Basis of Atrazine and Mesotrione Synergism for Controlling Atrazine‐ and HPPD Inhibitor‐Resistant Palmer Amaranth

Abstract

Palmer amaranth (Amaranthus palmeri S. Watson) resistant to atrazine [6‐chloro‐N‐ethyl‐N’‐(1‐methylethyl)‐1,3,5‐triazine2,4‐diamine] and 4‐hydroxyphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides was confirmed in a seed corn (Zea mays L.) production field in Nebraska, in 2014. Neither atrazine nor HPPD inhibitors (mesotrione [2‐(4‐mesyl2‐nitrobenzoyl)‐3‐hydroxycylohex‐2‐enone], tembotrione {2‐[2‐chloro‐4‐(methylsulfonyl)‐3‐[(2,2,2‐trifluoroethoxy) methyl]benzoyl]‐1,3‐cyclohexanedione}, or topramezone {[3‐(4,5‐dihydro‐3‐isoxazolyl)‐2‐methyl‐4‐(methylsulfonyl)phenyl](5‐hydroxy‐1‐methyl‐1H‐pyrazol‐4‐yl)methanone}) applied post‐emergence were able to control resistant Palmer amaranth even at greater than label rates. However, their tank mixtures even at lower than the label rate provided more than 90% control under greenhouse and field conditions. The objectives of this study were to investigate the effect of atrazine on mesotrione absorption and translocation when tank mixed or vice versa in atrazine‐ and HPPD inhibitor‐resistant Palmer amaranth from Nebraska. Tank mixing commercial formulation of atrazine at 560 g ha−1 increased 14C‐mesotrione absorption to 51% compared to 39% with 14C‐mesotrione alone. However, 14C‐atrazine absorption or translocation was not affected by mesotrione at 26 g ha−1 in the tank mixture. Similarly, mesotrione did not affect the metabolism of 14C‐atrazine in resistant or susceptible plants when tank mixed compared to 14C‐atrazine applied alone. Increased absorption of mesotrione when tank mixed with atrazine could be one of the reasons of atrazine and mesotrione synergism besides their biochemical interaction in the atrazine‐ and HPPD inhibitor‐resistant Palmer amaranth biotype from Nebraska. Core Ideas Atrazine applied in tank‐mixture increased mesotrione absorption. Mesotrione applied in tank mixture did not affect atrazine absorption and translocation. Atrazine metabolism was not affected by mesotrione applied in tank mixture.

Predominance of Metabolic Resistance in a Six-Way-Resistant Palmer Amaranth (Amaranthus palmeri) Population

“…However, some plants were not effectively controlled by this tank mixture. Such improvement in control can be attributed to the synergism often observed with tank mixing PS II- and HPPD-inhibitor herbicides ( Abendroth et al, 2006 ; Chahal et al, 2019a ). Additionally, the commercial pre-mixture of bromoxynil (PS II-inhibitor) and pyrasulfotole (HPPD-inhibitor) performed poorly in comparison to a tank mix of atrazine and mesotrione.…”

A systematic approach for finding herbicide synergies

“…To find which herbicides were synergistic we created a matrix of 24 herbicides, where 20 herbicides represented one mode of action each, and four herbicides had an unknown mode of action ( Table 2). The herbicides chosen for screening are widely used and included three molecules (clomazone, mesotrione and atrazine) for which well-documented cases of synergy are known as positive controls (Keifer et al, 1990, Abendroth et al, 2006, Bollman et al, 2006, Armel et al, 2007, Woodyard et al, 2009b, Walsh et al, 2012, Kohrt and Sprague, 2017, Chahal et al, 2019 To detect synergistic herbicide combinations, the 276 possible pairwise mixtures of 24 herbicides were made and screened using Arabidopsis thaliana grown on agar plates. For each possible combinations (276), a single sub-lethal concentration for each herbicide was used.…”

“…Three herbicides stood out in these synergies; three of the pairs included mesotrione, two included clomazone and two included atrazine. Two combinations (mesotrione-atrazine and clomazone-atrazine) (Keifer et al, 1990, Abendroth et al, 2006, Bollman et al, 2006, Armel et al, 2007, Woodyard et al, 2009b, Walsh et al, 2012, Kohrt and Sprague, 2017, Chahal et al, 2019 were previously reported in the literature, which showed that one-dose screening was sensitive enough to detect these known synergistic mixtures.…”

“…The herbicides chosen for screening are widely used and included three molecules (clomazone, mesotrione and atrazine) for which well-documented cases of synergy are known as positive controls. 10, 11, 15–20…”

Syngenta's contribution to herbicide resistance research and management

“…The post‐emergence application of a three‐way mixture of glyphosate (HRAC group G), mesotrione and S ‐metolachlor in addition to atrazine within 21 days of the pre‐emergence application is advocated for season‐long, overlapping residuality in glyphosate‐tolerant corn. The synergistic effect of atrazine and mesotrione in the four‐way mixture and other related products further protects against resistance evolution in key weeds 73,74 . Similar season‐long residual programs include a mixture of metribuzin (HRAC group C1) and S ‐metolachlor applied pre‐emergence followed by S ‐metolachlor + fomesafen (HRAC group E) 30 days later for controlling glyphosate‐resistant A. palmeri and A. quitensis populations in soybean in Argentina.…”