Also called:
southwest winter injury or southwest injury
Related Topics:
plant disease
scorch
Top Questions

What is sunscald?

How does sunscald differ from sunburn?

What factors influence sunscald occurrence and intensity?

Which plants are susceptible to sunscald?

How can sunscald be prevented and treated?

sunscald, common temperature-related injury of primarily young, thin-barked trees, shrubs, and other woody plants. The condition is usually characterized by elongated patches of dead bark forming on the sun-exposed trunk and branches of trees. The injured area may be discolored, sunken, dried, or cracked. In some cases the bark will peel, exposing underlying dead tissue in the damaged portion. Usually the wound heals naturally over time, although intervention can prevent and mitigate the damage. The symptoms of sunscald are similar to those seen in sunburn, another temperature-related injury in plants. Although the two terms are often used interchangeably, the mechanisms of injury underlying the two conditions are different.

Sunscald vs. Sunburn

Sunscald is primarily a winter injury, whereas sunburn occurs during summer. Sunburn is caused by intense sunlight and high temperatures, especially when young trees with thin bark are exposed to such elements suddenly and for long periods, often because of transplantation from low-light-intensity nurseries or pruning of neighboring trees and removal of shade. Because the two injuries look similar, knowledge of weather patterns during the approximate time of damage can help identify the root cause. Sunburn is more likely in places with mild winters.

Causes and consequences

In sunscald the cambium—the layer of actively dividing cells below the bark and between the xylem (wood) and phloem tissues—is damaged by rapid and short-term fluctuations in bark temperature. During cold but sunny winters, intense sunlight heats the bark and the underlying cambium of trees. A study of New Hampshire fruit trees found that the difference in cambial temperatures between the north (less sun-exposed) and south (more sun-exposed) sides of peach trees can be as high as 13 °C (about 55 °F). This was about equivalent to the temperature difference measured between the south sides of trees and the surrounding air. When the ambient temperature drops, either because of sudden cloud cover or the setting of the Sun, the bark and underlying tissues freeze rapidly, shrink, and crack. It is not clear whether cell death and tissue damage in sunscald are due to intracellular freezing and ice crystal formation or to the deacclimation (dehardening) of the heated plant tissue and a consequent reduction in its cold tolerance.

Damage mostly occurs on the south and southwest sides of trees in the Northern Hemisphere, where the incident sunlight is the most direct and intense during midday—hence, it is also called southwest winter injury. Similar injuries occur on the north and northwest surfaces of trees in the Southern Hemisphere, although reported instances are rare. Such factors as moisture stress, pathogenic fungal infestation, the cultivar planted, the age and size of trees, and frost-induced or physical damage to roots influence the occurrence and intensity of sunscald injury.

Besides stressing the tree, sunscald-damaged areas can act as entry sites for insects and pathogens (bacteria and fungi), which can lead to diseases. Calluses that form naturally to cover the wound may disfigure the wood and decrease its timber value.

Sunscald in Fruits and Vegetables

Sunscald injury is the term also used to refer to the damage caused when unacclimated fruits and vegetables are exposed to bright sunlight for long periods, either before or after harvest. Damage in the form of discoloration and necrotic spots can occur on the surfaces of fruits and vegetables, reducing their commercial value and causing economic losses for farmers.

Susceptible plants

Sunscald injury has been reported in aspen, ash, mountain ash, birch, honey locust, Japanese snowbell, linden, maple, tuliptree, willow, and many fruit trees, including apple, peach, cherry, crabapple, walnut, and plum. Young, thin-barked trees are the most susceptible, as are trees with heavily pruned lower branches. Trees that have been recently transplanted to open areas from nurseries where they were shaded by nearby trees are also vulnerable because of their sudden exposure to direct sunlight. Deciduous trees are more prone to scalding, whereas the trunks of evergreen trees are usually protected by their low-hanging branches and year-long foliage.

Treatment and control

After sunscald injury has occurred, removing loose bark, leaving the wound uncovered, and spraying fungicides when needed can aid in callus formation and wound healing. Controlling and preventing sunscald injury involves wrapping young tree trunks in reflective materials (commercial tree wraps); applying white latex paint (sometimes dissolved in water in a 1:1 ratio) or an antidesiccant (to retard the loss of plant moisture); avoiding heavy pruning; and planting susceptible trees in more protected locations. Although commercial wraps are widely recommended, they can harbor pathogens and pests if left on the trees for too long. Shading the south and southwest sides of trees (in the Northern Hemisphere) can also help. Research suggests that much sunscald injury can be avoided by watering trees thoroughly in dry autumns before the soil freezes and applying mulch to retain soil moisture and regulate its temperature.

Manjishtha Bhattacharyya The Editors of Encyclopaedia Britannica