What is bacterial canker?
Bacterial canker is caused by the bacterium Pseudomonas syringae pv. Syringae. It affects cherry most of all, but it can also damage plum, greengage, damson, peach, apricot and almond trees. The bacteria are spread by rain, they overwinter on the trunks and branches, and in spring, they enter the tree through a wound. It develops and grows, producing a toxin called syringomycin which destroys tree tissue. The risk of infection by bacterial canker is increased by wounds on trunk or branches, which may be caused by frost damage, or by pruning in the dormant season (read more about pruning cherries here). Even the natural scars caused by leaf fall represent a possible route of infection.
Bacterial canker can be recognised from the sunken patches on trunk and branches that exude a sticky gummy substance. Affected branches show dieback – when leaves and shoots wilt and die during the growth season. In addition, dark purple, brown or black spots on the leaves become holes or “shotholes” when the dead tissue falls away. The entire leaf may turn yellow and fall. Some cherry varieties are more susceptible to bacterial canker, such as Royal Ann, Bing, Lambert, Napoleon, Sweetheart and Van. Other varieties are more resistant, such as Corum, Regia, Rainier, Sam and Sue.
How to treat bacterial canker
To control bacterial canker, if you manage to identify it when there is just a small spot on a branch, you can remove the infected tissue using a curved canker knife, making sure that none of the material falls onto the ground, and disinfecting the knife after you have finished. If the bacterial canker is more extensive and is affecting a single branch, cut out this branch, dispose of it in a sealed bag in the non-recycling bin, and sterilize your saw or pruning tools. Seal the wound with Heal and Seal. If the condition has gone too far and the canker can be seen on several branches and the trunk, cut the tree down, and remove the stump and as much of the root system as possible. Dispose of the tree in sealed bags placed in the non-recycling bin. Do not plant another tree of the same family, as the infectious agent may survive in the soil.
Preventing bacterial canker in cherry trees
For bacterial canker, more than treatment, there are some preventive measures worth consideration before planting trees. If you want to plant new trees, make sure that there is no old cherry tree in the vicinity, which could cause infection. Choose a location that is less affected by frost. Check soil pH and ensure that soil does not contain ring nematodes (read more about how to obtain a soil analysis here). Ensure that soil conditions remain good, in particular moisture and nutrients. Prevent the growth of weeds and grass around the tree. Never prune during dormancy: it is best to prune after harvest, in dry weather.