Mesotrione use for selective post-emergence control of glyphosate-resistant Conyza spp. in black oats

: Background: Achieving satisfactory weed control levels in black oat ( Avena strigosa ) fields is often difficult, owing to the limited number of registered herbicide molecules. Objective: To determine novel options for selective, post-emergence chemical weed control in this crop. Methods: Eight herbicide treatments were sprayed in the field onto black oat plants at the tillering stage to evaluate crop safety and control of Conyza spp. and Gamochaeta americana , two major weeds of Brazilian black oats. A separate trial was conducted to further assess herbicide safety in a controlled-environment setting, and mesotrione selectivity was then re-evaluated separately in the greenhouse during the following growing season. Crop phytotoxicity was determined using biomass production both in the field and in greenhouse trials. Results: Mesotrione, bentazon, 2,4-D, and a 2,4-D+bentazon tank-mix produced light symptoms of crop phytotoxicity, from which plants quickly recovered. Conyza spp. control was achieved via applications of mesotrione (192 g a.i. ha -1 ), metsulfuron-methyl (3.9 g a.i. ha -1 ), and a 2,4-D + bentazon tank-mix (502.5 g a.e. ha -1 + 720 g a.i. ha -1 , respectively), whereas proper G. americana control was only achieved via applications of either mesotrione or metsulfuron-methyl. Biomass accumulation by black oat plants in the greenhouse was similar across mesotrione-treated and untreated plants. Conclusions: Mesotrione could become an option for selective, post-emergence weed control in black oat fields, aiding in the fight against multiple herbicide-resistant Conyza spp. populations which are widespread across major black oat-growing areas in Brazil.