A Palmer Amaranth Population Resistant to HPPD Herbicides.

Palmer amaranth continues to be a serious weed problem in crop production fields in Kansas.  Seeds were gathered from Palmer amaranth plants in Stafford County that were treated postemergence with pyrasulfotole&bromoxynil 1:8 ratio at 245 g/ha and from plants in the same field not treated with a HPPD inhibiting herbicide.  Reports from evaluation of these populations in greenhouse and field dose response experiments indicated the populations were 8 to 11 times more resistant to pyrasulfotole&bromoxynil than a susceptible population.  The objective of this research was to evaluate other HPPD inhibiting herbicides mesotrione, tembotrione, and topramezone for control of progeny of crosses surviving application of pyrasulfotole&bromoxynil.  Crosses were made with male and female plants surviving pyrasulfotole&bromoxynil treatment to develop homozygous resistant plants.  Progeny were treated and crosses from surviving plants were made.  Progeny of these crosses were used in greenhouse dose response experiments.  Dose response curves were developed using AMR 8.0 based on visual evaluations 3 weeks after treatment. The rates required to give 50% control of the resistant progeny were 231 g of mesotrione, 56 g of tembotrione, and 35 g of topramezone. The S population was very susceptible to these HPPD inhibitors resulting in resistance indexes of 30 for mesotrione, 26 for tembotrione, and 80 for topramezone.  The Palmer amaranth biotype evaluated in these experiments is resistant to several of the HPPD inhibiting including isoxaflutole, and to atrazine and is not controlled with thifensulfuron or imazamox herbicides which will make control strategies of this population much more complex for crop producers.  Work must be conducted to fully understand the inheritance of this HPPD resistant trait in Palmer amaranth and to determine the mechanism of resistance.