Cankers

A canker is a sunken wound in bark caused by fungal and bacterial diseases. The wound is an open sore filled with dead plant tissue. Okay, so it’s not the prettiest thing we’ve talked about, but it is important to know where cankers come from, how to prevent them, and how to treat them. Your trees will thank you!

Butternut tree infected with fatal canker disease (Mike Ostry, US Forest Service) CC BY 3.0 us Public Domain

Canker causes & identification

Some cankers are obvious and some are not. They are caused by fungal and bacterial microorganisms that infect the cambium layer of trees and shrubs. Cankers are very slow to heal and often do not heal at all. If the wound travels laterally, the sap found in the xylem and phloem cannot move and the branch dies.


Foliage on infected branches often turns yellow or brown and wilts. Cankers can encircle (girdle) and kill limbs or an entire tree. Common canker diseases include: Eutypa dieback, pitch canker, fire blight, Fusarium wilt, chestnut blight, pine blister rust, anthracnose diseases and sudden oak death.

Healthy bark (left) protects the tree. Cankers (right) allow problems to enter. (Hannahzd3) CC BY-SA 4.0

How to prevent cankers

The best prevention method is planting resistant cultivars. Also, installing plants best suited to the local microclimate helps them to be strong enough to fight off pests and disease on their own. Good cultural care, such as proper pruning, watering and feeding will also help prevent disease. Whenever dead or diseased limbs are seen, they should be removed and destroyed right away. Avoid heavy feeding, since that stimulate vulnerable new growth. Sunburn and overwatering can both make plants susceptible to various canker diseases. Many healthy trees have these pathogens present. Trees that are stressed become susceptible to disease.

Canker on a birch tree (MarkusHagenlocher) CC BY-SA 3.0

Treating cankers

Once a fungal disease has taken hold, getting rid of it can be difficult. Most fungicide treatments are ineffective against fungal cankers since the pests are safely inside the plants they infect, and the same is true for bacterial cankers. Maintaining healthy plants allows them to fight for themselves.