What is it?
Edema (or oedema) is a disorder that can occurs in orchids, houseplants, greenhouse plants, and other plants experiencing certain environmental conditions. It is usually nothing more than a cosmetic problem for the hobby grower, but can be costly to commercial growers, particularly those with greenhouse crops.
What does it look like?
Edema appears as the formation of small, see-through blisters that contain fluid. These blisters primarily develop on the lower side of older leaves, typically starting at the edges of the leaf. They can also occur on stems and occasionally on flowers. When observed against light, the affected areas appear lighter in color than the surrounding leaf tissue. Over time, the blisters may grow larger or merge, eventually bursting and leaving behind tan-colored scars with a cork-like texture. In some cases, the affected leaves may wither, curl, and eventually drop off. If extensive, the blistering and scarring may affect the plant’s ability to carry out photosynthesis.
What causes it?
Edema is caused by an imbalance in the water flow through the plant. Most plants lose 95% or more of the water they have absorbed through transpiration through the leaves. In situations where the transpiration is stifled without a similar reduction in uptake through the roots, the blisters may form. There are several environmental factors that can hinder effective water loss, such as very high relative humidity, low light intensity, cool air temperatures, and inadequate ventilation, especially right after watering. Using heat mats to compensate for lower air temperatures can be an issue, especially if the humidity is high. Treatments that include oils, such as neem-, or paraffinic insect-smothering oils, may also disrupt regular water loss enough to cause edema.
What can you do about it?
Edema is generally not fatal, but it does diminish the visual appeal of plants. Once the symptoms appear, there is no cure, so prevention is the key. Water plants early in the morning to allow the medium to drain before nightfall, especially if nighttime temperatures are significantly lower than days, as that is accompanied by high relative humidity. Decrease the frequency of watering during overcast periods or when there is low light intensity.