Posted: 02/03/2016 at 19:12
Pseudomonas syringae is a strong possibility. A bacterial infection not at all helped by cold and wet (a slight tiny wee small problem for those of us in the UK then!) I have had several Japanese acers affected but pruning well below the damaged area seems to help. Unfortunately acers don't really like being pruned either..
There aren't really any chemical treatments left that we're allowed to use, but copper sulphate can still probably be found, although you wouldn't dream of using it as a fungicide/bactericide of course..