Characterization of Podosphaera macularis derived from the hop cultivar ‘Strata’ and resistance to powdery mildew

Authors: BLOCK, MARY - Oregon State University; WISEMAN, MICHELE - Oregon State University; Gent, David - Dave

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2021
Publication Date: 3/24/2021
Citation: Block, M., Wiseman, M.S., Gent, D.H. 2021. Characterization of Podosphaera macularis derived from the hop cultivar ‘Strata’ and resistance to powdery mildew. Plant Health Progress. 22(2):154-156. https://doi.org/10.1094/PHP-11-20-0095-BR.
DOI: https://doi.org/10.1094/PHP-11-20-0095-BR

Interpretive Summary: Powdery mildew is an important disease of hop and can be challenging to manage. Development of hop cultivars with resistance to the disease are important to long term control of the disease. However, changes in the population of the fungus that causes powdery mildew can render resistant cultivars susceptible. In this research, we identified strains of the hop powdery mildew fungus capable of causing severe disease on the newly released cultivar, Strata, which was considered resistant at the time of its release. The strains of the powdery mildew fungus found on Strata were indistinguishable from those that cause severe disease on the cultivar Cascade, with both causing significantly more disease on Strata and Cascade plants as compared to other strains of the fungus. A diverse set of powdery mildew strains were able to cause disease on Strata, but only at low levels. Only strains that cause severe disease on Cascade were able to cause appreciable levels of disease on Strata. This research will inform disease risk assessment for growers and guide plant breeders on which strains of the fungus to use when screening progeny derived from Strata for their powdery mildew reaction.

Technical Abstract: Hop powdery mildew is one of the most destructive diseases of hop. Host resistance is the most efficient means of managing the disease, and breeding efforts have produced multiple cultivars with resistance to powdery mildew. One such cultivar is Strata, which was developed by Oregon State University and produced on approximately 309 ha in Oregon in 2020. The patent application for Strata submitted in 2017 stated the plant was resistant to powdery mildew based on an initial greenhouse screening and 8 years of observations at three locations in western Oregon. In 2019, powdery mildew colonies were reported in a commercial yard of Strata near Woodburn, Oregon. Two clonal isolates of Podosphaera macularis were isolated via single chain transfers from colonies and characterized by inoculating each of seven differential cultivars. Cultivars with Rb, R3, and R5 were susceptible to the isolates. The susceptibility of Strata to powdery mildew was characterized by inoculating seven isolates of P. macularis with known virulences onto leaves of Strata. Sporulation appeared on Strata leaves with all of the differential isolates independent of their specific virulence, but only at trace levels with of sporulation. Strata-derived isolates were phenotypically indistinguishable from Cascade-adapted isolates when inoculated onto the cultivars Cascade, Strata, and Symphony, as only Strata-derived and Cascade-adapted isolates could cause severe disease on Cascade and Strata. Like Cascade-adapted isolates, Strata-derived isolates also lack virulence on hop plants possessing the R-gene R6. These finding should inform both disease risk assessment in the field and future breeding tactics and strategies involving Strata. Further characterization of Strata is warranted to determine if its partial resistance to powdery mildew is under the same genetic control as that of Cascade.