Downy mildew is definitely more of a problem than it used to be, and last year with it's damp conditions, followed by this year's cold late spring, have made matters worse. The spores are airborne so rotating the crop (which you should be doing anyway) won't help much. F1 variety Santero, from Suttons, is allegedly mildew resistant. Other things you can do are space the crop well, so the leaves don't touch, and watch any chives or welsh onion on your plot like a hawk - they may be harbouring the disease and infecting your onions.
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