Characterization of multiple herbicide-resistant waterhemp (Amaranthus tuberculatus) populations from Illinois to VLCFA-inhibiting herbicides

Authors: STROM, SETH - University Of Illinois; CONZINI, LISA - University Of Illinois; MITSDARFER, CHARLIE - University Of Illinois; Davis, Adam; RIECHERS, DEAN - University Of Illinois; HAGER, AARON - University Of Illinois

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2019
Publication Date: 5/27/2019
Citation: Strom, S.A., Gonzini, L.C., Mitsdarfer, C., Davis, A.S., Riechers, D.E., Hager, A.G. 2019. Characterization of multiple herbicide-resistant waterhemp (Amaranthus tuberculatus) populations from Illinois to VLCFA-inhibiting herbicides. Weed Science. 67(4):369-379. https://doi.org/10.1017/wsc.2019.13.
DOI: https://doi.org/10.1017/wsc.2019.13

Interpretive Summary: The evolution of multiple herbicide resistance in agricultural weeds is rapidly undermining our ability to protect food crops from substantial yield loss due to weed interference, threatening national and global food security. One avenue of investigation that requires more attention is how the evolution of resistance by a weed species to one herbicide mode of action (MOA) can affect its susceptibility to compounds from another herbicide MOA, a phenomenon known as ‘cross resistance.’ We conducted a field study in 2016 and 2017 at a site in Champaign County, Illinois, infested by a common waterhemp (Amaranthus tuberculatus (Moq.) Sauer) population resistant to five different herbicide MOA: 4-hydroxyphenylpyruvate dioxygenase (HPPD), photosystem II, acetolactate synthase, and protoporphyrinogen oxidase-inhibiting herbicides plus the growth regulator herbicide 2,4-D. Field experiments were designed to quantify the efficacy of five Group 15 herbicides (chloracetamides), acetochlor, metolachlor, S-metolachlor, dimethenamid-P, and encapsulated acetochlor on this population. Field studies demonstrated that the waterhemp population was not controlled by Group 15 herbicides other than acetochlor. Greenhouse experiments supported this finding and indicated that other HPPD-resistant populations from Illinois demonstrated reduced sensitivity to Group 15 herbicides, with S-metolachlor ineffective at a field use rate. Understanding the metabolic pathways conferring cross-resistance among different herbicide groups will be important for making future recommendations as to which herbicides are most likely to be effective in controlling resistant weed populations.

Technical Abstract: Field experiments were conducted in 2016 and 2017 at a site in Champaign County, Illinois, containing a population of waterhemp (Amaranthus tuberculatus (Moq.) Sauer) (CHR) previously characterized as resistant to 4-hydroxyphenylpyruvate dioxygenase (HPPD), photosystem II, acetolactate synthase, and protoporphyrinogen oxidase-inhibiting herbicides plus the growth regulator herbicide 2,4-D. Two field experiments were designed to investigate previous observations of the efficacy of Group 15 herbicides, including a comparison of active ingredients at labeled use rates and a rate titration experiment. Waterhemp density and visual control were evaluated at 28 and 42 d after treatment (DAT). Overall, acetochlor provided the greatest control of CHR (75%) at 28 DAT while metolachlor, S-metolachlor, dimethenamid-P, and encapsulated acetochlor provided less than 30% control. In the rate titration study, acetochlor provided greater control than equivalent rates of either pyroxasulfone or S-metolachlor. Subsequent dose-response experiments to S-metolachlor and acetochlor preemergence (PRE) were conducted in the greenhouse and included three multiple-resistant waterhemp populations: M6 (progeny from CHR), NH40 (progeny obtained from Mclean Co., IL), and ACR (Adams Co., IL), in comparison with a known sensitive population (WUS). Both the M6 and NH40 populations demonstrated higher survival rates (LD50) to S-metolachlor and acetochlor than ACR and WUS. Based on biomass reduction (GR50) following S-metolachlor PRE application, resistant-to-sensitive (R/S) ratios were 7.5 and 12.9 for M6 and NH40, respectively, in comparison to WUS. In response to acetochlor PRE, values of 6.1 and 6.8 were calculated for M6 and NH40, respectively, and complete control was achieved at or below a field use rate. Field studies demonstrated the CHR population is not controlled by various Group 15 herbicides other than acetochlor. Greenhouse experiments supported this finding and indicate that both HPPD-resistant populations from Illinois demonstrated reduced sensitivity to Group 15 herbicides, with S-metolachlor ineffective at a field use rate.