Further Development of “in field” Tests for Resting Spores of Clubroot and the Development of Control Based on Detection

Oilseed rape (OSR) is widely grown in the UK and has a high economic value for many arable farmers, frequently forming part of cereal rotations. As a member of the Brassica family, OSR is susceptible to the clubroot pathogen (Plasmodiophora brassicae). Once soil has been contaminated, clubroot spores can remain viable for several years causing any subsequent oilseed or vegetable brassica crop to be at risk of infection. With the development of new detection methods based on identifying Plasmodiophora brassicae DNA levels in soils, the presence or absence of clubroot risk can be determined; however, these are limited by the need for processing in a laboratory.
The primary aim of this work was to validate a clubroot lateral flow device (LFD) for use in fields by OSR growers, therefore avoiding the need for specialised laboratories. Quantification of spores by lateral flow devices (protocol developed and reported earlier by Kennedy et al, 2013; AHDB Horticulture final report FV349) was tested against a molecular quantification method (quantitative PCR; also validated and reported earlier by Kennedy et al., 2013) to establish whether a commercially viable diagnostic test could be provided for growers. In addition, the effect of oilseed rape cultivar on disease development was investigated through pot-based bioassays, as was the effect of P. brassicae resting spore density on seedling infection levels. An additional objective was to study the effect of clubroot on yields of resistant and susceptible oilseed crops.
In a comparison of spore quantification by lateral flow device (LFD) and quantitative PCR, it was found that the LFDs overestimated the number of clubroot spores in soil samples. However, qPCR proved to be a reliable assay after appropriate soil sampling. In the pot-based bioassays, there was increased damage observed on OSR roots at higher soil spore concentrations in both a resistant and a susceptible cultivar; it was notable that galling (at very low levels) was observed during glasshouse trials on the main resistant cultivar of OSR currently used in the UK: cv. Cracker. Of other cultivars tested, a range of disease severity was observed.
The relationship between clubroot resting spore density at planting and plant infection at harvest and yield was examined by comparing OSR seed weight from fields with and without clubroot presence. There was a good relationship between the clubroot detection in soil at planting and subsequent infection on plants at harvest (r2 = 0.726). This indicates that clubroot quantification at sowing in soil using a diagnostic test is a good indicator of subsequent plant infection. However, this does not necessarily allow for a determination of likely yield loss for oilseed rape. At sites where clubroot resting spores were detected at planting, there were consistently higher yields where clubroot resistant varieties were used in comparison to clubroot susceptible varieties.