A canker is equivalent to
an open wound in the bark of a tree or woody shrub. It
is a disease caused primarily by a fungus but
occasionally by bacteria that enters through the bark by
way of a wound. Cankers usually form a split in the bark
which then roles back upon itself along the edges.
Once formed, there is no
"cure" for a canker. There is only prevention by
avoiding making wounds in the tree if possible. Of
course, you cannot control high winds, ice loads or
other natural occurrences that will damage the bark of a
tree. You can, however, avoid improper pruning cuts,
running into trees with lawn mowers and other power
equipment and planting trees too deep in the ground.