Great care is needed when planting begonia tubers, since they have a personality change at this stage. Normally thriving on wet conditions, they hate them at germination time. Too much water at planting will rot the tubers. Watering should be done before planting to ensure correct wetness of compost.
HOW WET SHOULD COMPOST BE?
The same as required for germinating cuttings: so that water just appears between the fingers when squeezed hard in hand, but is drawn back into compost when grip released. Think of a freshly-wrung-out face cloth.
This tiny begonia tuber is all that is left of the one you saw recently in a 5 inch pot.
After removing the old diseased (grey mould - botrytis) stem and treating wound with yellow sulphur powder, I also carefully removed the old dry compost from around the tuber. Now there is hardly anything left. I now regret doing this; it might have been better to leave the old compost on.
Prepared some fresh compost in a 3.5 inch pot as described above and planted tuber up to neck. Placed in propagator with others.
Will have to watch out for botrytis re-appearing in humid conditions.