Daily water probably isn't the cause of your powdery mildew. But it's not a good schedule for grapes or even a lawn for that matter. Could you go to a weekly or twice weekly watering?

Grapes would do best with a deep watering every couple weeks. Once a week on shallow soil.

Study the info linked below and you will be an expert on PM of grapes.

Here is a link that might be useful: grape PM

Also its probably best to water the roots and not the foliage.

I had some powdery mildew last year.

This year I may try spraying with a solution of milk. I may also try EM (effective organisms).

Definitely not into soaking my food with fungicides.

I have been growing grapes for about 20 yrs now(Calf. & AZ) and have learned a lot, mostly the hard way. First and formost do not water the vines from above Big NG. Do not water with a sprinkler under them as it will drive the spores. Put them on drip system or deep water with the hose. They get wet and are not dry by night you will get fugus or if it rains and they do not dry quickly, you will get fungus. If you have lots of humindity you are going to get fungus. Though I don't remmber Idaho haviing tons of humidity in the Coer d' Alene area , but I don't no where you are at. Keeping them aired out is perfect.
As for the fungus during the dorment season you can spray and it won't effect the fruit. I use a dorment spray 3 times during the winter and if I had fungus the year before I will spray with a funguside before leaf break. Keep the leaves dry, maybe even a little dusty dirty. Once there is leaf break I do not spray @ all. Hope this will help

spray with wettable sulfur or dust with sulfur every 2 weeks from bud break till veriason, assuming your grapes arent sulfur sensitive.