High Elevation Gardening Web Log "Blog"

Powdery mildew often appears as white, powdery blotches on the leaves, shoots and buds of the host plant. Plants are adversely affected by infection as the fungi interfere with photosynthesis. Leaves may turn yellow or brown and drop from the plant. Powdery mildew may also cause stunting and distortion of leaves and buds.

Fortunately, this disease rarely kills but repeated infections may weaken the plant.

Once plants are infected, powdery mildew can be difficult to control but there are many cultural practices that you use to reduce or prevent infection.

The simplest way to limit the disease is to grow disease-resistant cultivators or species. If you consistently have problems with powdery mildew on summer squash, try growing the cultivars ‘Sungold’ or ‘Sunray’.

If resistant varieties aren't available, avoid planting susceptible plants in shady locations. Plants should receive a minimum of 6 hours of sun daily. Mugo pine is susceptible to powdery mildew when planted in shade but rarely gets infected when grown in full sun.

Do not over fertilizer especially with nitrogen because this practice encourages excessive growth. Tender, succulent growth is more prone to infection.

Provide good air circulation by properly spacing plants and pruning when needed. Dense foliage creates pockets of high humidity which are for ideal for spore germination and infection.

Avoid overhead watering especially late in the day to reduce humidity around the plant.

In fall, cleanup and discard any infected material, and prune out all dead and diseased branches to reduce the amount of spores that can start infections next spring. Do not compost infected plant debris.

With established plants that are repeatedly infected, a spraying regiment may be required. Before spraying, consider whether spraying is even needed. On lilac, it is not necessary to control infection because the disease usually occurs late in the season and rarely has much affect on the overall health of the plant.

There are two types of powdery mildew sprays: preventatives that ward off infections and eradiacants that kill disease once it is established. A preventative spray will do little once infection has occurred and an eradicant will have no affect if sprayed before you see any signs or symptoms. Regular spraying of a preventative is required to avoid infection.

Sulfur dusts and horticultural oil sprays are effective treatments to prevent infection. Care must be taken when using sulfur as it can burn leaves on hot, sunny days. It should not be used in conjunction with an oil spray.

Research has shown that a simple fungicide made by mixing 1 Tbsp. baking soda, 2.5 Tbsp. horticultural oil and 1 gallon water can prevent infection. Use sparingly as excessive use of baking soda may damage plants and can have deleterious effects on soil structure.

Plant based insecticidal oils such as neem oil or jojoba oil can be used to eradicate mild to moderate infections.

Commercially produced, synthetic fungicides that control powdery mildew are also available. To be effective, these products must be used at the first sign of infection. Few products will cure a severely infected plant. A combination of the proper cultural practices in conjunction with a fungicide is the best way to achieve acceptable control.

Remember, when applying a fungicide, always read the label carefully and follow the instructions exactly. Make sure that the host plant is listed on the label.

By Hattie Braun
The author is the Master Gardener Program Coordinator for Coconino Cooperative Extension. If you have a gardening question, call the Master Gardener hotline at 774-1868 ext. 19 or visit our Web site: highelevationgardening.arizona.edu.