No abstract

Conyza bonariensis is a major weed infesting zero-tilled cropping systems in subtropical Australia, particularly in wheat and winter fallows. Uncontrolled C. bonariensis survives to become a problem weed in the following crops or fallows. As no herbicide has been registered for C. bonariensis in wheat, the effectiveness of 11 herbicides, currently registered for other broad-leaved weeds in wheat, was evaluated in two pot and two field experiments. As previous research showed that the age of C. bonariensis, and to a lesser extent, the soil moisture at spraying affected herbicide efficacy, these factors also were investigated. The efficacy of the majority of herbicide treatments was reduced when large rosettes (5-15 cm diameter) were treated, compared with small rosettes (<5 cm diameter). However, for the majority of herbicide treatments, the soil moisture did not affect the herbicide efficacy in the pot experiments. In the field, a delay in herbicide treatment of 2 weeks reduced the herbicide efficacy consistently across herbicide treatments, which was related to weed age but not to soil moisture differences. Across all the experiments, four herbicides controlled C. bonariensis in wheat consistently (83-100%): 2,4-D; aminopyralid + fluroxypyr; picloram + MCPA + metsulfuron; and picloram + high rates of 2,4-D. Thus, this problem weed can be effectively and consistently controlled in wheat, particularly when small rosettes are treated, and therefore C. bonariensis will have a less adverse impact on the following fallow or crop.

No abstract

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.