Aimee says: I've got onion rot! Nightmare! Lost 50 onions, may have saved 20. All that weeding gone to waste...

It may have come into my plot on the sets, or it may be in the site soil - lasts for 15 years. Signs are yellowing foliage and when you pull the onions up they have white fluff on the base. 

See the link for more info. Hope you aren't afflicted too.