Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2013
Publication Date: 6/20/2013
Citation: Agostini, A., Johnson, D.A., Hulbert, S., Demoz, B., Fernando, W., Paulitz, T.C. 2013. First report of blackleg caused by Leptosphaeria maculans on canola in Idaho. Plant Disease. 97(6):842. Interpretive Summary: Blackleg caused by Leptosphaeria maculans is one of the most important diseases of canola (Brassica napus) worldwide, and is widespread in the Canadian prairie provinces. However, this disease has been absent from Washington State and Idaho, which have been rated as blackleg-free. However, in summer, 2011, samples were obtained from the Bonners Ferry of northern Idaho, which showed blackleg symptoms on canola stems. The pathogen was identified morphologically and with molecular techniques. This finding has implications for the blackleg-free status of the Pacific Northwest, and could have large economic consequences if it moves to the Brassica seed production area of the Skagit Valley of western Washington.
Technical Abstract: Canola (Brassica napus L.) is produced in the dryland agriculture areas of eastern Washington State and northern Idaho, often in rotation with cereal cropping systems. Canola is also used as a rotation crop in irrigated circles in the Columbia Basin of Washington State and southern Idaho, where potato is the main cash crop. In 2011, 19,000 acres of canola were harvested in Idaho and 10,500 acres in Washington. One of the major diseases of canola around the world is blackleg, caused by Leptosphaeria maculans (aggressive) and L. biglobosa (non-aggressive). Both Washington State and Idaho have been considered blackleg-free, and production of rapeseed in Idaho is subject to government regulations. Canola seed originating from outside of Washington and Idaho should have a phytosanitary certificate. This disease is widespread in Canada, the US Midwest and South, and is the major disease of canola in these areas. In Aug. 2011, a sample from a canola field in Bonners Ferry, Idaho was brought for diagnosis to Washington State University. The canola stems showed the typical grey to dark-grey lesions with black pycnidia. The pycnidia and conidia were examined microscopically, and found to be similar to descriptions of Phoma lingam (1). Samples were sent to the University of Manitoba for confirmation with PCR. The pathogen was cultured out of stems on V8 juice agar amended with streptomycin and 23 single pynidiospore isolates were made from the cultures. DNA was extracted from the cultures using methods in (2) and a multiplex PCR was performed with species-specific primers for L. maculans and L. biglobosa. The reaction produced a 330 bp amplicon for L. maculans and a 440 bp amplicon for L. biglobosa. Based on this, all 23 isolates were identified as L. maculans. The source of the seed used in the infested fields is not known at this time. This disease is seedborne, and may pose a threat to the two major vegetable and oilseed Brassica seed production areas of Washington: the Skagit River valley of western Washington and the Columbia basin area of central Washington. In addition, the susceptibility of Pacific Northwest varieties of canola and other Brassica oilseeds are largely unknown.