Lettuce downy mildew

What you see

Pale angular patches on older outer leaves, squared off by the veins, with a white down beneath in the early morning. Affected tissue turns papery brown and grey mould often moves in behind it.

Conditions

Cool, damp and still - 10-15C with long leaf wetness. The classic autumn, winter and early spring lettuce disease, and severe under cloches and fleece.

Treatment

Strip the outer affected leaves - the heart is often perfectly usable. Ventilate covers during the day. Copper protects new growth.

Prevention

Wide spacing, morning watering at soil level, and lifting cloches on mild days. Grow resistant varieties - lettuce breeding for Bremia resistance is very good.

Months: October, November, December, January, February, March, April

Plants

Lettuce, Rocket, Endive

Remedies

Bordeaux mixture (copper), Bacillus subtilis biofungicide

The thing people get wrong

Winter lettuce under a cover is grown in exactly the conditions this disease wants, and the fleece that is protecting the crop from cold is what is giving it the disease. On any mild day, taking the cover off for a few hours does more for the crop than leaving it on - lettuce tolerates far more cold than gardeners expect, and far less damp.