Quantifying Resistance to Isoxaflutole and Mesotrione and Investigating Their Interactions with Metribuzin POST in Waterhemp (Amaranthus tuberculatus)

Abstract

Greenhouse experiments were conducted to quantify resistance levels to the 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione (MES) and isoxaflutole (IFT) in NEB (Nebraska HPPD- and atrazine-resistant) and SIR (Stanford, IL, HPPD- and atrazine-resistant) waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] populations. These populations differ in their field-use histories and resistance levels to MES. Foliar growth responses were compared with ACR (HPPD sensitive; metabolic atrazine-resistant) and SEN (sensitive to HPPD and photosystem II [PSII] inhibitors). A greenhouse dose–response study was conducted with each herbicide at two POST timings: early (EPOST) (5 cm; 4 to 5 true leaves) and POST (10 cm; 8 to 9 true leaves). At the EPOST timing, SIR was 10-fold resistant to IFT and 32-fold resistant to MES, while NEB was 4-fold resistant to IFT and 7-fold resistant to MES when compared with ACR. At the POST timing, SIR was 17-fold resistant to IFT and 21-fold resistant to MES, while NEB was 3-fold resistant to IFT and 7-fold resistant to MES when compared with ACR. Results overall indicated greater fold-resistance levels to MES relative to IFT at each timing. However, POST treatments to SIR showed contrasting effects on resistance levels relative to EPOST. To investigate potential management strategies for resistantA. tuberculatuspopulations, a POST interaction study was conducted using combinations of metribuzin and either IFT or MES. A metribuzin rate (191 g ai ha−1) causing an approximately 20% biomass reduction was chosen for interaction studies and combined with varying rates of either IFT or MES. Results indicated 52.5 g ai ha−1of MES combined with metribuzin displayed a synergistic effect on biomass reduction in SIR. However, other combinations of either MES or IFT and metribuzin resulted in additive effects on biomass reduction in both HPPD-resistant populations. These results provide insights into the joint activity between HPPD and PSII inhibitors for controlling metabolism-based, multiple herbicide–resistantA. tuberculatus.

Metabolic Pathway of Topramezone in Multiple-Resistant Waterhemp (Amaranthus tuberculatus) Differs From Naturally Tolerant Maize

“…Dose-response analysis demonstrated that the SIR population is more resistant to POST topramezone than the NEB population, which is supported by enhanced topramezone metabolism (Figure 3 ) and metabolite formation (Figure 4 ). The greater fold-resistance of SIR may be related to prior usage of topramezone to control the SIR population (along with mesotrione and tembotrione; Hausman et al, 2011 ), in contrast with NEB ( Kaundun et al, 2017 ), and is also consistent with higher fold-resistance levels of SIR to mesotrione and isoxaflutole applied POST relative to NEB ( O’Brien et al, 2018 ). Moreover, NEB was never pressured with topramezone in the field yet significant levels of resistance were observed, suggesting that the gene(s) selected by mesotrione and/or tembotrione confer cross-resistance to topramezone in the NEB population ( Kaundun et al, 2017 ).…”

“…Four waterhemp populations were subjected to dose-response analysis with topramezone in the greenhouse. Two are sensitive to foliar HPPD-inhibiting herbicides (SEN and ACR) and two (SIR and NEB) are resistant ( Hausman et al, 2011 ; Kaundun et al, 2017 ; O’Brien et al, 2018 ). Maize hybrid DKC 63-14 RR was also included for comparison, but data are not shown since this hybrid did not exhibit visual injury symptoms at any topramezone rate tested (24 g ha −1 is a field-use rate in maize).…”

“…At the lower rates of topramezone examined, ACR was the most sensitive population and NEB was less resistant than SIR. Although both SEN and ACR are sensitive to HPPD-inhibiting herbicides ( O’Brien et al, 2018 ), only the SEN population was utilized to generate resistance indices (RIs) for NEB and SIR (Table 1 ). Each population displayed GR 50 values well below the field-use rate of topramezone in maize, ranging from 7.3 g ha −1 for SIR to 0.2 g ha −1 for ACR (Table 1 ).…”

“…Dose-response curves were generated as described in Section “Materials and Methods” and used to determine the 50% growth reduction and resistance index values listed in Table 1 . SIR and NEB are HPPD inhibitor-resistant populations while ACR and SEN are sensitive to HPPD inhibitors ( O’Brien et al, 2018 ).…”

“…The four waterhemp populations investigated in this research are the same as those described by O’Brien et al (2018) . Two are sensitive to HPPD-inhibiting herbicides (SEN and ACR) and the two others (SIR and NEB) are resistant to POST applications of mesotrione, tembotrione, topramezone, and atrazine ( Hausman et al, 2011 ; Kaundun et al, 2017 ).…”