UC Davis

Identification of potential weed management tools for California tree nut orchard systems is an ongoing endeavor. Proper weed management reduces competition with the crop and facilitates harvest. Many weed control programs include the use of preemergence and postemergence herbicides. However, selection pressure has led to herbicide-resistant weeds which require additional options. A potential tool for orchard weed management is pyroxasulfone, an HRAC/WSSA group 15 herbicide that is an inhibitor of very long-chain fatty acid synthesis. Pyroxasulfone is registered as a preplant incorporated or preemergence herbicide, in corn, soybean, and cotton in some Midwestern states of the United States. However, there is limited published literature on the use of pyroxasulfone in tree nut orchard systems. A series of crop safety and weed control efficacy experiments were carried out for the characterization of pyroxasulfone in California orchard crops. A suspension concentrate (SC) formulation of pyroxasulfone was evaluated in fallow field studies initiated in fall 2020 and carried out into summer 2022 near Davis, CA. Studies were conducted to evaluate the weed control efficacy of pyroxasulfone at 145, 219, and 293 g ha-1 rates. In addition, an experiment was conducted in the summer of 2021 to evaluate herbicide efficacy in response to two incorporation timings. Single application and sequential applications experiments evaluated the use of a water dispersible granule (WDG) formulation of pyroxasulfone or pyroxasulfone (SC) at multiple rates in comparison to commercially used standards flumioxazin, indaziflam, oxyfluorfen, pendimethalin, penoxsulam + oxyfluorfen, and rimsulfuron. Experiments were conducted in a fallow field, a vineyard and in almond and walnut orchards near Arbuckle, Davis, and Winters, CA in spring 2021 and spring 2022. A two-year crop safety experiment was conducted to evaluate repeated applications of above-label rates including pyroxasulfone at 1,199 g ha-1 and S-metolachlor at 14,010 g ha-1 on 1-2-yrs-old tree nut crops in spring 2021 and spring 2022. Both formulations of pyroxasulfone SC and WDG performed similarly to commercial standards with up to 95% control of broadleaf and grass weeds. No significant differences in weed control were found among treatments in the incorporation timing study. Crop injury was not observed in the vineyard, established orchard, or young orchard studies and there were no treatment effects on tree trunk diameter of almond, pistachio, and walnut in the two-year crop safety study. These results indicate a potential for pyroxasulfone in California tree nut orchard systems which would be a new mode of action and benefit to manage herbicide-resistant weeds in these crops.