Powdery Mildew on Oak Trees: What to Do

Managing Powdery Mildew on Large Oaks

Your oaks are dropping young green leaves with a gray powdery coating—classic powdery mildew. It’s a fungal disease that thrives in humid conditions with cool nights and warm days. On large trees, it appears late season and rarely causes significant harm because leaves have already done most of their photosynthesis.

Management Options

• No spraying needed for big trees. Fungicides only protect new growth; coating a 3-story canopy is impractical.
• Cultural practices: Prune to improve light and air flow. Rake and destroy fallen leaves to reduce spores. Avoid fertilizing unless a soil test shows a deficiency.
• If treatment is desired (small trees): Use fungicides like triadimefon, triforine, thiophanate-methyl, or propiconazole at first sign. A baking soda spray (1 tbsp soda + 2.5 tbsp oil + 1 gal water + ½ tsp soap) every 5–7 days can help.

Monitor next year—outbreaks are weather-dependent and may not recur.