Most summers in western Oregon provide excellent conditions for vigorous vine growth and powdery mildew infections. Many growers are nervous about finding even a little powdery mildew on their grape clusters during the summer, as it can destroy fruit and wine quality. However, what can be done if an infection occurs? Here are some recommendations:

Pamper your grapes with a refreshing water bath. Followed by a soothing mineral oil treatment. And a complete cover.

This article will explain the process:

https://extension.oregonstate.edu/crop-production/wine-grapes/how-deal-vineyard-powdery-mildew-outbreak