CHARACTERIZATION OF HPPD RESISTANT PALMER AMARANTH. C. R. Thompson*, D. Peterson, N. G. Lally; Kansas State University, Manhattan, KS (413)


Palmer amaranth, Amaranthus palmeri 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 7 to 11 times more resistant to pyrasulfotole&bromoxynil than a susceptible population.  The objective of this research was to determine the mechanism of inheritance, and to determine the level of resistance of this population to mesotrione, tembotrione, and topramezone.  Surviving ‘R’ plants were crossed with susceptible plants and progeny treated with pyrasulfotole&bromoxynil to determine if the resistant trait was passed through pollen.  Palmer amaranth is a true dioecious species.  Surviving progeny, assuming heterozygosity and that the trait was transferred via pollen, were crossed and 167 progeny were treated with a 2X rate (604 g) of pyrasulfotole&bromoxynil to evaluate segregation for the trait.  Visual evaluations 10 d after treatment suggest 33% of the plants were recovering from treatment and were rated 40% injured or less.  Thirty eight percent of the plants were killed from the treatment.  Initial crosses also 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 5.4 g of mesotrione, 4.9 g of tembotrione, and 0.4 g of topramezone. The S population was very susceptible to these HPPD inhibitors resulting in resistance indexes of 54 for mesotrione, 55 for tembotrione, and 63 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.  Further work must be conducted to fully understand the inheritance of this HPPD resistant trait in Palmer amaranth.