From BASF: The Chemical Company Website
By - Paul Pilon
Many growers observe edema (oedema) on various crops during production. Although some of the symptoms may resemble those observed with other disorders, edema is a non-infectious (not transmittable) physiological disorder and is not the result of insect feeding, plant pathogens, bacterium, or plant viruses. Growers most commonly associate the occurrence of edema with ivy geraniums; however, there are numerous crops such as begonia, cleome, cole crops (broccoli, cabbage, and cauliflower) Cuphea, ferns, jade, palms, pansy, peperomia, and sweet potato vine (Ipomoea) which are also susceptible to edema.
The occurrence of edema on most plants does not adversely affect its growth or development, but may detract considerably from its appearance and consequently may reduce its perceived quality and adversely affect its marketability.
Symptoms of edema vary depending on the plant species and the plant parts affected. Edema is often described as small water-soaked blisters, bumps, warts, or galls. Initially, edema develops as small translucent, fluid filled blisters on the undersides of lower or older leaves. As these blisters rupture, the exposed surface often turns rust-colored, brown or tan with a corky texture. From a distance, these corky spots may resemble mite feeding or thrips damage. Severely affected leaves on certain plants may become cup-shaped or turn yellow and drop prematurely. Plants with only mild symptoms of edema often recover; once more favorable growing conditions occur, the new growth generally develops normally.
Edema is essentially a bursting of cells walls and is caused by maintaining greenhouse environments that are not ideal for susceptible crops. Essentially, the plant roots are taking up water at a faster rate than the leaves are transpiring; this results in some of the plant cells to rupture while this occurs. Overwatering susceptible crops or keeping the growing media moist while the air temperatures are cool and humid is commonly associated with edema development.
The best strategy to prevent the occurrence of edema is to carefully manage the greenhouse environment while producing susceptible (sensitive) crops. Here are several steps that can prevent edema from occurring:
- When possible, select cultivars that are less susceptible to this disorder.
- Use a well drained growing mix.
- Provide a wide plant spacing to increase light intensity and air movement.
- Do not over-water. Keep plants on the dry side during extended periods of low light and cool temperatures.
- Increase air circulation around susceptible crops using horizontal air flow fans.
- Reduce air humidity by venting.
- Avoid over-fertilizing plants.
- Use calcium nitrate at least once every three feedings to help thicken cell walls, making the plants more resistant to edema.
Edema sensitive crops are commonly grown during the winter months or times of the year when the light levels are the lowest; therefore, plant transpiration rates are also low. During these times, the outside temperatures are the coldest and consequently, this often translates into lower greenhouse temperatures. Together these factors provide perfect conditions for edema to occur. High moisture, both in the air and in the growing mix, results in an imbalance between the water in the plant and water in the air, which causes the plants to transpire abnormally.
By understanding the environmental conditions which promote the development of edema growers can often manage these variables to reduce this physiological disorder from occurring and increase the appearance and marketability of sensitive crops.