Q What is responsible for the white dusty covering on brassica leaves?

A Dusty, whitish coverings of leaves and stems is a sign of crucifer powdery mildew (Erysiphe cruciferarum). This spreads by airborne spores. Swedes and Brussels sprouts are the usual victims in gardens, but other plants can be attacked.

Sprouts develop black spots that ruin their appearance and they need extra trimming before eating. In severe attacks the leaves turn yellow and fall. This disease is worst in the south-east.

Caption: Powdery mildew can cause leaves to turn yellow and fall off

Q What can be done about powdery mildew on brassicas?

A Hot, dry seasons are favoured by powdery mildew. It especially affects plants on light or sandy soil. This can be helped by extra watering. Wider spacing of plants also reduces the disease's severity.

Q Are there any ways to avoid powdery mildew on brassicas?

A Many brassica varieties are said to be resistant. In practice, such varieties still may be affected. Late-sown crops are often less affected. Late sowing of turnips and swedes (in June) may reduce disease. Excess nitrogen makes plants more susceptible. Powdery mildew survives the winter on old plant material. Burn, bury or completely compost old plants to prevent carrying over the disease into the new year.