Washington State University

Plant Pathology

Various photos of powdery mildew infection on hops

Above: Powdery mildew caused by Podosphaera macularis is a major hop disease.

Focus on Mildews

The plant pathology component of our project focuses on the most important diseases in hops. Powdery mildew caused by Podosphaera macularis and downy mildew caused by Pseudoperonospora humuli are among the most significant production concerns for growers. Both types of mildew can affect flowers and cones, and either can threaten and diminish the economic viability of hop production.

Development and implementation of effective disease management strategies require judicious use of fungicides currently registered for hop, development of new cultural control strategies, registration of new fungicides with unique modes of action, and commitment to the areawide adoption of sound fungicide resistance management strategies by the hop industry. 

Second Year Projects/Progress

In our second year of the project, we continued to look at fungicide timing, including impacts of last spray date. We found that fungicide applications were required through at least late July to minimize powdery mildew and maximize cone color quality and alpha/beta acid content. Additional late-season sprays may be of value under moderate or high disease pressure conditions or when harvest is anticipated to be late.

Click here for more details on the second year of our project.

FIrst Year Projects/Progress

In the first year, our research and outreach focused on hop cone disease model development. Important factors included noting disease levels on leaves and tracking temperature, rain, fungicide timing, and last spray date. We noted that young cones were more susceptible to disease, with both bracts and bracteoles being impacted, while bracts appeared to retain some level of susceptibility all the way to harvest maturity. Controls through late July proved most impactful, with later controls showing no significant effect on yield, aroma, bittering acids or storage index.

Click here for more details on the first year of our project.

Disease Modeling

Our outreach efforts have focused on the development of value-added products for the hop industry and are driven by the Washington State University Agricultural Weather Network (http://weather.wsu.edu). AgWeatherNet is a system of 136 weather stations distributed throughout Washington state but concentrated in the irrigated regions east of the Cascade Mountains. The weather data collected by these remote station are the raw materials that drive an extensive portfolio of crop, pest, and disease models.

Click here for more details on our hop disease modeling.



Hop Disease Images

Hop cone with downy milder

Hop cone exhibiting downy mildew damage

Hop bines with downy mildew

Hop bines showing browned cones and downy mildew damage

Hop plant leaf with downy mildew spots

Black, angular legions on the leaf of a hop plant indicating downy mildew

Washington State IPM Coordinator, WSU Irrigated Agriculture Research & Extension Center, Prosser WA 99350, 509-786-9287