Club root is an infection of the roots of brassicas and related plants by a soil-dwelling micro-organism related to the slime moulds, leading to massive swelling, distortion and severely retarded growth.
It affects cabbages, cauliflowers, turnips, swedes, radishes and ornamental relatives such as Cheiranthus (Wallflowers), Matthiola (Stocks), Aubrieta (Aubretia) and cabbage-family weeds such as Capsella bursa-pastoris (Shepherd’s purse).
Club root can infect whenever the soil is moist and warm, so most new infections occur from mid-summer until late autumn.
There are no chemical controls available to gardeners to treat club root and unfortunately once club root takes hold it’s very difficult to get rid of.
This is the advice given by the RHS (Royal Horticultural Society) and is basically just good garden hygiene and housekeeping. I would follow these non-chemical controls to eradicate the problem:-
If you buy in your brassica plants, take great care that they come from a guaranteed club root-free source. Be particularly careful in accepting plants from gardening friends, who with the best of intentions may be an unwitting source of infection.
If the disease is known to be present, try to give your new plants a head start by growing them on in healthy soil to a larger than normal size before planting out, so that they begin growth in the affected soil with a larger than usual healthy root system.
Grow transplants in pots of at least 9cm (3½in) diameter, which give plants a head start with a larger than usual healthy root system.
Beware of spreading contaminated soil on tools, wheelbarrows or footwear.
Club root is reduced (but not eliminated) by raising the soil pH by liming. On acid soils, lime at the rate of 500g per sq m (15oz per sq yd), with lighter dressings of 270g per sq m (8oz per sq yd) in future years.
Along with the liming regime, take care to improve drainage, by making raised beds for example.
Keep down susceptible weeds like shepherd’s purse, charlock, wild radish.
Choose brassica cultivars that show some level of club root resistance, more and more are available each year via the gardening seed catalogues.
Hope this information is of help to you in eradicating this problem.