Clubroot

Symptoms

On the roots of brassicas (cabbage, cauliflower, oilseed rape, radish), characteristic swellings and thickenings form, deforming the root system. The damaged roots no longer take up water and nutrients properly, so the plants wilt during the day, especially in heat, grow poorly, turn yellow and become stunted. Over time the galls rot, releasing huge quantities of resting spores into the soil. The galls take the form of spindle-shaped, club-shaped or knobbly thickenings that often merge into large, tuberous masses on the taproot and the lateral roots. The disease affects only brassicaceous plants (Brassicaceae), including weeds such as shepherd's purse and wild mustard, which also harbour and spread the pathogen. The above-ground symptoms are non-specific: wilting (at first recovering at night, later permanent), stunted growth and dwarfing, small, loose heads, and the leaves are sometimes discoloured blue-green or reddish, so a reliable diagnosis requires lifting the plant from the soil and examining the roots. Young seedlings may die off completely. The rotting galls give off an unpleasant smell and release billions of resting spores, which contaminate the soil for as long as a dozen or more, up to twenty years. The development of the disease is favoured by acidic, warm and moist soils.

Treatment

The disease cannot be cured on an infested site, so diseased plants must be removed and destroyed. The key measures are liming the soil, which by raising the pH above about 7.2 inhibits the pathogen's development, and improving drainage; on a heavily infested field, brassica cultivation is abandoned for many years. There is no effective fungicide that cures clubroot; control relies solely on soil and sanitation measures. Liming is carried out well in advance (ideally several months before cultivation), so that the reaction stabilises at pH 7.2–7.4; carbonate lime, dolomite lime, or faster-acting oxide (burnt) lime may be used. An alternative that combines liming with action against the spores is calcium cyanamide (nitrogen lime), also applied in advance. Uprooted plants are removed together with the whole root system and destroyed (burning or disposal); they must not be composted, because the resting spores survive composting and passage through the digestive tract of animals. Tools, footwear and machinery should be cleaned of soil so as not to carry the pathogen to healthy sites. On heavily infested fields the only solution is a multi-year break from growing brassicas and a switch to unrelated species.

Prevention

A long crop rotation (5–7 years without brassicas), liming the soil to a neutral or alkaline reaction, improving soil structure and drainage, growing resistant cultivars and using healthy seedlings while avoiding the spread of infested soil on tools. Because of the many-year viability of the spores, the break from cultivation should be as long as possible (7 years and more), and no brassicas at all are placed in the rotation, including radish, oilseed rape or mustard as a catch crop. The systematic destruction of cruciferous weeds, which keep the pathogen going between vegetable crops, is essential. Seedlings are raised in a clean, disinfected substrate, so that the young plants reach a disease-free field. Improving drainage limits standing water, in which the motile zoospores reach the roots. Resistant cultivars (available especially for oilseed rape and some cabbages) effectively reduce infection, but their resistance can be overcome by pathotypes of the pathogen, so it is worth changing the sources of resistance and not relying on them alone. As a supplementary measure, trap plants are used (sowing brassicas that stimulate spore germination and then destroying them before infection), which only partly lowers the reservoir of spores in the soil.

How to tell it apart from look-alikes

Control — organic and chemical methods

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