Sunburn and Sunscald

It is a quirk of nature that sunburn and sunscald have opposite causes, but the damage looks much the same! Both typically result in a vertically elongated strip of cambium killing, manifested as a strip of dead, peeling bark and ultimately exposed, dead wood. In the northern hemisphere, the damage appears on the south to west sides of the tree.

Sunburn

Sunburn usually occurs in the heat of summer. It is due to extreme heating by a combination of ambient temperature and solar insolation, and is more likely under moisture stress or high temperatures. It is quite common on planted landscape trees, but much less common in natural forests ​[1]​. It can damage any aerial plant part, but the most long-lasting damage is to bark.

It may take some months after injury to bark before the damage becomes apparent ​[3]​. The bark becomes discolored, later dries and begins to crack, then starts to peel and eventually leaves a strip of bare, dead wood. As the tree continues to grow, the damaged area appears sunken and there may be callus rolls on the sides. Sunburn is most common on the exposed lower stem, but if the crown is open and thin, damage may extend to the upper stem and even branches.

Sunburn often occurs when trees grown under shaded, protected conditions are exposed suddenly to direct solar radiation they are not acclimated to ​[2, 3]​. Often this is because trees in nurseries are closely spaced and perhaps intentionally shaded. When they are outplanted, exposed bark is vulnerable to sunburn. It could also occur if shading objects, such as other trees, are removed from the south to west sides of the tree. Damage often appears on the southwest side in the northern hemisphere because temperatures are usually highest when the sun is in that direction.

The likelihood and severity of sunburn are relatively high:

• in young saplings;
• in species with thin bark;
• when exposure of trees to direct sunlight is increased;
• under drought stress;
• in hot, dry weather periods (heat waves);
• in trees planted in mid to late spring or summer (best planting time is autumn to late winter).

Sunburn at base of Fraxinus sp.

Sunburn on the southwest side of Acer x freemani. Most trees in a large street planting had such damage, most of which occurred a few years before these photos. There was some branch dieback and mortality.

Sunburn on the southwest side of Acer x freemani. Most trees in a large street planting had such damage, most of which occurred a few years before these photos. There was some branch dieback and mortality.

Acer x freemaniwith sunburn of the upper stem, in the canopy.

Acer x freemani with sunburn and presumably consequent dieback. This view is from the north; stem damage is on the south-southwest side. Most trees in a large street planting had sunburn, most of which occurred a few years before these photos. Only some sunburnt trees had dieback; mortality was rare.

Schizophyllum commune associated with sunburn injuries on young Acer x freemani (maple cultivar).

Schizophyllum commune (split-gill fungus) associated with sunburn injuries and killing bark and sapwood beyond the injury on young Acer x freemani (maple cultivar).

Sunscald

Sunscald occurs in winter. It is killing of cambium and phloem of stems or branches, typically due to warming by solar exposure followed by rapid and deep freezing.

Like sunburn, it is especially common when exposure of tree stems to the sun increases, such as removal of trees to the south/west, road construction, or a blowdown event. This suggests that trees can acclimate to the conditions leading to sunscald.

Sunscald occurs on cold, sunny days when solar exposure heats the bark. The warmed bark deacclimates, decreasing its ability to withstand freezing temperatures ​[3]​. When the sun goes down or behind a cloud, the bark freezes rapidly. The rapid and large temperature drop kills the tissues. It is also reported that the reverse sequence can cause it, freezing night temperatures followed by unusual warming during the day ​[2]​.

Sunscald is recognized long after by long, narrow scars, mostly below the live crown, on the southern or southwestern side (in the northern hemisphere).  There is often evidence of a recent disturbance near the affected trees on that side that increased exposure, such as harvesting of adjacent trees. It typically affects groups of trees and is most severe on trees with thin bark.

Differential Diagnosis

If sunburn and sunscald cause damage that looks the same, how do you tell them apart? Consider the circumstances. If you’re in a wildland forest, you’re very likely looking at sunscald. If you’re in a place where winter temperatures are mild, or the trees are recently planted, you’re likely looking at sunburn. If you’re seeing the first evidence of symptoms, that should give you a clue as to whether it was winter or summer damage. Consider weather patterns that occurred at the approximate time of damage. Otherwise, the diagnosis may remain uncertain.

Consequences of Sunburn and Sunscald

Ideally, the tree produces callus on both sides of the dead strip, seals the dead wood, and life goes on. But life is often not ideal, is it?

Wood borers, canker pathogens and decay fungi may enter through such lesions. Wood borers are insects that, uh, bore through wood. They contribute to its decomposition and facilitate invasion by decay fungi.

Decay fungi that could invade such injured areas are many and varied. They could remain restricted to the dead strips of wood, or they could invade further, either expanding the area of dead bark or creating an extensive column of decay inside the tree.

One of the common invaders of sunburn injuries to street trees seems to be Schizophyllum commune, the ‘split-gill fungus’. This fungus causes a ‘wound decay’ in a modern classification of decay diseases, and a ‘sap rot’ in the traditional system. It is ubiquitous in the northern hemisphere ​[4]​. It frequently infects fresh wounds of various sorts and also invades cut branches and stems while the sapwood is still alive. It can be an important pathogen in fruit orchards ​[4]​. It apparently can invade further beyond the wounded area, especially when the tree is stressed.

Some canker-causing fungi secondary, opportunistic pathogens that don’t do much to vigorous trees, but can take advantage of any weakness. They kill tissues when they can. A good example is Cytospora canker. Such fungi can infect through sunburn or sunscald injuries, and cause significant further damage when the tree is stressed.

Prevention

Providing light shade and reducing temperature (for sunburn) will reduce the likelihood of damage, but are rarely practical.

Increasing the albedo of exposed stems decreases absorption of solar radiation. A simple and effective way to do that is to paint it with whitewash, made by mixing white latex paint and water 50:50 ​[2]​. Wrapping stems in light-colored material can do the same ​[3]​.

Where sunburn or sunscald are a concern, postpone pruning of lower branches for a few years to maintain shade on the lower stem. Keep soil moist during hot periods by irrigation and/or mulching ​[2]​.

More general information is available on abiotic diseases and injury.