- Author: Anne E Schellman
Over the past few months, gardeners have asked the UCCE Master Gardeners for help with their grapes. They want to know:
- What's this white powdery substance on my grape plants?
- Why are my grapes so small?
- What's causing my grapes to split?
- What can I do to “save” my grape plants?
The culprit is a common grape disease called powdery mildew. This fungus leaves a telltale white powdery coating on plants. It also deforms leaves, shoots, and grapes. Young grapes can be stunted or scarred, and sometimes split open.
We've told gardeners that unfortunately, powdery mildew can't be eradicated. For now, gently hosing down plants weekly with water will help to wash off and kill the spores.
In winter, prune grapes and remove and destroy infected materials. During spring, use fungicides to protect grapevines. Timing is important. Read about how and when to prevent and control this disease in the publication Pest Notes: Powdery Mildew.
Small grapes are a result of too many clusters of grapes on a vine. The clusters will need to be thinned. Sometimes gardeners have trouble doing this. It may feel like you are throwing away perfectly good fruit! However, thinning out grape clusters is a necessary task that should be done in early spring during the first three to four weeks after fruit has set.
Study up on grapes now! Then you'll know what you need to do next year. For information on pruning, thinning, and growing grapes, visit The California Garden Web page Growing Grapes in Your Backyard.