Canker in trees are defined as areas of dead bark and underlying wood on twigs, branches or trunks and can be caused by both fungi and bacteria or through environmental and manmade factors such as extreme temperatures or hailstones or damage from lawnmowers, chemicals or insects.
Some fungi can aggressively attack trees and cause cankers but usually the fungi that cause them inhabit the surface of the tree and cause disease by gaining access through these natural or man made wounds particularly when the tree is under stress.
The canker itself makes the tree affected highly vulnerable to disease from bacteria, fungus and insects.
How do I recognise it?
Canker usually presents itself in a few different ways:
- In young trees, look out for patches of roughened or cracked bark, especially around wounds and branch stubs
- Callus formation in multiple layers or in ridges
- Small pimple-like fungal spore forming structures either in the centers or around the hardened bark or calluses.
There are three types of canker that can affect trees. These are:
These are caused by fungi which cause disease when the tree is under environmental stress and low in vigour during the dormant season. In the growing season, the callus tissue walls off the canker build and prevent further spread. Although cankers are not persistent, more cankers can form under continued stress leaving the tree vulnerable to more serious diseases.
Wounds and branch stubs are invaded by fungi during the tree's dormant period. The tree forms callus around the infection site during the growing season but the fungus invades more tissue the following dormant period.
These are the most dangerous type as there is little or no callus growth but fungus invasion is so rapid, the tree tissue at the edge of the advancing fungus is killed rapidly and unable to form a protective barrier against further spread. Branches or whole trees can sometimes be seriously affected or even killed in a single season.
If canker occurs on branches, they should be pruned to 3-4 inches below the canker. However, cankers appearing on the main trunk of young trees sadly indicate that the entire tree should be removed.
Perennial and diffuse cankers do not go away. Such cankers are present for the life of the tree which and shorten the tree's life considerably.
Once a tree has canker, it is essential to remove as much of the canker fungi as possible from the tree to avoid infection and spread.
image courtesy of www.rhs.org.yk