You might try touching the rhizome right at the base of the original fan. Based on its appearance, I'd expect you will find that it is soft and mushy. If so, scoop out all the mushy stuff with a spoon or something similar. Many people will then either sprinkle Comet or pour a 10:1 water:bleach solution on the remaining parts of the rhizome. I just leave the remaining parts exposed to the sun, but I live in a relatively dry part of the country.
You may find that the entire rhizome is mushy, in which case, it's too late. But, it doesn't hurt to try. If there are any good parts (they'll be hard enough that you can't cut through them with a spoon) left, you need to remove the rotten parts so the rhizome can heal.
