Oh, oh, not good that it's on the central parts of the plant... on the outside it would have been easier just to have removed an outer infested leave or two and then followed the following procedure.... The best cure known to me so far, would be to remove as much of infested tissue as possible, apply Dragon's Blood (100% croton lechleri) to the infested areas and then sprinkle these same areas generously with cinnamom powder...
Professional growers had some time back access to some very efficient, but very chemical and very toxic remedies. In Europe, or more precisely in the EU, it's now not only prohibited to use these chemicals, possession/storage of them. is strictly forbidden and considered a criminal offence.
However, croton lechleri and cinnamom have helped me to save quite a few plants. But better hurry, as in my experience there is a tipping point, where the rot has infested so many parts of the plant, that it might be beyond repair!
Good luck!
