Powdery Mildew

Powdery mildew is one of the most recognizable plant diseases. While it looks like someone dusted your tree with flour, it is actually a superficial fungus living on the surface of the plant tissue.

What causes the problem? Unlike many fungi that need rain to spread, powdery mildew spores are carried by the wind and thrive in high humidity combined with dry, warm weather—making the coastal and valley microclimates of Southern California ideal breeding grounds.

Long-term consequences: In mature trees, powdery mildew is rarely fatal. However, because it covers the leaf surface, it reduces photosynthesis. Severe infections can cause leaves to curl, turn yellow, and drop prematurely. On young trees or new growth, it can cause severe stunting and distortion.

Disease Profile

• Pathogen TypeFungus
• Primary TargetsSycamores, Crape Myrtles, Oaks
• Environmental TriggerHigh Humidity, Warm Days
• Key IndicatorWhite/Gray Powdery Dust
• LethalityLow (Aesthetic/Stress)

White Powder

Distinct white or light gray powdery spots appear on the upper or lower surfaces of leaves, eventually coating the entire leaf.

Distorted New Growth

Newly emerging leaves and shoots may appear twisted, curled, or stunted as the fungus attacks the tender tissue.

Premature Leaf Drop

Heavily infected leaves may turn yellow or brown and fall off the tree earlier than normal in the season.

What Can Be Done?

Managing powdery mildew involves changing the microclimate around the leaves and protecting vulnerable new growth.

• Canopy ThinningProfessional pruning improves air circulation and sunlight penetration, making the environment less favorable for the fungus.
• Fungicide SpraysPreventative treatments applied as buds break in spring can protect the new foliage before the fungus establishes itself.
• Balanced NutritionAvoiding high-nitrogen fertilizers that push excessive soft growth helps reduce the tree's susceptibility.