Boron Sensitivity and Storage in Brassica Napus and Its Potential for Clubroot Management

Clubroot [Plasmodiophora brassica] severity in canola (Brassica napus) can be suppressed by addition of boron (B), but excess B produces phytotoxicity. However, some lines of a closely related species, B. rapa, are relatively insensitive to high levels of B. Assessment of 150 accessions of B. napus and B. rapa treated with 0, 8 and 16 kg B ha-1 as disodium octaborate tetrahydrate (trade name Solubor) identified several lines that were relatively insensitive to high levels of B. In 2015, 88 lines were planted in a clubroot-infested plot in Ontario, Canada in a replicated split-plot design, where the subplot treatments were 8 kg ha-1 of B versus a non-treated control. Boron-insensitive lines generally had slightly lower clubroot severity than sensitive lines, even in the control with no added B. Application of B reduced clubroot slightly in the most sensitive lines, but produced a large and significant reduction in severity in the most insensitive lines. Assessment of five insensitive lines under controlled conditions supported the observation that application of B substantially reduced clubroot severity in insensitive lines. Comparison of two sensitive and two insensitive lines each of B. napus and B. rapa using Boron K-edge X-ray absorption near-edge structure (XANES) spectra from a synchrotron-based soft-X-ray beamline showed that some insensitive lines were able to extract B efficiently from a soil that contained very low levels of B. The spectra of sensitive and insensitive lines differed when additional B was applied, which demonstrated that the insensitive lines stored B (and other elements such as S and P) differently than the sensitive lines. Differences in the amount and type of storage likely affect the susceptibility to clubroot, and merits further study.