Help! Possible pathogen on hornbeam

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This is on a European hornbeam (carpinus betulus) cultivated for bonsai.

I cut off part of a branch that I wanted to shorten and noticed weird black and brown bumps along a line on the branch. When I looked at the cross-section, there was a darkened segment that looked like the wood was rotting.

Does anyone have any idea what it might be? Could it be harmful? If so, what should I do to remedy this? A person on reddit said it might be damage from beetle larvae.

I am reluctant to cut off that entire branch as it’s pivotal to the design of the tree, but I could cut a little lower and carve out the part that looks dead/diseased. Would that be enough to stop the possible pathogen from spreading, or is cutting off the entire branch the only option?

Thank you in advance for your help!

Hey there,
I think it might be canker, or at least it looks like it.

I have no experience with the disease, only what I’ve read. The usual treatment in landscaping is to cut below the infected part to healthy tissue, but in bonsai that pretty much makes the tree useless in some cases.
Maybe someone has a less destructive idea on how to deal with it.

Yep, sure looks like canker. I’ll try and carve out the infected bits. Honestly, this is on the second trunk, and if I cut it off, I might as well lose the whole tree. Don’t wanna end up with a boring tree with no character.

I know the feeling, been there , done that.

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Unfortunately all you can do is keep cutting away at the infected branch until you think you have got it all out but looking at the second photo, it could go all the way to the roots. Trees are more susceptible to it when stressed. When you have cut it all away pay special attention to sealing the wound to prevent further attacks. It can also spread to and from other trees. Is it present on a tree in your garden or one possibly close by? It can be spread by insects and on the wind so it’s pretty darn good at getting around. I’ll keep my fingers crossed for you.

It’s just the one tree, far as I can tell. I carved it out, it didn’t go too far down beyond where I cut it, and it’s all removed now. I’ll be applying a systemic fungicide too.

But in case I didn’t cut all of it off (might not be visually apparent if it’s still there), will the fungicide kill off anything I missed?

It should. I would give it a couple of doses just to make sure. I would also put it into quarantine to guard my other trees.

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Thanks! Kinda impossible to quarantine the tree as is its a grow bed, but even on this tree it looked pretty well localized. I’ll keep checking my other trees every day for signs of infection to be on the safe side.

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