White blister

Albugo candida

also called white rust, white blister rust

Glistening white, blister-like pustules on the undersides of brassica leaves, with pale yellow blotches showing on the upper surface.

Water mold Disfiguring Airborne sporesRain splashSpread in water Spreads to neighbors

What you'll see

Shiny, raised white pustules, like little blisters, mostly on the leaf undersides.
Corresponding pale green or yellow blotches show through on the upper leaf surface.
Pustules split to release a powdery white mass of spores.
Infected shoots and flower stalks may swell, twist and distort.
On the crop it disfigures edible leaves and heads, though the plant survives.

Why it happens

A downy-mildew relative — not a true fungus but an oomycete — specific to the cabbage family. It survives on debris and related weeds and can be carried on seed; its spores spread on wind and rain splash and germinate in a film of surface water.

Cool, wet, humid conditions favor it, so it is worst in damp springs and autumns and on crowded plants where the leaves stay wet.

Treating it

Pick off and destroy affected leaves as soon as blisters appear.
Remove and bin badly infected plants rather than leave them to shed spores.
Improve spacing and airflow, and water at the base to keep foliage dry.
Keep brassica weeds such as shepherd’s purse cleared from nearby ground.
No fungicide is normally warranted; cultural control is usually enough.

Keeping it away

Rotate brassicas so they don’t follow themselves on the same ground.
Clear all brassica debris and control related weeds after harvest.
Sow at sensible spacing for good air movement, and avoid overhead watering.
Grow resistant varieties where white blister is a regular problem.
Use clean seed from a reputable source.

Who gets it

Cabbages, Brussels sprouts, broccoli, cauliflower, swede, turnip and other brassicas, along with related ornamentals such as candytuft.