Detection of Plasmodiophora brassicae (causal agent of Clubroot) in vegetable production fields of the Pacific Northwest (2013)

Research report to the Oregon Processed Vegetable Commission

Cindy Ocamb
OSU Dept. of Botany and Plant Pathology

Clubroot, caused by Plasmodiophora brassicae, affects all species of the Crucifer family and produces durable resting spores which can persist in soil for 18 years or longer after an infected crop.  Because the pathogen cannot be cultured in the laboratory, traditional testing of soil for P. brassicae has been based on plant bioassays (soil baiting).  PCR-based assays have been developed in recent years that are highly specific to P. brassicae.  We evaluated PCR protocols for detection of P. brassicae in Willamette Valley field soil.  We found that extraction of DNA  and subsequent detection of P. brassicae using a PCR screen adapted from Cao et al. (2006) has been straightforward and gave consistent results when testing plant tissues but not when testing western Oregon field soil.  While we have had some limited success in detecting P. brassicae spores in DNA extracts from soil, we have determined that the presence of soil extract in a PCR reaction can mask the detection of P. Brassicae in our positive controls.  This indicates that powerful PCR inhibitors are in the soil are co-extracted with the DNA, making testing with the current protocols unreliable for Willamette Valley field soils.