First report of paraquat-resistance Conyza sumatrensis in Bolivia and efficacy of herbicides for its control

Conyza sumatrensis is one of the main weeds affecting extensive agricultural crops in the department of Santa Cruz, Bolivia, causing yield losses due to difficulties in control with herbicides used in conventional management. In 2020, isolated Conyza sp. plants were detected in the town of San Pedro, raising suspicions of resistance to the herbicide paraquat. Subsequently, this phenomenon was observed in other regions. The objectives of this study were to determine the resistance of C. sumatrensis to paraquat and to evaluate the efficacy of different herbicides for its management. To this end, two greenhouse trials were conducted in 2023. The first experiment consisted of dose-response bioassays on whole plants using paraquat, while the second trial tested the herbicides 2,4-D, dicamba, fluroxypyr, and paraquat at doses of 720, 240, and 100 g ae ha⁻¹, and 400 g ai ha⁻¹, respectively. The dose-response analysis revealed a high resistance index (RI) in the Cuatro Cañadas biotype (R), with an RI of 24.8 compared to the Yapacaní biotype (S). The herbicides 2,4-D and dicamba proved to be effective for managing this weed. This study presents the first report of C. sumatrensis resistance to paraquat in Bolivia and proposes alternative herbicides for its control. Integrated weed management strategies should be implemented to mitigate resistance cases and delay the development of resistance to currently effective herbicides.