PPDL Picture of the Week for
August 29, 2011

Southwest Injury/Sunscald

Rosie Lerner, Extension Consumer Horticulture Specialist; Janna Beckerman, Assistant Professor of Plant Pathology; Purdue University

Sunscald, also called southwest injury, develops on the south and southwest sides of the trunk or on the upper surfaces of limbs exposed to sun, particularly in thin-barked trees like maple, young crabapples, flowering fruit trees, and mountain-ash. Usually, a pre-existing injury exists on the side where the damage develops. The cambial temperature of south to southwest facing trees can reach into the 60-degree range while the shaded portion remains at freezing (32° F). Damage to the cambium (the thin, formative layer beneath the bark of the tree that gives rise to new cells and is responsible for secondary growth) can result in dieback or even death. This heating results in the tree losing its dormancy, which is followed by lethal freezing when the sun sets. Sunscald, coupled with drought, can result in vertical frost cracks and death of the cambium. Frost cracks also provide an infection court for decay and canker pathogens. Prevention is the best method for contending with sunscald. If possible, provide shade by strategically placing other plants or structures on the south sides of thin barked trees and shrubs. Tree wrapping with reflective or light-colored material may be effective in preventing sunscald and bark cracking.

The current recommendation for newly planted, thin bark trees is that they should be wrapped for at least two winters or until mature bark is established. Regardless of how many winters you wrap your trees, care must be taken to remove the wrapping in the spring to prevent moisture from collecting between the bark and the wrapping. During the spring, wrapped bark may provide an infection court for disease when the weather warms up.

Trees can function pretty well indefinitely with even major trunk injury, depending on what other stresses come their way. The trees in these photos are no doubt a bit stunted and yellow, and may also be under considerable heat and drought stress. Stressed trees are more likely targets for borers and other problems that can cause further damage or eventual plant death.

Click image to enlarge

Fig. 1 Southwest/Sunscald Injury on Maples

Fig. 1. Southwest/Sunscald Injury on Maples (Photo credit: Rosie Lerner)

Fig. 2. Southwest/Sunscald Injury on Maples

Fig. 2. Southwest/Sunscald Injury on Maples (Photo credit: Rosie Lerner)

Fig 3. Southwest/Sunscald Injury on Maples

Fig. 3. Southwest/Sunscald Injury on Maples (Photo credit: Rosie Lerner)

Fig 4. Southwest/Sunscald Injury on Maples

Fig. 4. Southwest/Sunscald Injury on Maples (Photo credit: Rosie Lerner)

Fig 5. Southwest/Sunscald Injury on Maples

Fig. 5. Southwest/Sunscald Injury on Maples (Photo credit: Rosie Lerner)

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service