<i>Conyza bonariensis’</i> Resistance to Glyphosate, Diclosulam, and Chlorimuron: Confirmation and Alternative Control for the First Case of Multiple and Cross-Resistance in Uruguay

<i>Conyza bonariensis</i> L. (hairy fleabane) is a significant weed in production systems, especially due to its evolving resistance to various herbicides. In Uruguay, control failures of <i>C. bonariensis</i> have been reported following the use of glyphosate and ALS inhibitors. The objective of this study was to investigate the occurrence of the multiple and cross-resistance of <i>C. bonariensis</i> to glyphosate, chlorimuron, and diclosulam, and to assess the efficacy of alternative herbicides against these resistant biotypes. Seeds were collected from sites where plants had survived herbicide applications during the 2019/2020 and 2020/2021 seasons. Following initial screenings, biotypes were selected to establish independent dose–response curves for glyphosate, diclosulam, and chlorimuron. For each herbicide, four biotypes of <i>C. bonariensis</i> were tested: one susceptible (S), two putatively moderately resistant (MR) biotypes, and one putatively highly resistant (R) biotype. In each assay, eight herbicide doses were used (1/32; 1/16; 1/8; 1/2; 1; 2; and 4X for S and MR biotypes, and 1/8; 1/2; 1; 2; 4; 8; and 16X for R biotypes) based on the recommended dose (1x) for each herbicide, with four repetitions per treatment. Each assay was completely replicated twice. Resistance was confirmed through testing in two plant generations (G1 and G2). The findings reveal high levels of multiple and cross-resistance in <i>C. bonariensis</i> to glyphosate, diclosulam, and chlorimuron. In general, herbicides with alternative action mechanisms effectively controlled <i>C. bonariensis</i> exhibiting multiple and cross-resistance. This study confirms the first case of <i>C. bonariensis</i> cross-resistance to diclosulam and chlorimuron, and the first occurrence of multiple and cross-resistance to glyphosate, diclosulam, and chlorimuron in this species.