Edema on the underside of a Peperomia Leaf

March 23, 1996; updated 10/29/00 7:12:16 PM

Edema (Oedema). Small, wartlike, sometimes corky swellings form on the leaf tissue of many plants - cabbage, tomatoes, geraniums, begonia, camellias, peperomia, citrus, etc. due to the uptake of more water by the roots than can be given off by the leaves, Certain cells become engorged with water and swell.

This condition is more common from late fall into early spring especially with houseplants and greenhouse crops. Houseplants frequently arrive in the Grand Junction, Colorado office (CSU Extension) during this winter period with leaves having rust like spots on the underside of the leaves, or leaves mottled with patches of various greens as well as water-soaked blemishes around enlarged dark-green to black cells.

Rust spots are commonly thought to be caused by rust fungi that invade the plant through stomates or wounds. Water-soaked spots are frequently caused by bacteria that invade houseplant leaves via the same openings. A common winter problem, however, that shows the same symptoms, however, is not caused by a fungus or bacteria but is due to an environmental problem called edema.

Edema is a condition that results when the plant takes up more water through the roots than the leaves can give off in the form of water vapor through the stomates. The excess water accumulates in the leaf cells, causing them to enlarge and often burst. As the cells become engorged with water, swellings may appear on the upper or lower surface of the leaf. The afflicted cells often have a dark green bacterial-like water-soaked appearance surrounding these swollen cells. Thin leaves as found on Schefflera and Citrus may have a mottled appearance.

Peperomia, rubber plants and other indoor plants with thick leaves normally develop edema on the underside of the leaves as shown on the accompanying photograph. The first indication of the problem is usually dead rust-colored spots. As the problem intensifies, the underside of these leaves will be covered with small dots or patches of dead rust-like spots. Houseplants with thin leaves will often develop water- soaked spots but close examination will reveal dark raised water-engourged cells within this water-soaked halo. The upper surface of these leaves will often be mottled or have indentations above the engourged cells that appear on the lower surface.

Prevention Edema is best prevented by increasing the temperature and ventilation around the plant to increase moisture loss through the stomates. It is not, however, advisable to move the plants into direct sun or leaf burning may occur. As a rule, a rise in temperature of the air surrounding the plant will increase the rate of transpiration and thus help prevent engorged cells and blisters from forming. Increasing the ventilation also hastens transpiration by lowering the humidity at the leaf surface. One should, however, avoid cold drafts.

Plants that are overwatered are highly susceptible to edema. At the same time, plants that are properly watered but located in low light or cool areas are equally susceptible to this problem.

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This page was developed by Curtis E. Swift, Ph.D. at cswift@coop.ext.colostate.edu Comments are welcome.