Powdery mildew is a fungus that reveals itself as a powder-like coating over a plant's leaves. Rarely will it kill a plant but some perennials like phlox and shrubs such as lilacs, crape myrtles and roses can be damaged.
A heavy infestation of powdery mildew can cause a plant to lose its leaves, diminishing its vigor and causing it to not flower quite as much.
If you're having this problem in your garden there are two approaches you can try. The first is prevention. You should remove and dispose of infected plants and leaves by burning them or putting them in the garbage. Don't put infected leaves in your compost because you will just harbor spores for another round next season.
The second line of defense is to take action by spraying. But before using a conventional fungicide, try some of the new safer alternatives. I use a sulfur-based product. Since it's not a synthetic fungicide I don't have to worry about it damaging the environment. Spray your phlox about every 7 to 10 days when mildew is a problem.
One home remedy that some people have found useful as a preventative to powdery mildew is using this formula:
1 heaping teaspoon of baking soda
1 tablespoon of dormant oil (found in garden centers with the other garden chemicals)
1/2 teaspoon of insecticidal soap or dishwashing soap
1 gallon of water
It's important that a plant is well hydrated before applying this solution. Water deeply a couple of days before spraying and don't spray during the heat of the day.