Syngenta 'RISQ' test: a novel in-season method for detecting resistance to post-emergence ACCase and ALS inhibitor herbicides in grass weeds

P>This paper describes a novel method for detecting resistance to post-emergence acetolactate synthase and acetyl CoA carboxylase herbicides in grass weeds. The method relies on seedlings being collected from farmer's fields and sent for testing to a central laboratory. These seedlings are plated onto agar containing discriminating rates of herbicides and survivorship is recorded after 10 days and compared with standard sensitive and resistant populations. The test has been validated in the glasshouse, growth cabinet and phytotron on Lolium spp. seedlings at the 1-3 leaf stage and has resulted in very low levels of false positives and negatives of resistance. Based on several pre-determined target site and non-target site resistant Lolium spp. populations and three commonly used herbicides, clodinafop-propargyl, pinoxaden and iodosulfuron + mesosulfuron, we demonstrate that the results from the agar-based seedling assay correlate very well with classical whole-plant pot tests. The method has been successfully applied to Lolium spp. seedlings collected from two UK fields in 2009 and proved transferable to other grass weeds. As it is applied very early in the season, it provides an opportunity for predicting herbicide efficacy prior to field application and thus allows for an informed choice of herbicide for effective weed control. To distinguish this cost effective, simple and early season bioassay from the many existing ones, we propose to refer to it as the Syngenta 'Resistance In-Season Quick' test.