Glyphosate-resistant hairy fleabane (Conyza bonariensis) documented in the Central valley

In recent years, growers and pest consultants have reported poor control of the weed hairy eabane in some areas of the Central Valley. Hairy eabane seeds were collected from Esparto, Fresno and Reedley, Calif., and greenhouse-grown seed- lings were treated at several differ- ent glyphosate rates and compared with an untreated control. None of the Esparto or Fresno plants survived glyphosate rates greater than 0.78 pounds acid equivalent per acre (lb ae/ac), while some of the plants from Reedley survived even the high- est rate of glyphosate tested (12.4 lb ae/ac). The dose required to reduce plant dry weights by 50% (GR50) of the Esparto plants ranged from 0.28 to 0.30 lb ae/ac, whereas the GR50 of the Fresno and Reedley plants ranged from 0.26 to 0.61 and 0.92 to 2.88 lb ae/ac, respectively. This study showed that the hairy eabane plants from Reedley were much more tolerant of glyphosate than either of the other two biotypes and, based on the GR50, the level of resistance ranged from 3- to 10-fold greater.