Boulder County Master Gardeners

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How to tackle powdery mildew

By Gregory Robl, Colorado State University Master Gardener

August brings the sultry “dog days” of summer when even watering the garden seems a task in the oppressive heat. The tomatoes, squash, cucumbers and melons are soaking up water and sunlight, and if they didn’t need my attention, I’d likely just stay in the hammock, read mysteries, and sip cold drinks. But I want to see the literal fruits of my labors, so I’m sticking with the garden work.

A couple weeks ago I welcomed a heavy rain, and watching the rain pour dpowdery mildewown for the better part of two hours, I could nearly feel the plants’ thirst finally slaked in the way that a garden hose cannot replicate. As the days got hot again, I continued the watering routine. After a week, I checked to see how the squash, cucumbers, and melons were doing. The fruit was all tucked under leaves the size of dinner plates—shaded on the cool, damp soil. When I looked a couple days later I noticed the leaves were no longer forest green, and the surfaces did not shimmer in the sunlight. A gray colored powder covered most of the tops of the leaves.

Yes, powdery mildew had befallen my garden. To my disappointment, the undersides of the leaves were infected, too. Powdery mildew is common, and it can be severe in hot, dry climates. I added it all together: the heavy rain that day plus the hot and sunny days that followed, and, well, it made sense. But I can’t blame the weather exclusively. My gardening practices were conducive to a powdery mildew infection.

I had planted my vegetables and melons in mounded circles, and the pine trees nearby shade the vegetables during the middle of the day. When the seeds first germinated, it looked like the small plants would have plenty of room to grow, but they were, in fact, too close together.

Although powdery mildew thrives in the heat, it needs high humidity, too, and the microclimate in my vegetable garden was just perfect for it: I had watered the plants regularly, so the soil was consistently damp; the leaves had grown large enough to shade the soil and each other; and by watering with the garden hose, I’d left water drops on the plant surfaces. All were contributors to higher humidity in the garden.

But, what to do now? To reduce relative humidity, I’ll stop overhead watering, and then I’ll selectively cut out some of the affected foliage so that the air can circulate better. Because the fungus prefers young, succulent growth, I won’t apply any more nitrogen fertilizer this season in order to inhibit new growth.
Even though the fungus is developing mycelium which is threads of fungus, the mildew will not actually grow into the plant tissues, but will remain on the surface only. Altering my cultural practices should both prevent the fungus from developing further and preclude the use of chemicals.

I’ll likely enjoy a decent crop of vegetables and melons later this month, and next year I’ll know better how to prevent powdery mildew. It’s hot, so for now, I’ll roll up the garden hose and head back to my police mystery and iced lemonade.

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