Powdery mildew - Outdoors
Click for larger image Powdery mildew on lilac (Syringa)

Powdery mildew is caused by a fungus and is seen as a light gray or whitish powder on the surface of leaves. It occurs following warm days and cool nights often being seen in the fall and spring. The disease is considered more unsightly than harmful. Death of the plant is rare.

Symptoms and Diagnosis

Even though each species of powdery mildew attacks only a narrow range of hosts, there are 11,000 species of the powdery mildew fungi, and many ornamentals are hosts. Superficial powdery patches appear on leaf surfaces, young stems, flowers, and even fruit. The powder is composed of mycelium and colorless chains of spores on upright stalks. Later there may be dark "pepper-like" spots among the powder (the spots are spore-producing bodies). As the disease progresses, leaves may be dwarfed, curl, turn yellow, and drop off. Flowers may be deformed. Fruit crops may be reduced, with the fruit misshapen and covered with powdery patches.

Life Cycle

Powdery mildew commonly winters over as mycelial mats in dormant buds or on plant stems and fallen leaves. During humid and warm spring weather (with cool nights), infected buds open and spread conidia (spores) to new host tissue. These conidia do not require free moisture in order to germinate as many fungi do. A new generation may be produced every 72 hours, if conditions are right.

Integrated Pest Management Strategies

1. Purchase resistant varieties. This is the best strategy for avoiding powdery mildew.

2. Pruning. Prune out diseased tissue and destroy it. Gather and destroy dead leaves and stems in the fall. This is the source of next season’s inoculum.

3. Keep plants in good vigor. Space the plants for good air circulation. Give the plants plenty of sun, as too much shade seems to make the problem worse. Try to avoid overhead watering and do not handle the plant when the foliage is wet as this will spread the disease. Water early in the day.

4. Fungicides. Use fungicides as a preventive treatment before a rain for maximum protection. Spray both sides of leaves and flowers well with sulfur, mancozeb, maneb, chlorothalonil (Daconil), cinnamaldehyde (Cinnamite), or copper-containing fungicides.

Organic Strategies

Strategies 1, 2, and 3 are strictly organic approaches. Of the fungicides mentioned in Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.

More images:

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Powdery mildew on squash (Cucurbita)
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Powdery mildew on summer squash (Cucurbita pepo 'White Bush Scallop')
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Powdery mildew on summer squash (Cucurbita pepo 'White Bush Scallop')
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White patches on the older leaves of this summer squash (Cucurbita pepo 'White Bush Scallop') are caused by powdery mildew; the mottling on the newer, inner leaves is caused by a virus
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Powdery mildew on phlox (Phlox)
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Powdery mildew on phlox (Phlox)
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Powdery mildew on phlox (Phlox)
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Powdery mildew on phlox (Phlox)
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Powdery mildew on phlox leaves and stem (Phlox)
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Witches'-broom on ninebark (Physocarpus) caused by powdery mildew
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Witches'-broom on ninebark (Physocarpus) caused by powdery mildew
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Powdery mildew on underside of cherry leaf (Prunus) showing the dark perithecia (cleistothecia) or fruiting bodies
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Close-up of powdery mildew on cherry leaf (Prunus) showing the dark perithecia (cleistothecia) or fruiting bodies
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Close-up of powdery mildew on cherry leaf (Prunus) showing the dark perithecia (cleistothecia) or fruiting bodies
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Powdery mildew on underside of oak leaves (Quercus)
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Powdery mildew on azalea (Rhododendron)
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Powdery mildew on zinnia (Zinnia)
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Powdery mildew on flowering dogwood (Cornus florida). J. Hartman, UKY, Bugwood.org
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Powdery mildew on rose (Rosa). C. Calderon, USDA APHIS PPQ, Bugwood.org
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Powdery mildew on peony leaves (Paeonia)
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Powdery mildew on peony leaves (Paeonia)


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