Control of waterhemp (Amaranthus tuberculatus) at multiple locations in Illinois with single preemergence applications of VLCFA-inhibiting herbicides

Abstract

Very-long-chain fatty acid (VLCFA)-inhibiting herbicides have been widely used for preemergence control of annual monocot and small-seeded dicot weed species, such as waterhemp, since their discovery in the 1950s. VLCFA-inhibiting herbicides are often applied in combination with active ingredients that possess residual activity on small-seeded broadleaf weeds, which can make their contribution to preemergence waterhemp control difficult to quantify. Bareground field experiments were designed to investigate the efficacy of eight VLCFA-inhibiting herbicides applied at their minimum and maximum labeled rates for control of Illinois waterhemp populations. Four different locations were selected, two of which contained previously characterized VLCFA inhibitor-resistant waterhemp populations in Champaign County (CHR) and McLean County (MCR). Two locations with VLCFA inhibitor-sensitive waterhemp populations included the University of Illinois South Farm in Urbana, IL and the Orr Research Center in Perry, IL. Soils at the CHR, MCR, and Urbana locations contained greater than 3% organic matter, but less than 3% organic matter at Perry. Non-encapsulated acetochlor and alachlor controlled CHR and MCR 28 d after treatment (DAT), whereas other VLCFA-inhibiting herbicides resulted in 61 and 76% control of CHR and MCR, respectively. In contrast, all VLCFA-inhibiting herbicides resulted in 81 and 88% control of the Perry and Urbana populations, respectively, 28 DAT. Waterhemp control decreased by 42 DAT, especially for the VLCFA inhibitor-resistant CHR and MCR populations. Overall, VLCFA-inhibiting herbicides remain effective for controlling sensitive waterhemp, but most are not effective for controlling VLCFA-inhibitor-resistant waterhemp populations. Proper herbicide stewardship and integrated weed management practices should be implemented to maintain VLCFA-inhibiting herbicide efficacy for waterhemp management in the future.

Inheritance of resistance to S-metolachlor in a waterhemp (Amaranthus tuberculatus) population from central Illinois

“…By contrast, segregation of F 2 -♀ lines best fits a two-locus model of 9:7 (R:S), consistent with two complementary genes wherein one homozygous recessive locus (rr) interacts with dominant allele(s) at a different locus (R-) controlling resistance (Bernardo 2014; Falconer 1989). The most plausible biochemical explanation for the single, dominant gene model is that S -metolachlor resistance in SIR is conferred by a single P450 gene encoding a P450 enzyme that rapidly detoxifies the herbicide (Strom et al 2020, 2021). However, the two-gene model described above is consistent with a single P450 gene that primarily confers resistance, but a second recessive gene (if homozygous) decreases the magnitude of S -metolachlor resistance by an unknown mechanism.…”

“…It is possible a VLCFAE inhibitor–resistant population existed at the McLean County, Stanford, Illinois, site before field scouts noticed the lack of chemical control, particularly because S -metolachlor is typically not used alone for managing A. tuberculatus (Strom et al 2022). Furthermore, resistant waterhemp may have occurred before Group 5 (PSII inhibitors; e.g., atrazine and simazine) and Group 27 herbicides (HPPD inhibitors; e.g., mesotrione, topramezone, tembotrione) no longer controlled weed escapes (Hausman et al 2011; Strom et al 2019).…”

“…The DK 3-2 line as well as maternal- and paternal-derived F 1 and F 2 lines (Supplementary Table S1) provide unique genetic stocks for identifying the precise molecular mechanism conferring VLCFAE-inhibiting herbicide resistance. Although S -metolachlor resistance in A. tuberculatus is primarily conferred by enhanced metabolism—where P450 (primary) and GST (secondary) enzymes play important detoxification roles (Strom et al 2020, 2021)—it is also possible transcription factors may affect expression of these metabolic genes or that altered target-site genes encoding the large family of plant VLCFAE elongases (Böger 2003; Haslam and Kunst 2013; Krähmer et al 2019; Trenkamp et al 2004) may play a secondary role. Ongoing work in our lab utilizing proteomic, transcriptomic, and tissue-specific expression analysis in roots, etiolated shoots, and leaves is aimed at identifying which specific metabolic enzyme confers VLCFAE-inhibitor resistance in A. tuberculatus .…”

“…Resistance to pyroxasulfone in L. rigidum is metabolism based and is characterized by upregulation of glutathione S -transferase ( GST ) and cytochrome P450 ( P450 ) genes (Busi et al 2014, 2018; Busi and Powles 2016). Resistance to S -metolachlor in an A. tuberculatus population from Stanford, Illinois (SIR) occurs via P450- and GST-catalyzed detoxification mechanisms acting in concert (Strom et al 2020, 2021). However, research has not been reported that clearly establishes the genetic basis and inheritance of S -metolachlor resistance in A. tuberculatus .…”

Quantifying resistance to very-long-chain fatty acid-inhibiting herbicides in Amaranthus tuberculatus using a soilless assay

“…Biomass and survival data per study were pooled since the O'Brien test for homogeneity of variance for repeated experiments was not significant. Survival and biomass data were analyzed using a three-parameter logistic regression model y ¼ d 1þexpfb½logðxÞÀ logðeÞ�g [62], where d is the upper limit, b is the slope of the curve, and e is the 50% reduction in seedling survival (LD 50 ) or 50% reduction in seedling aboveground dry biomass (GR 50 ), in the 'Analysis of Dose-Response Curves', or drc package, in R (Version 3.4.3) and RStudio (Version 1.2.1335) [63]. Lethal dose estimates of 50% (LD 50 ) and growth reduction estimates of 50% (GR 50 ) for each population were obtained from each analysis, respectively.…”