Sunscald is a condition which strikes deciduous trees planted in cool and high altitude regions. When a tree is sunscalded, it causes injury or death to tissues in the tree, resulting in damage to the bark and underlying tissue. Depending on the severity of the damage, a tree may simply look unsightly, or it may die. Fortunately, there are several solutions which can be used to avoid or reduce sunscald, which is also sometimes called southwest disease.
A combination of sun and extremely low temperatures cause sunscald. Sunny winter days stimulate growth of the cells in the tree, which are normally dormant during the winter to avoid injury. When night falls and temperatures cool down, the cells die off, creating a layer of dead tissue. The condition typically manifests first in the form of cracking or sunken bark, which eventually falls off to reveal a layer of dead tissue in the tree trunk.
In the northern hemisphere, sunscald most commonly appears on the southwest side of trees, because this is the side which will be exposed to sun for the longest period of time in the winter. In the southern hemisphere, of course, this condition manifests to the northwest. After a bout with this disease, a tree will have a scar in its bark, with the underlying tissue being clearly visible.
One way to prevent sunscald is to wrap trees in the winter. While the wrapping will not insulate the trunk, it will reflect the rays of the sun. If a tree is wrapped, the bark should be brushed down first to remove loose pieces of bark and insects, and the wrap should be removed in the early spring so that it does not girdle the tree during the spring growth period. Some people prefer to paint their trees with reflective white paint to avoid sunscald, especially in the case of commercial orchards.
Some gardeners prefer to arrange their gardens in ways which will reduce or prevent sunscald. For example, an evergreen shrub could be planted on the southwest side of a deciduous tree to provide shade, ensuring that the rays of the winter sun will not fall on the bark of the tree. Gardeners can also choose to plant evergreen trees instead of deciduous ones; the year-round foliage of evergreens protects the bark from sunscald.
In extreme cases, sunscald can kill a tree, but in most instances it simply looks unsightly. Gardeners who decide to remove damaged trees may want to consider using measures to prevent sunscald if they replant in the same spot.