Resistance to herbicides in the broadleaf weeds Raphanus raphanistrum, Sisymbrium orientale, and Sonchus oleraceus as determined by random weed surveys in south-eastern Australia

Context Broadleaf weeds infest crops and reduce crop yield. The use of herbicides to control these weeds results in the evolution of herbicide resistance. Aims There is limited information about the extent and distribution of herbicide resistance in broadleaf weeds in south-eastern Australia, so a survey was conducted to provide this information for three key broadleaf weeds of this region. Methods A survey of three broadleaf weeds of grain crops collected wild radish (Raphanus raphanistrum), Indian hedge mustard (Sisymbrium orientale), and sowthistle (Sonchus oleraceus) across south-eastern Australia between 2013 and 2017. Samples were screened for resistance to commonly used herbicides. Key results A total of 1760 fields were visited with 74 R. raphanistrum, 95 S. orientale, and 488 S. oleraceus seed samples collected. Resistance to the ALS-inhibiting herbicide chlorsulfuron was most common in these species, present in 66% of S. oleraceus, 34% of S. orientale, and 17% of R. raphanistrum samples tested. R. raphanistrum and S. orientale also had samples resistant to atrazine, diflufenican and 2,4-D. No resistance to glyphosate was found in any species. Multiple resistance to two or more herbicide modes of action was present in all three species. Conclusions Herbicide resistance was identified in all three weed species in all the states where they were collected. All species had resistance to more than one herbicide mode of action. Implications While the frequency of resistance in these broadleaf weeds in south-eastern Australia is low to most herbicides, the accumulation of multiple resistance means that alternative management strategies will be required to effectively manage these weeds.