White blister

What you see

Glossy white, chalky pustules on the underside of the leaf that burst like small blisters, with corresponding yellow blotches above. Stems and flower heads can become grotesquely swollen and distorted.

Conditions

Cool, wet, humid weather; worst in autumn and mild wet winters. Spores need free water to germinate and are splashed and blown short distances.

Treatment

Remove and bin affected leaves - do not compost. There is no effective home treatment once it is established, so the crop is managed rather than cured.

Prevention

Three-year rotation. Wide spacing. Remove brassica weeds such as shepherds purse, which harbour it. Clear all crop residue promptly.

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

Plants

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

Remedies

Bordeaux mixture (copper)

The thing people get wrong

The white pustules look exactly like a fungal rust and get treated with sulphur, which does nothing - Albugo is an oomycete, biologically closer to algae than to fungi. It is also almost entirely cosmetic on leafy kale and cabbage: strip the marked outer leaves and the heart is perfectly good. The one time to take it seriously is on rocket, where it ruins the whole crop.