Oh dear this sounds bad. However I would not use patio cleaner which would do more harm (to soil life) than supposed good. I wonder whether you mean white rot? If so it does indeed stay a long time in soil BUT:
no dig contains its spread because infected areas are not dispersed around by cultivations
a one year break, with brassicas growing eg cabbage, kale, Brussels, has been shown to diminish its vigour; you could even grow some oriental leaves from August sowing, after all onions are cleared
spread an inch or two of compost every autumn/winter to increase soil health
I would then try onions again, just pull any diseased ones from May, as soon as you see the yellowing leaves, hopefully it will be only a few, and plenty of good ones.