Q What causes white spots on brassicas?

A If small, white, smooth, shiny patches like scales appear beneath the leaves, white blister (Albugo candida) is probably to blame. It starts off as green blisters, which then form white warts, eventually turning powdery. It occasionally distorts the heads of cauliflowers, cabbages and broccoli, and disfigures Brussels sprouts. It may attack stems and flowers as well. Seedlings can be affected. White blister looks serious but seldom reduces growth or yield.

Caption: White blister looks unattractive but seldom affects yield or growth

Q What can I do about white blister on brassicas?

A It is worst where airflow is poor and the atmosphere humid. It spreads by rain splash, wind and insects carrying its spores from plant to plant. Sheltered gardens are likely to be more affected than open allotments. Warm, wet autumn weather is the most likely period for attacks. Remove the worst-affected plants by burning, burying or binning. Keep plants spaced out, especially in seedbeds. Resistant spores can remain in the soil, so grow cabbage-family plants in a different spot each year.

Q Can I spray against white blister on brassicas?

A There are no longer any approved fungicides that gardeners can use to control white blister. Some Brussels sprout varieties are thought to be partly resistant