I've been browsing the web & found an interesting article on controlling onion white rot (http://www.ipm.ucdavis.edu/PMG/r584100511.html)

It suggests that you can greatly reduce the number of spores surviving in the soil by treating the soil with garlic extract when no onion related crops are anywhere near the soil. The idea is that the garlic oil tricks the white rot spores into germinating & they then die without reproducing because there are no onion/garlic/leek roots to infect.