Symptoms are observed both above ground and below ground. Overall, plants are declining, showing a stunted growth and a yellowing of the leaves. They will tend to wilt in dry weather, but recover under wetter conditions. Leaves may also turn purple. Below ground symptoms include the development of knotted swellings on the roots and a loss of the smaller roots (also called root hairs). Over time, the swellings lead to a severe distortion, ending up in club-shaped aspect of the roots instead of the normal fine network (thereby the common name of the disease). Growth and yield are severely reduced and very badly affected plants may die.
The symptoms of the disease are caused by the infection of the roots by the soil-dwelling pathogen Plasmodiophora brassicae. It is an obligate parasite that affects, among other plants, a series of important crops, such as Brussels sprouts, cabbages, cauliflowers, turnips, and radishes. The strategy of the fungus is to produce dormant spores that can contaminate soil for up to 20 years. In the presence of susceptible plant roots, these spores germinate and infect the root hairs, causing the swellings in the roots that give the disease its name. These swellings then produce more spores that are released to the soil, completing the cycle. The disease is favored by moist and warm soils. Club root is reduced (but not eliminated) by raising the soil pH by liming.
The only organic control available is to raise the pH of the soil to a more alkaline 7.2 by mixing oyster shell or dolomite lime into the soil in the fall (small gardeners and farmers). Simple and affordable soil test kits are available to check pH often.
Always consider an integrated approach with preventive measures together with biological treatments if available. Soil fumigations are not recommended because they are not 100 percent effective. The raising of the pH (7.2) through addition of limestone (calcium carbonate CaC03) and hydrated lime (calcium hydroxide Ca(OH)2) before planting is a way to reduce the incidence of the disease.