Variation in boron tolerance and clubroot severity in Brassica napus and B. rapa lines in a field trial in 2015.

In a previous study, boron (B) applied at 4 kg ha^-1 reduced clubroot (Plasmodiophora brassicae Wor.) incidence and severity, but higher rates produced severe toxicity on canola (Brassica napus L.). Variability in B-tolerance and clubroot severity were assessed on lines of B. napus and B. rapa in a field trial on muck (~70% organic matter) soil infested with P. brassicae at the Muck Crops Research Station in Bradford, Ontario in 2015. Seedlings of 88 accessions were grown in a greenhouse for 3 weeks, then transplanted to the field. Boron (Solubor, 20.5% B) was applied at 8 kg B ha-1 in 1500 L ha^-1 of water using a backpack CO2 sprayer 3 days after transplanting. Boron was reapplied 5 days after the initial application because leaching by heavy rainfall had reduced the effective rate of applied B. Toxicity was assessed 5 days after the second boron application and categorized by degree of leaf cupping and marginal burning (0-3 scale). Plants were harvested 9 weeks after seeding. Clubroot severity was rated (0-3 scale) and phytotoxicity and clubroot severity indices were calculated for each line. The above-ground growth (fresh and dry weight) of selected B-susceptible and B-tolerant lines was assessed. There were differences in boron tolerance among accessions. The only difference in clubroot severity was observed in the selected subset of B-tolerant accessions, where application of B reduced clubroot severity and increased fresh weight. Also, the water content and fresh weight of B-treated and nontreated plants from the B-tolerant and B-susceptible subsets differed.