Tree canker is a localized disease resulting in areas of dead tissue on the bark, branches, or trunk. This damage is typically caused by various fungal or bacterial pathogens that gain entry through a wound in the bark. The pathogen kills the underlying cambium and wood, leading to the characteristic lesion. Since canker development follows an injury, immediate action is necessary once a visible lesion is identified to prevent the spread of the disease.
Recognizing Canker Symptoms
Canker lesions commonly appear as distinct, sunken, or flattened areas of bark that are often darker in color than the surrounding healthy tissue. On some tree species, the diseased area may ooze sap or resin. This weeping is a common sign of bacterial cankers.
The tree attempts to defend itself by forming a raised ring of callus tissue around the infection site, which can make the canker look target-shaped or swollen at the margins. Look for signs of branch dieback, where twigs or limbs above the canker suddenly wilt and die because the disease has blocked the internal water-conducting tissues. Cankers on young, thin-barked trees are typically more visible and sunken, whereas those on older trees with thick bark may be harder to spot, with dieback being the first noticeable symptom.
Essential Steps for Lesion Removal
The most effective treatment for a canker on a branch or twig is prompt physical removal of the infected wood. Prune the branch several inches below the visible canker, cutting back into healthy wood that shows no discoloration. For smaller twigs, a margin of 6 to 10 inches beyond the symptom is often recommended to ensure the entire infection is removed.
Tool sanitation is paramount to avoid spreading the pathogen to healthy parts of the tree or to other plants. Pruning tools, such as loppers or saws, must be sterilized between every cut using a solution of 70% rubbing alcohol or a mixture of one part household bleach to ten parts water. The infected wood should be disposed of immediately by burning or bagging for trash collection; it should never be composted, as this can spread the fungal or bacterial spores. Fungicides and bactericides are generally ineffective for treating established cankers, and physical removal remains the primary action for homeowners.
Preventing Future Canker Development
Cankers are opportunistic diseases that exploit trees already under stress. Long-term management focuses on reducing environmental stressors and promoting tree vigor. Improper water management, including drought stress or over-watering leading to poor soil drainage, is a common predisposing factor. Providing supplemental, deep watering during dry periods is a simple way to boost a tree’s natural defenses.
Mechanical wounding creates easy entry points for pathogens. Maintaining a proper mulch ring helps prevent “mower blight” or damage from string trimmers near the trunk. Pruning should be done correctly, making cuts at the branch collar without leaving stubs, and is best performed during dry weather to reduce the chance of spore spread. Avoiding the over-application of nitrogen fertilizer is also important, as excessive growth can sometimes make tissue more susceptible to infection.
Poor soil health and nutritional imbalances can weaken a tree over time, making it a target for canker pathogens. Ensuring the tree has adequate nutrients and is planted in a suitable location for its species will help it naturally resist infection. Maintaining consistent health through proper cultural practices is the most effective defense against canker disease.
Determining When a Tree Cannot Be Saved
Homeowners must recognize the limitations of DIY canker treatment, especially when the lesion is on the main trunk. If a canker has expanded to girdle, or completely circle, more than 50% of the trunk or a major scaffold limb, the tree’s internal transport system is compromised beyond recovery. Girdling prevents the movement of nutrients and water, which ultimately leads to the death of the tree above the lesion.
If a canker is high in the canopy or if the tree is large, making removal of the infected branch unsafe or impractical, consult a certified arborist. Significant wood decay associated with the canker may indicate a structural safety risk, where the tree could fail in a storm. An arborist can assess the extent of the internal damage and provide a professional recommendation on whether the tree can be saved or if it needs to be removed to prevent injury or property damage.