Determination of multiple herbicide resistance in populations of hairy fleabane (Conyza bonariensis) in the Central Valley of California and evaluation of alternative chemical control /by Marcelo Luvizotto Moretti.

Glyphosate-resistant (GR) populations of hairy fleabane (Conyza bonariensis) were documented in the Central Valley (CV) in 2007. However, poor control of these populations was also observed with paraquat, a common contact herbicide also widely used in the CV. Therefore, it was hypothesized that hairy fleabane may have evolved resistance to both glyphosate and paraquat. Greenhouse studies were conducted in 2010 and 2011 to determine the response of different populations of hairy fleabane in the CV to both herbicides. Plants were grown in pots and sprayed at the 5- to 8-leaf stage with 0.01, 0.1, 0.5, 1, 2, 4, 8, 16 and 32-fold the recommended label rate of either herbicide separately (1.1 kg ae ha-1 and 0.5 kg ai ha-1, respectively)as well as a non treated. The experimental design was a randomized complete block with six replicates. Plants mortality and aboveground dry weights were recorded. Results showed that some of the populations had resistance to both herbicides, with levels of resistance varying among herbicides and populations. In a separate study, glyphosate and paraquat- resistant plants were treated with other postemergence herbicides to determine mortality. Glufosinate, saflufenacil-alone or in combinations with other tested herbicides, and glyphosate with 2,4-D or carfentrazone controlled both biotypes of hairy fleabane. These herbicides could be utilized as alternate treatments to manage glyphosate and paraquat resistant biotypes