Identifying and Controlling Powdery Mildew on Trees

Powdery mildew is a disease common in trees that appears as a white powdery substance on the surface of leaves. The powdery appearance comes from millions of tiny fungal spores, which are spread in air currents to cause new infections. Powdery mildew attacks all kinds of landscape plants, including trees. Fortunately, although the disease is disfiguring, it rarely kills a tree. 

Almost any tree species can be affected by powdery mildew, but the most common are maple, basswood, dogwood, lilac, magnolia, crabapple, catalpa, and oaks. 

Identification

Powdery mildew disease is caused by many different species of fungi, with Erysiphe cichoacearum reported to be the most common culprit. 

According to the University of Minnesota: 

• Powdery mildew appears as superficial growth on plant surfaces and is seen as white to gray powdery spots, blotches or felt-like mats on leaves, stems, and buds.
• Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.
• The disease is often most severe on young leaves, water sprouts, and green shoots.
• Once severely infected, leaves may turn yellow and fall prematurely during the growing season.
• In some plants, leaves turn purple to red around the infection.
• In late summer/early fall, tiny round orange to black balls form within white fungal mats.
• Most prevalent when outdoor conditions consist of cool temperatures with high humidity; however, it can be seen in warm, dry conditions as well.
• The disease is most severe on plants or plant parts in shaded areas with poor air movement (interior or lower branches).

Biology of the Fungus

Some powdery mildew fungi survive winter inside structures known as chasmothecium, which contain the spores. In spring, the chasmothecium rupture to release spores that are then spread by the wind. Other species of powdery mildew survive the winter as a dormant fungus in the infected buds or shoot tips. In spring, these spores start new infections on new plant growth. As the growing season progresses, news spores are produced and transferred to new plants on the wind. 

Prevention

Powdery mildew is rarely a tree killer, but it can disfigure specimens in the landscape. It is a product of moist conditions and is usually seen in the wetter spring and fall seasons. In many areas, powdery mildew is virtually unavoidable during the most humid parts of the period from spring through fall. Once dryer weather returns, the fungus usually retreats.

It may not be necessary to treat the fungus at all, but certain measures may prevent it from becoming prevalent. This humidity-loving fungus can be controlled only if moisture can be controlled. Don't plant trees in heavily-shaded areas and provide plenty of space for air movement and growing room. Prune trees and shrubs to improve air movement between the branches. Additional methods for controlling powdery mildew:

• Choose disease-resistant varieties whenever possible. Mildew-resistant cultivars are available for many plants.
• Do not overcrowd plants. Adequate spacing improves air circulation and reduces powdery mildew infection.
• Prune the tree or shrub to increase light penetration and improve air circulation throughout the canopy. But avoid excessive pruning of infected plants—do your pruning during inactive periods. 
• Avoid fertilizing trees and shrubs when they are suffering from powdery mildew. Fertilizing stimulates new growth and may hasten the spread of the fungal infection. 
• Do not compost infected branches or leaves. The spores will remain in the compost and may infect other plants. 

Controlling Powdery Mildew

Commercial fungicides will kill powdery mildew, but many experts advise using these toxic chemicals only on specimen plants that are highly prized since the fungus rarely kills trees. A somewhat effective non-chemical treatment is to spray the plants with a mixture of household baking soda and water.