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Preventing Powdery Mildew

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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved

Are things looking a little gray around your yard? Do your plants seem a little fuzzy as you view them? Is your lawn changing color before you eyes and losing it bright green color. If so, you likely have powdery mildew or at least your plants do. (No, people do not get powdery mildew.)

With record heat and record drought, how can there be such a problem with this fungal disease. On of our claims in Utah has always been that we have few fungal disease problems in Utah because of our warm, dry climate.

While that is true of most diseases, powdery mildew has made some remarkable adaptations to our desert climate. The reason is that the spores have adapted and can grow even when it is very warm and dry.

Most fungal spores land on the leaf of the plant and then must have free moisture on the leaf for a period of four to six hours. The moisture can be in the form of the dew or sprinkler irrigation. The spore then has time to grow and infect the plant. Once the plant is infected, the disease is a parasite and gets it moisture and food from the host plant. The best method of controlling fungal diseases has always been to keep the plants dry.

So why is powdery mildew such a problem with our warm dry conditions? The disease has made a remarkable adaptation by being able to carry a few molecules of water with it as a sort of canteen. Unlike most other fungal spores, when this spore lands on a leave it has the ability to grow and infect the plant without needing any additional moisture. Therefore, the disease infects plants even under very dry conditions.

Since it is so prevalent, how can you protect your garden? First and foremost is to select plants that are not susceptible to the disease.

Highly susceptible plants include common zinnias, euonymus, Johnathan apples and apples that are crossed with this variety. Phlox, monarda, sweet peas, many kinds of roses and other flowers. Even lawns are starting to show the problem particularly in shady areas.

In the vegetable garden, peas, squash, melons and even tomatoes get the disease. Fortunately, it is not likely to kill the plants as it usually shows up late in the season.

At this time of year, the best approach is to do nothing as far as spraying fungicides. The cost and trouble are not worth it for because the plants will soon die from the frost or drop their leaves. If you have badly affected twigs on the woody plants, cut them off when you prune and clean the garden. Remove badly affected annuals, cut back the stems on damaged perennials, and discard the infected plants rather than trying to compost them.

Make note what plants have the problems. The easiest solution is to avoid the plants that get the problem. Select Profusion Zinnias or another mildew resistant variety. Remove roses that are highly susceptible to the disease and replace them with mildew resistant varieties. (Most shrub roses are resistant.)

Fungicide sprays are preventive. Apply those next spring on your apples and other plants that might develop a problem before they show symptoms.

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