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.
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.
You may see the following symptoms:
X) 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. X) On leaves: Small brown spots appear which are often round and fall out later 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 paint promptly with a wound paint to protect the wound from re-infection.
Literature from Royal Horticultural Society (===https://www.rhs.org.uk/advice/profile?pid=86===)
Our risk model in FieldClimate is based on following conditions:
Risk Model based on daily values
if soil water tension all day is smaller than 25 and rain is more than 2 mm and leaf wetness duration is longer than 6 hours risk increases by 20% else
if soil water tension all day is higher than 40 risk decreases by 20% or
if no leaf wetness all day risk decreases by 10% or
if no hour with more than 70% rel humid risk decreases by 10%