Onion White Rot

I managed to harvest my Overwintered Senshyu Japanese Onions this Sunday but have lost six of the biggest to white rot, I've had similar problems with spring onions in past years but for some reason it's never given me problems with larger onions.

So I'm thinking of growing all my onions in containers or flower buckets next year using a mixture or coir and compost, coffee grounds, and perhaps some Blood fish & bone to start off with, then a comfrey tea feed every 2 - 3 weeks.

Anyone grown onions in containers and managed to get a good crop?
Any suggestions ?