powdery mildew on squash (6974 bytes)

Powdery Mildew

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture, plant pathology

Powdery mildews are one of the most common and easily recognized diseases to occur in Colorado.

If you've noticed a white talcum powder-like growth on the tops of lilac, rose, columbine or squash leaves, you've probably seen powdery mildew. Later in the season, small black pepper-like structures may appear within the white powdery material. This is part of the powdery mildew life cycle. These structures serve an overwintering function and will survive on leaf material that falls to the ground.

Powdery mildew is a fungus that grows well in arid climates, such as Colorado's. It can occur on almost any plant including flowering plants, grasses, vegetables such as squash and cucumbers, trees, shrubs and even weeds. Fruit production on these plants may decrease slightly if powdery mildew is present.

Several weather or environmental conditions favor powdery mildew disease. Unlike most plant pathogenic fungi, the fungi that cause powdery mildew do not need water on leaves to infect the plant. It does, however, need relatively high air humidity. That's why the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Young succulent growth usually is more susceptible than older plant tissues.

Several practices will reduce or prevent powdery mildews. Before purchasing a plant, ask if it is powdery mildew resistant. If resistant varieties aren't available, avoid planting in low, shady locations.

Once the disease becomes a problem:

Powdery mildew is not always easy to control, but you can do several things to manage the disease.

  • Avoid overhead watering to reduce relative humidity.

  • Clean up and dispose of all leaves and vegetable debris that falls to the ground in autumn.

  • Increase air circulation. If plantings are dense, selectively prune to open the area up and reduce relative humidity.

  • Consider using a fungicide labeled for powdery mildew, but use in conjunction with the above-described practices. Follow exactly instructions on fungicide label. Several "alternative" pesticides work well to help prevent the disease. Sulfur dusts and horticultural spray oils are two non-toxic, effective alternatives that will protect uninfected leaves from the fungus. (Be sure to follow label directions; sulfur can burn leaves if used when the air temperature is above 90 F.) Synthetic fungicides such as Daconil 2787, Maneb and Zineb also can be used to protect foliage from getting this disease.

None of these products have great eradicative properties. So don't expect to take a severely infected plant, spray it and find that all mildew disappears. The products are meant to protect leaves from infection rather than to eradicate infections.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010