It infects root tissues, stimulating abnormal growth of affected parts. This clubbed tissue significantly restricts growth and can also release spores which can be transferred to other fields. Within the soil, these spores can last over ten years, infecting any subsequent brassica crop in the rotation. While some fungicides are available, these are limited in activity and not available in many countries. Resistant varieties are also limited in their availability to resist all clubroot variants.
Good rotations (every 7 years); practices that minimize the spread of infected soil; better drainage and control of all brassica family weeds are all recommended methods of minimising clubroot.
Calcium nutrition through the use of calcium nitrate fertilisers is also essential to minimise clubroot. The use of calcium nitrate has been shown to reduce clubroot incidence from 90% to 40%.
Alongside this, liming to maintain a pH above 7.2 as part of an integrated control programme which utilises calcium-based fertilisers will also help to suppress the pathogen.
The positive effect of calcium nitrate compared to liming is probably because it is not possible to achieve a homogeneous pH throughout the soil when using lime.
Thus, a growing root may pass through lower pH sites, where the clubroot spores may be active. Furthermore, by utilising nitrates rather than ammonium-nitrate, the pH of the soil rhizosphere is increased or maintained. This helps provide a shield-like protection at the surface of the root, reducing risks.
In addition, calcium, from highly soluble calcium nitrate, further minimizes the risk of club root infection. Other fertilisers and other calcium sources (e.g. CAN) are not as effective as calcium nitrate. There is an indication from some trials that in certain situations, boron can also play a role in minimizing clubroot.