Master Gardener: Preventing, treating powdery mildew | Home & Garden

I went away for the weekend and came home to find some my plants covered with what looks like a white powder. What should I do? — L.H.

It sounds like you are describing powdery mildew. Many plants can be affected by powdery mildew including azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber, and squash. How severely it affects the plant depends on several factors including the variety of the plant, its overall health, and the weather conditions. Unfortunately, there’s a lot of powdery mildew in our area right now due to our wet, relatively cool spring.

Powdery mildew becomes apparent when you notice a whitish substance covering of leaves of your plant. This fungus gets its nutrients through small root-like appendages called haustoria. The haustoria penetrate the outer layer of the leaf’s surface allowing it to access nutrients in the leaves. This causes the leaves to turn brown, die, and eventually fall off if left untreated.

The fungus can spread from one infected plant to another, but it may not, depending on the particular strain of powdery mildew you have. Some are specific to only closely related plants.

Even though there are a variety of powdery mildews, they follow the same life cycle. The fungi overwinter as small black spore bearing structures (cleistothecia) or as fungal threads called mycelium. Overwintering occurs in leaf debris, stems, or dormant buds on plants. In the spring when temperatures get above 60 degrees, the overwintering structures begin to produce spores which are moved by the air to an appropriate host plant. The disease is spread by not only wind but by splashing rain to other locations on the plant or other plants. It only takes about 48 hours for a newly germinated fungal spore to begin producing new spores which contributes to its ability to spread quickly.

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