Club root is the thickening and distortion of the roots of Brassicas by the soil fungus Plasmodiophora brassicae. Roots are distended and swollen and may appear to be shaped like fingers or tumors. Infected plants will exhibit day wilting (wilting during the day, recovering at night), and general poor production. Root Brassicas like radishes and turnips may show black lesions.
All Brassicas are prone to this fungal disease: arugula, bok choy, broccoli, Brussels sprouts, cabbage, canola, cauliflower, ching chiang, collards, cress, daikon, gai lan, horseradish, joi choi, kale, kohlrabi, mustards, pac choi, radish, rape, rutabaga, tatsoi, toy choy, turnip, wasabi, and watercress.
The fungus can lay dormant in soil as a cyst for as long as 8 years. Acidic, wet, cool, and poorly drained soils are the most attractive environments. There is no treatment for infected soil, so prevention is absolutely key.
Crop rotation is essential in preventing club root from becoming established in the soil. Brassicas must never be planted in the same soil for 5 to 7 years, and this is true between the varieties – don’t plant broccoli where turnips have grown, for instance.
Maintaining a high pH (above 7.2) by annually applying lime is good for Brassicas and makes the soil less suitable for the fungus. Use good finished compost, and raise healthy transplants – but crop rotation is the fundamental prevention. Use calcitic lime if soil magnesium levels are moderate to high, and dolomite lime if magnesium is low. Quicklime may be incorporated a week before planting as a last resort. Brassicas are heavy users of boron, and boran appears to help plants fight this disease. Supplement Brassica beds up to 2 parts per million of boron. Do not over-apply.
Make sure to dispose of any infected plants in the garbage or fire pit – not the compost.