Club root is an infection of the roots of brassicas and related plants by Plasmodiophora brassicae, a soil-dwelling  micro-organism related to the slime moulds, leading to massive swelling, distortion and severely retarded growth.

It affects vegetable brassicas such as Brussels sprouts, cabbages, cauliflowers, turnips, swedes and radishes, their 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.

You may see the following symptoms:

• Above ground: Stunted growth, purplish foliage and wilting in hot weather (wilted plants may revive temporarily under cooler conditions)
• Below ground: The root system becomes massively swollen and distorted, with a loss of the finer roots
• Growth and yield are severely reduced and very badly affected plants may die

Non-chemical control

• If you buy 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 infected plants
• If the disease is known to be present, try to give plants a head start by growing them on in healthy soil to a larger than normal size before planting out, or as transplants in pots of at least 9cm (3½in) diameter, so that they begin growth in the affected soil 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
• Some cultivars show some degree of resistance: Brussels sprouts 'Cryptus F1' and 'Crispus F1', calabrese ‘Monclano F1’, cauliflower ‘Clapton F1’ and 'Clarify F1', cabbages ‘Kilaxy F1’, 'Kilazol F1' and ‘Kilaton F1’,  kale ‘Tall Green Curled’ and swede ‘Marian’ show resistance

Chemical control

There are no chemicals available to gardeners to treat club root.

Plasmodiophora brassicae is loosely described as a fungus but is in fact more closely related to the slime moulds. It produces resting spores that can contaminate soil for up to 20 years. In the presence of susceptible plant roots, these resting spores germinate and infect the root hairs, causing the distortion. The pathogen produces more resting spores in the affected tissue, which eventually rots and releases them back to the soil, ready for the cycle to start over again.