Brassica downy mildew

What you see

Pale yellow blotches on the upper leaf, bounded by the veins, with a fine white-grey felt on the underside. Seedlings can be killed outright; on mature plants it disfigures the leaf and, on cauliflower, greys the curd.

Conditions

Cool, damp and still - autumn and spring, and any crowded seed tray. It thrives at 10-15C with long leaf wetness, so it is the classic propagation-bench disease.

Treatment

Increase ventilation immediately and space or thin seedlings. Remove affected leaves. Copper protects but does not cure.

Prevention

Sow thinly, ventilate frames and tunnels, water in the morning at soil level, and rotate brassicas on a three-year cycle.

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

Plants

Cabbage, Cauliflower, Broccoli, Curly kale, Rocket, Radish, Brussels sprouts

Remedies

Bordeaux mixture (copper), Bacillus subtilis biofungicide

The thing people get wrong

Most brassica downy mildew starts on the propagation bench, not in the bed - a crowded, unventilated seed tray in a mild damp spring is a perfect incubator, and the infection travels out to the garden inside apparently healthy transplants. Sowing half as thickly is a more effective control than anything you can spray later.