   Sunscald of Trees - May 8, 2013 Jeff Schalau, Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County Have you ever seen dead or damaged areas on the trunk of a tree? I certainly see this often in orchards and landscapes that I visit. Sometimes trunk damage is caused by weed eaters, lawn mowers, or wildlife. However, it can also be caused by sunscald. Sunscald (also called “southwest injury”) occurs on the south or west side of the tree trunk. Recognizing the causes, symptoms, and taking early corrective actions can prevent sunscald. Sunscald is damage caused by rapid temperature fluctuations during the winter months. Sunscald occurs when bark is exposed to warm daytime temperatures followed by freezing temperatures at night. Research has shown that the south side of a tree can be as much as 77°F warmer on a cold winter day than the north side of the tree. The newly activated cells lose some of their cold-hardiness and are injured when temperatures drop below freezing during the nighttime hours. Resulting damage includes discoloration and cracking of the bark. The portions of trunks and branches facing south and southwest warm the most because they get the most direct sun and because they get sun later in the day when air temperatures are warmest. A mounting body of evidence suggests the sensitive cambium cells (and maybe some phloem cells) are killed because they cannot adjust to these rapid temperature fluctuations. Phloem is a zone of vascular tissue in the inner bark that carries food to where it is needed in the tree. The cambium is located just inside the phloem and outside the wood or xylem and is a thin layer of cells that divides to form new phloem to the outside and new xylem to the inside (see images in the Utah State University Cooperative Extension publication that accompanies the on-line version of this column). Mild symptoms of sunscald may appear as discolored reddish or brownish bark. More advanced symptoms are sunken and/or split bark which peels back exposing the wood in the trunk. Depending on the extent of the damage and time elapsed since the injury occurred, wood boring beetle larvae may cause additional damage. Reflective surfaces such as light-colored buildings, fences, and block walls can also contribute to sunscald. Over time, the damage can lead to dead branches and possibly tree death. Young trees are more susceptible than mature ones due to their thinner bark. It becomes even more of an issue when a young tree is moved from the protected environment of a nursery to the planting site without some gradual hardening-off. Sunscald may also be mistaken for other plant problems or diseases. Water stress can also make a tree more susceptible to sunscald. To properly diagnose sunscald, you should consider contributing factors: recent weather, irrigation method and effectiveness, pruning, planting dates, and potential for root injury. Some trees have naturally exfoliating bark. In our area, these trees include sycamore, Arizona cypress, birch, and some species of pines. To test if damage has occurred, make a small cut into the suspected area with a sharp knife. If the inner bark is soft and green, then it is still alive. If it is brittle and falls off to expose that wood, then damage has occurred. If you still suspect damage but are unsure, evaluate the location of the plant and the potential for direct exposure of the bark to the sun. You can also look for other individuals of the same species in the area to see if they are also affected. The following trees are susceptible to sunscald: Apple, Ash, Aspen, Birch, Cherry, Cottonwood/Poplar, Crabapple, Honeylocust, Linden/Basswood, Maple, Mimosa, Mountain-ash, Oak, Peach, Tuliptree, Willow, and Walnut. Avoiding sunscald is the best situation. Branches that partially shade the trunk will also reduce the incidence of sunscald. Once injury occurs, the trees respond by forming callous tissue which slowly grows back over the affected area. This may heal a tree having slight damage, but often, sunscald damage leads to overall decline of the tree. Susceptible plants can be protected by wrapping the trunk and lower limbs with white tree wrap, which can be purchased at home and garden stores. White tree wrap reflects light away from the trunk and limbs and keeps the bark surface temperature cooler. Tree wrap should be applied in late fall before the risk of freezing temperatures and removed the following spring after the risk of freezing nights has passed. Tree wrap not removed in early spring may harbor pests during the growing season. Use tree wrap for two to three winters after planting. Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. Additional Resources Sunscald Injury or Southwest Winter Injury on Deciduous Trees, Utah State University Extension Service, Utah Forest Facts http://extension.usu.edu/files/publications/factsheet/NR_FF_021pr.pdf  |