Canker Rot

Can you see a crack in the trunk or branches of your tree? It may be canker rot.

Canker on beech tree (Roger Griffith, Public Domain)

Canker rot is a collection of fungal diseases that eat away at the interior of tree trunks and branches, weakening the tree and setting the stage for other pests and diseases. Canker rots can also girdle your tree and kill it. While most commonly seen in ornamental trees, canker rot can occur in apple and other fruit and nut trees.

Interior of oak tree killed by canker rot (USDA Forest Service, Public Domain)

Canker rot identification

Cankers are open wounds or lesions. Cankers can be a few inches long and wide or several feet long, depending on the fungal species. The bark surrounding these cankers dies, becoming discolored, often lighter or orangish. And it is tightly bonded to the canker. After a year or two, the dead inner bark turns black and stringy. It may look like a sooty bark canker. But canker rot can also have lens-shaped lighter areas in the bark. Unlike other canker diseases, canker rot affects both bark and inner tissue.

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

Canker rots can also cause swelling, sunken areas, gnarled bark, and conks. Conks are shelf-shaped fungal fruiting bodies. After spores are released, the conk will dry out and darken. It may remain on the tree or fall off.

Canker rot conks (Joseph OBrien, USDA Forest Service, Bugwood.org) CC BY 3.0 US

If you were to see inside your tree, you would see that the heartwood and sapwood have become discolored. Instead of warm, rich yellowish-browns of healthy wood, you would see gray, orange, or even pink-tinged wood, often extending three or more feet beyond the canker.


Canker rot lifecycle

The fungi responsible for canker rot usually enter trees through pruning cuts and wounds. Fungi attach to the wood and then move to the cambium to access the water and nutrients flowing through the vascular bundle. The fungi also move to the bark, where they eject spores carried by wind to nearby trees.


How to control canker rot

As always, healthy trees are better able to protect themselves. So, select species suitable to your microclimate, plant them at the proper depth, irrigate and fertilize your trees properly, and monitor for signs of problems. Other actions you can take to reduce the chance to canker rot occurring in your trees include:

• Make sure pruning cuts are clean and do not use sealants.
• Whitewash exposed trunks and limbs to prevent sunburn damage.
• Remove tree supports as soon as they are not needed.​​

Canker rot on ash tree (Roger Griffith, Public Domain)

Trees with canker rot may fall over at any time. Large trees weigh several tons and can be extremely dangerous. If you suspect canker rot, call a licensed arborist right away.