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Identification of boron tolerant canola cultivars and assessment of clubroot (Plasmodiophora brassicae) severity following a drench application of boron.

McLean, A., Gossen, B.D. and McDonald, M.R. 2016. Identification of boron tolerant canola cultivars and assessment of clubroot (Plasmodiophora brassicae) severity following a drench application of boron. Can. J. Plant Pathol. 38: 139.

Abstract

Clubroot substantially reduces seed yield and oil content in infected canola, (Brassica napus L., B. rapa L.). Resistant cultivars limit clubroot severity however few chemical treatment options exist. Previous studies have demonstrated that concentrations of boron above 4 kg ha-1 significantly reduce clubroot but also induce phytotoxicity in canola. Our research sought to identify boron tolerant canola cultivars and assess clubroot severity among tolerant and intolerant cultivars. A total of 150 canola cultivars were seeded and grown in a greenhouse for three weeks in May 2015. Seedlings from 88 cultivars were hand transplanted into organic soil infested with Plasmodiophora brassicae Woronin, in Bradford Ontario. Cultivars received a drench application of 8 kg ha-1 boron delivered at a rate of 1500 L ha-1. Phytotoxicity was assessed five days after boron application with a visual scale (0-3) based on the extent of leaf cupping and marginal burning. Mean cultivar phytotoxicity was calculated using a phytotoxicity index. Plants were harvested nine weeks after seeding and clubroot severity assessed using a scale (0-3) based on the total proportion of root galling. Mean cultivar clubroot severity was calculated using a disease severity index. Fresh and dry top weights were recorded in cultivars demonstrating the highest and lowest boron tolerance. Cultivars with highest boron tolerance showed a significant reduction in clubroot severity compared to boron susceptible cultivars in both treated and untreated plots. No significant difference was found between the mean top weights of boron tolerant and susceptible cultivars when compared to their untreated counterparts.

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