The only way that you might be able to save the Dracaena is to cut off the upper part of the plant above the part infected by the fungus and repot it after applying some rooting hormone on the cut. The cut should be as high as possible to make sure that you left behind all of the fungal mycelia. I recently did the same thing to a Dracaena that had gotten too tall for the room that it was in, and it worked quite well, although it took a few months for it to root. Dracaenas are extremely hardy plants tolerant of all kinds of abuse.