Edema (Leaf Blisters)
Small, wart-like bumps or water-soaked blisters, usually on the undersides of older leaves, are a physiological water-pressure disorder rather than a pest or disease — and it isn’t contagious.
Common causes
Roots absorbing water faster than leaves release it
Warm, wet soil paired with cool air, high humidity, or still conditions stops leaves from releasing water as fast as the roots take it up, causing cells to swell and rupture. Improve air circulation and let the soil dry a bit more between waterings.
Overwatering after a dry spell
Letting soil dry out fully and then giving a heavy, thorough watering sends a surge of water to the leaves faster than they can transpire it. Water more consistently in smaller, more frequent amounts instead.
Low light with high humidity
Dim, humid conditions slow transpiration even when watering itself is reasonable. Increasing light intensity, without moving the plant too abruptly, helps leaves keep pace with root uptake.