Confirmation ofS-metolachlor resistance in Palmer amaranth (Amaranthus palmeri)

Chad Brabham,Vijay K Varanasi,Jason K Norsworthy,Tom Barber,Michael M Houston

doi:10.1017/wet.2019.44

Abstract

AbstractS-Metolachlor is commonly used by soybean and cotton growers, especially with POST treatments for overlapping residuals, to obtain season-long control of glyphosate- and acetolactate synthase (ALS)–resistant Palmer amaranth. In Crittenden County, AR, reports of Palmer amaranth escapes followingS-metolachlor treatment were first noted at field sites near Crawfordsville and Marion in 2016. Field and greenhouse experiments were conducted to confirmS-metolachlor resistance and to test for cross-resistance to other very-long-chain fatty acid (VLCFA)–inhibiting herbicides in Palmer amaranth accessions from Crawfordsville and Marion. Palmer amaranth control in the field (soil <3% organic matter) 14 d after treatment (DAT) was ≥94% with a 1× rate of acetochlor (1,472 g ai ha–1; emulsifiable concentrate formulation) and dimethenamid-P (631 g ai ha–1). However,S-metolachlor at 1,064 g ai ha–1provided only 76% control, which was not significantly different from the 1/2× and 1/4× rates of dimethenamid-P and acetochlor (66% to 85%). In the greenhouse, Palmer amaranth accessions from Marion and Crawfordsville were 9.8 and 8.3 times more resistant toS-metolachlor compared with two susceptible accessions based on LD50values obtained from dose–response experiments. Two-thirds and 1.5 timesS-metolachlor at 1,064 g ha–1were the estimated rates required to obtain 90% mortality of the Crawfordsville and Marion accessions, respectively. Data collected from the field and greenhouse confirm that these accessions have evolved a low level of resistance toS-metolachlor. In an agar-based assay, the level of resistance in the Marion accession was significantly reduced in the presence of a glutathione S-transferase (GST) inhibitor, suggesting that GSTs are the probable resistance mechanism. With respect to other VLCFA-inhibiting herbicides, Marion and Crawfordsville accessions were not cross-resistant to acetochlor, dimethenamid-P, or pyroxasulfone. However, both accessions, based on LD50values obtained from greenhouse dose–response experiments, exhibited reduced sensitivity (1.5- to 3.6-fold) to the tested VLCFA-inhibiting herbicides.

Highlights

Very-long-chain fatty acid (VLCFA)–inhibiting herbicides (Group 15) are labeled for use in numerous crops for residual control of annual grasses and small-seeded broadleaves
VLCFAs are synthesized by a four-enzyme complex, and the substrate specificity of this complex is determined by B-ketoacyl-CoA synthase, otherwise known as fatty acid elongase (FAE) (Joubes et al 2008; Li-Beisson et al 2010)
Acetochlor and dimethenamid-P, averaged over rates and herbicides at 14 d after treatment (DAT), reduced Palmer amaranth density by 93%, whereas S-metolachlor only reduced density by 71% when compared to the nontreated control (49 plants m–2) (Table 1)

Summary

Introduction

Very-long-chain fatty acid (VLCFA)–inhibiting herbicides (Group 15) are labeled for use in numerous crops for residual control of annual grasses and small-seeded broadleaves. Dose–response experiments were conducted to determine difference in sensitivity between susceptible and resistant accessions to S-metolachlor (Dual Magnum), acetochlor (Harness), dimethenamid-P (Outlook), and pyroxasulfone (Zidua SC; BASF, Research Triangle Park, NC).

Results

Conclusion