White rust
A disease that forms white, chalky blisters on the undersides of leaves of mustard greens, amaranth, radish and other leafy crops.
Also known as: White blister, Albugo
Symptoms and identification
Raised, white, powdery or chalky pustules on the lower leaf surfaces, with yellow patches above. Stems and flower heads may become swollen and distorted.
Causes
The water mould Albugo spreads in cool, damp weather with heavy dew. It survives on crop debris and weeds of the same family.
Organic treatment
Remove and bin affected leaves early. Harvest healthy leaves promptly. Copper-based sprays can protect healthy plants during damp spells.
Prevention
Space plants well, water in the morning at soil level, rotate crops, and remove cruciferous weeds such as wild mustard.
Chemical treatment (use with care)
Use a fungicide labelled for white rust on that crop, following the label and the waiting period before harvest.