Plant Problem Diagnostic
Powdery Mildew: How to Identify and Manage It
The short answer
Powdery mildew shows up as grayish-white, powdery patches on the surfaces of leaves, stems, and buds, as if the plant were dusted with baby powder. It affects a huge range of plants, from squash and cucumbers to zinnia, phlox, bee balm, and peony. Unlike most leaf-spot fungi, it does not need leaf wetness to infect, so the least-toxic priorities are cultural: space and prune plants for good air circulation, avoid overhead watering, remove infected leaves, and choose resistant varieties where available.
Least-toxic sprays such as sulfur, copper soap, or a Bacillus subtilis biological can protect healthy tissue, but no fungicide reverses existing infection. If mildew spreads rapidly on a valued plant despite good culture, or you are unsure of the diagnosis, contact your local Extension office before escalating.
How to identify powdery mildew
Look for white to gray powdery spots, blotches, or felt-like mats on the tops of leaves and on stems and buds. Heavily infected leaves may yellow, distort, and drop.
- White, powdery coating on upper leaf surfaces
- Spreads in warm conditions and does not require leaf wetness
- Affects both edibles and ornamentals like phlox, bee balm, and zinnia
Least-toxic management, in order
- 1Air circulation — Space plants, prune for airflow, and site them in sun to help leaves dry and slow spread.
- 2Avoid overhead watering — Water at the base to keep foliage and the surrounding humidity lower.
- 3Remove infected material — Pinch off and dispose of infected leaves to reduce spore load.
- 4Resistant varieties — Choose mildew-resistant cultivars where available for susceptible crops and flowers.
- 5Least-toxic sprays — Sulfur, copper soap, or a Bacillus subtilis biological can protect healthy tissue but will not cure existing infection.
When to escalate
Escalate to a protectant spray program only when a valued plant is repeatedly and heavily infected despite good spacing and sanitation. Because many different fungi cause powdery-looking growth, contact your regional Extension office if the diagnosis is uncertain before treating.
→ Fungicides prevent new infection but do not remove existing powdery mildew, so start any spray program before the disease takes hold.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Minnesota Extension — Powdery mildew in the flower garden — White powdery appearance, wide host range, spread without leaf wetness, and cultural plus least-toxic controls
- Clemson Home & Garden Information Center — Powdery Mildew on Landscape and Garden Plants — Air circulation, sanitation, resistant plants, and that fungicides do not cure existing infection