Q • I have a white coating on my zucchini giving it a cloudy appearance, and I’m worried it is going to spread to my other plants. What is it, and how can I stop it?
A • The fungus that causes this cloudy appearance is aptly named powdery mildew and is a common disease of many plants, including peonies, squash and zinnia.
Fortunately, powdery mildew is highly specialized to a specific host, so you don’t have to worry about powdery mildew transferring from your squash plants to your peonies. It is generally considered a seasonal inevitability, and most plants are able to stay healthy and productive despite its appearance; however, if you are seeing a lot of it, there are some steps you can take to slow its progress.
Prune off leaves as they develop the disease to help slow reinfection of new leaves, and increase air circulation by thinning or pruning. Fungicides that contain sulfur are effective in preventing powdery mildew development, but they will not cure the disease once it appears. A 40% dilute solution of milk thoroughly applied every one to two weeks can help to reduce the severity of powdery mildew.
For more information on controlling powdery mildew, check out our fact page here: bit.ly/2WoE7kT.
Write to the Missouri Botanical Garden’s Center for Home Gardening at [email protected] or the Horticulture Answer Service, 4344 Shaw Blvd., St. Louis, MO 63110.
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