Control of clubroot (Plasmodiophora brassicae) in oilseed rape using varietal resistance and soil amendments

Clubroot is a major threat to global brassica production. It has been an increasing problem in UK oilseed rape (OSR) crops due to the persistence of the soil-borne pathogen responsible for disease, Plasmodiophora brassicae, exacerbated by close rotations. Field surveys in the UK in the years 2007–2010 showed that clubroot was present in all areas of the UK where OSR was grown with 52% of the selected sites testing positive. Varietal resistance and soil amendment with calcium carbonate, calcium cyanamide and boron alone or in various combinations applied before sowing were assessed for the potential to manage clubroot. Soil amendments gave variable control between sites and years but showed some potential as part of a clubroot management strategy. Varietal resistance remained the more effective management option providing 50–95% disease control at three sites in England. However, this control was not consistently effective at sites in Scotland where resistant OSR varieties have been heavily used in rotations. Yield losses were demonstrated at 0.03 t ha−1 for every 1% increase in clubroot severity in the susceptible variety Kommando. Yield losses were only slightly lower per 1% increase in clubroot severity for the resistant variety Mendel at 0.028 t ha−1 despite lower disease levels. Losses in affected crops can therefore equate to over 50% of potential yield in severely infected crops. Soil testing for clubroot and lengthening rotations are important to the long term management of clubroot as varietal resistance and soil amendments can reduce clubroot severity but provide inconsistent results.