Location: Forage Seed and Cereal Research UnitTitle: Adaptation to partial resistance to powdery mildew in the hop cultivar Cascade by Podosphaera macularis
|Gent, David - Dave|
|MASSIE, STEPHEN - Oregon State University|
|TWOMEY, MEGAN - Oregon State University|
|WOLFENBARGER, SIERRA - Oregon State University|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2017
Publication Date: 6/30/2017
Citation: Gent, D.H., Massie, S.T., Twomey, M.C., Wolfenbarger, S.N. 2017. Adaptation to partial resistance to powdery mildew in the hop cultivar Cascade by Podosphaera macularis. Plant Disease. 101(6):874-881.
Interpretive Summary: In the U.S., the hop variety Cascade has been grown for two decades with little or no control measures needed for powdery mildew. Beginning in 2012, we observed powdery mildew on this variety, suggesting that its resistance may have been overcome by the powdery mildew fungus. Field surveys conducted during 2013 to 2016 indicated increasing occurrence of powdery mildew on Cascade, as well as an increasing number of fungicide applications applied to this cultivar, especially in Washington State. We found that powdery mildew that originated from Cascade plants was specifically adapted to Cascade, causing more severe disease on this variety than all others tested. Other sources of disease resistance not derived from Cascade provided immunity to the strains of the pathogen adapted to Cascade. Breeding strategies for powdery mildew may need to consider the potential for adaptation of the fungus to both qualitative and partial resistance in the host.
Technical Abstract: The hop cultivar Cascade has been grown in the Pacific Northwestern U.S. with minimal input for management of powdery mildew (Podosphaera macularis) for nearly 20 years due to the putatively quantitative resistance in this cultivar. While partial resistance is generally thought to be more durable than qualitative resistance, in 2012 powdery mildew was reported on Cascade in Washington State. Field surveys conducted during 2013 to 2016 indicated increasing prevalence of powdery mildew on Cascade, as well as an increasing number of fungicide applications applied to this cultivar. Nearly all isolates of P. macularis originating from the Pacific Northwest were able to infect Cascade in laboratory inoculations. However, the greatest number of colonies, most conidia produced, and the shortest latent period was observed with isolates derived originally from Cascade as compared to other cultivars. The enhanced aggressiveness of these isolates was only manifested on Cascade and not six other susceptible cultivars, further indicating a specific adaptation to Cascade by the fungus. Race characterization indicated Cascade-adapted isolates of P. macularis were able to overcome R-genes Rb, R3, and R5, but not other known R-genes. Therefore, multiple R-genes and other forms of quantitative resistance are expected to provide resistance to Cascade-adapted strains of the fungus. Breeding strategies for powdery mildew may need to consider the potential for adaptation of the fungus to both qualitative and partial resistance in the host.