Table grapes disease
My backyard grapevines looked healthy until about a couple of weeks ago. They are 3 years old. They now have gray black mildew like appearance on the vines the grapes still look healthy. Any diagnosis or treatment would be appreciated
Thank you for using Ask an Expert for help with your grapes.
I believe your grapes are infected with powdery mildew—a common fungus. The photo included, taken from the Oregon Extension Service publication Scouting for Grape Powdery Mildew quite similar to your symptoms. See here https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9067.pdf
This fungus also attacks the leaves and grapes. On the leaves, early in the infection, you could see tiny white powdery patches on the under surface. These later show as yellow/brown spots on the upper surface. On the fruit, a grey mass will coat the bunches.
To combat this disease, prune the vines to increase air circulation. Also, ensure that the grapes are in good sun and do not suffer drought stress. This makes them more susceptible to this and other diseases. Good cultural practices may be enough to control the disease in years when pressure in not severe.
When your vines and bunches are affected year after year, a preventive spray program might be needed. There are several materials available to the home gardener. Begin when new vines are six inches long and continue through the growing season, as recommended on product labels. Product names may vary, but look for these active ingredients. Read and follow all label directions, especially concerning use on food crops, number of sprays allowed per season, and how long before harvest to stop spraying.
- Bicarbonate-based products
- Sulfur-based products
- Bacillus subtilis strain QST 713 (commonly sold under the Serenade name)
For complete information on growing grapes in the Willamette Valley, see Growing Table Grapes. See here: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1639.pdf
Have a good gardening year,