Southwest injury, also known as sunscald, is a wintertime phenomenon affecting certain deciduous trees, particularly young ones with thin bark. This damage occurs on the southwest side of the tree trunk during sunny winter days when temperatures fluctuate above and below freezing. The process involves the following steps:

1. Daytime Warming: On sunny days, the southwest side of the tree trunk absorbs heat, causing the bark temperature to rise significantly. This warmth can reactivate the cambial tissue just beneath the bark, making it metabolically active. 
2. Nighttime Cooling: As temperatures drop rapidly in the evening, the now-active cambial cells lose hardiness. The sudden cooling can cause these cells to contract and die, leading to visible damage such as bark splitting or sunken areas.

This injury is most prevalent on the southwest side of the trunk due to its increased exposure to sunlight during winter months. Trees with thin bark, such fruit trees, are especially susceptible. Additionally, trees under stress from environmental factors, herbicide injury, or pest and disease issues are more prone to southwest injury.

Symptoms of Southwest Injury:

• Bark Cracking: Vertical cracks on the trunk, particularly on the southwest side. 
• Discoloration: Darkened areas on the bark, indicating damaged cambial tissue. 
• Bark Sloughing: Detached or peeling bark, revealing dead tissue underneath. 

Preventive Measures:

To mitigate the risk of southwest injury, consider the following strategies:

• Tree Guards: Install light-colored, preferably white, tree guards that extend from about 2 inches below ground level to 16–18 inches above ground. These guards reflect sunlight, keeping the trunk cooler and reducing the risk of injury. Ensure guards are ventilated to prevent girdling as the tree grows. ​
• Trunk Painting: Apply white latex paint to the trunk, especially on the southwest side, to reflect sunlight and maintain a consistent bark temperature. This method is particularly useful for young trees during the first few years after planting. ​
• Tree health: Ensure trees are well-watered and healthy going into winter, as drought-stressed trees are more susceptible to injury.

This article was posted in Apples, Other News and Resources and tagged Amaya Atucha, Apples, Southwest Injury, sunscald.