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Title: First report of Leptosphaeria maculans and Leptosphaeria biglobosa, causal agents of blackleg, on canola in Washington State

item Paulitz, Timothy
item Schlatter, Daniel

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2016
Publication Date: 3/1/2017
Citation: Paulitz, T.C., Knerr, A.J., Carmody, S.M., Schlatter, D.C., Sowers, K., Derie, M.L., Du Toit, L.J. 2017. First report of Leptosphaeria maculans and Leptosphaeria biglobosa, causal agents of blackleg, on canola in Washington State. Plant Disease. 101:504-505.

Interpretive Summary: Blackleg, caused by Leptosphaeria maculans, is the most important pathogen of canola world-wide. Until recently, the Pacific Northwest of the US was free of this disease. But it was reported in Oregon in 2014, and Idaho in 2015. Based on surveys, we found isolates that were identified as L. maculans by morphology, DNA sequencing and pathogenicity testing, although the disease was not widespread. We also identified L. biglobosa, a weakly virulent species.

Technical Abstract: Blackleg, caused by Leptosphaeria maculans, is endemic in the Canadian prairies and the Midwestern and southern USA (Roger Rimmer et al. 2007). The canola production region of the Pacific Northwest USA was considered free of blackleg until 2011, when L. maculans was found in canola fields in northern Idaho (Agostini et al. 2013). An outbreak of blackleg occurred in the Willamette Valley, OR in 2014-15 in diverse brassica crops and weeds (Ocamb et al. 2015). In spring 2015, blackleg was confirmed in >18 winter rapeseed and canola crops in Camas Prairie, ID (L. J. du Toit, personal observation). In eastern Washington, where 12,000 ha of dryland canola are grown, winter and spring canola fields were surveyed in 2015 (n = 25), including residues from 2014-15 crops. Lesions on winter canola leaves collected in April from a field in Okanogan Co. had pycnidia typical of the asexual state. Pycnidia were also found on canola residues. Single-conidia isolations onto water agar from cirrhi were transferred to PDA. The ITS1, 5.8s ribosomal RNA gene, and ITS2 sequences (Schroeder et al. 2006) of isolates Phl048 and Phl049 from Whitman Co. (Genbank Nos. KX037027 and KX037028) had 99% homology with that of L. biglobosa ICMP:13281 (KT309866.1), and the sequence of isolate Phl050 from Okanogan Co., WA (KX037029) was 99% homologous to that of L. maculans ATCC 46318 (JX499035.1). B-tubulin gene sequence homology and phylogeny identified Phl048 and Phl049 as L. biglobosa subsp. australiensis (Vincenot et al. 2008). Isolates Phl048 to Phl051 were each tested for pathogenicity on six cabbage plants (14-days-old) of the cv. Copenhagen Market. Each hypocotyl was punctured with a needle, and 10 ul of conidial suspension (106 CFU/ml) applied to the wound. Six plants were inoculated with isolate Phl031 of L. maculans as a positive control treatment; and water was applied to six plants as a negative control treatment. Plants were grown at 20°C on a 12 h/12 h day/night cycle, and observed 10 and 14 days after inoculation for symptoms. L. maculans isolates Phl050 and Phl051 caused a necrotic lesion, wilt and collapse of each seedling, and formed pycnidia in the lesion. L. biglobosa isolates Phl048 and Phl049 caused a superficial lesion with limited or no pycnidia. Re-isolates from the seedlings inoculated with Phl048 and Phl049 were confirmed by morphology and/or sequencing as L. biglobosa; and re-isolates from seedlings inoculated with Phl050, Phl051, and Phl031 were confirmed as L. maculans. Fungi were not recovered from negative control seedlings. A repeat of the test gave similar results. The hot, dry summer of 2015 probably limited pathogen dispersal in the Okanogan Co. canola crop and from infested residues in Whitman Co. The results indicate L. maculans and L. biglobosa occur in inland Washington at a low incidence. A 2006 Washington blackleg quarantine modified in 2015 requires testing brassica seed for P. lingam before planting to prevent introduction of blackleg, especially in northwest counties and the Columbia Basin where a significant amount of brassica seed is produced for the nation and export.