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.
- 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.
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.
- 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.
- 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.
Cabbages, Brussels sprouts, broccoli, cauliflower, swede, turnip and other brassicas, along with related ornamentals such as candytuft.
The Cottage Garden Companion — the gardener's apothecary at cottagegardencompanion.com