Its a disease. No cure once you have it. Be careful not to transfer any soil from the infected area to other parts of the garden, even on your boots. To prevent it another year try dipping the young plants into garden lime before you plant them, and also surround them with a small circle of the same. Don't plant any of the cabbage family in the same place for several years- this includes siberian wallflowers which can also be affected.

There is probably a chemical control but the lime treatment was taught me by an old gardener and it works.

Avoid it by never ever buying in any plant from the Brassica family and that includes Wallflowers. Grow all of them from seed and if you have not got Club root in your soil you would be very, very unlucky if you got it. It is soil based not seed.

there is a product on the market called PERLKA that I have used and so far this year I've got a healthy looking crop of brassica's after having clubroot in the soil for a few years. Try www.garden-supplies.co.uk