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Boron reduces development of clubroot in canola

Abstract Boron (B) slows the development of Plasmodiophora brassicae (clubroot) during infection of root hairs (primary infection) and the root cortex (secondary infection) of several vegetable Brassica spp., but the impact of B application on clubroot development in canola has not been assessed. The present study assessed the impact of B application rates (0, 0.25, 0.5, 1, 2, 4, 8, 16, 32 kg ha−1), application timing, and commercial formulations of B (Solubor, BoronMax, Boron) on primary and secondary development of clubroot in canola. Under controlled conditions, increasing rates of B application reduced root hair infection and subsequent development of primary and secondary infection. However, phytotoxicity to canola seedlings occurred at rates higher than 2 kg B ha−1. Application of 2 kg B ha−1 reduced overall root hair infection only slightly, from 81% to 65%, but delayed the development of each stage of primary infection. There were no substantial differences in reduction of incidence and severity of the disease by B whether it was applied before root-hair infection (pre-emergence) or before cortical infection (post-emergence), or split into two applications (pre-emergence + post-emergence). All three formulations of B exhibited similar responses. In field trials, 4 kg ha−1 was the most effective rate that produced no phytotoxic symptoms. Clubroot severity at 6 weeks after seeding was reduced by 64% compared with a nontreated control. This indicates that B application could be an important component of an integrated management programme to manage P. brassicae in canola.