UC Davis

Tiafenacil is a protoporphyrinogen oxidase (PPO) inhibitor that blocks chlorophyll biosynthesis and causes protoporphyrin accumulation, a highly photodynamic intermediate. The lipid peroxidation and cell membrane destruction caused by tiafenacil leads to plant death. Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Glufosinate leads to plant death by a massive accumulation of reactive oxygen species (ROS). Herbicide mixtures are commonly used in agriculture to increase weed control spectrum and reduce selection pressure for herbicide resistance. Tiafenacil is registered in the US for preplant use on annual crops such as corn and soybean, but not in orchard crops. Glufosinate is commonly used in orchards with a rate range of 650 to 998 g ai ha-1. Field studies were conducted to determine the crop safety of tiafenacil on young almond, walnut, prune, and pistachio trees, as well as the weed control efficacy on broadleaf and grass weeds relevant to California orchard crops. To evaluate crop safety, tiafenacil was applied at 74 and 148 g ai ha-1 three times per year at the base of prune, walnut, and pistachio trees that were less than one-year-old at the time of the first application. A similar almond experiment also included a 222 g ai ha-1 rate of tiafenacil in the protocol. In all four tree crop experiments, treatments were applied once in the spring of 2020, then reapplied three times during early 2021 and early 2022 so that plots were treated a total of 7 times during a three-year period. There was no visual injury on any of the young trees between 30 and 700 days after initial treatment. Similarly, there were no treatment effects on tree diameter even at the highest rate of tiafenacil. Although no yield measurement was taken because of the age of the trees, the relatively few fruits that formed appeared to be normal. In a separate study on weed control efficacy, tiafenacil at 12 g ai ha-1 performed statistically similarly with tiafenacil plus glufosinate in most instances, but control of both broadleaf and grass weeds numerically improved when tiafenacil was applied in mixture with glufosinate. In a greenhouse study, tiafenacil at 12 g ha-1 alone provided 98-100% control of barnyardgrass and 95-98% control of junglerice. There was no significant difference between tiafenacil alone or tiafenacil plus glufosinate; although in some instances, control of junglerice and barnyardgrass was numerically higher with the tankmix than with glufosinate alone. Most postemergence PPO inhibiting herbicides registered in tree crops have activity only on broadleaf weeds; however, these results indicate that tiafenacil has good activity on broadleaf and grass weeds relevant to California orchard crops and that crop safety was acceptable at up to 2- or 3-fold the expected use rate in tree crops. Although tiafenacil has some activity on grass weeds, mixing tiafenacil with glufosinate may be needed for the most reliable control of both broadleaf and grass weeds.