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Thus far, many of the tree diseases we’ve discussed first appear on leaves and shoots, but not all do. Some attack the bark itself, creating damaged areas that slowly weaken branches and trunks. These are known as tree canker diseases, and while there are several different types, they all affect the same critical part of a tree’s structure.
Quick Links:
- What are Tree Canker Diseases?
- Types of Tree Canker Diseases
- Tree Canker Disease Symptoms: How to Recognize Them
- What Causes Tree Canker Diseases?
- Treating Tree Canker Diseases
- How to Mitigate Tree Canker Diseases
What are Tree Canker Diseases?
Thus far, we’ve covered singular diseases, but tree canker diseases are a group of tree diseases that damage the bark and underlying tissue of branches or trunks. They are caused by both fungal and bacterial pathogens that enter a tree through wounds, natural openings, or stressed tissue.
The term “canker” refers to the visible result of that infection: a localized area of dead or damaged bark, which often appears as sunken, cracked, or discolored patches on branches or the trunk. As the disease progresses, the infected tissue can expand, sometimes girdling branches and interfering with the tree’s ability to move water and nutrients.
Because several different pathogens can cause canker disease in trees, the exact symptoms and severity vary, depending on the tree species and the organism involved. However, most canker diseases share one important characteristic: they tend to take hold when trees are already under stress, such as from drought, physical injury, poor soil conditions, or environmental damage.
Understanding what canker diseases are and what a canker looks like is the first step toward monitoring tree health and preventing more serious damage over time.
Types of Tree Canker Diseases
It’s important first to note that the number of tree diseases that cause cankers is extensive. With multiple variables, from bacterial and fungal pathogens to species and growing regions, covering all of them would be quite lengthy. Instead, we’ll focus on some of the more common canker tree diseases you’ll see in the Intermountain West and Pacific Northwest.
- Fire Blight Canker (Erwinia amylovora) – Bacterial
Fire blight can form cankers on branches and trunks of apples, pears, and crabapples. These cankers may appear dark and sunken and sometimes produce sticky bacterial ooze. - Cytospora Canker (Leucostoma / Cytospora species) – Fungal
One of the most common cankers on spruce and other conifers. It causes branch dieback and sunken bark areas, often accompanied by resin flow. - Nectria Canker (Neonectria species) – Fungal
A widespread canker affecting hardwoods such as maple, birch, and beech. It often produces target-shaped bark lesions and small reddish fruiting bodies on infected tissue. - Botryosphaeria Canker (Botryosphaeria species) – Fungal
Common in drought-stressed trees, including maples, oaks, and ornamental species. It produces elongated bark lesions and progressive branch dieback. - Bacterial Canker (Pseudomonas syringae) – Bacterial
Often seen in stone fruit trees like cherry, plum, and peach. It can cause dark bark lesions, gumming, and dieback of shoots and branches. - Thyronectria Canker (Thyronectria austro-americana) – Fungal
Most commonly associated with honey locust trees. It produces sunken cankers that may girdle branches and trunks, often leading to structural decline in infected trees. - Hypoxylon Canker (Hypoxylon species) – Fungal
A serious disease of stressed hardwood trees, especially oaks. Infected bark eventually sloughs off, revealing gray or silvery fungal mats underneath. - Diplodia Canker (Diplodia species) – Fungal
Common on pines and other conifers, this disease causes branch dieback and canker formation, particularly when trees are weakened by drought or environmental stress.
Although these pathogens differ, most canker diseases share an important trait: they tend to attack trees that are already weakened or injured. These infections often begin when pathogens enter trees through wounds, pruning cuts, frost damage, sunscald, or other stress points. Maintaining strong tree health is often the most effective long-term defense against canker disease. Everything from proper tree pruning and siting to tree sourcing and ongoing monitoring can help keep trees healthy and mitigate the risk of tree canker diseases.
Tree Canker Disease Symptoms: How to Recognize Them
You might be wondering whether, despite their different causes, cankers look the same. The short answer is: not exactly. That doesn’t provide much clarity (we’ll get into it a bit more in a moment), but it’s important to understand that the wide variety of causes and impacted trees means there are variations.
While cankers share some common characteristics, their appearance can vary depending on the tree species and the organism causing the infection. What they all have in common is that they represent areas where bark and the underlying tissue have been killed. These dead areas disrupt the tree’s ability to move water and nutrients through the affected branch or trunk.
Common Signs of Tree Canker Diseases
Even though different pathogens cause them, most cankers share several recognizable features:
- Sunken or discolored patches on bark
Infected areas often appear darker than the surrounding bark and may look slightly depressed or flattened. - Cracked or peeling bark
As the disease progresses, bark over the infected area may split, flake, or fall away. - Dead or dying branches above the canker
If a canker interrupts the tree’s vascular system, the portion of the branch above it may decline or die back. - Oozing sap or resin
Some cankers produce sap flow or sticky exudate as the tree responds to the infection. - Callus growth around the edges
Trees sometimes attempt to seal off the infected area by forming raised ridges of new tissue around the wound.
