- Clubroot, caused by Plasmodiophora brassicae, affects all species of the Crucifer family, including wild mustard. It is a fungus-like microorganism that can persist in soil in the form of durable resting spores for 18 years or longer after an infected crop. These resting spores can be spread through any means that transports soil or other growth media: wind, water, footwear and equipment, as well as through movement of infected transplants. Soils that are cool, wet (70-80% water-holding capacity), and acidic provide a favorable environment for the pathogen; conditions that are common at least part of the year in the Willamette Valley. The distinctive symptom of clubroot is abnormally large roots—including the taproot, fine roots, secondary roots—and sometimes portions of the underground stem. Roots which are severely clubbed have a reduced capacity to absorb minerals and water from soil. Plants may wilt or be stunted, and are likely to prematurely bolt.
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. These PCR-based diagnostic tests are currently planned for use in Canada to test field soils in canola biofuel production. No testing facilities in the Pacific Northwest are offering a PCR-based test for P. brassicae at this time. Though testing costs are currently rather expensive, development of robust sampling protocols should ameliorate costs to producers. The PCR test could provide reliable, rapid diagnosis for routine detection of P. brassicae in soil if testing hurdles we’ve encountered in development can be overcome.