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What is a Bacterial Canker? How to identify and treat it
Bacterial canker (Pseudomonas syringae pv. morsprunorum and P. syringae pv. syringae) is a bacterial disease that only seems to affect the Prunus species of trees. It appears to be a lot more common in plums and cherries, however, I have also seen it affect apricot and peach trees. It’s also worth noting that it does affect both the ornamental and edible varieties of Prunus.
Evidence of this disease is found from mid-spring through summer so be on the lookout and be ready to prune as and when required because there is no chemical treatment. It is more about spotting the early signs of an infection. These symptoms include sunken dead patches of bark and brown spots on leaves that turn into tiny holes (also known as shothole). You will need to remove affected branches back down to healthy growth with clean secateurs or loppers.
How to recognise Bacterial Canker
I have briefly described how to identify Bacterial Canker, but here I will talk a little more in depth on what specifically you need to look for. Watch out for small cankers and shallow lesions that develop in mid-spring. You may also notice dead areas of bark on the branches, sometimes with a sticky sap coming from them. The branch dies if the infection spreads around it. The production of the sap without dying bark, though, could be evidence of a different injury to the tree.
If the young shoots in the spring fail to come up or, alternatively, start to grow and then die back, the reason could be bacterial canker. It is important not to mistake the dying branches on your Prunus for blossom wilt which is another fungal disease. If the tree is severely infected, this disease could affect most of the shoots on it.
If you suspect a Bacterial Canker, be sure to check the leaves for small round, brown spots that appear in them. The spots eventually fall out of the leaves leaving tiny holes which are more commonly known as ‘shotholes’.
What can you do about it?
When it comes to preventing Bacterial Canker, it’s more about when you prune and seal the wounds to prevent infection in the first place. Ideally, you need to be pruning Prunus trees around July or August when the tissues can resist diseases more effectively. This will also help prevent another fungus disease (known as silver leaf disease) which affects plums, apricots, cherries, and also apples. It causes the same dieback at the end of stems, but the leaves also turn silver. Hence it’s the name.
Cut out all areas of the tree that have cankers on them. Make sure to prune back to healthy wood, sterilising your cutting tools frequently, this is key to preventing spread.
Paint the cut with a wound product to protect (the wound) and prevent re-infection from Bacterial Canker and other common diseases. Make sure you burn the cuttings you have taken or send them to the landfill – never compost them.
Unfortunately, there are no chemical fungicide sprays (that I know of) for domestic use for this disease. Luckily pruning correctly and at the right time of year seems to help a lot.