Powdery Mildew
Powdery mildew is a fungal disease that coats leaves and shoots with a white-to-grey powder.
Powdery Mildew at a glance
| Also known as | White mildew |
|---|---|
| Affected parts | Leaves, shoots, buds |
| Most at risk | Crepe myrtle, oak, maple, azalea, many broadleaf in humid, shady, still air |
| Main cause | Fungi that thrive in warm days, humid nights, shade, and poor airflow |
| Severity | Mild–moderate; distorts growth and weakens the tree if chronic |
| Reversible? | Yes — treatable and preventable |
What to look for
- Leaves: White or pale-grey powdery patches, usually on the upper surface first (sometimes underneath and on stems); new leaves may cup, curl, twist, or develop purple/reddish blotches; heavy infection yellows and drops leaves.
- Whole tree: New growth, shoot tips, and flower buds most affected; a dusty white look across young foliage; reduced flowering.
Overview
Powdery mildew is a fungal disease that coats leaves and shoots with a white-to-grey powder. Unlike most fungi it doesn't need wet leaves — it thrives in warm days with humid nights, shade, crowding, and stagnant air. It distorts new growth, reduces flowering, and weakens the tree over time. Some species (notably crepe myrtle) are especially prone.
Causes
Warm, humid, shaded, crowded conditions with poor airflow; shade-grown or over-fed soft growth; susceptible species/cultivars.
Commonly confused with
Mealybugs/woolly insects are discrete cottony lumps that squash; mildew is a flat powdery film. Downy mildew grows on the underside with yellow top blotches. Spray residue or hard-water marks rub off without spreading. True mildew spreads and reappears.
Which species are affected
Many broadleaf species; classically crepe myrtle, plus oak, maple, azalea, rose-family trees, and others; worse on shaded, crowded plants.
Treatment
Improve light and airflow immediately; remove badly infected leaves and prune to open the canopy. Spray sulphur or potassium-bicarbonate fungicide, a wettable-copper, neem, or a baking-soda solution (~1 tsp/quart water), repeating every 1–2 weeks through the susceptible season. Avoid wetting foliage late in the day. Don't over-feed nitrogen.
Prevention
Give full sun/appropriate light and good airflow; don't crowd trees; choose mildew-resistant cultivars where possible; avoid soft over-fed growth; sanitise tools.