I would also like to know the answer because one of mine is rotting too. It's very sad.

Are these winter storage plants dug up from the previous winter, seed,new nursery bought plants in pots, or over wintered in the ground with stalks or cut down ? Usually it is wet and cold that rots them if they do not stay dry before the growing season begins.

Ditto bamatufa. Before they get into good growth, the tubers are sensitive to rotting from too wet/too cool conditions. Warm/hot wet they love once they are in active growth. Live and learn. Here, I would put mine out in April or May on the south side of the house but move into the garage for heavy, prolonged rain events just to keep them drier--an overhang is useful in this regard.