Powdery Mildew on Hops
Powdery Mildew, cause by the pathogenic fungus Podosphaera macularis is a very common and important disease on hops. Powdery Mildew first affects foliage on hops, which decreases the plant’s photosynthetic activity and can lower yield. The Powdery Mildew fungus can also directly infect hops before harvest, causing a direct impact on yield.
Powdery Mildew is easy to identify on leaves, with white “powdery” looking mycelia visible. Early infection on hop cones can appear as reddish-brown discoloration. These symptoms indicate that infection of the plant has already occurred and the disease may be difficult to control at this stage.
Most fungicides labelled for Powdery Mildew on hops work best as a preventative application before infection occurs. To help growers know when is the optimal time to use preventative sprays, and at what intervals, researchers from the USDA and UC Davis have adapted the Gubler-Thomas Powdery Mildew Risk Index Model for grapes to be used on hops.
Powdery Mildew Model as a Management Tool
The adapted Gubler-Thomas Powdery Mildew Risk Index for hops is a useful tool to make smart decisions regarding preventative fungicide sprays based on the weather and optimum temperatures for conidial reproduction. The model uses these temperature thresholds to calculate a daily (0–100) risk index which can be used to adjust intervals between sprays.
- An index of 0–30 indicates low risk of disease. Chemical fungicides may be used at intervals of 14-18 days, or label max, and sulfur may be used at 14 day intervals.
- An index of 40–60 indicates moderate risk of disease. Chemical fungicides may be used at intervals of 10-14 days at this risk index, and sulfur, if used, should have an interval rate of 10 days.
- Above 60 indicates high risk which should results in a 7 day interval, or label minimum, for chemical fungicides, or sulfur.
Using the Powdery Mildew Risk Index to adjust spray intervals has been proven, in many cases, to reduce overall number of fungicide sprays compared to weekly rotations while maintaining effective control.
Calculating Daily Risk Index Levels
The adapted Gubler-Thomas Powdery Mildew for hops is intended to be a daily risk index which should be recalculated, as a running total throughout the production season. The rules for calculated are listed as follows:
- If fewer than 6 continuous hours of temperatures occurred between 61° and 81°F, subtract 10 points.
- If 6 or more continuous hours of temperatures occurred between 61° and 81°F, add 20 points.
- If temperatures reached 86°F for more than 15 minutes, subtract 10 points.
- If there are 6 or more continuous hours with temperatures between 61° and 81°F AND the temperature rises to or above 86°F for at least 15 minutes, add 10 points. (This is the equivalent of combining points 2 and 3 above.)
The total maximum risk index level is 100 and the minimum is 0. The maximum amount of points that can be added in a given day is +20 points, and the most that can be subtracted is -10.
Temperature data can be obtained from in field thermometers, or temperature loggers or through weather stations placed near the hopyard. Sensors placed within the plant canopy, at the leaf level will typically give the most accurate data, but ambient temperatures are still very useful when in-field sensors are not practical or reliable. Some weather station models will calculate daily risk. Growers seeking an option without weather stations should consider using the Pest Prophet mobile app.
Initiating the Risk Index Model
For hop producing areas where Powdery Mildew is common, we should assume overwintering and presence of the disease. For this reason, the best time to start calculating risk, and adjusting spray intervals is based on the growth of the plants. The model should start for the season either when 15 to 30 cm of growth from 50% of the plants is observed at bud break, or when 15 to 30 cm of regrowth is observed after pruning in the spring.
Using Pest Prophet to calculate Powdery Mildew Risk
The easiest way to begin using pest and disease models in your hopyard is through the Pest Prophet app. Using the Pest Prophet app, you can have Powdery Mildew Risk calculated automatically for your hopyard daily, without needing to add any expensive weather stations or hardware. It is simple and easy to integrate into your decision making process, and will pay for itself many times over in saved chemical costs.
To learn more about this app, and how you can get started using it for free, please visit www.pestprophet.com or start using the app.
Spanish 
English