There are fungicides that can be sprayed, but since you are growing your grapes to be eaten, treating them organically is the best choice. The simplest prevention is to plant mildew-resistant grape plants. American varieties are less likely to fall victim to mildew. The European grape varieties are much more vulnerable.
If you already have or prefer the European varieties, you can keep mildew from starting by dusting your plants with sulfur powder early in the season as the grape clusters just begin to form. Sulfur is an old remedy that is now coming back into fashion as people are becoming more discerning in the treatments used on edible plants.
Sulfur should be used on a day that isn’t windy and make sure you wear both eye glasses and gloves for protection. Sulfur dust isn’t toxic but it is very irritating and wearing a mask is also advised to protect from breathing in the irritating particles. I once forgot the glasses and my eyes stung and teared for hours.
There are a number of commercial fungicides available in garden centers. These will also work. But if you want to grow organic grapes – and if you're growing your own, why not make them as healthy as possible? – using sulfur is a good choice.
Grow healthy, handsome grapes as both productive and attractive additions to your landscape structures, fences or walls. Expect to do some regular pruning to keep the plants controlled. Use these tips to avoid damage from mildew fungus. Then you can have a great plant for design purposes and delicious, healthy fruit enough for your family, friends and even some for your neighbors.