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.
New disease alert: The bacterium Xanthomonas arboricola pv. pruni also affects Prunus species, in particular apricot, peach and plum but also cherry laurel (Prunus laurocerasus). The disease is not established in the UK but has been intercepted on imported plants of cherry laurel. Symptoms include shothole, fruit and leaf spotting (often with a distinct yellow halo), stem cankers and severe defoliation. Any suspected cases should be reported to the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal.