General Principles Information Note 3 (VDIN-003)
Charles W Averre, Extension Plant Pathologist
Ronald K. Jones, Extension Plant Pathologist (retired)

Edema (oedema, corky scab) is a common, noninfectious disease of many herbaceous and some woody plants grown outdoors and in greenhouses. Edema is often mistaken for an infectious disease or insect gall. The small edema spot generally do little damage to plants, but detract considerably from the appearance of ornamental plants and some vegetables and consequently may cause economic loss.

Plants very susceptible to edema include begonia, cabbage, cacti, eranthemum, ferns, geranium, jade, palms, pansies, peperomia, schefflera, violet, and tomato. The ivy and Irene cultivars of geranium are more susceptible than other types of geraniums. Edema may be a problem on the following vegetables: tomato foliage and fruit; bean foliage and pods; cabbage; broccoli; cauliflower; Brussels sprouts; potato foliage; most cucurbit fruit, foliage and stem. Probably all succulent tissues of vegetable crops are susceptible to edema.


Blisters of edema on the underside of leaves.

Symptoms of edema are variable and depend on the plant species, the plant parts affected, and tenderness of the tissue. Typically, symptoms appear on the succulent leaves, stems, flowers, and fruit of susceptible plants as single, minute, watersoaked "blisters," "wart," or galls. Symptoms are especially common on undersurfaces of leaves and occasionally will form on the larger veins and petioles. The single pimplelike blisters usually are about 1 to 2 mm in diameter, but frequently two or more spots will merge to form a line of spots or a large affected area form. Following rupture of the blisters, the exposed surface may become rust-colored, brown, or tan, with corky texture. Severely affected leaves of certain plants, such a geranium, may become cup-shaped or turn yellow and drop prematurely.

On cacti and other plants, pale yellowish green spots form on the shoots. These spots may remain smooth, greenish white, or watersoaked. However, these spots often result in irregular corky or rusty areas that may later become sunken.


Edema may be caused by any agent that stimulates an abnormal increase in the size and number of a group of inner cells. Edema can be induced by (1) spraying with some chemicals such as ammoniacal copper carbonate in an oil emulsion, (2) injuries resulting from wind-blown sand particles and sucking insects, (3) high light intensity (over 2000 foot-candles) for ivy geraniums, and (4) accumulation of water in the intercellular spaces.

The most common cause of edema is the presence of abundant, warm soil water and a cool, moist atmosphere. Under these conditions the roots absorb water at a rate faster than is lost through transpiration. Excess water accumulates in the leaf, some parenchyma cells enlarge and block the stomatal openings through which water vapor is normally released from the plant; thereby contributing to further water retention in the leaf. If this condition persists, the enlarged cells divide, differentiate a cork cambium, and develop elongate cork cells externally to form a periderm. The rupture of the epidermis by the enlarged inner cells and the periderm account for the raised, crusty appearance of older edema spots.


Changes in weather and cultural practices of growing plants usually will avoid edema. To reduce the risk of edema occurrence, the following may be helpful:

1. Avoid irrigation or watering during cool, overcast humid weather. For potted plants in greenhouses, remove saucers under pots, or discard any water that remains in the saucer 30 minutes after watering. Irrigate or water when air temperature are rising or humidity is low.

2. In greenhouses: a) reduce the humidity of the air by venting and increasing heat; b) improve air circulation; c) increase light intensity; d) space the plants farther apart; e) for potted plants use a well-drained potting medium for potted plants and avoid standing water in saucers under the pots.

3. Avoid overfertilizing, especially when the plants are growing slowly, such as during the winter months. Maintain fertility based on a soil test. Avoid low levels of potassium and calcium.

4. Avoid cultivars that are highly susceptible to edema under your growing conditions.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication but the status of registration and use patterns are subject to change by actions of the state and federal regulatory agencies. Last printed: 04/91

Published by North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted Nov. 2000 by Plant Disease and Insect Clinic