Powdery mildew was rampant again this year.  It is a complex issue as you will fins in the information from several sources below.  There are also some suggestions for control that I hope will help you.  
1.) Powdery mildew is a common fungi disease that affects many plant species including common vegetables and landscape plants. Caused by different types of host specific, obligate parasite fungi, powdery mildew is prevalent; overwintering in plant debris in and around a garden site.  Powdery mildew, like many fungi spread by water splashing and wind; developing during warm periods when the humidity next to plant leaves surfaces is above 90%. Humidity, warm days and cool nights are the environmental requirements for powdery mildew inoculum to develop and cause an infection. Some plants are more susceptible than others. You need to look for plant and seed varieties that have resistance to Powdery mildew.
2.) According to Dr. John Ball, this is also a very common disease of lilacs but powdery mildews (usually different fungi as these mildews are host specific) can occur on many different plants including grapes, roses and Virginia-creeper. Powdery mildew appear as white to grayish spots or patches on the leaf that often contain small pin-size black dots. These symptoms usually occur on the upper leaf surface. They also are more common on plants that are crowded and shaded, locations that provide the high relative humidity needed for the disease to develop. Since the disease occurs mostly on crowded plantings, the best solution is to selectively prune out (or even remove) plants to provide better air circulation and lower the humidity. There are also many fungicides labeled for control of this disease, but these need to be applied every 10-days throughout the growing season or at least throughout the hot, humid summer weather. Below, more about control.
3.) Control: If you have a problem with powdery mildew, a thorough cleanup of the foliage and stems in the fall will help because the disease overwinters on old, diseased foliage and stems. There are also several fungicides that can help control powdery mildew, check the labels of the ones you see available in your local garden center or hardware store. There is also a home remedy that seems to work fairly well. This treatment is based on baking soda with a little vegetable oil and dish detergent mixed in. Use 2 TBS baking soda, 1 tsp. of vegetable oil and liquid soap. Mix all of these in a gallon jug filled with water, shake well then spray it on your plants. There actually commercial fungicides based on the active ingredient of sodium bicarbonate in baking soda, on the market too (Dr. D. Graper).

4.) And more on control: 
Plants that are too close together have reduced air flow which enhances the spread of the mildew. Spores blow on the wind and will infect nearby plants. Control measures include using a fungicide as soon as the first symptoms are present. Do not wait because this fungus spreads very rapidly and is capable of covering a plant in one week. Copper and sulfur based fungicides work well. If a chemical fungicide is used, check the label to be sure that powdery mildew is one of the fungi controlled. Water plants only in the morning so leaves dry off, thin crowded plants and remove infected leaves, throwing them away. Also look for varieties that list resistance to powdery mildew (M. Roduner)  -