Answer: Grapevines are prone to a number of disease and insect problems so you'll have to do a little detective work to find the cause. As a rule, insects feeding on the leaves won't harm your vines too much. Usually by the time you notice the damage, the insects are long gone. As for the browning leaves, I suspect the triple digit temperatures over the past two weeks have stressed your plant out to the point where it is prematurely dropping some leaves. If the vine completely dried out, check the crown of the plant near the soil surface for outward clues of insects, or injury (perhaps cracking and splitting, lawn mower or weed whacker damage). If it appears intact and free of problems, then consider root injury - from nematodes, drought, soggy soil, or other stressful condition.
If the plant continues to decline, cut back the dead vines and remove the debris from the garden. Cut all the way back to the woody parts of the plant. Healthy new growth should follow the pruning. Be sure to water deeply and then allow the top of the soil to dry out before watering again. This should provide your plant with enough water to thrive but not so much that the roots suffocate (and the leaves turn brown).
Best wishes with your grapevine!
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