Sorry for being late to respond to this, but I have had the same problem. I grow some grapes with the most unfavorable conditions. Three years ago due to powdery and downey mildew, I almost gave up on grapes. I was down to zero yield. I am not kidding. I tried all of the low risk fungicides like oils, sulfur, potassium bicarbonate. It was all a waste of time. So I did two things. First, I pruned aggressively. Then latter on when dormancy was ending, I sprayed with Accelerate, Holocal, and Micro 5000. My production was good. The Powdery mildew was not a problem. after leaf out I noticed a small amount of downey mildew and I applied a small amount of Rebound Copper. This seemed to hold it in check. This year I repeated the Accelerate/Holocal/Micro 5000 combo and got great results. Hope this helps. I am just a sample size of one and its only been two years, but this is one of my best results at defeating powdery mildew with little effort.