Differential Susceptibility of <i>Conyza bonariensis</i> Biotypes to Glyphosate and ALS-Inhibiting Herbicides in Argentina

Abstract

The aim of this study was to compare control of three stages biotypes (rosette, bolting, and repro- ductive stage) of Conyza bonariensis glyphosate susceptible biotype (S) and tolerant biotype (T) to glyphosate with glyphosate and mixtures of acetolactate synthase (ALS)-inhibiting. For glyphosate, the dose-response curves confirmed that injury of the biotype T relative to biotype S was signifi- cantly lower for both rosette and bolting stages. Resistance index (RI) for this herbicide was approximately 4 for both weed stages. At bolting, for both biotypes doses much higher than the recommended dose were required. For acetolactate synthase (ALS)-inhibiting herbicides, at the rosette stage, control of both biotypes was excellent with doses significantly lower than the recommended rate. All herbicides within this group showed a very low I 50 relative to the recommended rate. These results indicate that biotypes difficult to control with glyphosate at the rosette and bolting stage may be controlled using acetolacte synthase (ALS)-inhibiting herbicides. Control for the reproductive stage was poor. A management program based on the combination of glyphosate with pre- and post-emergence acetolactate synthase (ALS) herbicides may be effective to control the weed.