This month we are taking a look at two physiological disorders commonly seen on foliage plants here in south Florida during the colder months. Physiological disorders are not caused by pathogens, but by environmental factors instead. Because these disorders are not caused by biotic agents, they are not infectious and will not spread.
Oedema (edema) occurs when roots take up water faster than it can be used by the plant or transpired through the leaves. Water pressure builds up in the mesophyll or internal cells of the leaf causing them to enlarge and form tiny swollen blister like areas. The swollen blisters are usually round, and can become corky with age. The tissue under the surface of the lesion, however, often remains green. Oedema lesions can resemble insect galls or scale insects, but scales can be scraped off of the plant surface while oedema blisters cannot.See above for a photo of oedema on orchid, and below for oedema on sapodilla (front and back of leaf).
Oedema is most common during winter months in south Florida (November-March) especially during extended periods of cooler, cloudy weather. The cooler air temperature slows transpiration and water evaporation by the leaves, and so excess water in the warmer soil is absorbed by the roots faster than it is lost from the leaves. Several tropical plant species are very susceptible to oedema. For example, Ixora, Peperomia, and some orchids may show symptoms 
of oedema any time throughout the year due to favorable conditions (i.e. excessive water, low air movement, etc.).Oedema lesions are permanent, but under favorable growing conditions, affected plants will produce healthy new growth. Although it may be tempting to remove affected leaves as soon as lesions appear, these leaves are still able to photosynthesize and so removing them may be harmful to the overall health of the plant. Once sufficient new growth is present, the older blistered leaves can be pruned out. The most important step in preventing oedema is to make sure plants are not over watered, especially during cooler weather. Water in the morning, as daily temperatures are rising. Spacing plants further apart to allow more air flow on leaf surfaces can also reduce oedema. For potted plants, avoid accumulation of water in saucers and use well-drained soil mix.
Mesophyll cell collapse is another physiological disorder that occurs after exposure to low water or air temperatures. The low temperature directly damages the mesophyll cells of the host, but symptoms may not appear for 6-8 weeks after damage occurs. Symptoms are localized or large areas of sunken or yellow spots on the foliage. These areas may become brown and necrotic with age, and saprotrophic (not pathogens) fungi may colonize the dead tissue. New leaves are more susceptible than older leaves. On orchid species, temperature of 35-45°F and water that differs by more than 25°F from leaf tissue can cause mesophyll cell collapse. Some studies also suggest that extreme fluctuations in day/night temperatures may lead to the condition.
Mesophyll cell collapse can be difficult to diagnose because symptoms develop many weeks after the damage occurs. If you suspect that the symptoms you are seeing are due to mesophyll cell collapse, you can check temperature records for the area or greenhouse for the last six weeks and look for cold temperatures or extreme fluctuations in high/low daily temperatures. Also, the pattern of symptoms seen can be clue as to whether the damage is due to mesophyll cell collapse. If the damage is due to a single cold temperature event, symptoms will be seen on a few newer leaves, but growth developing after symptom development should be healthy.
Like oedema, damage from mesophyll cell collapse is permanent, but several steps may be taken to prevent or minimize future damage. When watering plants, take care that water is not too cold (more than 25°F/4°C different than plant tissue). Sensitive plants may need extra protection from cold temperatures (heaters and physical protection from cold winds).
Several IFAS extension publications exist for these two physiological disorders as well as other disorders caused by environmental factors:
Publication #PP265 Physiological Disorders of Orchids: Mesophyll Cell Collapse. R.A. Cating and A.J. Palmateer.
Publication #PP244 Physiological Disorders of Orchids: Oedema. R.A. Cating, A.J. Palmateer, C.M. Stiles, P.F. Harmon and D.A. Davison.
Publication #PP248 Guidelines to Idenitification and Management of Plant Disease Problems: Part 1. Eliminating Insect Damage and Abiotic Disorders. Monica Elliott, Ken Pernezny, Aaron Palmateer, Nikol Havranek.
Also, here is a thorough treatment of abiotic diseases on houseplants from the American Phytopathological Society:
APSnet Feature: Non-infectious diseases of common houseplants. January 2001.
