A tree canker is a localized area of dead bark and the underlying tissue, often appearing as an open sore on a woody plant. It is not a specific disease but rather a symptom caused by fungal, bacterial, or sometimes non-living agents that damage the tree’s outer structure. Formation typically begins when a disease-causing organism gains entry through a wound, leading to a restricted area of necrosis. Cankers can affect the trunk, branches, and twigs, indicating that the tree’s natural defense system has been breached.
Identifying the Physical Signs
Cankers appear as distinct, localized areas on the bark that look different from the surrounding healthy tissue. A common sign is a sunken or flattened patch of bark that may be discolored, often appearing dark brown, reddish, or black. On younger or thin-barked trees, this sunken appearance is particularly noticeable because the tissue beneath the bark has died.
Some cankers, particularly those caused by bacteria or certain fungi, may show a gummy or wet exudate, often sap or resin oozing from the infected area. The tree defends itself by forming a boundary of new, healthy tissue, known as callus tissue, around the margin of the dead area. This reaction can create a raised ridge around the canker, resulting in a characteristic “target” shape, especially with perennial cankers that expand over multiple seasons. Dieback of branches or wilting of leaves above the canker may also signal a lesion that is disrupting internal functions.
Pathogens and Predisposing Factors
The formation of a canker is typically the result of an opportunistic pathogen, most often a fungus, exploiting a weakness in the tree’s defenses. Fungal genera such as Nectria, Botryosphaeria, and Cytospora are commonly associated with these lesions, though bacteria like Pseudomonas syringae can also be causal agents. These organisms usually cannot penetrate the healthy, intact outer bark, which serves as a protective barrier.
The pathogen usually requires a predisposing factor, which is an injury or stressor that provides an entry point. Common wounds include improper pruning cuts, insect feeding sites, mechanical damage from lawn equipment, or natural damage from hail or wind. Environmental stress also plays a significant role in weakening the tree. Factors such as prolonged drought, freeze damage, sun scald, or transplant shock lower the tree’s natural defenses, allowing the pathogen to colonize the damaged tissue.
How Cankers Damage Tree Structure
The primary damage is the localized death of the inner bark and the vascular cambium, the layer responsible for producing new wood and bark. This necrotic area directly interferes with the tree’s internal transport system. The phloem, which moves sugars from the leaves downward to the roots, is often destroyed first, starving the lower portions of the tree.
If the canker expands to completely encircle a branch or the main trunk, a process known as girdling occurs. Girdling effectively severs the flow of nutrients and water, leading to rapid decline and dieback of all tissue above the infected ring. Beyond biological interference, cankers also compromise the structural integrity of the wood, making the affected branch or trunk weaker and more prone to breakage during high winds or heavy snow.
Strategies for Managing and Preventing Cankers
Management focuses on sanitation and improving the tree’s overall health to allow it to compartmentalize the infection. The most direct action involves sanitation pruning: removing the infected branch several inches below the visible canker and disposing of the material to prevent spore spread. It is important to sterilize pruning tools between cuts, often using alcohol or bleach, to avoid transferring the pathogen to healthy wood.
Prevention focuses on minimizing wounds and reducing environmental stress, as healthy trees are better equipped to wall off infections. Ensuring the tree receives adequate water, especially during dry periods, and providing proper nutrition helps maintain vigor. Avoiding mechanical injury from lawn mowers or string trimmers is important, as is using correct pruning techniques that minimize open cuts and occur during the dormant season. Since chemical treatments are generally not effective on established cankers, focusing on cultural practices and prevention is the most productive strategy.