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

Abstract

S-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 following S-metolachlor treatment were first noted at field sites near Crawfordsville and Marion in 2016. Field and greenhouse experiments were conducted to confirm S-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–1 provided 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 to S-metolachlor compared with two susceptible accessions based on LD50 values obtained from dose–response experiments. Two-thirds and 1.5 times S-metolachlor at 1,064 g ha–1 were 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 to S-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 LD50 values obtained from greenhouse dose–response experiments, exhibited reduced sensitivity (1.5- to 3.6-fold) to the tested VLCFA-inhibiting herbicides.

Seed destruction of weeds in southern US crops using heat and narrow-windrow burning

“…In southern U.S. crop production systems, two of the most troublesome weeds are Palmer amaranth and barnyardgrass (Riar et al 2013;Schwartz-Lazaro et al 2018;WSSA 2017). Palmer amaranth has documented resistance to herbicides that inhibit microtubule assembly, very long chain fatty acid elongase, acetolactate synthase, 5-enolpyruvylshikimate-3-phosphate synthase, photosystem II, 4-hydroxyphenylpyruvate dioxygenase, and protoporphyrinogen oxidase (Brabham et al 2019;Heap 2019;Varanasi et al 2018). Barnyardgrass is the most problematic weed of rice (Oryza sativa L.), a crop that is routinely grown in rotation with soybean in the southern United States.…”

Effect of dicamba rate and application parameters on protoporphyrinogen oxidase inhibitor-resistant and -susceptible Palmer amaranth (Amaranthus palmeri) control

“…Special attention should be paid to Palmer amaranth mortality with dicamba in fields where PPO-resistance is suspected. Metabolic resistance to S-metolachlor was recently confirmed in the Marion population and it is possible that this metabolic resistance could be the cause of reduced mortality of Palmer amaranth treated with dicamba at this location (Brabham et al 2019). Because PPO-resistant populations have proven to be harder to control with dicamba, other weed control methods may need to be employed.…”

“…In 2018, metabolic resistance of Palmer amaranth to fomesafen was confirmed in Arkansas (Varanasi et al 2018a). Subsequently, metabolic resistance to the very-long-chain fatty acid inhibitor S-metolachlor was also identified in Arkansas (Brabham et al 2019). While no mechanisms of resistance to dicamba have been identified in Arkansas Palmer amaranth, the discovery of metabolic resistance mechanisms to other herbicides in Arkansas suggests that metabolic resistance to more herbicide sites of action could be building (Yu and Powles 2014).…”

Non-Target-Site Resistance to Herbicides: Recent Developments

“…NTSR mechanisms to very long chain fatty acid (VLCFA) inhibitors have been studied in ryegrass populations from Australia [122], U.K., and France [123], as well as Palmer amaranth from Arkansas [124]. Metabolic resistance to pyroxasulfone was reported in a rigid ryegrass population from Australia, where approximately 88% of parental applied herbicides was metabolized within 24 HAT [122].…”

“…Recently, resistance to s -metolachlor was documented in Palmer amaranth from Arkansas. A reduction in root growth was observed when the resistant accessions were kept for germination in agar solution containing a GST-inhibitor, NBD-cl, indicating the role of GSTs in mediating the resistance [124].…”