Resistance of two Arkansas Palmer amaranth populations to multiple herbicide sites of action

Abstract

Herbicide resistance may evolve at a rapid rate with the lack of proper adoption of best management practices (BMPs). This wide-spread resistance problem is particularly impactful for fields with problematic weeds such as Palmer amaranth (Amaranthus palmeri). Protoporphyrinogen oxidase (PPO)-inhibiting herbicides have been one of the remaining effective sites of action (SOAs) for the control of Palmer amaranth, but even these herbicides have begun to fail in many soybean fields across Arkansas, USA. The objective of this research was to determine which of the most commonly used PPO-inhibiting herbicides have the greatest effect on two putative PPO-resistant Palmer amaranth populations, compared to a susceptible standard, when applied preemergence (PRE) and postemergence (POST); as well as to test for multiple resistance to other commonly used herbicides. A dose-response study was conducted under greenhouse conditions that examined five PRE herbicides and four POST herbicides on one PPO-susceptible and two PPO-resistant populations. Complete control was not achieved at the 8X rate with any PPO-inhibiting herbicide at the PRE application; whereas for the POST application, complete control was not achieved until after the 32X rate for all herbicides. Furthermore, twenty-one different herbicides, representing various SOAs, were used to test various application timings (preplant incorporated (PPI), PRE and POST) on these populations at a 1X field rate. Both of the PPI herbicides (trifluralin and pendimethalin) provided 9.7% mortality of Palmer amaranth. Only two PRE herbicides (in 2 SOAs) showed greater than 85% mortality and three POST application herbicides (3 SOAs) all had greater than 95% mortality of both Palmer amaranth populations. Thus, it is likely that PPO-inhibiting herbicides or the other commonly used herbicides, which were tested, cannot be solely relied upon in the field. Thus, the use of multiple effective sites of action along with other integrated weed management tactics need to be a focus for management of this species.