Confirmation of Dicamba-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. A survey of Palmer amaranth escapes in dicamba-resistant 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 dicamba resistance in west Tennessee. Greenhouse dicamba dose response screen was conducted on 15 Tennessee accessions. Three accessions with relative resistance factor between 1.85-2.49 and one accession from Lauderdale County Tennessee with a relative resistance factor of 14.25 were found. The Lauderdale County 1 accession developed a higher dicamba resistance level than all others evaluated and can no longer be effectively controlled using dicamba. The history of Palmer amaranth escaping dicamba in the Lauderdale County 1 location from 2019 to 2021 in the field and in preliminary greenhouse screens would suggest that the dicamba resistance has passed between generations. This research documents the first findings of Palmer amaranth control failures, in cotton and soybean fields, due to the evolution of dicamba resistance.