Powdery Mildew on Red Maple: Identification and Management

White Coating on Leaves = Powdery Mildew

The white/gray powdery substance on your red maple foliage is powdery mildew—a common fungal disease (multiple species) that grows on leaf surfaces, not inside tissues.

Symptoms and Progression

• White powdery patches on leaves, stems, buds.
• Patches enlarge, merge, cover entire leaf surfaces.
• Young growth most susceptible; may curl/twist before powder appears.
• Severe cases: yellowing, browning, premature defoliation.

Impact on Tree Health

Rarely fatal. Most mature trees tolerate annual infections with no long-term vigor loss. Primarily aesthetic concern. However, repeated severe infections can weaken young or stressed trees.

Management Options

Cultural (First Line)

• Improve air circulation via selective pruning.
• Avoid overhead irrigation—water at base.
• Rake/destroy fallen leaves (reduces overwintering spores).
• Plant resistant cultivars when possible.

Chemical (If Warranted)

• Begin at first sign of mildew.
• Spray 7-14 day intervals (shorter in cool, damp weather).
• Cover both leaf surfaces thoroughly.
• Use spreader-sticker for better coverage.
• Effective fungicides: triadimefon, triforine, thiophanate-methyl, propiconazole.

Decision Framework

If tree is mature, healthy, and unsightliness is tolerable → no treatment needed. If young, stressed, or high-visibility specimen → treat early and consistently.