Bacterial canker is a disease of the stems and leaves of Prunus, especially plums and cherries, but also apricots, peaches and ornamental Prunus species. It causes sunken patches of dead bark and small holes in leaves, called ‘shothole’.
What is bacterial canker?
Bacterial canker is a disease caused by two closely related bacteria that infect the stems and leaves of plums, cherries and related Prunus species.
Cankers begin to form in mid-spring and soon afterwards shoots may die back. Shotholes appear on foliage from early summer.
You may see the following symptoms:
- On stems and spurs: Sunken, dead areas of bark develop in spring and early summer, often accompanied by a gummy ooze. If the infection spreads all round the branch it will die rapidly. However, it should be noted that gum production (gummosis) from the bark of Prunus species is actually quite common, and in the absence of dead, sunken bark is likely to have resulted from causes other than bacterial canker, for example physical damage or environmental stresses
- On emerging shoots: Shoots may either fail to emerge, or start to grow normally in spring before dying back rapidly. In severe cases this may affect a large percentage of shoots on a tree. Note that shoot dieback can also be caused by the fungal disease blossom wilt
- On leaves: Small brown spots appear, which are often round and eventually fall out to leave holes – as if the leaf had been hit by shotgun pellets, leading to the popular name of ‘shothole’
Where possible, carry out all pruning in July or August when tissues are most resistant. This is also the best time to prune in order to minimise the risk of infection by spores of the fungus causing silver leaf disease. Cut out all cankered areas, pruning back to healthy wood and painting promptly with a wound paint to protect the wound from re-infection. Details of wound paint products can be found in the ‘Fungicides for gardeners’ document below. Burn or landfill the prunings.
The cherries ‘Merton Glory’, ‘Merton Premier’, ‘Merla’ and ‘Merpet’ and the plums ‘Marjorie’s Seedling’ and ‘Warwickshire Drooper’ have some resistance.
There are no fungicide sprays available to amateur gardeners for the control of bacterial canker.
The bacteria Pseudomonas syringae pv. morsprunorum and P. s. pv. syringae are members of a large genus of plant-pathogenic bacteria. Several others occur in the UK but are relatively unimportant and, generally speaking, bacterial diseases of plants are more important in warmer climates. However, bacterial canker of Prunus is a serious disease and although more often confined to smaller branches, can sometimes kill larger branches or whole trees.
The species P. syringae exists as a large number of pathovars, (abbreviated to pv.), so-called because although all look the same, they have different, specific hosts. The pathovar morsprunorum is restricted to Prunus species, pv. syringae has a much wider host range, but both cause similar symptoms on Prunus.
The bacteria exist as surface dwellers (epiphytes) on leaves and, during wet weather in spring or early summer, can enter through the leaf pores (stomata), causing infections to develop in the young leaves. As the leaf matures these infections cease to expand and are revealed as small patches of dead tissue. As the leaf expands fully, the live tissues pull away from the dead patch, which drops out, leaving a ‘shothole’.
Cankers develop when the bacterial cells gain entry through wounds or leaf scars at the time of leaf fall. Cankers remain more or less dormant through summer, when tissues are resistant, and during autumn and winter when temperatures are low. In spring, the infections spread rapidly, killing the bark.
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