Temperature influences on powdery mildew susceptibility and development in the hop cultivar Cascade

Authors: Gent, David - Dave; CLAASSEN, BRIANA - Oregon State University; WISEMAN, MICHELE - Oregon State University; WOLFENBARGER, SIERRA - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/26/2021
Publication Date: 1/3/2022
Citation: Gent, D.H., Claassen, B.J., Wiseman, M.S., Wolfenbarger, S.N. 2022. Temperature influences on powdery mildew susceptibility and development in the hop cultivar Cascade. Plant Disease. 106(6):1681-1689. https://doi.org/10.1094/PDIS-10-21-2133-RE.
DOI: https://doi.org/10.1094/PDIS-10-21-2133-RE

Interpretive Summary: The disease powdery mildew is one of the most damaging and costly problems that hop producers must manage. The hop cultivar Cascade possessed partial resistance to powdery mildew that rendered this cultivar largely unaffected by the disease for nearly 20 years. However, new strains of the powdery mildew fungus emerged in the western U.S. coincident with expanded planting of the cultivar Cascade and these new strains of the pathogen can cause severe disease on this cultivar. Therefore, growers must now manage powdery mildew on a cultivar where they lack historical knowledge and where there is a limited research base. We conducted a series of detailed growth chamber experiments to understand more fully if the cultivar Cascade responds differently than other susceptible cultivars to high temperature events that occur before, concurrent to, or after infection with the powdery mildew fungus. Overall, we found that Cascade is generally less susceptible to powdery mildew than the highly susceptible cultivar primarily used to devise current disease management approaches. Moreover, the susceptibility of Cascade to powdery mildew was disproportionately reduced when exposed to high temperature before or during the early stages of infection. This research indicates that cultivar-specific risk assessments for powdery mildew are warranted, which could help producers refine their disease management programs for Cascade and reduce unnecessary inputs.

Technical Abstract: The hop cultivar Cascade possesses partial resistance to powdery mildew (Podosphaera macularis) that can be overcome by recently emerged, virulent isolates of the fungus. Given that hop is a long-lived perennial and that brewers still demand Cascade, there is a need to better understand factors that influence the development of powdery mildew on this cultivar. Growth chamber experiments were conducted to quantify the effect of constant, transient, and fluctuating temperature on Cascade as contrasted with another powdery mildew susceptible cultivar, Symphony. Exposure of plants to supraoptimal temperature (26 and 32°C) prior to inoculation led to more rapid onset of ontogenic resistance in intermediately aged leaves in Cascade as compared to Symphony. Cascade was overall less susceptible to powdery mildew when exposed to constant temperature ranging from 18 to 32°C after inoculation. However, cultivar also interacted with temperature such that proportionately fewer and smaller colonies developed on Cascade than Symphony at supraoptimal yet permissive temperatures for disease. Colonies became progressively more tolerant to temperatures of 26 to 30°C with increasing time from inoculation to exposure, as moderated by cultivar, the specific temperature, and their interaction. Subjecting plants to simulated diurnal temperature regimes at the time of inoculation or 24 h later indicated Cascade and Symphony responded proportionately similar on days predicted to be marginally unfavorable or marginally favorable for powdery mildew, although Cascade was quantitatively less susceptible than Symphony. In sum, this research indicates that Cascade is overall less susceptible to powdery mildew than Symphony, and supraoptimal temperature before, concurrent to, or after infection may interact differentially to moderate disease risk in Cascade. Therefore, cultivar-specific risk assessments for powdery mildew appear warranted.