Powdery Mildew 🦠
Fungal — order Erysiphales
At a glance
- Cause: Fungal, favored by moderate temps + high humidity + poor airflow
- Tell-tale sign: White, dusty, powdery patches on leaf surfaces
- Severity: Moderate — unsightly, weakening, rarely fatal
How to identify
Distinctive white-to-grey powdery patches that look like a dusting of flour or talc, usually starting on upper leaf surfaces and spreading to cover leaves, stems, and buds. Affected leaves may yellow, curl, distort, or drop. Unlike most fungal diseases, it doesn’t need wet leaves — it thrives in humid air with poor circulation, including warm days and cool nights.
What causes it
A group of host-specific fungi that grow on the leaf surface. Crowded plantings, shade, stagnant air, and high humidity all encourage it. (The species on your rose isn’t the same as the one on your squash, but the look and management are the same.)
Treatment & management
Following Integrated Pest Management:
- Improve airflow & light: Space and thin plants; prune crowded growth; indoors, add gentle air circulation.
- Remove the worst-affected leaves and dispose of them (don’t compost).
- Avoid overhead watering late in the day; keep humidity from pooling around foliage.
- Low-toxicity sprays if needed: potassium bicarbonate, horticultural/neem oil, or sulfur (follow labels; don’t combine oil and sulfur or spray in heat).
- Choose resistant varieties where available.
Prevention
Good spacing and airflow are 90% of prevention. Site plants in adequate light; avoid over-fertilizing with nitrogen (soft growth is more susceptible).
Affects (in this guide)
Western Sycamore (some seasons) and many garden ornamentals, edibles, and the occasional humid-grown houseplant.