Powdery Mildew
A dusty white or grey coating across the leaf surface is a common fungal disease that thrives in still, humid air with poor circulation.
Common causes
Poor air circulation
Crowded plants with little airflow between them create the still, humid microclimate mildew spores need to germinate. Space plants out and run a small fan nearby.
Overhead watering
Wetting the foliage, especially in the evening when it stays damp overnight, encourages fungal growth. Water at the soil level instead.
Established infection
Once visible, mildew spreads readily to nearby leaves and plants. Remove affected leaves, wipe healthy ones with diluted neem oil, and isolate the plant until it clears.