Susceptibility of Palmer amaranth (Amaranthus palmeri) to herbicides in accessions collected from the North Carolina Coastal Plain

Abstract

Palmer amaranth (Amaranthus palmeri S. Watson) populations resistant to acetolactate synthase-inhibiting (ALS) herbicides and glyphosate are fairly common throughout the state of North Carolina (NC). This has led farm managers to rely more heavily on herbicides with other sites of action (SOA) for A. palmeri control, especially protoporphyrinogen oxidase- and glutamine synthetase inhibitors. In the fall of 2016, seeds from A. palmeri populations were collected from the NC Coastal Plain, the state’s most prominent agricultural region. In separate experiments, plants with 2 to 4 leaves from the 110 populations were treated with field use rates of glyphosate, glufosinate-ammonium, fomesafen, mesotrione, or thifensulfuron-methyl. Percent visible control and survival were evaluated three weeks after treatment. Survival frequencies were highest following glyphosate (99%) or thifensulfuron-methyl (96%) treatment. Known mutations conferring resistance to ALS inhibitors were found in populations surviving thifensulfuron-methyl application (A122S, P197S, W574L, and/or S653N) in addition to a new mutation (A282D) which requires further investigation. Forty-two populations had survivors after mesotrione application with one population having 17% survival. Four populations survived fomesafen treatment, while none survived glufosinate. Dose-response studies showed an increase in fomesafen needed to kill 50% of two populations (LD50); however, these rates were far below the field use rate (less than 5 g ha-1). In two populations following mesotrione dose-response studies, a 2.4- to 3.3-fold increase was noted with LD90 values approaching the field use rate (72.8 and 89.8 g ha-1). Screening of the progeny of individuals surviving mesotrione confirmed the presence of resistance alleles as a higher number of survivors at the 1X rate compared to the parent population, confirming resistance to mesotrione. These data suggest A. palmeri resistant to chemistries other than glyphosate and thifensulfuron-methyl are present in NC, which highlights the need for weed management approaches to mitigate the evolution and spread of herbicide-resistant populations.

Safety and efficacy of linuron with or without an adjuvant or S-metolachlor for POST control of Palmer amaranth (Amaranthus palmeri) in sweetpotato

“…The current herbicide program for Palmer amaranth control in sweetpotato is heavily reliant on PRE herbicides including flumioxazin or fomesafen preplant, and S-metolachlor posttransplant (KM Jennings, personal communication; SC Smith and LD Moore, unpublished data). The evolution of protoporphyrinogen oxidase (PPO)-inhibiting herbicide-resistant Palmer amaranth biotypes have been reported in the United States (Heap 2020), including North Carolina (Mahoney et al 2020), which produces more sweetpotato than any other state (USDA-NASS 2020). Therefore, an urgent need exists for alternatives to flumioxazin and fomesafen to control Palmer amaranth in sweetpotato.…”

Susceptibility of Palmer amaranth accessions in North Carolina to atrazine, dicamba, S‐metolachlor, and 2,4‐D

“…acre -1 ) controlled all accessions. More recently, Mahoney et al (2020) surveyed 110 accessions collected from the North Carolina Coastal Plain in 2016 and reported that at least 96% of the accessions survived glyphosate (0.75 lb acre -1 ) and thifensulfuron-methyl (0.016 lb acre -1 ). Fomesafen (0.25 lb acre -1 ) and mesotrione (0.1 lb a.i.…”

“…acre -1 ) controlled these accessions 98 and 90%, respectively, while no accessions survived glufosinate (0.4 lb acre -1 ). Since Mahoney et al (2020) collected the Palmer amaranth accessions in 2016, evolved resistance to 2,4-D, dicamba, and S-metolachlor has been reported in the United States (Heap, 2021). Although Palmer amaranth with resistance to atrazine was first confirmed in the United States in 1995, resistance to this herbicide has not been reported © 2021 The Authors.…”

“…Crop, Forage & Turfgrass Management © 2021 American Society of Agronomy and Crop Science Society of America in populations of this weed in North Carolina (Heap, 2021). Therefore, studies were conducted to further evaluate sensitivity of Palmer amaranth accessions collected by Mahoney et al (2020) to atrazine, dicamba, S-metolachlor, and 2,4-D.…”

“…To determine Palmer amaranth susceptibility to atrazine, dicamba, and 2,4-D, seeds from 120 accessions (Figure 1) collected by Mahoney et al (2020) were planted in 2020 and 2021 in a greenhouse (35.788˚N, 78.694˚W) having 85/78 ˚F day/night and supplemental lighting for a 16-h photoperiod. Irrigation was applied overhead to maintain field capacity.…”