Fungus. Is there still the old (shrivelled and rotten) left-over from last years tuber there? In that case you can remove it together with the fungus and you are ready (and the tuber safe).
If not find out if there is any rotten debris in the tuber under the fungus. Remove all soft rotten material till you are in the healthy part of the tuber. Take some charcoal, powder it and add cinnamon (a natural fungicide) to it and rub it in. Keep your Sauromatum dry and plant next year. As the grow point of both plant and roots are at the top they only lose some food-reserve, which they easily survive as long as the rot stops.
Ruud