Why Tree Cankers Can Look Different
Although these symptoms are common, the exact appearance of a canker can vary depending on the pathogen involved. Some cankers, such as those caused by Pseudomonas and Thyronectria, form long, narrow lesions along branches. Others create more circular or target-shaped wounds, such as Nectria canker, in which the tree repeatedly forms callus tissue around the infection.
Some fungal cankers produce visible fruiting bodies on the bark, such as the reddish structures seen in Nectria or Thyronectria cankers. Bacterial cankers, on the other hand, often produce sticky sap or bacterial ooze, as seen in diseases like fire blight or bacterial canker on stone fruit trees.
Because the symptoms can overlap with mechanical injury, sunscald, or winter damage, accurate identification often requires examining both the bark damage and the tree's overall condition.
For homeowners, landscapers, and property managers, the most important step is recognizing when bark damage appears abnormal and monitoring the tree for branch dieback or expanding lesions, which may signal an active canker disease.
What Causes Tree Canker Diseases?
Tree canker diseases are caused by fungal or bacterial pathogens that infect the bark and underlying tissues of a tree. These organisms typically enter through wounds or weakened areas, such as pruning cuts, storm damage, insect feeding sites, frost cracks, or sunscald.
Once inside the tree, the pathogen begins killing localized areas of bark and cambium, resulting in the dead tissue we recognize as a canker. In many cases, the organisms responsible are naturally present in the environment and only become a problem when a tree is under stress. As such, there is no way to prevent them entirely, but understanding the causes can help mitigate what is within your control.
Common factors that increase the risk of canker diseases include:
- Drought or heat stress
- Mechanical injury to bark or branches
- Poor soil conditions or compaction
- Improper pruning practices
- Winter injury or sunscald
We cannot stress enough that tree care, maintaining overall tree health and reducing stress are among the most effective ways to prevent canker diseases from developing or spreading.
Treating Tree Canker Diseases
If you discover a canker on a tree, the first and most important step is to positively identify the underlying cause. Because cankers can be caused by several fungal and bacterial pathogens and can sometimes resemble physical injury or environmental damage, a correct diagnosis is important before taking action. In most cases, consulting a certified arborist or tree professional is the best course of action before taking any steps.
Once the cause is understood, treatment typically focuses on removing infected tissue and reducing the conditions that allowed the disease to develop.
Steps for Managing Canker Diseases
- Prune out infected branches
Remove affected limbs well below the visible canker, cutting back into healthy wood. This can help stop the disease from spreading further through the branch. - Disinfect pruning tools between cuts
This helps prevent the spread of pathogens from one branch, or tree, to another. - Remove and dispose of infected material
Do not leave diseased branches beneath the tree where pathogens can persist. - Reduce stress on the tree
Improve watering practices, soil conditions, and airflow around the canopy to support recovery. - Monitor the tree over time
Some cankers can recur or expand, so periodic inspection helps ensure the issue is being contained.
In some cases, healthy, established trees can compartmentalize smaller cankers and continue to grow normally. However, when cankers begin to girdle branches or spread into the trunk, professional evaluation is especially important to determine whether more aggressive treatment, or tree removal, is necessary.
How to Mitigate Tree Canker Diseases
Because canker diseases often take hold when trees are stressed or injured, mitigation focuses on reducing the conditions that allow pathogens to enter and spread. While not every canker can be prevented, consistent care can significantly lower the risk.
Key tree canker mitigation strategies include:
- Avoid unnecessary bark injuries, such as damage from lawn equipment, snow plows, or construction equipment
- Prune during the appropriate seasons (dormant season pruning) to reduce infection risk
- Maintain proper watering (including winter watering) and soil health to keep trees vigorous
- Select tree species suited to the site conditions, as what you plant is as important as where you plant (tree siting)
- Monitor trees regularly so new cankers can be identified early
In managed landscapes and urban forests, these preventative practices help keep trees healthier and less vulnerable to the pathogens that cause canker diseases.
In urban and community forests, trees often face a combination of stressors, including compacted soils, heat, limited rooting space, mechanical damage, and repeated pruning. These conditions can make trees more vulnerable to the pathogens that cause canker diseases.
When susceptible species are planted close together or when trees experience repeated stress, cankers can gradually weaken branches, reduce canopy performance, and increase maintenance needs across a site.
While a single canker may not always threaten the entire tree, repeated infections can lead to branch dieback, structural decline, and reduced long-term tree health, especially in managed landscapes where trees play an important role in providing shade, enhancing aesthetics, and delivering environmental benefits.
If you’re dealing with canker diseases or simply want to start with healthier, more resilient trees, SuperTrees can help. From selecting and planting the right trees at our retail or wholesale nursery to providing professional tree services, diagnosing tree diseases, and helping remove and replace declining trees to improve canopy diversity, we work with property managers, homeowners, and cities across the West to support stronger urban forests.
Reach out to our team to learn how we can help with your landscape or community tree plan!
