I have no personal experience growing grapes. However vineyards spray Wettable Sulfur as a preventative measure. It does not work though on leaves that are already affected.

I have had this problem on and off before. So. California was the worst (high humidity).I have had it here in AZ as well. Damp leaves or maybe spores from other plants have caused this. If you have grape set you should not use a fungicide, wait until the grapes are finished. It will look bad for a while. Try trimming out the extra leaves to open up the area so the sun will shine in and the air will circulate. I do it every year; I call it "giving the grapes a hair cut". Try to keep the leaves dry (no sprinklers). Monsoon can also cause this problem, which we have not seen yet. That is usually when I start to get a little powdery mildew, but by then the grapes are done and I can spray. Hope this helps

Thanx campv...I will start trimming they get lots of sun and I never water the leaves this poor grape has has something every yeart...but I still get big bunches of grapes This is a first with mildew..Just as long as I can still get the grapes and then treat the vine...
Thanx