Powdery Mildew Tree Disease

How to Eliminate Powdery Mildew in Your Landscape

Powdery mildew on rose foliage
Powdery mildew on rose foliage. (Mark Turner/Getty Images)

Powdery mildew is a common disease that appears as a white powdery substance on a tree leaf surface. The powdery appearance comes from millions of tiny fungal spores, which are spread in air currents to cause new infections. It attacks all kinds of trees. Trees most commonly affected by powdery mildew are numerous, but the most common are maple, basswood, dogwood, magnolia, crabapple, catalpa, and oaks.

 Almost any tree or shrub can get powdery mildew.

Identification

Powdery mildew is seen as white to gray powdery spots, blotches or felt-like mats on leaves, stems, and buds. The infection can be most visible on young leaves and new shoots. The infected plants may look to be covered with baby powder or cobwebs.​​

Severely infected leaves may turn yellow and fall early during the growing season. Leaves will begin to turn purple to red during the infection. As the disease progresses during late summer and early fall, tiny round orange to black balls form within white fungal mats.

When outdoor conditions turn to cool temperatures with high humidity, you will see an increase in disease symptoms. This mildew disease is more severe on tree parts in shaded areas with poor air movement.

Prevention

Powdery mildew is a product of moist conditions and is usually seen in the wetter spring and fall seasons.

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 the tree for effective air movement. The fungus is rarely a tree killer but can disfigure specimens in the landscape.

Control

If necessary, spray with wettable sulfur at the recommended rate specified on the label. Sulfur may injure tender foliage, especially in hot weather, so be careful. Do not use sulfur on walnuts, as injury may occur. Other fungicides can also be used under certain conditions including baking soda. Check with your local extension agent for these chemicals and application rates.