I'm not sure if fungal pathogens will propagate in a
hugel bed. I'm sure someone can help you out.
My plums are only a few years old but there is a copper based fungicide that might help you out. However the damage sounds fairly extensive. You might want to coppice or outright swap out for something else. Even if you do not cut the tree the old wood will force fruit into the periphery where the wood is newer which is not natural for plums. This is why I aggressively trim my plums. Then again most of stuff acclimatized here is used to
deer and moose feedings so I am not doing anything different than what a moose would do.
For the record my plums put on 18-24" of new growth last year depending on the tree because of the hard prune. They look great.
You would be surprised how much a new plant can grow in three years time. In my
experience plums and apricots take drier conditions than other fruits. Given the wet climate it may time to consider something more disease resistant.