If your plants' leaves have a white or gray dust on them, powdery mildew is probably attacking your garden. It's a common disease, and different strains of powdery mildew attack a wide range of plants.
The bad news is that there's no good treatment of powdery mildew. Happily, though, you can control powdery mildew.
How to Control Powdery Mildew
Keep Your Garden Clean: Powdery mildew spores can overwinter in your garden -- so remove all affected fallen leaves from your garden during and at the end of the season. If you see leaves that are just beginning to show the disease, pluck them off the plant and throw them away to help keep powdery mildew from spreading to healthy plants.
Encourage Air Flow: Powdery mildew loves still, humid conditions, so keep spaces between your plants for air to flow through. Divide perennials every few years to keep them loose and open and prune trees and shrubs so they don't get too thick. Also: Consider airflow when you choose spots for your plants. Avoid planting mildew-prone plants in especially sheltered spots.
Apply Fungicides: Fungicides can effectively control powdery mildew from spreading, but they're not as good at killing the disease once it's established. So use fungicides -- either traditional or organic -- in wet, humid periods before you see a problem or just as the plants first start to show symptoms. Look for these products at your local garden center, nursery, or online.
Choose Varieties that are Resistant to Powdery Mildew
Some plants have been bred to resist the disease better than others. While they can certainly suffer from the disease, powdery mildew tends to affect them less than on mildew-prone varieties. Here are some top-performing mildew-resistant varieties:
Blue Stocking bee balm
Cherokee Brave dogwood
Donald Wyman crabapple
Hopi crape myrtle
James McFarlane lilac
Marshall's Delight bee balm
Miss Kim lilac
Natchez crape myrtle
Prairifire crab apple
Sexy Rexy rose
Sheer Bliss rose
Topaz Jewel rose
Violet Queen bee balm