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Rain brings new threat to gardensThank goodness for the rain. It has provided a refresher for the plants and a break in the heat for the rest of us. But don't start your rain dance just yet. With all of the extra moisture and high humidity another problem might be just around the corner. Powdery mildew is a fungal disease that affects many host plants including ornamentals, shrubs and trees. If left untreated powdery mildew can be severely detrimental to a plant depending on its variety, time of the infection and the weather conditions. Plant damage may range from an unsightly whitish powdery coverage on the foliage to drying out and browning of infected leaves. By diagnosing and treating the fungus in its early stages it is easy to control and the plant will be unaffected by this nuisance. Powdery mildew fungi obtain nutrients from infected plants through small root-like organs that penetrate the outer or epidermal layer of cells. Unlike other fungi, the powdery mildew fungi can attack when very little moisture is present on the leaf surface and even under low humidity levels. There are certain species of plants that are more susceptible to the disease than others. In this area of Oklahoma ash, crape myrtle, lilac, oak, photinia, roses and zinnia are all likely to be affected, and may be the first plants to show symptoms. Humid conditions with widely fluctuating temperatures between the day and night will increase the chances of powdery mildew flare-ups. It will also be more of a problem in crowded plantings that have poor air circulation or in shaded damp areas. Young succulent growth that could come from a fall growth flush is more likely to be affected than older plant tissue. There are several cultural practices that can be done to control powdery mildew. By pruning dense shrubs, like photinia, better air circulation can be achieved. This should be done by removing some of the interior branches of the shrub or thinning out crowded plantings. Dead leaves provide an excellent spot for the fungus to harbor. Disposing of infected or dropped leaves will help maintain a more sanitary environment for the plant. Homeowners can also help prevent powdery mildew by doing a little bit of research before purchasing plants. There are certain varieties of plants that have resistance to powdery mildew. If those varieties are not available, try to avoid planting in shady damp areas if possible. If these cultural tips fail, or are not aggressive enough to stop the disease, a fungicide may need to be applied. There are several products that homeowners can purchase. Be sure to read the product label and follow the instructions very carefully before applying. If powdery mildew has been a problem in previous years a spray application may need to be applied in the spring as new growth develops or in the fall as conditions become ideal for the fungus. With minimal maintenance and some attention to detail powdery mildew will not be a problem. But if left untreated, it can get carried away and make for a difficult scenario. So keep your eyes peeled for this garden nuisance. For more advice on this topic, or any of your gardening questions please feel free to contact the OSU Master Gardeners at 405-713-1125. 9/1/08 Date: 8/25/08
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