Club root is a fungal disease of brassicas and related plants such as wallflowers that results in the roots becoming swollen and distorted. They cannot take up water and nutrients adequately, so the leaves become discoloured and the plant is stunted. The spores can remain in the soil for 20 years and are easily spread from place to place. It is best to grow your own brassica seedlings, as the disease is most frequently introduced in imported soil. Brassicas prefer a slightly alkaline soil, and club root cannot live in alkaline conditions. So always dust the soil with lime before growing any member of the cabbage family. Practising crop rotation reduces the chance of the disease becoming established. Failing that, grow your brassicas in pots for as long as possible before planting out. You should, find that they manage to produce a crop before the disease takes hold.