Canker diseases on shade and forest trees: Part 1

Canker diseases on shade and forest trees are quite common but can be a threat to tree health as branch dieback and tree mortality can occur. Because there is no chemical treatment, it can be difficult to deal with a canker disease once it infects a tree.

In addition to foliage and root diseases, canker disease pathogens can attack tree trunks and branches. Cankers are usually fungal, but can be caused by bacteria as well. Some well-known examples include: chestnut blight, Phomopsis canker of juniper, beech bark disease and thousand canker disease of walnut.

Cankers are essentially “sores” that erupt on the branch or trunk of a tree and kill woody cell tissue in a localized area. Typically, cankers are described as “sunken areas” that protrude through broken or blistered bark. Some cankers (such as cacterial canker of cherry) are very noticeable – which is typical of perennial or target cankers.

Other types of cankers (such as chestnut blight) are much harder to spot (referred to as diffuse cankers), because they do not create the ridges of callus around the typical target canker.


Bacterial Canker of Cherry | Photo by MSU Extension


Chestnut Blight Canker | Photo by State University of New York College of Enviornmental Science & Forestry

Cankers often enter a tree via openings in the bark caused by natural or manmade wounds. Lawn mowers, weed trimmers or weather-related damaged (hail, wind, etc.) are all good examples of injuries that can occur. Once inside the tree, cankers can burst through at any spot as the pathogen moves through the tree’s tissues.

Depending on the type of canker, their size and shape can vary. However, cankers often start as small lesions and grow larger over time. As they grow, they can kill the water conducting (xylem) and food conducting (phloem) systems of a tree branch resulting in stunted, off-color or dead foliage. And if the canker spread, it can possibly move to kill other limbs or even the entire tree.

Cankers are very difficult to treat and there may not be a suitable chemical treatment available. Trees that are stressed may be more susceptible to attack and because no viable chemical treatment is available, increasing tree vigor through fertilization and watering are often recommended. Improved tree vigor will often help improve the tree’s condition but may not be a total cure for the problem.           

In part two of this article, some specific canker diseases will be discussed and how Michigan residents can help be on the lookout for canker-related issues. Through programming offered by Michigan State University Extension, such as the Eyes on the Forest and Sentinel Tree Network, volunteers can help be on the lookout for many different types of tree problems.

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