Wednesday, Jul. 01, 2009
Master Gardener: Early detection key to fighting powdery mildew
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closeQ: Large white spots seem to be forming and covering some of the leaves of my zucchini plant. What might they be and what should I do about it?
—Daniel Cocagne, San Luis Obispo
A: To nongardeners the expression “zucchini pests” refers to well-meaning neighbors who set bags of their excess summer squash by your front door when you’re not home. I mean, everyone can grow zucchini, right?
Actually, while relatively easy to grow, summer squash, like every other plant, does have a variety of natural pests. Particularly annoying is powdery mildew, a common leaf-infecting fungus. There are several species, each preferring a specific plant host.
On squash and cucumbers, among others, the disease first shows up as white, powdery spots on leaves. These are actually thin layers of fungal tissue as well as spores, which are carried by wind to other plants to spread the disease. The spots enlarge and destroy the leaf, exposing the vegetables to sunburn, leading to reduced yield, and just making the plant really ugly.
Unlike most fungi, the spores of powdery mildew can germinate in the absence of free water and, in fact, regular sprays of water may inhibit its development. This fungus thrives in the warm, dry conditions of the Central Coast.
Prevention is the best method of control. Plants should be located in a well-ventilated area with full sun and watered and fertilized appropriately. The area should be kept clear of dead and damaged plant debris that can serve as a source of infection.
Fungicides can be used if necessary, but control may be impossible if extensive damage has occurred. Horticultural oils and biological fungicides can be protective if used before damage has occurred. Dusting with sulfur is sometimes helpful. Contact your Master Gardeners for advice before applying any fungicides and for further discussion of disease prevention.
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