I was really curious about it since I have never seen it before, so I asked about it for you on another forum and was told by a Florida grower that he has had it happen too. Just cut off anything that is icky and they will likely recover.
I suspect it won't happen regularly. This kind of fungus is going to be really dependent on warmth and humidity.
If your drainage was really terrible, the tubers would have rotted - this is more about to much heat and humidity making just the right conditions for the fungus to happen.
