Oedema, often spelled 'edema', is a physiological disorder that develops when a plant absorbs water faster than it can be lost normally from leaf surfaces. The term itself means 'swelling', which describes its initial stages. Excess moisture builds up in the plant and causes swellings that appear initially as pale-green or water-soaked blisters or bumps, primarily on the undersides of leaves. Although oedema mars the beauty of plants it rarely kills them.
The cells eventually erupt, then the spots turn yellow, brown, brownish-red, or even black. They often have a crusty or corky appearance, and can occur in irregular patches, or streaks, as well as distinct spots. Sometimes the spots may be mistaken for fungal rust, but these are not signs of a contagious disease.
Corky spots most frequently appear on the underside of leaves or veins. Older leaves are more affected than younger ones. Severely affected leaves may turn yellow and drop off (especially lower ones). In Ficus (the figs), a milky sap may leak out. Upon drying it leaves a brown spot on the raised area. Corky streaks may appear on plant stems and occasionally other parts.
While no plants are immune, fleshy-leaved plants are more likely to suffer from oedema than those with thin-leaves. It occurs in both indoor and outdoor plants, but is much more common indoors. Some indoor plants often affected are geranium (especially the ivy-leaved types), clivia or Kaffir lily, peperomia, jade plant, members of the Ficus family, schefflera, epiphyllum (orchid cactus), English-ivy, and begonia. Some outdoor plants such as vegetables in the tomato family and the cabbage family can also be affected.
Oedema may occur when plants have been over-watered during periods of high humidity. It can also occur when soil is moist and air is also moist or cool. These conditions are most likely to occur with houseplants indoors in late winter or early spring, especially during cloudy weather. Such conditions allow water to be absorbed by the plant but lost only slowly by means of normal transpiration through the leaf surface. Occasionally oedema does appear on plants that have been kept relatively dry and have grown under conditions of low humidity.
Leaves affected with edema never clear up, but several steps can be taken to prevent further symptoms in new leaves: