Powdery Mildew

By Jeff Rugg

February 15, 2012 4 min read

Q: Last summer, I planted several varieties of squash and cucumber that were supposed to be resistant to powdery mildew. They did OK for a while, but then the mildew did a pretty good job of killing most of the leaves toward the end of the season. What did I do wrong?

I really like fresh summer squash and want to plant them again this summer, but I am beginning to give up hope of a successful crop. As I plan my garden for this year, do you have any suggestions?

A: You did the right thing in getting varieties that were resistant to powdery mildew. If you know you have had a problem in the past, it is wise to search for varieties that are resistant. The problem is that the word "resistant" doesn't mean they never are going to have the problem. It just means that they are going to withstand better than nonresistant varieties.

In some years, having resistant varieties may be enough to prevent all or almost all of the disease. Last year in your area, the conditions were proper for a heavy dose of the disease; your plants did better than those without resistance.

Diseases need three things to cause a problem. First, the disease spores must be present. There needs to be a susceptible host and then the proper environmental conditions for the spores to infect the host. If the conditions are proper for only a day or two, then the disease may not infect many leaves. But if the conditions are proper for days on end, then nonresistant plants may be killed and resistant plants may be infected. The longer the conditions are good for the disease the more of a problem it will become.

Rotate your vine crops to a new part of the garden this year, but keep them in as much sun as possible. They are more prone to mildew in the shade. Water them with more efficient drip irrigation so the leaves stay as dry as possible. Monitor your plants for the disease, and if only a leaf or two become infected, you can cut them off. But you can't keep doing this all season.

You can spray the top and bottom of the leaves with Bayer Advanced's Natria multi-insect control. That may sound strange, but the product label also states that it can be used as a powdery mildew control on fruits, vegetables, roses, shrubs, houseplants and ornamental plants.

Because squash are also susceptible to several insect problems -- from squash bugs and cucumber beetles to aphids and spider mites -- this is an excellent product to use on squash and many other vegetable garden plants. Because the active ingredient is canola oil, it can be used by organic gardeners. It can be used as a dormant oil spray, as well as an in-season oil spray. It even works on insect and mite eggs.

Last summer, I gave several gardeners samples of this product, and they all reported having success against mildew on cucumbers. The infection did not spread to new leaves that were sprayed.

Jeff Rugg's weekly column, "A Greener View," can be found at creators.com.

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