Clubroot (Plasmodiophora brassicae) is a fungal infection that affects the brassica family which causes swollen, distorted roots and can stun plant growth. Clubroot normally strikes from mid-summer to autumn when the soil is warm and moist, and once you are infected, can remain in the soil for up to 10 years.
Clubroot can be easily spread by walking from one infected plot to another. Another common way clubroot is spread is by plants themselves, so it is important to buy plants from clubroot free sources. You can help reduce the affects of clubroot by regular liming to raise the pH level of the soil and also improving soil drainage. Unfortunately, there are no chemicals available to treat clubroot.
Some varieties of the brassica family do show some resistance; for instance Tall Green Curled (kale), and Clapton (cauliflower), so it is worth investing time to look for these varieties.