Multiple herbicide-resistant palmer amaranth (amaranthus palmeri) in Connecticut: confirmation and response to post herbicides

Authors: AULAKH, JATINDER - Agricultural Experiment Station, Connecticut; CHAHAL, PARMINDER - Fmc Corporation; KUMAR, VIPAN - Kansas State University; Price, Andrew; GUILLARD, KARL - University Of Connecticut; GHIMIRE, SHURESH - University Of Connecticut

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2021
Publication Date: 1/22/2021
Citation: Aulakh, J., Chahal, P.S., Kumar, V., Price, A.J., Guillard, K., Ghimire, S. 2021. Multiple herbicide-resistant palmer amaranth (amaranthus palmeri) in Connecticut: confirmation and response to post herbicides. Weed Technology. 35: 457–463. https://doi.org/10.1017/wet.2021.6.
DOI: https://doi.org/10.1017/wet.2021.6

Interpretive Summary: Palmer amaranth is the most troublesome weed in many agricultural and horticultural crops in the United States. Previously undiscovered, Palmer amaranth is the newest pigweed documented in Connecticut. In a single dose experiment, the Connecticut Palmer amaranth biotype survived the field-use rate of glyphosate and imazaquin herbicides thus exhibiting resistance qualities. Thus, additional experimental objectives were (1) to determine the level of resistance to glyphosate and acetolactate synthase (ALS) inhibitor herbicides in the Connecticut biotype using whole-plant dose-response bioassays, and (2) evaluate the response of the Connecticut biotype to alternative postemergence herbicides commonly used in Connecticut cropping systems. A previously known glyphosate-susceptible Palmer amaranth biotype from Kansas was included for comparison. Based on the effective dose required to control 90% of plants (ED90), the Connecticut biotype was 10-fold resistant to glyphosate when compared with the Kansas susceptible biotype. Furthermore, the Connecticut biotype was highly resistant to ALS–inhibitor herbicides with only 18% control with a 16-fold imazaquin herbicide rate. It was also cross resistant to other ALS–inhibitor herbicides. This research reports the first case of Palmer amaranth from Connecticut with multiple resistance to glyphosate and ALS inhibitors. Growers should proactively use all available weed control tactics, including the use of effective preemergence and alternative postemergence herbicides (tested in this study) for effective control of the Connecticut Palmer amaranth biotype.

Technical Abstract: Palmer amaranth is the most troublesome weed in many agricultural and horticultural crops in the United States. Previously undiscovered, Palmer amaranth is the newest pigweed documented in Connecticut (CT). It was first discovered in the fall of 2019 in a pumpkin field in Hartford County. In a single dose experiment, the Connecticut Palmer amaranth (CT–Res) biotype survived the field-use rate of glyphosate (840 g ae ha-1) and imazaquin (137 g ai ha-1) thus exhibiting resistance qualities. Thus, additional experimental objectives were to determine the level of resistance to glyphosate and acetolactate synthase (ALS) inhibitors in the CT–Res biotype using whole-plant dose-response bioassays, and (2) evaluate the response of the CT–Res biotype to alternative POST herbicides commonly used in CT cropping systems. A previously known glyphosate-susceptible Palmer amaranth biotype (KS–Sus) from Kansas was included for comparison. Based on the effective dose required to control 90% of plants (ED90), the CT–Res biotype was 10-fold resistant to glyphosate when compared with the KS–Sus biotype. Furthermore, the CT–Res biotype was highly resistant to ALS–inhibitor herbicides with only 18% control with 2,196 g ai ha-1 of imazaquin. It was also cross resistant to other ALS–inhibitor herbicides including: chlorimuron–ethyl (13.1 g ai ha-1), halosulfuron–methyl (70 g ai ha-1), and sulfometuron–methyl (392 g ai ha-1). Response of CT–Res Palmer amaranth to selected POST herbicides indicated 75% to 100% control at 21 DAT with recommended rates of 2, 4-D, carfentrazone, clopyralid, dicamba, glufosinate, lactofen, oxyfluorfen, and mesotrione herbicides. Atrazine (2,240 g ai ha-1) controlled the CT–Res biotype only 52% probably because of reduced sensitivity to atrazine. This research reports the first case of Palmer amaranth from Connecticut with multiple resistance to glyphosate and ALS inhibitors. Growers should proactively use all available weed control tactics, including the use of effective preemergence and alternative POST herbicides (tested in this study) for effective control of the CT–Res biotype.