The first this to do to prevent this is to avoid spraying your plants with water. When they need watering direct the water to the soil with watering wands or soaker hoses to soak the soil while keeping the leaves. stems and flowers dry. HERE’S A LINK to the dangers of spray watering.
It sounds like you have picked up powdery mildew, a fungal disease. You can spray it with Fung-o-nil a garden fungicide. The better approach would be to start spraying in spring with the fungicide to prevent the disease. They will come up next spring just fine in spite of the damage the mildew has cause this year.