Bacterial Canker - xanthomonas populi
There are two common forms of canker, fungal canker and bacterial canker.
Bacterial canker is less common than fungal canker, and generally only affects fruit trees in the UK.
For example: Apples, Cherry, Pears and Plums.
Infections have the potential to kill entire sections of the affected trees, meaning prompt treatment is called for once symptoms are seen.
The first sign of the canker is sunken areas of dead bark around the area where the branch meets another branch or trunk of the tree, later these areas ooze with a gummy substance.
If the canker girdles stems, wilting and die-back occurs.
Another indication is small holes in the leaves of the plant, this is known as shot-holing.
Shot holes are caused by bacteria entering the stomata and forming lesions.
As the leaves mature these lesions stop growing, and the infected tissue falls out leaving a hole.
The bacteria lives on the surface of the leaves during summer,and is dispersed with rainwater into openings in the bark such as leaf scars and pruning wounds, and appear in spring in the form of cankers.
Carefully prune out the affected part of the tree to 300mm (12") below the site of the canker infections in summer, when bacterial populations are low, and re-infection is less likely.
Spray with a half strength Copper fungicide or Bordeaux mixture at 2-3 week intervals during the summer and a full strength application before autumn leaf drop.