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Efficacy of two fumigants against clubroot (Plasmodiophora brassicae) in three field trials.

Robson, J., Gossen, B.D., Al-Daoud, F., and McDonald, M.R. 2017. Efficacy of two fumigants against clubroot (Plasmodiophora brassicae) in three field trials. Can. J. Plant Pathol. 39: 107.

Abstract

Seed treatments, biocontrol agents and soil amendments do not provide effective management of Plasmodiophora brassicae Woronin (clubroot) on canola (Brassica napus L.) in Canada. Field trials were conducted in a naturally infested mineral soil and a high organic content (muck) soil site in Ontario to assess the efficacy of two soil fumigants, metam sodium (Vapam HL, Busan 1236) or chloropicrin (Pic Plus) at selected rates on clubroot. Efficacy was assessed in plant bioassays with a susceptible cultivar of Shanghai pak choi. Also, propidium monoazide (PMA) assisted qPCR was used to quantify the viability of resting spores in the soil after treatment. Disease pressure at the mineral soil site in 2014 was very low; there were no clubroot symptoms in any treatment that received metam sodium or chloropicrin, and only 8% severity in the nontreated control. In trials in 2014 and 2015 at the muck soil site, chloropicrin reduced clubroot severity when applied at 128 kg a.i. ha-1 or higher. Metam sodium was only effective at 150 kg a.i. ha-1 or higher in one year (2015) at this site. However the lack of efficacy in 2014 may have been due to ineffective sealing of the metam sodium treatments within the soil after application in 2014. Unexpectedly, the fumigants did not have a consistent effect on resting spore viability assessed with PMA-PCR. We conclude that fumigants can reduce clubroot, but that efficacy depends on both rate of application and effectively sealing the fumigant in the soil after application.

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