This sounds like the dreaded onion White Rot. I have this on my allotment – but strangely very patchy. Some spring onions have had it, but others in the same row haven’t. Similarly, I have raised onions (Kelsae) from seed and some have it and some not. Same with shallots. Some leeks were affected last year. It shows as a white fungal growth at the base of the bulbs leading to rotting.
I understand from the Hessayan book that it could be 8 years before it clears – sorry to be the bearer of bad tidings if this is what it is.
