What Is a Tree Canker?


Tree cankers are caused by diseases and mechanical damage, according to Cornell University. You may see cankers form on branches and trunks. Understanding what a tree canker is and what causes it to form can help you provide the right type of treatment. Leaving a tree canker on a tree untreated can cause future pest problems and other health issues.

Tree Cankers

  • Tree cankers are areas of dead wood. Symptoms of tree cankers include branch dieback, wilting foliage and defoliation. The canker itself may appear discolored compared to the surrounding bark, swollen or deformed. When cankers appear on conifer trees, the trees may leak clear resin that dries white. If there are tiny bumps around the canker, there is a good chance that the tree has a fungal disease, according to Iowa State University.

Fungal Cankers

  • Fungal diseases cause both target-shaped and diffuse cankers, according to Cornell University. Target-shaped cankers are typically caused on deciduous trees. Fungi invade living or dead branches and grow in the fall and winter months. When the tree comes out of dormancy, it responds to the infection by growing layers of tissue around the infected areas, which causes the target shape. Diffuse cankers are the most deadly canker types. The fungal diseases that cause these cankers grow rapidly beneath the bark area. The tree has little chance to bounce back from the infection.

Mechanical Cankers

  • Mechanical cankers occur when gardeners have made wounds into the base of the tree with lawn mowers. These wounds are the perfect entryway for fungal diseases and insects. It is extremely important to protect trees from mechanical cankers by applying mulch around the tree. Spread a 3- to 4-inch layer of mulch 4 inches from the tree and out to the dripline. The mulch will protect the tree from your mower's blades and it will also help increase soil moisture.


  • Treat your tree's canker by cutting around the canker with a sharp knife. Dip the knife in a mixture of 70 percent denatured alcohol and 30 percent water to prevent spreading any disease. Trace around the canker to create an oval shape. The cut should go 1/2 inch into the bark of the tree. By removing the diseased wood, the tree gets a chance to grow healthy bark around the canker area.

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