Sunscald and Tree Health - April 12, 2006
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
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 freezing temperatures at night followed by warm daytime temperatures. While the exact mechanism of injury is unclear, it is thought that the sensitive cambial cells are killed because they cannot adjust to these rapid temperature fluctuations.
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 most susceptible: maples, mimosa, walnut, pecan, and deciduous fruit trees. Branches that partially shade the trunk will also reduce the incidence of sunscald.
Avoiding sunscald is the best situation. 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.
The following steps can be taken to avoid sunscald:
1. Plant healthy trees that are well-adapted to the local climate.
2. Avoid excessive pruning of lower branches for a few years after planting.
3. Keep trees adequately irrigated.
4. Paint exposed trunk and limbs with a 50 – 50 mixture of white latex paint and water. Light colored tree wraps can also be used, but can also be hiding places for plant pests.
5. Apply 4-6 inches of coarse organic mulch on the soil surface to retain irrigation water and minimize reflected light and heat. Do not place mulch against the stem.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at email@example.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: April 6, 2006
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