Sun scald (flora)

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.

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.

When sun scald appears on trees it is most frequently a result of reflected light off the snow during winter months. The damage in this case will appear as sunken or dead bark on the trunk of the tree, then later in the tree’s life the bark might fall away revealing dead tissue in the tree's cambium layer. This damage will typically be found on the south west facing side of the tree’s trunk. It can be found on other sides of the tree if there is light reflection from other sources, like man made structures or reflective rock faces. After a tree is afflicted by sun scald it becomes much more vulnerable to decay organisms. The plant will create walls around the affected area, but sometimes it is not enough to block the infections. The leaves of the tree are also affected by sun scald, particularly on a bright sunny day following a period of warm cloudy humidity. The damage to the leaves will start as bronzing of the epidermis between the veins of the leaf, and if the sunny conditions persist the tissue of the leaf will die.

Sun scald on fruit appears when a fruit is exposed to direct sunlight after being in the shade for an extended period of time. The damage to fruit will often lead to the death of the fruit by consumption from insects, animals, bacteria, or fungi. This is the case if the defenses of the fruit are rendered useless, which is the case when the outer skin is damaged to the point that the cell walls are either gone, or so thin that the plant’s enemies can get through it. Some times the sun scalding is a more internal damage and, although it destroys the fruit in a productive sense, the fruit is able fend off attacks and recover if placed back into an ideal setting.

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Wikipedia