Forty-three Spanish populations of hairy fleabane, sampled from perennial crop locations, were studied under controlled and field conditions to confirm and characterize glyphosate resistance. In the initial screening, under controlled conditions, significant differences in glyphosate response between locations and among plant progenies within location were observed. From the initial screening, six populations (five potentially resistant [R] and one susceptible [S]) were selected, and a dose–response experiment was conducted to determine the resistance factor. The resistance factor was close to 10× for the most resistant population. In addition, the glyphosate response of R and S populations was dependant on phenological stages: the glyphosate rate required for control increased as a function of plant age. Finally, the resistance was confirmed with field trials conducted in five locations (one S and four R previously studied under controlled conditions). The field trials were also used to find alternative solutions for Spanish farmers to control resistant hairy fleabane populations.
