Sunscald of hardwoods

Micro-habitat(s)

Distribution

North America

Damage, symptoms and biology

Sun can be a tree's best friend or, under adverse conditions, an enemy. At some times of year, it can cause sun-scald cankers on the stem or trunk and large branches of many hardwood trees; hardwoods are those trees that loose their leaves in winter. Sun-scald occurs during both winter and summer when portions of the cambium are killed due to temperature stresses; the cambium is the very thin layer of cells directly under the bark through which the water and nutrients flow between leaves and roots. Kill the cambium and the bark over it dies and falls off. Trees with thin, dark-colored bark are especially prone to this damage.

Summer sun-scald is heat injury to the exposed bark during the summer months. The bark will die and a canker forms around the damaged area. The wound or cankered area is an excellent location for decay and other harmful fungi to enter the tree and cause further damage or permanent defect. Winter sun-scald is injury from rapid changes in bark temperature during cold sunny winter days. Exposed darker bark becomes much warmer on the sunny side of the tree than the air temperature during the afternoon, but cools very rapidly after sunset. These temperature changes can result in bark injury that can also result in canker formation. Both summer and winter sun-scald injuries tend to occur on the south to southwest sides of the tree. That is because this is the direction from which the warm afternoon sun comes. Trees are most susceptible when they are growing in the open, when nearby trees or structures are removed causing new exposure, or when they are moved from a shaded nursery or other location to an exposed site.

Other information

Keep this fact in mind when moving or transplanting a tree from one location to another and try to keep sun exposure the same in the new site as it was in the old. I would even go so far as to mark the north side of the tree before moving to ensure it continues to face that direction in its new location. Since cankers or dead bark will reduce water and nutrient flow to the tree top, crown dieback is usually associated with this damage.

Winter sun-scald can be prevented or reduced by wrapping the trunk with a commercial tree wrap, plastic tree guards or other light-colored material. The wrap will reflect the sun and keep the bark at a more constant temperature. Put the wrap on in the fall and remove it in the spring after the last frost. Newly planted trees that could be susceptible because of location, should be wrapped for at least two winters and thin-barked species, like maple, up to five or more winters.

Summer sun-scald can be reduced by painting stems white. You will often see this treatment on trees in fruit orchards. I do not recommend this treatment for urban trees but if you feel you must, be sure to use a product that will not kill the bark. For example, avoid oil-based paints. Visit your local garden center and ask for a product designed for this purpose. If your tree has suffered from sun scald, remove all dead bark immediately that it becomes evident. Use a sharp knife and carefully cut it back to living tissue in the shape of a vertical oval. Do not treat the excised area with wound dressing or paint. This often does more harm than good.