Canker is a general term used to describe a number of different diseases that cause unsightly wounds on the stems and branches of trees, usually as the result of fungal or bacterial infection. Cankers are rarely life-threatening, but can cause death of branches if the damaged tissue completely encircles a branch. Equally, deep cankers will weaken the stem or branch and can predispose the tree to damage by the wind. Some fungal cankers can be treated with fungicides.
This apple canker (Nectria galligena) threatens to encircle this apple sapling, and will likely be terminal.