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1 April 2015 Characterization of PPO-Inhibitor–Resistant Waterhemp (Amaranthus tuberculatus) Response to Soil-Applied PPO-Inhibiting Herbicides
R. Joseph Wuerffel, JULIE M. YOUNG, JOSEPH L. MATTHEWS, BRYAN G. YOUNG
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Abstract

Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen plus s-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance.

Nomenclature: Flumioxazin; fomesafen; glyphosate; s-metolachlor; sulfentrazone; tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer (syn. rudis) AMATA.

Weed Science Society of America
R. Joseph Wuerffel, JULIE M. YOUNG, JOSEPH L. MATTHEWS, and BRYAN G. YOUNG "Characterization of PPO-Inhibitor–Resistant Waterhemp (Amaranthus tuberculatus) Response to Soil-Applied PPO-Inhibiting Herbicides," Weed Science 63(2), 511-521, (1 April 2015). https://doi.org/10.1614/WS-D-14-00108.1
Received: 21 July 2014; Accepted: 1 October 2014; Published: 1 April 2015
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