Sun scald

Sun scald is the freezing of bark following high temperatures in the winter season, resulting in permanent visible damage to bark. Fruits may also be damaged. In the northern hemisphere, it is also called southwest injury.

Causes

The reason the sun can cause so much damage to trees is because of dormancy. When a tree is dormant in the winter it can be reactivated by warm weather. In the northern woods trees are exposed to the most sunlight and heat on the southwest facing side, so this side is heated during warm sunny winter days to the point that it can be awoken from dormancy. The temperature required to wake up a tree depends on plant species and length of day, but it is typically just above freezing. Once active, the cells on the southwest side of the plant are unable to return to dormancy by nightfall, at which time the temperature returns to levels capable of killing active cells. Fluctuating winter temperatures can also cause frost cracks, which result from the expanding and contracting of the tree trunk.