First report of Verticillium dahliae Kleb. causing wilt symptoms in canola (Brassica napus L.) in North America

Abstract

In Canada, Verticillium wilt of canola (Brassica napus L.), caused by Verticillium longisporum, was first confirmed in Manitoba in 2014. Verticillium dahliae, however, has not been reported to cause this disease. In 2016, a field survey in Alberta revealed canola plants with vascular wilt symptoms. Symptomatic plants were collected and infected stem tissue was cultured on agar medium, with one isolate designated A1-SS05 identified as a putative Verticillium spp. based on its colony characteristics. The colony was off-white in colour with a felt-like surface. On potato dextrose agar, the underside of the colony was dark only in the central area with radiating ridges. On Howell’s medium, only the test isolate exhibited polyphenol oxidase activity. The isolate formed conidiophores with 4–5 verticillate phialides. Mean conidial length was 5.96 µm (range of 4.65–6.64 µm) and mean width was 2.37 µm (range of 1.71–2.79 µm). The isolate also produced irregularly elongated chain-like microsclerotia of various sizes. Based on these criteria, isolate A1-SS05 was tentatively identified as V. dahliae. Sequencing of PCR products amplified with the primer sets ITS5/4 and VeruniF2/VeruniR3 revealed 99 and 100% identity, respectively, with V. dahliae sequences in GenBank, confirming the identity of this isolate. The isolate was pathogenic on the canola cultivar ‘Westar’, causing wilt symptoms, stunting, and leaf yellowing and senescence. Microscopic analysis of infected vascular tissues revealed irregular shaped microsclerotia, as were observed in pure culture. This is the first report of V. dahliae causing Verticillium wilt of canola in North America.