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. Edema also occurs on woody plants such as camellia, hibiscus, and yew when soil is waterlogged and transpiration is impaired
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) Growers may confuse these symptoms with 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 referred to as intumescence. High light, high humidity and poor air circulation is 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 only mild symptoms of edema often recover. As soon as more favorable growing conditions occur, 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 and are best discarded.
Edema is often confused with two-spotted mite or thrips feeding damage on ivy geranium. As mites feed on ivy geraniums, the plants develop edema-like symptoms that often spread to the youngest leaves. Stippling from mite feeding is not seen on ivy geraniums. To distinguish mite feeding injury from edema, use a 10 x to 20x hand lens, to look on the underside of leaves for the two-spotted mites. (Photo 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)
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.
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 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 on drip watering systems, where all plants in the same line are watered, whether or not they all need to be watered. 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.
Growers can try to prevent edema by changing some of their cultural practices. 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. Water when air temperature is rising and humidity is low. Do not water susceptible varieties or crops on cloudy days.
Reduce humidity levels in the greenhouse by heating and venting in the evening and early 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.
Because ivy geraniums are so susceptible to edema, the following tips may help reduce this disorder on them.
- 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.