Why Has Old Begonia Stem Not Fallen Off?

It's not the Botrytis creeping all over this old broken begonia stem that's the problem. This has already been treated with kitchen bleach (carefully so as not to get any on the compost), a treatment which worked a treat on the Triphylla Fuchsia back in October. No. It's the fact that it's still firmly attached to the tuber; it has not fallen off as it should have done.

Normally these old stumps can be found lying on the compost some time in December, leaving behind a nicely-healed wound (I'm not sure this is formed in the same way as an abscission layer in a leaf petiole (stem)). Had this happened, fungal infection would not be a problem, since the scar (scab) would prevent entry of spores (a fungus's equivalent of seeds). However, tearing away the stump could open the wound which, in the moist warm conditions needed for germination, could create problems.

It's as if the tuber has been forced prematurely into dormancy before it could finish its work, probably by the sudden influence of plunging temperatures. Even worse: the tuber could have been frozen - it might be dead.

I don't know what will happen now things have warmed up a little - I was hoping to start propagating some begonias later this month. If there's no improvement within a couple of weeks, I might gently break it off and treat the wound with yellow sulphur powder to kill any unwanted disease spores. But I don't want to do this. There is always a chance things could go wrong.