Onion downy mildew

What you see

Pale oval patches part-way up the leaf, which in humid mornings carry a violet-grey furry bloom. The leaf collapses and folds over at the patch. Bulbs stay small and store badly.

Conditions

Cool, damp, still conditions with long dew periods - a mild wet spring is ideal. It overwinters in the soil and in infected bulbs and sets.

Treatment

Remove the worst leaves. Copper protects the remaining foliage. Lift the crop early rather than nursing it - bulbs from infected plants do not keep.

Prevention

Wide spacing and a sunny, airy site. Rotate four years. Buy clean sets. Do not plant onions downwind of last year's onion bed.

Months: March, April, May, June, November, December

Plants

Onion, Shallot, Garlic, Leek, Spring onion

Remedies

Bordeaux mixture (copper)

The thing people get wrong

The important consequence is not in the garden but in the store: onions from a downy-mildew crop rot in the net in November even if the bulb looked sound at lifting. If mildew got into the leaves, plan on eating that crop fresh through autumn rather than counting on it for winter.