North Dakota leads the United States in canola (Brassica napus L.) production (4). A canola field with a distinct patch of dead plants spreading over an area of approximately 0.4 ha was detected in Cavalier County, North Dakota, in early September 2013. Numerous spots within the patch had plant mortalities >80%. Dead plants pulled from the soil had roots with severe galling and clubbing. Clubbed roots were brittle and disintegrated easily when pressed between fingers. Root and soil samples collected at several locations within and outside the affected patch were pooled in separate groups. All plants collected in the patch were symptomatic but those collected outside were not. In the lab, total genomic DNA from three symptomatic and two healthy root samples was extracted using standard procedures and freehand slices were prepared for observation with a compound microscope. Also, DNA from pooled soil samples was extracted using FastDNA Spin Kit for Soil (MP Biomedicals, Solon, OH). Round resting structures ranging from 2.2 to 4.2 μm in diameter were observed by microscopic examination of symptomatic root tissues. These structures resembled those typically produced by Plasmodiophora brassicae Woronin. This initial identification was later confirmed through PCR analysis using the species specific primers TC1F/R and TC2F/R (1). PCR products of 548 bp (TC1F/R) and 519 bp (TC2F/R) were produced in the three symptomatic and two infested soil samples, confirming the presence of P. brassicae. PCR amplicons were not detected in healthy root and soil samples. Pathogenicity tests were conducted in greenhouse to fulfill Koch's postulates. Briefly, five square plastic pots (10 × 10 × 13 cm) were filled with a 10-cm layer of Sunshine Mix #1 potting mix (Fison Horticulture, Vancouver, BC, Canada) and then 1 g of ground root galls (approximately 5 × 105 resting spores) was spread evenly on its surface and covered with 2 cm of soilless mix. A similar number of pots were filled only with soilless mix and used as controls. All pots were planted with two seeds of canola cv. Westar and incubated in greenhouse conditions at 21°C and 16 h light daily. The experiment was conducted twice. Four weeks after planting, all plants in the inoculated pots had developed galls while plants in control pots were symptomless. Presence of P. brassicae resting spores in the newly developed galls was confirmed by microscopic observations and PCR. Based on the symptoms, morphology of resting spores, PCR reactions, and pathogenicity tests, we confirm the presence of P. brassicae on canola. While P. brassicae has been reported as widespread in North America (2), to our knowledge, this is the first report of clubroot on canola in North Dakota and the United States. Clubroot became the most important disease affecting canola production in central Alberta, Canada, within 5 years of its discovery in 2003 (3); since then, the disease has been detected in Saskatchewan and Manitoba (3), Canadian provinces that share borders with North Dakota. Considering the difficulties in management of clubroot, measures should be initiated to limit the spread of the disease before it could pose a threat to United States canola production.
References: (1) T. Cao et al. Plant Dis. 91:80, 2007. (2) G. Dixon J. Plant Growth Regul. 28:194, 2009. (3) S. Strelkov and S. Hwang. Can. J. Plant Pathol. 36(S1):27, 2014. (4) USDA-NASS, Ag. Statistics No. 81, 2012.
