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Powdery Mildew

Powdery Mildew

By Therese Ciesinski, GardenSMART

Every year, powdery mildew does a number on my garden phlox and beebalm. Long before the plants even bud, their leaves yellow and fall off from the ground up, leaving lanky, bare stems. Remaining leaves are covered with a dusty powder. It doesn’t kill the plants, and they do flower, though they are ratty to the point where they spoil the look of the garden. I’ve wondered what I could do about it.

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Healthy garden phlox.

What is powdery mildew?

Powdery mildew is a fungal disease – or, more accurately, diseases. Many types of plants are susceptible to powdery mildew – perennials, vegetables, trees, shrubs, even weeds – but the type that attacks my phlox isn’t the same one that attacks cucumbers.

Which plants does it affect?

Phlox, lilacs, roses, spirea, beebalm, zinnias, dahlias, apple and crabapple trees, pumpkins, squash, watermelon, grapes, blueberry, and too many more to count. Also broad-leaved trees and shrubs such as rhododendron, dogwood, and crepe myrtle.

Powdery mildew spreads via wind and water splashed on leaves. It causes a dusty, grayish-white mold to grow on plants. It starts as fuzzy white circles, then expands to cover entire leaves, stems, petals, and eventually the entire plant so that it looks like it’s been sprayed with a grey dust. Before the white fuzz even appears, foliage can be discolored and distorted. On some types of plants that’s the only sign of the disease.

Unlike many fungal diseases, powdery mildew grows as happily in dry conditions as it will in wet.

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Powdery mildew on phlox. Photo by Elizabeth Bush, Virginia Polytechnic Institute and State University,

When does it occur?

The fungus itself starts growing on leaf debris once daytime temperatures reach 60 degrees F. When the weather is humid, the fungus grows. Then, when the weather is less humid, the fungus spreads. Humid days and cool nights are perfect conditions for powdery mildew.

Is it harmful to plants?

Somewhat. It won’t kill them, but it can stunt growth, decrease flowering, and possibly reduce winter hardiness. When it comes to ornamental plants, what people find most objectionable about powdery mildew is that it’s just plain ugly.

Is it treatable?

It is easier to prevent powdery mildew than it is to control it once it shows up. There is no cure. Here are some practices that can help avoid it:

  • Look for plant varieties that are resistant to powdery mildew.
  • Plant in full sun.
  • Provide adequate air circulation. Don’t jam plants together, or plant them where there isn’t sufficient airflow.
  • Avoid using too much fertilizer. Young, succulent growth is most susceptible to powdery mildew.
  • Avoid overhead watering, which wets the foliage and can cause disease spores to splash up from the soil against the leaves. It also raises the humidity around plants.
  • Practice good garden sanitation. The disease spores overwinter on plant matter left in the soil, so cutting down and cleaning up debris from affected plants can reduce future outbreaks. Don’t put any of it in your compost pile.
  • If you notice it early on only a few leaves, it's possible to stop the fungus from spreading by removing the affected leaves and any plant debris lying around.

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Powdery mildew on a lilac leaf. Photo by Whitney Cranshaw, Colorado State University,

What about chemical control?

There are many fungicides, both organic and synthetic, that treat powdery mildew. They should be applied as a preventative before disease symptoms appear, or started the minute you see the first fuzzy evidence, because once powdery mildew gets a foothold, it’s tough to control. Prompt action won’t reverse the damage, though it can slow its spread. Remember that pollinating insects can be harmed by fungicides.

A good source of information on resistant plant varieties and selecting and safely using fungicides is the Clemson University Cooperative Extension fact sheet Powdery Mildew (HGIC 2049).

I don’t love my phlox so much that I would turn to fungicides. In fact, I’m simply tearing most of it out and replacing it with perennials more suitable to my growing conditions.

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