Troubleshooting · Powdery mildew
A fine white dust coating the leaves — a fungal problem that thrives in warm, still, humid air and spreads faster than most gardeners expect.
Powdery mildew is a fungal disease that thrives in warm, humid, still air. That combination is what most backyard gardens deliver in late summer. It spreads fast but is manageable if caught early.
Care profile
Likely causes
Each cause below has a quick check you can do in about thirty seconds. Stop as soon as one matches. You’ve found your answer.
Dense foliage, crowded plants, and still air trap humidity against leaves, giving the fungus its ideal conditions.
Quick check
Are plants touching each other or pressed against a wall?
What to do
Prune to open up the canopy. Space plants further apart. Outdoor beds benefit from a single well-timed thinning cut more than any spray.
Leaves that go to bed wet will still be wet at 10 p.m. when the temperature drops, creating exactly the film of moisture the fungus needs.
Quick check
Are you watering from above, and doing it in the afternoon or evening?
What to do
Water at the base of the plant, in the morning, so the soil absorbs it and any stray leaf water dries off by noon.
Some plants (especially squash, bee balm, phlox, and lilac) are so susceptible that even with perfect culture they'll get mildew every year.
Quick check
Is this plant known for powdery mildew every year regardless of conditions?
What to do
Replace with a mildew-resistant variety next season. For the current plant, apply a weekly baking-soda spray (1 teaspoon per quart of water with a drop of dish soap) or neem oil for mild cases.
This page is beginner-friendly general guidance, not professional horticultural, medical, or veterinary advice. For pet-exposure questions, contact the ASPCA Animal Poison Control Center at (888) 426-4435. For persistent plant-health issues, your local university cooperative extension office is the best free expert in the country. See our full disclaimer for details.