Answer: Based on your description I think your grapes may be suffering from powdery mildew, however there are many fungal problems that can affect this plant. A major step toward prevention is to plant the vine in a full sun location with good air circulation, prune it so it is open to the air, and do a thorough clean up of all the prunings and plant debris and any fruit as well as fallen leaves each fall. I would suggest you consult with your local county extension to verify the diagnosis and based on knowing that, determine if a preventive spray program would be advisable depending on your local growing conditions, weather, and the specific variety of grape you are growing. In the meantime, you may find the following information helpful.
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