Plasmodiophora brassicae diversity in the UK and implications for clubroot management

Clubroot disease caused by the soil-borne pathogen Plasmodiophora brassicae is a major threat to oilseed rape (OSR) crops worldwide with yield losses conservatively estimated to be >15% in the UK. Fungicides are not available to control the disease and agronomic practices such as liming, adjusting crop rotations and improving field drainage offer limited success. An integrated approach to managing the disease is required, which should include the use of resistant cultivars. However, the (Mendel) resistance is not effective against all pathotypes of P. brassicae and is being eroded where repeated deployment of resistant varieties has occurred. The objective of the study was to determine the diversity of P. brassicae pathotypes within the population and quantify the prevalence and distribution of Mendel-resistance breaking strains. Soil was collected from 75 individual fields which were considered high risk for clubroot. Bioassays were carried out comprising susceptible OSR cultivar Tolken, clubroot resistant OSR cultivar Mentor and Chinese cabbage susceptible controls. The plants were assessed for clubroot symptoms after 8 weeks and data were converted to a disease severity index (0-100%). The data were standardized by expressing disease severity in the resistant cultivar as a percentage of the severity in the susceptible cultivar. Pathotype determination was carried out on soil from a random sub-set of 25 fields, using differentials of the European Clubroot Differential Set (ECD). Plants were established, maintained and assessed as in the bioassays. In the bioassay tests the resistant variety remained free of clubroot disease for 18% of soils. In 17% of the soils tested, 30% -100% of the pathogen population was able to produce clubroot symptoms on the resistant variety and were thus considered to have overcome the Mendel resistance. Clubroot severity and the level of sensitivity to Mendel resistance varied from field to field but the distribution of Mendel resistance breaking strains was widespread and occurred in 15 out of 17 regions sampled throughout the UK. Twenty different pathotypes were identified using the ECD set but it was not possible to identify a single dominant pathotype from the limited number of soils tested in the project. The findings from this study highlight the need for an enhanced range of control options and sustainable integrated clubroot management strategies.