Clubroot is an infection of the roots of brassicas – broccoli, Brussels sprouts, cabbage and cauliflower, including swede and turnip – causing swollen and distorted roots and stunted growth. It also attacks radishes and other plants closely related to brassicas, including wallflowers, aubrieta and stocks (Matthiola).
The swollen roots, and the lack of finer feeder roots, means that affected plants are unable to take up water and nutrients from the soil.
Clubroot is always worse on overly wet or waterlogged soils and on acidic (low pH) soils.
Clubroot infections start when the soil is moist and warm, and tend to occur from mid-summer to late autumn.