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First Report of Blackleg Caused by Leptosphaeria maculans on Canola in Idaho

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 and southern Idaho, where potato is the main cash crop. In 2011, 7,700 ha of canola were harvested in Idaho and 4,200 ha 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 and Idaho have been considered blackleg-free, and production of canola 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 and the U.S. Northern Plains, Midwest, and South, and is the major disease of canola in these areas. In August 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 gray to dark grey lesions with black pycnidia. The pycnidia and conidia were examined microscopically, and found to be similar to descriptions of Phoma lingam, the anamorph of L. maculans (2). 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 22 single pynidiospore isolates were made from the cultures. DNA was extracted from the cultures using methods described in Fernando et al. (1) and a multiplex PCR was performed with species-specific primers for L. maculans and L. biglobosa. The reaction should produce a 330-bp amplicon for L. maculans and a 440-bp amplicon for L. biglobosa. Based on this, all 22 isolates were identified as L. maculans. The susceptible cultivar Westar was inoculated with the isolates, by wound inoculating 7-day-old cotyledons with a concentration of 10⁷ spores/ml. Plants were kept in a moist chamber at 23°C. After 14 days, plants were rated for disease with a 0 to 9 scale, where 0 = no infection and 9 = tissue collapse and appearance of pycnidiospores. Isolates with rating ≥5 are considered virulent. All isolates produced a rating of 7 to 9, indicating a high level of virulence. 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 is largely unknown.

References: (1) W. G. D. Fernando et al. Plant Dis. 90:1337, 2006. (2) S. Roger Rimmer et al. Compendium of Brassica Diseases, APS Press, 2007.