I borrowed the following from a web site on the subject of Powdery Mildew. It said what I wanted to say, and said it well. Hopefully this explanation will help others deal with this pesky stuff that always seems to attack some plants more than others - like lilacs and peonies for instance:
"Once powdery mildew appears, it can be difficult to treat, depending on the type and how severe the problem. For this reason, prevention is important. Avoiding susceptible cultivars, locating plants in full sun, providing suitable air circulation, and practicing proper maintenance (i.e. water, fertilizer, etc.) is usually adequate. Watering in the morning hours may also help.
But even with the best of precautions taken, powdery mildew may still strike. Although fungicides can help when applied early on, heavier infections may need to be treated with horticultural oil or neem oil. You can also use a homemade solution—mixing together a tablespoon each of baking soda, horticultural oil (or canola), and liquid dish soap (without bleach) with a gallon of water. Spray on your peonies every 10-14 days throughout the summer months. Do not spray the solution during hot and sunny days."
(The web site I borrowed from is: http://www.gardeningknowhow.com/flower/powdery-mildew-peonies.htm)