Glyphosate resistance in Sonchus oleraceus: determining the spatial extent of resistance in Australia's northern cropping region.

Abstract

Common sowthistle (Sonchus oleraceus) is a major weed of conservation tillage systems in Australia. The major problem associated with the weed is that it reduces stored soil moisture in fallow, impacting the subsequent crop. Glyphosate has been a reliable control option for the weed, but resistance to glyphosate has now been confirmed in populations growing in the northern cropping region of Australia. To determine how widespread this problem is in the northern cropping region, a survey commenced in 2014 and will conclude in 2016. Common sowthistle seeds have been collected from cropping properties in the region, as part of a non-targeted sampling approach. Plants were grown to a two to four leaf stage, and then treated with a commercial glyphosate formulation at various rates, to determine the discriminating dose that distinguishes between resistant and susceptible populations. Following determination of this rate, screening tests were performed using field collected seed obtained from cropping properties throughout the region. Close to 20% of the populations tested have exhibited resistance to glyphosate (i.e. less than 80% of seedlings were killed by treatment with glyphosate at the discriminating dose). Preliminary results indicate that glyphosate resistant populations are clustered in one location within the region, where long term use of conservation tillage practices has resulted in reliance on glyphosate for sowthistle control. By determining the geographic extent of glyphosate resistant common sowthistle in the northern region, the results of this survey will assist industry stakeholders to respond with suitable management solutions for the control of this problematic weed.