Edema

This disorder appears as numerous small bumps on the lower side of leaves or on stems. The "bumps" are tiny clusters of cells that divide, expand, and break out of the normal leaf surface. At first they form greenish-white swellings or galls. Later, the exposed surface of the swellings becomes rust-colored and has a corky texture. Edema is caused by overwatering, especially during cloudy, humid weather.

Control: Sidestep this disorder by watering less frequently, keeping the leaves dry, improving lighting on indoor plants, and improving air movement around the plants by avoiding overcrowding.