Geoff, my understanding is that white rot is spread physically i.e. if you dug up a chunk of infected soil and spread it in an unaffected area, you will increase it. Don’t do that though 😉
I’ve had trouble with it, but it doesn’t put me off growing onions because:
1) I cleared an area of our plot (weeds, rubbish) that was unused for many years; this is the main onion growing bed.
2) We harvest and then fry for about ten minutes, any infected onions, followed by freezing. The yield is less but we still gain something. Any trimmings go in the landfill bin, not the green composting.
3) I’m going to try “Golden Bear” next year as it, allegedly, has white rot resistance!
Hope this cheers you up/ increases your options a bit, as it can be very disappointing to see your hard work and care end up in a failed crop. Cheers.