Oedema or edema, also known as intumescence. This physiological disorder usually appears during periods of low light – from now through the fall, winter and into the spring. If you’ve noticed salt-like crystals, odd tumour-like growths or water-soaked spots on either side of your plant leaves this disorder could be the culprit (Figure 1). The disorder affects a wide variety of tropical plants and succulents. It can also affect greenhouse crops such as lilies, cyclamen, sweet potato vine, geranium, begonia, petunia, calibrachoa, ornamental peppers and other Solanaceae family plants.
The damage caused by oedema or intumescence can look very similar to thrip or spider-mite feeding damage. However, scouting will usually reveal low numbers or an absence of these pests. Typically new growth is fine once the source of the problem has been corrected, but older leaves may have to be removed.
The root cause of this disorder is still up for debate and it seems to be species specific, meaning that certain cultivars are more susceptible. There are two main environmental conditions that are thought to be the cause. The naming convention is related to the cause of the disorder, so uncertainty about the cause is the reason why the two names are often interchanged.
- Oedema: Low transpiration rates
Transpiration is related to the relative humidity in the greenhouse. If the relative humidity is high, the plant has a hard time transpiring. A recently irrigated plant in this environment will have roots that readily take up water, and leaves that cannot release it since the air around the plant is already very humid (Figure 2). Humidity is related to temperature, so this can also happen when the media the plant is grown in is warm and wet, and the air surrounding the plant is cooler.
These conditions are perfect for the formation of oedema. The water will pool in leaf tissue giving a swollen like appearance until cells become either malformed (crusty salt-like appearance) or the area around them collapses leaving a necrotic spot. Depending on the plant species, oedema can be seen on both the top and bottom of affected leaves.
- Intumescence: Lack of UV-B light
This is directly related to lower light levels seen during the seasons when this disorder typically presents. The lack of UV-B light (mainly provided through the sun) due to overcast weather and/or insufficient on supplemental lighting options is usually the cause of cell malformation or necrotic spots in this case. Here, the disorder presents more often on the top side of leaves where the light would typically hit the plant canopy, but it has been seen on lower leaf sides as well. Tropical plants appear to be particularly sensitive to a shortage of UV-B light.
Make sure your climate control system is in good working order. Regardless of what you grow, this is a good practice to get winter ready. Check humidity sensors, heating pipes, fans, irrigation lines and alarms to ensure everything is working well.
Watch the relative humidity closely! Make sure that irrigation is cut back in a humid greenhouse on cool days. Use fans and open vents to keep air circulating throughout the greenhouse. The combination of warm, wet media and stagnant cooler, humid air is the perfect recipe for oedema formation.
Ensure your supplemental lighting has some UV or Blue+FR wavelengths. There are many lighting solutions to help get the daily light integral that your plants require. Since intumescence is thought to be caused by a lack of UV-B light especially, sunny days are the easiest way to get this requirement in. When Mother Nature doesn’t want to co-operate supplemental lighting can help (Figure 3). HID lights, including HPS and plasma fixtures, typically offer some UV wavelengths. There is a fine line between just enough and sunburn, so bulbs or fixtures may have protective coverings. In terms of LED fixtures, new research is showing that a combination of Blue and Far Red (FR) wavelengths are able to reduce the risk of intumescence injury in tomatoes, but the jury is out on whether this holds up in other crops. If you’ve had problems with intumescence in the past, it might be worth looking into whether your crop is receiving the light it needs.