My tree's leaves are covered in a white powdery substance. What is it?

The white powdery coating on your tree's leaves is almost certainly powdery mildew, one of the most common and easily recognized fungal diseases of trees and shrubs. It's caused by various species in the order Erysiphales, and different host trees have their own specific powdery mildew strains.

Commonly affected trees:

• Dogwood, crabapple, oak, maple, and sycamore are highly susceptible
• The fungus thrives in warm days (60-80°F) with cool, humid nights
• Unlike most fungal diseases, powdery mildew does NOT need wet leaf surfaces — high humidity alone is sufficient

The good news:

Powdery mildew is rarely fatal to established trees. It's primarily a cosmetic issue, though severe infections can reduce photosynthesis, stunt growth, and cause premature leaf drop on young or stressed trees.

Management options:

1. For established trees, treatment is usually unnecessary. The infection often clears up naturally as temperatures shift.
2. Improve air circulation by thinning crowded branches — see our pruning best practices.
3. Avoid excessive nitrogen fertilization, which produces the tender new growth that mildew loves.
4. For young or valuable specimen trees, fungicide sprays containing myclobutanil or potassium bicarbonate can be applied preventively.
5. In fall, rake and dispose of fallen leaves to reduce overwintering spores.

For more on leaf discoloration issues, check out Why Are My Tree Leaves Turning Brown or Yellow?.