Assessment of the response of Plasmodiophora brassicae in contaminated horticultural land, using lime‐based fertilizer concentrations

Infection of brassica crops with the clubroot pathogen, Plasmodiophora brassicae, can result in stunted plant growth and wilting, which can severely affect crop yield. Determining P. brassicae infection within a field prior to crop planting has long posed a problem for choosing appropriate control treatments. The options for control of this pathogen are limited and in the UK and are based on adjusting pH with soil amendments. In this study quantitative polymerase chain reaction (qPCR) was investigated for measurement of this pathogen in different control treatments. The qPCR was capable of reliably quantifying P. brassicae at levels greater than and including 103 resting spores/g soil. The assay was used to study the effect of lime‐based products (LimeX) on the incidence of the clubroot pathogen in field trials with broccoli crops grown on contaminated land. The results showed that variation occurred in clubroot resting spore levels in treated and untreated plots during the crop growing period. In year one there was a 96% decrease in spore load during the growth of the crop. Treatment with LimeX resulted in a greater marketable head weight of broccoli in 2 years of the field trials, and significantly reduced gall numbers on the roots in 1 year. The rate of lime (calcium carbonate) application was not found to have a significant effect in this study, however a greater reduction in clubroot was observed at higher LimeX concentrations.