Plasmodiophora brassicae accesses the plant through its roots. Infected plants produce either no crop or only a poor crop.
Either grow from seed only, or buy plants from a reputable source and inspect the roots carefully before planting. Choose resistant varieties. Keep weeds down, especially brassica family weeds, and mulch with compost to help boost the number and range of soil organisms, some of which can help to suppress club root. Wash boots and tools well to avoid transferring the disease to other parts of the garden. Liming the soil to increase the pH and improving soil drainage will help, but will not eradicate the disease.
Destroy all infected plants as soon as seen, ideally by burning, and wash boots and tools well to avoid transferring the disease to other parts of the garden. Plasmodiophora brassicae can remain active in the soil for more than 20 years, even without a host crop, so avoid growing cabbage family plants in that same area of ground again.