Oedema

Oedema in plants is the extended swelling in plant organs caused primarily by an excessive accumulation of water. This occurs since the cell walls are composed of flexible cellulose. Mildly affected plants often recover from edema, putting out symptomless new growth, with the arrival of more favorable growing conditions in late spring and early summer.

Symptoms
Symptoms of edema appear as bumps or blisters initially on the undersides of lower or older leaves on a plant. They may then turn brownish or tan and become corky. Severely affected leaves will often turn yellow and fall off the plant. The corky spots sometimes resemble spider mite or thrips damage.

Prevention
Use a well drained growing medium. Increase light intensity by spacing plants farther apart. Avoid over-fertilizing plants, especially when the plants are growing slowly and avoid growing cultivars that are highly susceptible. Do not over water, and keep plants on the "dry side" during extended periods of low light and cool temperature. Water when air temperature is rising or humidity is low.