Multiple resistance detection to glyphosate and other herbicides in Conyza sumatrensis and the evaluation of chemical control herbicides

In South China, Conyza sumatrensis is a common broadleaf weed, and often causes serious damage in orchards and uncultivated areas. Whole plant dose-response experiments were conducted to evaluate the resistance levels of the suspected resistant population (GZ-R, collected from Guangzhou, Guangdong Province) to glyphosate, paraquat and diquat. The fragments of the glyphosate target enzyme gene, EPSPS2, were sequenced and compared between GZ-R and the susceptible population (QY-S, collected from Qingyuan, Guangdong Province). In addition, the control efficacies of five post-emergence herbicides (including bentazone, fluroxypyr, etc) on GZ-R and QY-S population at different leaf stages were also assessed. The results showed that GZ-R population had evolved moderate and high levels of resistance to glyphosate and paraquat, respectively. In addition, GZ-R population had also evolved cross resistance to diquat. The LD₅₀ values of GZ-R population to glyphosate, paraquat and diquat were 7.2, 72.3 and 6.6 folds than QY-S population, respectively. Compared to QY-S population, the mutation of EPSPS2 site 106 from Pro to Thr was found in GZ-R population. At the recommended field dosages, when bentazone, fluroxypyr and MCPA-sodium were sprayed at the 4-5 leaf stage, the mortality rate of C. sumatrensis plants was 100%. However, the mortality rate declined significantly to 44.4%-91.7% when the above three herbicides were sprayed at the 6-7 and 10-12 leaf stages. In comparison, at the the recommended field dosages of glufosinate-ammonium and saflufenacil, the mortality rates maintained at 100% regardless of the leaf stage. In summary, glufosinate-ammonium and saflufenacil can be applied for controlling glyphosate- and/or paraquat- resistant C. sumatrensis at its early growth stage.