Canker disease is a common and often serious affliction that impacts a wide range of woody plants, including trees and shrubs. It represents a localized, dead area on the bark or stem, posing a significant concern for plant health and longevity. Understanding this disease involves recognizing how it damages the plant and the conditions that allow it to take hold. This knowledge is fundamental for protecting landscape investments.
Defining Canker Disease
A canker is a necrotic lesion, an area of dead tissue that forms on the bark of a stem, branch, or twig. The infection kills the thin layer of living cells beneath the bark, known as the cambium, along with the phloem and sometimes the underlying sapwood tissue. This localized destruction disrupts the plant’s vascular system, preventing the movement of water and nutrients.
The plant attempts to seal off the infection by producing new tissue, called callus, around the perimeter of the dead zone. This defensive reaction often results in a raised ring or ridge of growth surrounding the sunken, diseased bark. If the canker expands rapidly, it can completely encircle the stem or branch, a process called girdling. Girdling cuts off the flow of resources, leading to the death of all plant parts beyond the infection point.
Primary Causes and Contributing Factors
Canker diseases are caused by two main categories of agents: biotic pathogens and abiotic stress factors. The primary biotic agents are various species of fungi, such as Nectria, Cytospora, and Botryosphaeria. Certain bacteria, like Pseudomonas syringae, can also cause canker formation. These pathogens do not penetrate healthy, intact bark but require an entry point to begin an infection.
Pathogens gain access to the plant’s internal tissues through wounds, which can be natural openings or damage from external sources. Common entry points include pruning cuts, insect feeding sites, bark cracks, and mechanical injuries. The presence of a pathogen alone is often not enough to cause a serious canker, as the plant’s natural defenses can usually fight off minor infections.
The true vulnerability arises from predisposing abiotic stress factors that weaken the plant’s immune response. Environmental conditions such as severe drought, excessive moisture, nutrient deficiencies, or sudden temperature fluctuations significantly reduce the plant’s ability to compartmentalize the infection. Wounds caused by winter injury, sunscald, or hail also create easy access points for opportunistic fungi. Maintaining good plant vigor is therefore a strong defense against the onset and spread of canker disease.
Recognizable Symptoms and Progression
The most immediate sign of a canker is the appearance of a discolored, sunken, or flattened area on the bark of a branch or trunk. This lesion represents the area where the pathogen has killed the underlying cambial tissue. The color of the affected bark may range from brown to black, depending on the tree species and the specific pathogen involved.
As the plant tries to heal, a distinct, raised ring of callus tissue may form around the infection, giving some cankers a characteristic target-like or concentric ring appearance. Oozing or gummy exudate (sap or resin) is often visible weeping from the lesion, particularly on fruit trees or conifers. This is a reaction by the plant attempting to seal the infected area.
A more severe symptom is the wilting and dieback of foliage and branches above the canker site. This dieback occurs because the expanding canker has girdled the stem, blocking the transport of water and nutrients. Leaves on affected branches may turn yellow or brown, shrivel, and often remain attached to the dead branch for some time. Early identification of these visual cues is important for successful intervention.
Management and Prevention Strategies
Since established cankers are difficult to cure, the most effective approach is prevention and cultural control. This involves minimizing stress on the plant by ensuring proper watering during dry periods and providing balanced fertilization. Avoiding injury to the bark from lawn equipment or improper pruning techniques will also reduce opportunities for pathogen entry.
If a canker is observed on a branch, the infected wood should be pruned out immediately. The cut must be made several inches below the visible canker and into healthy wood to ensure all diseased tissue is removed. It is important to sterilize pruning tools between every cut using a disinfectant, such as a diluted bleach solution or rubbing alcohol. This prevents accidentally spreading fungal spores to healthy parts of the plant or to other trees.
For cankers on the main trunk, removing the entire tree may be necessary if the infection is extensive, especially to protect surrounding plants. Chemical treatments, such as fungicides or bactericides, are generally not effective as a cure for existing cankers because the pathogen is protected beneath the bark. They are only occasionally used as a preventative measure following pruning, but cultural practices remain the most reliable method of control.