Shallots - basal rot

I've just thrown away about half my harvested shallot crop, having realised that rather than ripening and drying many of the bulbs were going soft and rotting at the base. Some had a thick black tarry liquid at the base. (No sign of white fluff though.)  Longor, which has the longest and deepest bulbs is by far the worst affected variety. Is it due to all the rain in recent weeks?  Any advice on causes or what do in future years would be appreciated. I was thinking of trying rounder varieties grown as high on the surface as possible.

G&R

Had a similar problem with mine this year. I grow round varieties, the Red Sun were worse than the Golden Gourmet.