Characterization and Management of Auxin-Resistant Palmer amaranth in Tennessee

Palmer amaranth has a long history of evolving resistance to herbicides to the point it has become a significant row crop production obstacle. Cotton and soybean growers were offered new technologies in 2016, expanding in-crop herbicide options to include dicamba or 2,4-D. Within three years of commercialization, dicamba use in these crops increased ten-fold and growers began to report Palmer amaranth escapes in west Tennessee auxin-tolerant production systems. A survey of Palmer amaranth escapes in dicamba and 2,4-D-tolerant cotton and soybean fields in Tennessee was conducted in the fall of 2021 with the objective of determining if poor control was due to environmental phenomenon or development of auxin resistance in west Tennessee. Field experiments were conducted across Tennessee in locations where growers witnessed poor control following these herbicides and in Georgia and Texas to characterize potentially resistant populations and compare these with known susceptible populations. Greenhouse findings confirmed three Palmer amaranth accessions with relative resistance factor to dicamba between 1.85-2.49 and one population from Lauderdale County, Tennessee with a relative resistance factor of 14.25. In field studies across multiple locations in Tennessee, the labelled rate of dicamba or 2,4-D controlled Palmer amaranth ≤60%. The addition of malathion insecticide in field experiments did not improve Palmer amaranth control, ruling out certain cytochrome p-450’s as a resistance mechanism.