Chemical control and herbicide resistance of hairy fleabane in Jordan

Two field experiments were conducted to evaluate the effectiveness of 12 herbicides in controlling hairy fleabane [Conyza bonariensis (L.) Cronquist] in a date palm orchard located in the central Jordan valley during the spring of 2017. Results showed that C. bonariensis resists paraquat (2.5, 5 and 7.5kgha-1), oxadiazon (5kgha-1) and oxyflourfen (3.3kgha-1) herbicides applied at normal or higher than the recommended rates. None of the three herbicides was significantly effective against the weed and treated plants continued growing normally similar to those of untreated control. Higher rates (10-fold of the recommended rates) of the same herbicides failed to control the weed. The effect of other tested herbicides on the weed was varied with bromoxynil plus MCPA (buctril(R)M), 2,4-D-iso-octyl ester, glyphosate, glyphosate trimesium and triclopyr were most effective and completely controlled the weed at recommended rates of application. Testing paraquat, oxadiazon and oxyflourfen using the normal recommended and 10-fold higher rates on two populations of C. bonariensis grown from seeds of the date palm and al-Twal (another site in the Jordan Valley) weed populations and grown in pots under glasshouse conditions showed that Date palm population was resistant to the three herbicides at both application rates while al-Twal site population was highly susceptible and completely controlled at normal and high rates of the three herbicides. It is concluded that certain populations of C. bonariensis developed resistance to paraquat, oxadiazon and oxyflourfen but control of this weed was possible using other herbicides of different mechanism of action. Herbicide rotation or other nonchemical weed control methods have been suggested to prevent or reduce the buildup and spread of resistant populations of this weed species. These results represent the first report on herbicide resistance of C. bonariensis in Jordan.