Edema (oedema), a common physiological disorder, appears as bumps, blisters or crystal-like growth on the undersides of lower or older leaves on many types of greenhouse plants in the spring. Edema is not a disease like a bacterium or a virus, and it is not transmittable from one plant to another.
Edema commonly occurs on ivy geraniums, and cultivars vary in their susceptibility. It also occurs on sweet potato vine (ipomoea), begonias, cacti, ferns, palms, pansy, cleome and greenhouse tomatoes, as well as cole crops such as broccoli, cabbage and cauliflower. Some succulent plants such as jade and peperomia are particularly susceptible, but almost any broadleaved plant may be affected.
According to research conducted by Kansas State University in 2009, edema blisters form on ivy geraniums when water and solutes build up underneath or possibly within cells, causing the epidermal cells to stretch and collapse. The cells do not rupture, as was previously thought.
Bumps may 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, especially on ivy geraniums. To rule out pest damage, growers are advised to use a handlens and check carefully on the undersides of leaves along midveins for spider mites and in growing points for thrips.
Mildly affected plants often recover from edema, eventually putting out symptomless new growth in late spring and early summer. Some plants are severely affected, however, dropping significant numbers of leaves and having badly distorted remaining leaves. These plants are probably not worth saving, as they will not recover in time for sale.
In the past, growers had been advised to fertilize ivy geraniums once every three feedings with calcium and potassium nitrate to thicken the cell walls, making plants more resistant to edema. Research by Kansas State University reported that supplemental calcium did not have any effect on edema on ivy geraniums in any of their experiments.
According to traditional beliefs, edema on ivy geraniums occurs when the growing media remains warm and moist and the greenhouse air is cool and moist. The plant roots absorb water at a faster rate than is transpired through leaf cells, causing the leaf cells to rupture. Long-standing suggestions for managing edema included the following:
Smith is a frequent contributor to Greenhouse Management and an extension floriculture specialist for the UMass Extension Greenhouse Crops and Floriculture program.