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. This problem usually occurs toward the end of the season in August or September. This year (now) powdery mildew is a problem because of the warm humid conditions late in the summer. Factually most of the garden is mature or ripening.
Since these mildews attack several different types of plants, the fungus from a disease plant may infect other types of plants in the garden or flowerbed. As some of you know (myself included)) mildews can spread in a matter of a few days.
The bad news is that there’s no good treatment of powdery mildew. Happily, though, you can control powdery mildew if it happens earlier in the year.
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 especially if you had the fungus this year. Look for these products at your local garden center, nursery, or online.