Identification, mapping, and chemical control of fleabane resistant to glyphosate, chlorimuron, paraquat and 2,4-D

Monitoring herbicide-resistant weeds makes it possible to study the evolution and spread of resistance, providing important information for management. The objective of this study was to map fleabane accessions in the states of Paraná (PR) and Mato Grosso do Sul (MS), Brazil, to identify herbicide-resistant accessions and their response to soybean pre-plant chemical burndown management strategies. Fleabane seeds were collected in agricultural areas in PR and MS, in 2018, 2019, and 2020. Initial screening was performed for glyphosate, chlorimuron, paraquat, 2,4-D, saflufenacil, and glufosinate efficacy. Subsequently, dose-response experiments were conducted. Field experiments were carried out in three locations, where accessions of multiple-resistant Sumatran fleabane were identified. Herbicides were used in single or sequential (seq.) applications at three plant heights (<5 cm, 5 to 10 cm, and >10 cm). After preliminary screening, accessions were classified as putative resistant (<80% control for all 4 replicates), segregated (<80% control for 1 to 3 replicates), or susceptible (>80% control for all 4 replicates). There was no evidence of resistance to glufosinate or saflufenacil in any of the 461 accessions, while 65 showed possible resistance or segregation only for glyphosate, 235 for glyphosate + chlorimuron, 79 to glyphosate + chlorimuron + paraquat, 59 to glyphosate + chlorimuron + 2,4-D, and 23 with 4-way resistance (glyphosate, chlorimuron, paraquat and 2,4-D). Of these 23 accessions, 7 were analyzed using dose-response curves (F2 generation), all from PR, confirming 4-way resistance to glyphosate, chlorimuron, paraquat, and 2,4-D. To control resistant Sumatran fleabane, an application should prioritize smaller plants. Despite resistance to 2,4-D, double mixtures containing this herbicide were among the most effective treatments in plants <5 cm in height. Sequential application is needed for plants >5 cm in height, it was recommended glyphosate + synthetic auxin followed by glufosinate or glyphosate + saflufenacil.