Seasonal Effects on the Efficacy of Some Postemergence Herbicides on Hairy Fleabane (Conyza bonariensis).

Herbicide-resistant populations of hairy fleabane [Conyza bonariensis (L.) Cronq.] have been documented in California and alternative postemergence herbicides are being sought for their control.  However, the seasonal effects on the efficacy of these postemergence herbicides is not well determined.  Therefore, experiments were conducted to determine the efficacy of postemergence herbicides on glyphosate-susceptible (GS), glyphosate-resistant (GR), and glyphosate-paraquat-resistant (GPR) hairy fleabane plants.  The experiments included a combination of growth chamber, greenhouse, and field studies and were conducted in the spring and fall.  Three herbicides (saflufenacil, glyphosate or a mixture of saflufenacil and glyphosate) were used under different environmental conditions that included three temperature regimes (15/10º C, 25/20º C, 35/30º C day/night).  Four herbicides (saflufenacil, glyphosate, glufosinate and pyraflufen) were used with three light regimes (100%, 50%, 30%, 0% of full sun).   Efficacy of the herbicides differed between the two seasons.  Better control was obtained with saflufenacil and glyphosate when used alone at the 15/10º C and 25/20º C than at 35/30º C.  The mixture of saflufenacil and glyphosate was effective at all temperatures in both seasons.  Light regime had no effect on the efficacy of saflufenacil.  Saflufenacil-alone was more effective in the fall than in spring.  Glyphosate was effective on all biotypes during the fall, but only on the GS biotype in the spring.  Glufosinate was very effective with all biotypes at all light levels in both seasons.  Pyraflufen alone resulted in no mortality in spring and very little mortality in the fall.