Submitted to: California Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2008
Publication Date: 7/1/2008
Citation: Shrestha, A., B.D. Hanson, K.J. Hembree. 2008. Glyphosate-resistant hairy fleabane(Conyza bonariensis)documented in the Central Valley.Calif. Agric. 62:116-119. Interpretive Summary: The research presented in this paper is the first documented case of glyphosate-resistant hairy fleabane in the United States. The researchers found that a population of hairy fleabane collected near Reedley, California had at least a 3-fold level of resistance to glyphosate, the active ingredient in RoundUp and other important herbicides. This results of this research also suggested that growth stage of the weed at herbicide application greatly influenced the susceptibility to the herbicide in both the resistant and susceptible populations. The manuscript includes discussion on detection and management of herbicide resistant weeds, particularly in light of recent reports of several glyphosate-resistant species in perennial cropping systems in California.
Technical Abstract: In recent years poor control of hairy fleabane (Conyza bonariensis) with glyphosate has been reported by growers and pest consultants in some areas of the Central Valley. Since glyphosate-resistance in a related species horseweed (Conyza canadensis) was recently documented in similar locations, we suspected glyphosate-resistance in hairy fleabane. Hairy fleabane seeds were collected from Davis, Fresno, and Reedley, CA and seedlings were treated with glyphosate rates of 0 (untreated), 6.5, 11, 22, 44, 88, 176, and 352 fl oz/ac. Greenhouse experiments comparing plants from Davis and Reedley, and from Fresno and Reedley were each conducted twice. Interactions (P<0.05) occurred between glyphosate rates and the experimental run for plant dry weights. Therefore, GR50 values for each experimental run were calculated separately. In these experiments, none of the Davis or Fresno plants survived glyphosate rates greater than 22 fl oz/ac while some of the plants from Reedley survived even the highest rate of glyphosate tested (352 fl oz/ac). The GR50 of the Davis plants was 8 fl oz/ac whereas the GR50 of the Fresno and Reedley plants ranged from 7.5 to 17.1 and 26 to 82 oz/ac, respectively. This study showed that the level of glyphosate-resistance in hairy fleabane was influenced by growth stage of the plants at the time of glyphosate application. The hairy fleabane plants from Reedley were much more tolerant of glyphosate than either of the other two biotypes and the level of resistance ranged from 3- to 10-fold depending on growth stage at application.