A modified disease risk index for hop powdery mildew

Authors: Gent, David; RICHARDSON, BRIANA - Oregon State University; MASSIE, STEPHEN - Washington Hop Commission

Submitted to: PhytoFrontiers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2024
Publication Date: 3/25/2025
Citation: Gent, D.H., Richardson, B.J., Massie, S.T. 2025. A modified disease risk index for hop powdery mildew. PhytoFrontiers. 5(1):30-41. https://doi.org/10.1094/PHYTOFR-10-24-0117-R.
DOI: https://doi.org/10.1094/PHYTOFR-10-24-0117-R

Interpretive Summary: The disease powdery mildew continues to be ongoing challenge for hop producers. The variety Cascade is one of the most widely grown varieties in the U.S. and until 2012 powdery mildew did not cause appreciable damage on Cascade. Since then, new strains of the hop powdery mildew fungus appeared that can cause severe disease on Cascade. Therefore, we conducted experiments to adapt an existing disease risk index that was originally developed for varieties highly susceptible to powdery mildew to Cascade. Over four years of field testing, we demonstrate that timing fungicide applications using the modified disease risk index can save one to two applications as compared to regular applications. Importantly, use of the modified risk index did not compromise disease control. We also demonstrate that fungicide applications made after mid-July do little to influence disease control or crop quality metrics. Combining these findings point the way forward for implementing an effective, minimal input fungicide program for powdery mildew management on this variety. Depending on the year, this could halve the number of fungicides that may be applied as compared to applications made on a regular basis for both the foliar and cone phases of powdery mildew.

Technical Abstract: The hop powdery mildew fungus, Podosphaera macularis, has overcome host resistance when cultivars with a given form of resistance are broadly deployed in the landscape, as most recently experienced in the partially resistant hop cultivar ‘Cascade’. Hop is a long-lived perennial grown under production contracts, therefore growers of Cascade hops must now manage powdery mildew because of contractual obligations. We developed and validated a modified powdery mildew risk index to aid in matching fungicide application intervals for foliar powdery mildew to weather favorability specifically for this cultivar. The modified risk index preserves the form and logic of the original hop powdery mildew risk, but lowers a cardinal temperature from 30 to 28°C and introduces a low temperature rule that reduces estimated daily risk points when temperatures are =4°C for =2 hours. During eight-location years of validation in commercial yards in Washington State, the modified disease risk index gave disease control comparable to fungicide applications made on regular intervals yet with up to two fewer applications in years predicted to be less favorable to powdery mildew. We separately evaluated the benefit of fungicide applications made after bloom for managing the cone phase of powdery mildew. In five years of studies, fungicide applications ceasing in mid-July yielded disease control and hop brewing quality metrics similarly to applications made into mid-August. Thus, an effective and minimal input program for the cultivar Cascade in Washington can be achieved by adjusting fungicide applications intervals appropriately in spring and omitting unnecessary fungicide applications four to six weeks before harvest.