Powdery Mildew on Cherry

Powdery Mildew, cause by the pathogenic fungus Podosphaera clandestina is a very common and important disease on cherries, especially in the Pacific Northwest. Powdery Mildew affects leaves on cherries, which decreases the plant’s photosynthetic activity and can lower yield. The Powdery Mildew fungus can also directly infect fruit 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 Washington State University and UC Davis have adapted the Gubler-Thomas Powdery Mildew Risk Index Model for grapes to be used on cherries.

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 70° and 85°F, subtract 10 points.
• If 6 or more continuous hours of temperatures occurred between 70° and 85°F, add 20 points.
• If temperatures reached 95°F for more than 15 minutes, subtract 10 points.
• If there are 6 or more continuous hours with temperatures between 70° and 85°F AND the temperature rises to or above 95°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 orchard. 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 after new leaves are observed in the orchard.

Source:

Grove, Gary G., Robin J. Boal, and L. H. Bennett. 2000. Managing powdery mildew of cherry in Washington orchards and nurseries with spray oils. Plant Health Progress Online.