Get powdery mildew under control

Warm, sunny, dry days followed by cool moist nights; I like these weather conditions. To me, this is the perfect weather for being outside, for growing plants, and being able to sleep at night with the windows open.

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Unfortunately, these are conditions favored by one of the diseases that attack some of our ornamental plants. This disease is powdery mildew. Powdery mildew can be active from May until October, but is most prevalent in May and June.

Normally when we hear the word mildew, we think of high humidity and wet conditions. Powdery mildew likes the opposite conditions. It likes sunny, dry days followed by cool moist nights. This year with the dry weather and cooler nights in May and early June, powdery mildew had the perfect growing conditions. I have gotten many calls about this disease.

There are many different fungi in the powdery mildew group; some are quite host-specific, while others can infect a wide range of plants. The symptoms are a whitish, powdery growth on the top of leaves, and stems and flowers. This growth will attack new leaves and shoots as they are emerging, causing them to be distorted. Once a leaf or shoot has been infected, the fungus penetrates the outermost layer of plant cells to obtain its nutrients. This feeding is the cause of the leaf distortion. The white powdery substance is the mycelium or reproductive part of the fungus. The mycelium can be dispersed by wind or water to plants of the same species.

There are numerous plants that can be affected by powdery mildew, including:

- Houseplants: African violets and grape ivy

- Annuals: zinnias

- Perennials such as gerbera daisy, phlox and verbena

- Woody ornamentals: roses, crape myrtles and euonymus

- Trees: dogwoods and oaks.

If you have plants in the landscape that are susceptible to or have powdery mildew, what can you do to reduce the disease?

The first practice is to improve the air circulation around your plants. This can be accomplished by increasing the distance between plants. Most of the time, we crowd our plants at planting. By using proper planting distances, we can improve the air flow around the plants.

Also, you need to put susceptible plants out in the open more. You don't need to plant them back in corners where there is no air movement. Another way to improve air flow is to remove some limbs from trees and shrubs.

The second practice is sanitation. You can remove the infected parts of the plant to help deter the spread of the disease. This works very well at the first signs of the infection. Once a plant is covered with powdery mildew, this is not an option. At the end of the growing season, you can rake up the leaves that were infected and dispose of them to help reduce the incidence of the disease next year.

The third way to help deter powdery mildew infection of your plants is to use varieties of plants that are resistant to powdery mildew. There are a number of crape myrtle varieties resistant to this disease. These varieties all have Indian names such as Acom, Tuskeegee, Zuma, etc. These varieties are resistant to aphids as well. If you have older varieties of crape myrtles, you can make it into a tree form. By letting it take a tree form, this gets the foliage up higher, and the disease is not as noticeable.

The last option is chemical control. You can use a fungicide to help control powdery mildew. You need to apply the fungicide at the first sign of the disease, and then follow the application instructions on the label. The fungicide will protect the foliage that hasn't been infected by the fungus. Some of the fungicides that will control powdery mildew are chlorothalonil (daconil), myclobutanil, and triforine.

There are some organic products that you can use as well: sulfur, neem oil and jojoba oil. When you use any of these products, you need to follow the instructions on the label.

Powdery mildew is a disease that we see every year. Usually, it doesn't kill the plant, but can affect the way our plant looks for the rest of the year.

So next year get started early for control of this disease.

Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at charlesp@uga.edu. The extension's Web address is www.ugaextension.com/columbia.