While powdery mildew and Botrytis bunch rot are two separate diseases, they often go hand-in-hand in California’s vineyards. A severe powdery mildew problem, if left untreated, can lead to Botrytis, causing double the headache for grape growers.
Becoming familiar with the treatment options available for both diseases can minimize yield loss and quality reduction for this high-value crop.
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Powdery mildew doesn’t require rain to thrive — just moderate temperatures, like those in the north coast regions of California. Severe powdery mildew also impacts the Central Valley, but the pressure is typically higher in years with cooler summers, as heat knocks down powdery mildew.
Because infection can do serious harm before symptoms are visible, it is critical to set up well-designed preventive programs. If insufficiently controlled, symptoms of powdery mildew can include red, blotchy areas on dormant canes, spots on the upper surface of leaves, powdery or dusty areas, and powdery masses on the entire berry surface.
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Botrytis Bunch Rot
Wet weather and berry injury are the key contributing factors for Botrytis bunch rot, and frequent spring rains can bring on early-season shoot blight. Diseased berries turn dark, making them easy to see in white varieties. The pulp softens and the skin splits or slips off the pulp when touched, a phenomenon known as “slip skin.” The fungus then grows out of the pulp exposed by the split skin, turning these berries into the main source of spores that infect healthy berries. Diseased fruit left on the vine will overwinter and provide inoculum the following spring.
Relevant: Early Season disease control is critical for wine grape growers
Proactive Measures for a Quality Harvest
Powdery mildew and Botrytis can be managed with proactive, vigilant treatment. By implementing best practices, growers can tackle these two problematic diseases at once, helping achieve a high-quality, profitable harvest.
The best overall prevention practice for Botrytis infection is leaf removal. Removing leaves in the fruit zone opens a window to allow light penetration, increased air flow over the surface of the berries and reduced relative humidity.
Leaf removal provides approximately 50 percent control for powdery mildew, and fungicides have been found to control the other 50 percent. Applications are recommended early, up to veraison. After that, the berries become less susceptible to powdery mildew infection.
Research has found that both Luna Experience® and Luna Sensation® from Crop Science provide excellent systemic control against both powdery mildew and Botrytis. This allows growers to maximize efficiency for their fungicide applications and time in the field. Growers also boost return by selecting a fungicide that is highly effective on both diseases.