My dad had an allotment in the 80s and one of his "things" was growing giant onions. Not for shows or anything, but just for pleasure, trying to beat his own records.
Anyway he grew a crop of non-giant onions as well every year which fitted into his rotation, but the giant onions had their own permanent bed. I don't remember him getting white rot - I'll have to ask him how he prevented it, because it sounds like he was just asking for trouble.