Confirmation and Alternative Control Solutions for Glyphosate and 2,4-D Resistance in Carduus acanthoides L. from Argentina

Carduus acanthoides L. is an invasive species native to Europe and distributed in other parts of the world, including North and South America. C. acanthoides is a common weed in fields intended for the production of wheat and soybeans. In Cordoba, Argentina, control failures of this species have been reported in Roundup Ready (RR) soybean crops where glyphosate and 2,4-D (2,4-Dichlorophenoxyacetic acid) have frequently been applied, although there are no confirmed reports worldwide of resistance to glyphosate and 2,4-D in this species. The objectives of this work were to confirm resistance to glyphosate and 2,4-D in a C. acanthoides population from Córdoba, Argentina through dose-response assay in greenhouse and biochemical such as the accumulation of shikimic acid and ethylene, quantify the foliar retention and the efficacy of glyphosate and 2,4-D with the use of adjuvants, and seek alternative herbicides for the control of this species. Mature seeds of a C. acanthoides population suspected of having multiple resistance (R) to glyphosate and 2,4-D used in this research were collected from 25 plants in a soybean field (RR). The statistical analysis was done to determine the glyphosate and 2,4-D dose resulting in a reduction in growth (GR50) and plant survival (LD50) by 50% in each C. acanthoides population. Dose-response tests showed multiple resistance to both active principles. The resistant population (R) had LD50 values of 1854.27 and 1577.18 g ae ha-1 (grams of acid equivalent per hectare), while the susceptible (S) population had LD50 values of 195.56 and 111.78 g ae ha-1 for glyphosate and 2,4-D, respectively. Low accumulations of shikimic acid (glyphosate) and ethylene (2,4-D) at different doses in the R population compared to the S population support the results observed in the dose-response curves. No significant differences in leaf retention were observed for glyphosate and 2,4-D in the R and S populations. However, the use of adjuvants increased the retention capacity of herbicides in both populations. Ten alternative herbicides with seven different action mechanisms (MOAs) were evaluated and the most effective active principles were dicamba, bromoxynil, atrazine, tembotrione, flazasulfuron, glufosinate, and paraquat. These findings are the first evidence of glyphosate and 2,4 D resistance in C. acanthoides. This research provides valuable insights into crop rotation options and the management of C. acanthoides in soybean cultivation.