White Spots on Oak Leaves: Likely Powdery Mildew

Cosmetic Fungus on Wisconsin Oaks

The white patches starting at the leaf center and spreading outward—and the whitish spots on other trees—are classic powdery mildew. It’s a superficial fungus that rarely harms healthy, mature oaks.

What to Do

• Confirm by comparing with online images of powdery mildew on oak.
• Rake and destroy fallen leaves in autumn to reduce overwintering spores.
• No fungicide needed unless the tree is a high-value specimen; then apply a labeled protectant at bud break next spring.

Your other species (beech, ash, birch) are unaffected because powdery mildew is host-specific.