I just sprayed my grapes again with a fungacide this morning. We keep having rain and humid weather I'm trying to keep this crap under control. Next step I'll just have to nit pik all the marked leaves off. Probably should have done that then sprayed.
I still have plenty of graped I just hate to see so much work go to waste.
What kind of grapes are you growing? If you're growing any vinifera, or some hybrids, you really have to watch for downy and powdery mildew. Both of these mildews are exceptionally common in Eastern North America.
It kind of sounds like downy mildew to me, as it tends to be spread by rainfall... although it is pretty well impossible to tell just through a brief description. If you look at the leaves you might notice "oil spots" on some which seem pale green/yellow and almost translucent. On the bottom of the leaf you will see a faint white/grey "downy" growth. This is the early stage of the fungus. The leaves will eventually form necrotic spots and the whole leaf may go brown and fall off. It can affect the fruit as well... they will shrivel up into little hard mummies. Severe infections can pretty well defoliate the vine and destroy the fruit.
For good control, you need to start a fungicide spray program when the shoots have 5 unfolded leaves. Then follow the manufacturers schedule for re-applying. There should be plenty of info online which should help you determine whether you might be experiencing downy mildew in your vines.