white rot

A fungus that you can do absolutely nothing about. I had the same problem last year on the allotment. This year I dug out a meter width and refilled it with rotted grass, compost and horse manure and had a wonderful crop of onions. The adjacent shallot patch had 3 bulbs with white rot so I will repeat the process. This patch will remain the onion bed for the foreseeable future. Just have to be very careful with re-contamination.