Answer: Grapevines are generally vigorous and disease free if grown in full sunshine and moist soil. I'd pick off the affected leaves. Your vines will produce lots of healthy new growth all summer long. If your plant does develop a disease, you'll need to have it positively identified so you'll know how to control it. Take a few of the affected leaves to your local nursery or master gardener clinic for positive identification of the problem, as well as control strategies.
Best wishes with your grapevines!
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