Edema (oedema) is not a disease like a bacterium, or a virus and it is not transmittable from one plant to another. Edema is a physiological problem occurring mainly on ivy geraniums but is also found on sweet potato vine (ipomoea), begonias, cacti, ferns, palms, pansy, cleome and cole crop vegetables like broccoli, cabbage and cauliflower. Some fleshy leaved plants such as jade and peperomia are particularly sensitive to conditions which lead to the development of edema although almost any broadleaved plant may be affected.
Edema on leaf. Photo by Joseph LaForest, University of Georgia, Bugwood.org.Edema, is a bursting of the cell walls on the underside of the leaf. Symptoms of edema appear as bumps or blisters initially on the undersides of lower or older leaves on a plant. They may then 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. To rule out pest damage, 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, putting out symptomless new growth, with the arrival of more favorable growing conditions in late spring and early summer. Some plants however are severely affected, have dropped significant numbers of leaves and have badly distorted remaining leaves. These plants are probably not worth saving as they will not recover in time.
Edema is caused by maintaining a greenhouse environment that is not ideal for growing susceptible plants such as ivy geraniums, often combined with over watering. Edema occurs when the growing media remains 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. This rupturing of the leaf epidermis and the inner cells causes the raised, crusty appearance on the underside of the leaf.
There are several cultural practices that contribute to edema. First, most growers grow their ivy geraniums in baskets hung above the benches where the air is most humid and with poor air circulation that reduces the transpiration rate. Secondly, many growers are now using saucerless hanging baskets for their ivy geraniums. This type of container retains water after each irrigation including periods of cloudy and overcast weather which results in overwatering. Third, hanging baskets are often on automatic watering systems, all watered at the same time. However, not all hanging baskets dry out at the same rate and therefore, some plants are overwatered. Lastly, during cool, cloudy weather, humidity is high in the greenhouse and plant transpiration rates are low. These factors combine to create the perfect conditions for edema to occur.
What can growers do to prevent edema on susceptible plants? The main method is to carefully manage the greenhouse environment. Begin by using a well drained growing media. Increase light intensity by spacing plants farther apart. Avoid over-fertilizing plants, especially when the plants are growing slowly and avoid growing cultivars that are highly susceptible. Do not over water, and keep plants on the "dry side" during extended periods of low light and cool temperature. Water when air temperature is rising or humidity is low.
Anything a grower can do to improve drainage and air circulation around plants will help prevent edema. Reduce humidity by venting the greenhouse first thing in the morning, even if that means turning up the heat. Make sure there is adequate air flow, whether from fan jets or horizontal air flow fans. Air movement is important 24 hours a day. Do not use saucerless hanging baskets. Instead, use containers that have snap-on saucers, but do not put the saucer on until the crop is nearly finished, or if possible, until point of sale. This will ensure maximum drainage of each basket. When using an automatic watering system, place varieties with similar growth vigor on each line or section, again to eliminate over watering. Lastly, properly manage media pH and soil fertility. Make sure media pH for ivy geraniums is 5.5. Fertilize once every three feedings with calcium and potassium nitrate. Calcium will thicken up the cell walls, making ivy geraniums more resistant to edema.
Reprinted from "Tips for Managing Edema on Spring Crops" by Tina Smith, University of Massachusetts, Extension Floriculture Program http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/edema.htm
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