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Change in virulence of Plasmodiophora brassicae at a site in Ontario.

Al-Daoud, F., Gossen, B.D., and McDonald, M.R. 2016. Change in virulence of Plasmodiophora brassicae at a site in Ontario. Can. J. Plant Pathol. 38: 541.

Abstract

Clubroot disease on canola and Brassica vegetables, caused by Plasmodiophora brassicae Woronin, is a major problem worldwide. Clubroot resistance is breaking down at many sites due to the emergence of more virulent pathotypes of P. brassicae. A study was conducted to assess changes in the virulence of P. brassicae at the Muck Crops Research Station (MCRS) in King, ON. Clubbed roots of canola (Brassica napus L.) cv. ‘ACS-N39ʹ were harvested from the MCRS in 2011 and 2014. Resting spores extracted from these clubs were used to inoculate canola cultivars selected for differences in disease reaction: susceptible, moderately resistant and resistant to P. brassicae pathotype 6. In a growth room, seedlings were inoculated with 1 × 107 resting spores mL−1 within 1 week of seeding, and 6-week-old plants were uprooted and examined for clubroot symptoms. A disease severity index (DSI, %) was calculated (mean ± SE). There were no differences in severity produced by inoculum from 2011 and 2014 on the susceptible line ACS-N39 (2011 and 2014, 80 ± 5%) or resistant cv. ‘45H29' (2011, 1 ± 1%; 2014, 5 ± 2%). However, severity increased on the intermediate cv. ‘InVigor 5030' (2011, 0%; 2014, 66 ± 4%), and on the previously resistant cvs. ‘45H73' (2011, 10 ± 5%; 2014, 71 ± 5%), and ‘45H21' (2011, 0%; 2014, 85 ± 4%). Therefore, we conclude that the virulence of P. brassicae at the MCRS has increased from 2011 to 2014.

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