Occurrence of clubroot (Plasmodiophora brassicae) in spring oilseed rape; Estimation and prediction of crop loss

Clubroot was detected in 148 (78%) out of 190 fields assayed for soil-borne inoculum with a bioassay method in a Swedish oilseed growing area. Subsequent assays of fields where no Brassica crops were grown in the following years indicated a significant decrease in disease incidence. Using the rate of decline observed, it was estimated that the level of soil infestation declines to below the detection level after a period of 17 years. The half-life of the spore inoculum was estimated as 3.6 years. In field experiments where increasing amounts of infested soil were supplied as inoculum and distributed prior to planting, relationships between yield and disease incidence and between yield and soil infestation measured by the bioassay were observed. A good relationship between disease incidence and degree of infestation according to the bioassay was also observed. Results from a field experiment site naturally contaminated with Plasmodiophora brassicae showed that the highest level of infestation (90% plants infected plants) decreased yield by 50%. At lower levels of infestation (<20% plants infected), yield decreases were estimated as 300 kg ha(-1).