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30 July 2019 Characterization of Multiple Herbicide–Resistant Waterhemp (Amaranthus tuberculatus) Populations from Illinois to VLCFA-Inhibiting Herbicides
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Abstract

Field experiments were conducted in 2016 and 2017 in Champaign County, IL, to study a waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] population (CHR) resistant to 2,4-D and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-, photosystem II–, acetolactate synthase (ALS)-, and protoporphyrinogen oxidase–inhibiting herbicides. Two field experiments were designed to investigate the efficacy of very-long-chain fatty-acid (VLCFA)-inhibiting herbicides, including a comparison of active ingredients at labeled use rates and a rate titration experiment. Amaranthus tuberculatus density and control were evaluated at 28 and 42 d after treatment (DAT). Nonencapsulated acetochlor, alachlor, and pyroxasulfone provided the greatest PRE control of CHR (56% to 75%) at 28 DAT, while metolachlor, S-metolachlor, dimethenamid-P, and encapsulated acetochlor provided less than 27% control. In the rate titration study, nonencapsulated acetochlor controlled CHR more than equivalent field use rates of S-metolachlor. Subsequent dose–response experiments with acetochlor, S-metolachlor, dimethenamid-P, and pyroxasulfone in the greenhouse included three multiple herbicide–resistant (MHR) A. tuberculatus populations: CHR-M6 (progeny generated from CHR), MCR-NH40 (progeny generated from Mclean County, IL), and ACR (Adams County, IL), in comparison with a sensitive population (WUS). Both CHR-M6 and MCR-NH40 are MHR to atrazine and HPPD, and ALS inhibitors and demonstrated higher survival rates (LD50) to S-metolachlor, acetochlor, dimethenamid-P, or pyroxasulfone than ACR (atrazine resistant but HPPD-inhibitor sensitive) and WUS. Based on biomass reduction (GR50), resistant to sensitive (R:S) ratios between CHR-M6 and WUS were 7.5, 6.1, 5.5, and 2.9 for S-metolachlor, acetochlor, dimethenamid-P, and pyroxasulfone, respectively. Values were greater for MCR-NH40 than CHR-M6, and ACR was the most sensitive to all VLCFA inhibitors tested. Complete control of all populations was achieved at or below a field use rate of acetochlor. In summary, field studies demonstrated CHR is not controlled by several VLCFA-inhibiting herbicides. Greenhouse dose–response experiments corroborated field results and generated R:S ratios (LD50) ranging from 4.5 to 64 for CHR-M6 and MCR-NH40 among the four VLCFA-inhibiting herbicides evaluated.

© Weed Science Society of America, 2019.
Seth A. Strom, Lisa C. Gonzini, Charlie Mitsdarfer, Adam S. Davis, Dean E. Riechers, and Aaron G. Hager "Characterization of Multiple Herbicide–Resistant Waterhemp (Amaranthus tuberculatus) Populations from Illinois to VLCFA-Inhibiting Herbicides," Weed Science 67(4), 369-379, (30 July 2019). https://doi.org/10.1017/wsc.2019.13
Received: 10 December 2018; Accepted: 19 March 2019; Published: 30 July 2019
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