Abstract
Protoporphyrinogen oxidase (PPO) inhibitors are one of the few remaining postemergence herbicide options for controlling Palmer amaranth in soybean growing areas of Mississippi, USA. Most Palmer amaranth populations in Mississippi are resistant to both glyphosate and acetolactate synthase inhibitors. Resistance to PPO inhibiting herbicides in Palmer amaranth has very recently been reported in Arkansas, Tennessee, and isolated pockets of Mississippi. A significant proportion of reports of PPO inhibitor failures in Mississippi are not considered to be resistance-related at this time. Therefore, the objective of this research was to evaluate factors affecting the efficacy of fomesafen on Palmer amaranth including: quality of spray carrier (water), formulations, adjuvant, rainfastness, and nozzle type. All water samples and formulation combinations provided >95% control of Palmer amaranth 3 WAT. Some combinations of water samples and formulations did not result in complete control of the treated plants, with one or two individuals surviving 3 WAT. Formulation 1 provided 99% control compared to 95% from formulation 2. Irrespective of combinations of herbicide, adjuvant and height, control of Palmer amaranth was ≥91%. Formulation 1 provided 94% control compared to 88% from formulation 2. The adjuvant x height interaction was significant, owing to a 10% reduction in control of larger plants (86%) compared to smaller plants (96%) in presence of COC. COC provided better control (93%) than NIS (88%). Simulated rainfall applied ≥60 min after herbicide application did not adversely affect efficacy on Palmer amaranth when formulation 1 was applied in combination with NIS, with control ranging from 94% to 100%. Formulation 1 with COC provided ≥93% control at all rainfall application times, except 30 min after herbicide treatment, which resulted in 79% control. Formulation 2 provided better control with COC (79% to 100%) than NIS (71% to 90%), in general, across the rainfall treatments applied at various times following herbicide application. All nozzle and weed height combinations resulted in 89% or better control of Palmer amaranth. In summary, water quality, formulation, adjuvant, rainfastness, or nozzle type did not affect the activity of fomesafen under optimal application conditions in the greenhouse.
Highlights
Widespread distribution of glyphosate-resistant (GR) weeds in soybean [Glycine max (L.) Merr.]-growing areas across Mississippi has economically affected soybean planting and follow-up crop management operations
This research, especially, with water quality analysis, is an “out-of-the-box” approach to eliminate factors that may result in the mis-diagnosis of non-performance of a Protoporphyrinogen oxidase (PPO) inhibitor as a case of resistance
An aircraft applicator from DeSoto County, Mississippi has indicated his readiness to switch from a city water supply to a well at his mixing facility to save on costs, based on results on water quality provided to him from this research
Summary
Widespread distribution of glyphosate-resistant (GR) weeds in soybean [Glycine max (L.) Merr.]-growing areas across Mississippi has economically affected soybean planting and follow-up crop management operations. New multiple herbicide-resistant crop technologies, involving dicamba and 2,4-D resistance, with associated formulations have been deregulated (transgenic traits by USDA)/registered (herbicide formulations by EPA), but their performance on large production fields is not clear. In 2016, several growers across the states of Arkansas, Missouri, Tennessee, and to a lesser extent in Mississippi treated fields planted to dicamba-resistant cotton (Gossypium hirsutum L.) and soybean with unauthorized/off-label/unregistered dicamba formulations, thereby, injuring large swaths of non dicamba-resistant soybean fields in the midsouthern US [2]. Labeled applications of registered dicamba formulations on dicamba-resistant soybean drifted (volatile/vapor drift and/or physical drift due to droplet movement owing to temperature inversion and other factors) off-target and injured an alarmingly large area, 1 million ha across the midwestern and southern U.S [3] and 1.5 million ha across the U.S (https://ipm.missouri.edu/IPCM/2017/10/final_report_dicamba_injured_soybea n/), of non dicamba-resistant soybean. Effectiveness of these technologies could be short-lived as it has been
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