putatively based on the gene designated R6. In 2013, isolates capable of causing severe disease on cultivars with R6-based resistance were confirmed in Oregon and became widespread during 2014. Surveys of commercial hop yards during 2012 to 2014 documented that powdery mildew is now widespread on cultivars possessing R6 resistance in Washington and Oregon, and the incidence of disease is progressively increasing. Pathogenic fitness, race, and mating type of R6-virulent isolates were compared to isolates of P. macularis lacking R6-virulence. All isolates were positive for the mating type idiomorph MAT1-1 and were able to overcome resistance genes Rb, R3, R5, whereas virulent isolates also invariably could infect differential cultivars reported to possess R6 and R4, though R4 has not yet been broadly deployed in the U.S. R6-virulent isolates were not detected from the eastern U.S. during 2012 to 2015. In growth chamber studies, R6-virulent isolates of P. macularis had a significantly longer latent period and produced fewer lesions on plants with R6 as compared to plants lacking R6, indicating a fitness cost to the fungus. R6-virulent isolates also produced fewer conidia when compared to isolates lacking R6-virulence, independent of whether the isolates were grown on a plant with or without R6. Thus, it is possible that the fitness cost of R6-viruelence is exacted irrespective of the host genotype. In field studies, powdery mildew was suppressed by at least 50% on plants possessing R6 as compared to those without R6 when co-inoculated with R6-virulent and avirulent isolates in 2014. R6-virulence in P. macularis appears to be race-specific and, at this time, imposes a measurable fitness penalty on the fungus. Resistance genes R1 and R2 appear to remain effective against R6-virulent isolates of P. macularis in the Pacific Northwestern U.S.