Exploring chemical control of 2,4-D-resistant wild radish (Raphanus raphanistrum) with auxin-related compounds

Abstract Synthetic auxin herbicides were developed and commercialised sixty years before their mode of action was definitively elucidated. Although evolution of resistance to auxinic herbicides proceeded more slowly than for some other herbicide chemistries, it has become a major problem in the dicotyledonous weeds of many cropping areas of the world. With the molecular characterisation of the auxin perception and signalling pathway in the mid-2000s came a greater understanding of how auxinic herbicides work, and how resistance may develop in weeds subjected to repeated selection with these herbicides. In wild radish ( Raphanus raphanistrum L.) populations in southern Australia, resistance to multiple herbicides, including synthetic auxins such as 2,4-D, has reduced the number of chemical control options available. The aim of this study was to determine if compounds involved in auxin biosynthesis, transport and signalling are able to synergise with 2,4-D and increase its ability to control 2,4-D-resistant R. raphanistrum populations. Although some mild synergism was observed with a few compounds (abscisic acid, cyclanilide, tryptamine), the response was not large or consistent enough to warrant further study. Similarly, alternative auxinic herbicides applied pre- or post-emergence were no more effective than 2,4-D. Therefore, whilst use of auxinic herbicides continues to increase due to the adoption of transgenic resistant crops, non-chemical control techniques will become more important and chemical control of 2,4-D-resistant R. raphanistrum should be undertaken with alternative modes of action, using mixtures and good stewardship to delay the development of resistance for as long as possible.