Palmer amaranth is the latest pigweed species documented in Connecticut; it was identified there in 2019. In a single-dose experiment, the Connecticut Palmer amaranth biotype survived the field-use rates of glyphosate (840 g ae ha^-1) and imazaquin (137 g ai ha^-1) herbicides applied separately. Additional experiments were conducted to (1) determine the level of resistance to glyphosate and acetolactate synthase (ALS) inhibitors in the Connecticut-resistant (CT-Res) biotype using whole-plant dose-response bioassays, and (2) evaluate the response of the CT-Res biotype to POST herbicides commonly used in Connecticut cropping systems. Based on the effective dose required for 90% control (ED₉₀), the CT-Res biotype was 10-fold resistant to glyphosate when compared with the Kansas-susceptible (KS-Sus) biotype. Furthermore, the CT-Res biotype was highly resistant to ALS-inhibitor herbicides; only 18% control was achieved with 2,196 g ai ha^-1 imazaquin. The CT-Res biotype 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). The CT-Res Palmer amaranth was controlled 75% to 100% at 21 d after treatment (DAT) with POST applications of 2,4-D (386 g ae ha^-1), carfentrazone-ethyl (34 g ai ha^-1), clopyralid (280 g ae ha^-1), dicamba (280 g ae ha^-1), glufosinate (595 g ai ha^-1), lactofen (220 g ai ha^-1), oxyfluorfen (1,121g ai ha^-1), and mesotrione (105 g ai ha^-1) herbicides. Atrazine (2,240 g ai ha^-1) controlled the CT-Res biotype only 52%, suggesting the biotype is resistant to this herbicide as well. Here we report 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 PRE and alternative POST herbicides (tested in this study), for effective control of the CT-Res biotype.

Nomenclature: 2, 4-D ester; atrazine; carfentrazone-ethyl; chlorimuron-ethyl; clopyralid, dicamba; glyphosate; glufosinate; halosulfuron-methyl; imazaquin; lactofen; mesotrione; oxyfluorfen, sulfometuron-methyl; Palmer amaranth, Amaranthus palmeri S. Watson