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Integrated Pest Management Program

Department of Plant Science and Landscape Architecture, Department of Extension

Fact Sheets > Greenhouse > General

Non-infectious plant disorder -- Edema (Oedema)


Edema (oedema) is a common physiological disorder affecting a number of greenhouse crops including begonia, ivy geraniums, cactus, cleome, ivy, ipomoea and annual thunbergia. Broccoli, cabbage, cauliflower and tomato (leaves or fruit) can also suffer from this disorder. Houseplants with fleshy leaves such a jade, peperomia and schefflera may be prone to edema during favorable environmental conditions.


Symptoms vary depending upon the plant species or cultivar affected. Bumps, blisters or water-soaked swellings form on the underside of leaves. These blisters are at first small, about 1 to 2 mm in diameter. They then turn tan or brown and become corky. (See photo 1) Severely affected leaves turn yellow and drop from the plant. Sometimes, stems and petioles become infected.

On certain cultivars of Ipomoea (sweet potato vine) white, crusty eruptions resembling grains of salt, develop along the leaf veins. (See photo 2). Often, these symptoms are mistakenly thought to be an infectious disease. On susceptible cultivars of Cleome, tan eruptions form on the upper leaves, leaf petioles and stems. Affected leaves curl and become distorted. This disorder is also known as “ intumescence.” High light, high humidity and poor air circulation are reported to encourage intumescence (Photo 3). On susceptible cultivars of thunbergia, small, tan eruptions, resembling small insect galls, form on the underside of leaves. A yellow spot may be seen on the upper surface of the leaf.

Plants with mild symptoms of edema often recover. As soon as more favorable growing conditions occur in the late spring or early summer, new growth will recover. However, some plants may be so severely infected, with significant leaf drop and distorted growth that they will not be saleable so are best discarded.
Edema is often confused with two-spotted spider mite or thrips feeding damage on ivy geraniums. As spider mites feed on ivy geraniums, the plants develop edema-like symptoms that often spread to the youngest leaves. Stippling or flecking from spider mite feeding does not occur on ivy geraniums. To distinguish spider mite feeding injury from edema, use a 10 x to 20x hand lens, to look on the underside of leaves for the two-spotted spider mites. (See Photos 4, 5, 6) Edema can also be confused with thrips injury. Use a hand lens to look for the small, yellow thrips larvae on the underside of the leaves. As thrips feed upon the ivy geraniums, white scarring and leaf distortion may be noticeable, especially on the youngest leaves. (Photo 7)

Favorable Conditions

Edema is thought to be caused by an imbalance of the plant’s water uptake and water loss. It develops when the plants roots absorb water at a faster rate than it is transpired through the leaf cells. The enlarged leaf cells divide and then rupture. This rupturing of the leaf epidermis and inner cells causes the raised blisters commonly seen on the underside of leaves.

In the greenhouse, susceptible varieties of ivy geraniums often develop edema in the late winter or early spring. Several cultural practices may contribute to the development of this physiological disorder. Ivy geraniums are most often grown in hanging baskets above benches. In many greenhouses, this is where the air is most humid with poor air circulation that reduces the plant’s transpiration rate.

Many growers use hanging baskets without saucers, so that excess water often remains in the bottom of these containers. In addition, hanging baskets are often irrigated by use of drip watering systems, where all plants in the same line are watered, whether or not they all need to be watered. In this situation, it is easy for a few plants to be over watered.

During cool, cloudy weather conditions, humidity levels are high whereas transpiration rates are low. So, environmental conditions are ideal for edema to develop, even when growers modify their cultural practices in an attempt to prevent this disorder.


Changing these cultural practices may help to prevent oedema.

  • Select a growing medium that drains well.
  • Space plants further apart so they receive more light.
  • Keep plants on the dry side during cool, cloudy growing conditions.
  • Avoid over-fertilizing plants, especially on susceptible cultivars that are growing slowly.
  • Water when air temperature is rising and humidity is low.
  • Do not water susceptible varieties or crops on cloudy days.
  • Do not use saucerless hanging baskets.
  • Use containers with snap on saucers that you put on just before crop is sold.
  • Reduce humidity levels in the greenhouse by heating and venting in the evening and early in the morning.
  • Use horizontal air flow (HAF) fans to keep air moving in the greenhouse.
  • Place plants with similar water needs on the same irrigation line to reduce the probably of over watering.

Here are some specific tips to help reduce oedema on ivy geraniums.

  • Keep your pH levels at 5.0 to 5.5.
  • Feed once every three feedings with calcium and potassium nitrate. (Calcium helps to thicken up cell walls, making the ivy geraniums more resistant to edema.)
  • Maintain moderate light levels, of 4,000 foot candles (maximum) that are lower than the light levels preferred by zonal geraniums.
  • Avoid high daytime temperatures.
  • Keep good records, so you know which varieties you grow are most susceptible, under your growing conditions. It may be possible to substitute a more resistant variety.

Table 1: Susceptibility of Ivy Geranium cultivars to edema

Most susceptible


Most resistant


Madeline Crozy

Sugar Baby



Double Lilac White

Balcon Princess


Salmon Queen

King of Balcon


Sybil Holmes

Balcon Imperial



Balcon Royale


Beauty of Eastbourne

Table 1. From: White, J. W. (Ed) 1993. Geraniums IV. Ball Publishing. Batavia, IL.  412 pp.

Table 2: Susceptibility of Ivy Geranium cultivars to edema

Most susceptible


Most resistant





Blanche Roche

Van Gogh







Plants grown in Rutgers University Greenhouses. Grown and evaluated in a glass greenhouse. Table 2. from:  Wulster, G. 1996. Minimizing Edema (Oedema) Problems on Ivy Geraniums. Rutgers Cooperative Extension Fact Sheet.

Leanne Pundt, Extension Educator, University of Connecticut, Revised June 2011

Averre, C.W. and R.K. Jones. Edema. General Principles Information Note 3 North Carolina State University.

Heimann, M.F. and G.L. Worf. 1996. Plant Disorder: Oedema. University of Wisconsin Extension, Cooperative Extension  2 pp.

Smith, T. 2004. Tips on Managing Edema on Spring Crops. UMass Extension Floral Notes. March-April 2004. 16(5): 3-4.

Smith, T. and J. Bartok. 2003. Reducing Humidity in the Greenhouse.

Troubleshooting Edema. Oglevee Articles

White, J. W. (Ed) 1993. Geraniums IV. Ball Publishing. Batavia, IL 412 pp.

Wulster, G. 1996. Minimizing Edema (Oedema) Problems on Ivy Geraniums. Rutgers Cooperative Extension Fact Sheet.

By: Leanne Pundt, Extension Educator, University of Connecticut, Revised 2011

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.