How to recognise it
Depressed areas appear on the stem from which a light brown gum is excreted, often to be found in the angle between branches. Infected leaves develop dark brown spots about 2mm across in summer, which can then be knocked out (eg by wind) to give the leaf a ’shot-holed’ appearance.
Where an entire branch is ‘girdled’ by the canker, the foliage above the infection can become brown and withered, often similar in appearance to attacks of fireblight.
Why it’s a problem
Severe infections can girdle branches or even the trunk and cause the death of tissues above the infection.
It is mainly carried by wind-blown rain droplets infecting leaf scars and pruning wounds in autumn and winter. Therefore it can spread readily between plants. New infections do not occur in spring or summer.
Where you are likely to find it
On species of Prunus, such as ornamental and fruiting plums, cherries, almonds, peaches and apricots.
How to prevent it
Take care when tying trees against supports, as bark damage can provide an entry point for the disease.
How to get rid of it
Carefully cut out any infected tissue and destroy (eg through burning). Apply paint to seal the wound and spray in August, September and October with a copper compound (eg Bordeaux mix, or copper oxychloride for plums and cherries) to help reduce the disease. Infected plant parts cannot be ‘cured’.
Is it good for anything?!
Other useful information
There are many other conditions which can cause the production of gum (including bark split virus and drought) so signs of this shouldn’t be assumed to be bacterial canker.