Powdery mildew
Dusty white-to-gray circular patches are powdery mildew, the signature disease of rex begonias.
Diagnosis
Powdery mildew
What's happening
Powdery mildew is a fungus that thrives in the exact conditions rex begonias enjoy — warm temperatures, high humidity, and still, stagnant air. It starts as small powdery white spots on the upper leaf surfaces and stems, then spreads into larger patches that distort and weaken the foliage if left unchecked. Poor airflow and crowding let it move fast from leaf to leaf.
How to fix it
Isolate the plant from your others immediately. Remove and discard the worst-affected leaves with sterilized snips, then treat the remaining foliage with a fungicide such as neem oil, coating tops and undersides and repeating every 7–10 days until it clears. Crucially, fix the conditions that invited it: improve airflow with a gentle fan, space the plant out from neighbors, avoid wetting the leaves when watering, and water in the morning so any moisture dries quickly.
What fixes it
- Neem oil for pests — Neem oil works as a gentle fungicide for powdery mildew — coat both leaf surfaces and repeat weekly until it's gone.
If that doesn't fix it
This is general guidance based on common symptoms; individual plants vary.
Read the full Rex Begonia care guide →
Reviewed June 2026 · how we check this