Abstract
AbstractStudies to evaluate the effect of application time of day (TOD) and protoporphyrinogen IX oxidase (PPO)-inhibiting herbicide–resistant Palmer amaranth on the efficacy of commonly used herbicides was conducted in Tennessee in 2017 and 2018. Treatments of fomesafen, lactofen, acifluorfen, paraquat, glufosinate, glufosinate plus fomesafen, paraquat plus fomesafen, and paraquat plus metribuzin were applied to PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) Palmer amaranth at sunrise and midday. Control of Palmer amaranth with acifluorfen, glufosinate, and glufosinate plus fomesafen was greater with the midday application. However, control of Palmer amaranth with paraquat-based treatments was greater with the sunrise application. TOD effects on PPO-inhibiting herbicides and paraquat-based treatments were more prominent for the PPO-R Palmer amaranth biotype. The TOD effect observed when applying glufosinate in early morning hours on PPO-S Palmer amaranth can be minimized by adding fomesafen to the tank mix. However, this strategy did not provide consistent performance on PPO-R Palmer amaranth. The percentages of living Palmer amaranth plants and control were greater when paraquat plus metribuzin was applied to both biotypes. These results highlight the necessity of at least two effective herbicide sites of action for POST applications intended for controlling PPO-R Palmer amaranth. In addition, the timing of herbicide applications can affect their activity in both PPO-R and PPO-S Palmer amaranth populations.
Highlights
Palmer amaranth resistance to protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides (WSSA Group 14) has complicated chemical control tactics in the mid-South (Giacomini et al 2017; Heap 2018; Johnston et al 2018; Schwartz-Lazaro et al 2017)
Management of glyphosate and acetolactate synthase inhibitor–resistant Palmer amaranth has relied on PPO-inhibiting herbicides applied PRE and POST for control
Coupled with Palmer amaranth’s prolific growth and ability to spread, the aforementioned reliance has selected for PPO-resistant (PPO-R) Palmer amaranth biotypes throughout Arkansas, Illinois, and Tennessee (Copeland et al 2018a; Heap 2018; Varanasi et al 2017; Ward et al 2013)
Summary
Palmer amaranth resistance to protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides (WSSA Group 14) has complicated chemical control tactics in the mid-South (Giacomini et al 2017; Heap 2018; Johnston et al 2018; Schwartz-Lazaro et al 2017). Management of glyphosate and acetolactate synthase inhibitor–resistant Palmer amaranth has relied on PPO-inhibiting herbicides applied PRE and POST for control. POST herbicide applications that include multiple, effective sites of action for control of PPO-R Palmer amaranth are a valuable resistance-management strategy. Coetzer et al (2001) reported increasing relative humidity from 35% to 90% increased glufosinate efficacy on control of Palmer amaranth, redroot pigweed (Amaranthus retroflexus L.), and common waterhemp [A. tuberculatus (Moq.) J.D. Sauer]. Increases in temperature increased glufosinate injury to Amaranthus spp. Temperature regimens increasing from 26/21 C (day/night) to 31/26 C increased visual injury from 51% to 71%, respectively, 14 d after treatment with glufosinate at 410 g ha−1 (Coetzer et al 2001)
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