What can sunken or discoloured areas on a tree’s bark indicate?

Sunken or discoloured areas on a tree’s bark can indicate several different issues, ranging from infectious diseases to environmental or physical damage.

• Canker: This is a common plant disease caused by various fungi and bacteria. Symptoms include round-to-irregular sunken, swollen, flattened, cracked, or discoloured areas on the trunk, limbs, or twigs. These cankers may enlarge and girdle a branch, eventually killing the foliage beyond the affected site.12
• Sunscald: This is a temperature-related injury, often occurring on the south or southwest sides of trees in the Northern Hemisphere. It is characterized by elongated patches of dead bark that may appear sunken, discoloured, dried, or cracked.3
• Physical or Environmental Injury: Similar symptoms can result from non-pathogenic causes, including damage from hail, mechanical wounds from tools or vehicles, or girdling wires.2

Distinguishing between these causes is important for proper management. For example, tissues directly infected by pathogens often show a gradual change in colour or consistency, whereas physical injuries typically present with a well-defined, abrupt transition between healthy and affected tissue.2