Symptoms of Sunscald
Sunscald is characterized by an elongated area of dead bark typically found on the south or southwest side of tree trunks, branches, or both (Figure 1). The area may be sunken with dried, cracked bark that peels off to expose dead wood (Figure 2). Sunscald is also called southwest winter injury because it is commonly found on that side of the tree.
Recently planted trees, young trees, and trees with thin bark are more susceptible to sunscald. Deciduous trees are more prone to sunscald damage than evergreen trees because evergreens usually have lower branches to shade the trunk. Susceptible species include maple, linden, mountain-ash, honeylocust, birch, walnut, crabapple, flowering cherry, fruit trees, aspen, ash, tuliptree, Japanese snow-bell, and willow.
Cause of Sunscald
Sunscald happens during cold winter weather and is caused by sudden temperature changes of the bark. On a sunny, cold winter day, cold hardy tissues in the bark on the south to southwest side of the trunk are exposed to direct sunlight and warm up. The warmed bark deacclimates, decreasing its ability to withstand freezing temperatures. When the sun goes down or behind a cloud, the temperature of the bark drops quickly to below freezing and the bark tissues are unable to reacclimate or regain cold hardiness quickly enough to withstand freezing. Living bark tissues are damaged by the freezing temperature, which leads to sunscald injury (Figure 3). For more information on freezing damage, see the WSU Extension Home Garden Series on Cold Temperature Injury of Landscape Woody Ornamentals.
Research has shown significant differences in the temperature of the cambium on the north and south sides of trees. One study of fruit trees in New Hampshire during the winter revealed temperature differences of 50 to 55°F between the north and south side of peach trees and 30 to 35°F between the north and south side of apple trees (Eggert 1944).