Abstract
Pyroxasulfone is a very long chain fatty acid (VLCFA) inhibitor (group 15) with potential for preemergence weed control in orchard crops and would be beneficial for reducing selection pressure for resistance to more commonly used herbicides, such as glyphosate. Pyroxasulfone is registered and widely used in corn and soybeans in the United States as a solo product and in several premix formulations. However, there is limited published information on the performance and safety of pyroxasulfone in tree nut orchard systems. To assess the potential fit for pyroxasulfone in this crop sector, a series of crop safety and weed control efficacy experiments were carried out in key California orchard crops and in fallow fields with common orchard weeds. The non-crop studies were conducted near Davis, CA, to evaluate winter annual weed control with pyroxasulfone in comparison with pendimethalin and indaziflam. In addition, an irrigation incorporation experiment was conducted in summer to evaluate the effects of incorporation timing on pyroxasulfone, pendimethalin, and indaziflam efficacy as a measure of relative stability. Orchard weed control experiments were conducted in spring in an almond orchard near Arbuckle, CA, and a walnut orchard near Davis, CA. The orchard experiments included pyroxasulfone, pendimethalin, and indaziflam in various rate combinations with glufosinate. To evaluate crop safety, 2-year experiments were conducted to evaluate repeated applications of above-label rates of pyroxasulfone at 1199 g·ha−1 and S-metolachlor at 14,010 g·ha−1 on 1- to 2-year-old almond, pistachio, and walnut trees near Davis. Pyroxasulfone performed similarly to commercial standards with up to 95% control of broadleaf and grass weeds with pyroxasulfone and indaziflam providing 96% to 100% control of annual bluegrass, and pyroxasulfone (293 g·ha−1) being the only herbicide to suppress (>70%) common lambsquarters at 60 days after treatment (DAT). Other weeds controlled by pyroxasulfone include swinecress, redroot pigweed, yellow nutsedge, and Italian ryegrass. No differences were found among treatments in the incorporation timing study indicating adequate stability on the soil surface under summer conditions. Crop injury was not observed in the established orchards or the young orchard studies and there were no treatment effects on tree trunk diameter of almond, pistachio, and walnut in the 2-year crop safety experiments. These results indicate a potential for pyroxasulfone in California tree nut orchard systems that would be an additional site of action and beneficial for management of herbicide-resistant weeds in these crops.
Orchard crops contribute substantially to the California economy; in 2021, almonds (Prunus dulcis) alone brought in $5.03 billion, and pistachios (Pistacia vera) and walnuts (Juglans regia) contributed $2.91 and $1.02 billion, respectively (California Department of Food and Agriculture 2022). There are several reasons to practice proper weed management in tree nut crops, but two of the most important are to reduce competition with the crop and to facilitate harvest (Hanson et al. 2017). In young orchards, weeds compete for nutrients and water that can limit young tree growth and long-term productivity (Goff et al. 1991). Weed presence can also interfere with harvest practices in almond and walnut in which the nuts are mechanically shaken from the tree, swept into windrows in the orchard alley, and are left to dry for 7 to 10 d before they are picked up for processing (Carbo and Connell 2017). Weeds and plant debris can interfere with equipment operation and make it more difficult to recover the nuts.
Weed control programs in conventionally managed tree nut orchards in California typically include tree strip applications of preemergence (PRE) herbicides in early winter followed by postemergence (POST) herbicides in spring, mowing of the alleyways during spring and summer, and a full orchard floor treatment with POST herbicides before harvest (Buchner et al. 1998; Connell et al. 1996; Hanson et al. 2017). The use of broad-spectrum herbicides with the same mode of action consecutively has led to resistance in weed species such as annual bluegrass (Poa annua), barnyardgrass (Echinochloa crus-galli), hairy fleabane (Erigeron bonariensis), horseweed (Erigeron canadensis), Italian ryegrass (Lolium multiflorum), and junglerice (Echinochloa colona) all of which are commonly found in California orchards (Hanson et al. 2014; Heap 2023). Development of herbicides with additional modes of action can help reduce further selection for cross- and multiple-resistant phenotypes (Gressel and Segel 1978). In specialty crops such as tree nuts, new herbicide options often are developed by expanding labels of products already registered in major crops.
Pyroxasulfone was introduced into the pesticide market in 2011 (Australian Pesticides and Veterinary Medicines Authority 2011) and is currently widely used in corn (Zea mays) and soybean (Glycine max) in the United States (Nakatani et al. 2016) among other crops in various solo and premix formulations. Pyroxasulfone is an inhibitor of VLCFAs, belonging to the Herbicide Resistance Action Committee/Weed Science Society of America group 15 herbicides (Nakatani et al. 2016; Tanetani et al. 2009). Resistance to VLCFA inhibitors is limited so far with only 13 weed species having documented resistance in the United States (Heap 2023; Kumar and Jha 2015; Strom et al. 2019). Pyroxasulfone has had experimental uses in PRE and POST weed control programs; however, results have demonstrated greater weed control efficacy with PRE applications as compared with other VLCFA inhibitors (Lee 2018; McNaughton et al. 2014; Stephenson et al. 2017).
Previous studies have evaluated the crop safety and weed control efficacy of pyroxasulfone compared with atrazine, S-metolachlor, and other commonly used PRE herbicides in cotton, corn, field pea (Pisum sativum), rice (Oryza sativa), soybean, and wheat (Triticum aestivum) production systems (Belfry et al. 2015; Geier et al. 2006, 2009; Godwin et al. 2018; King and Garcia 2008; King et al. 2007; Kleemann et al. 2016; Stephenson et al. 2017; Tidemann et al. 2014; Walsh et al. 2011; Webb et al. 2020). Given the demonstrated weed control spectrum and broad use in many annual crops, pyroxasulfone also could be useful in orchard crops. In addition, as a group 15 herbicide, pyroxasulfone would provide an alternative site of action for currently known herbicide-resistant weeds in orchards. Currently, napropamide is the only group 15 herbicide registered for use in California vineyards and almond orchards but is not widely used (California Department of Pesticide Regulation 2023).
Few pyroxasulfone studies have been conducted in tree nut cropping systems; therefore, the objectives of this research were to evaluate the crop safety and weed control efficacy of pyroxasulfone in irrigated California tree nut orchard production systems.
Materials and Methods
Weed control experiments.
The suspension concentrate (SC) formulation of pyroxasulfone was evaluated for crop safety and control of broadleaf and grass weeds. Pyroxasulfone (Zidua® SC; BASF Corporation, Research Triangle Park, NC, USA) was compared with commercial preemergence standards indaziflam (Alion®; Bayer Crop Science LP, Research Triangle Park, NC, USA), and pendimethalin (Prowl H2O®; BASF Corporation). In all experiments, assessments were conducted in reference to nontreated control plots. Crop safety assessments were conducted every 7 d up to 30 DAT and followed by assessments every 15 d between 30 and 120 DAT. Visual weed control assessments were conducted every 15 d up to 90 DAT and followed by assessments every 30 d 90 to 180 DAT.
Fallow field experiment studies were conducted at the Plant Sciences Field Facility of the University of California, Davis (UCD) (38.53, −121.78). Weed control efficacy with pyroxasulfone at 146, 219, and 293 g·ha−1 was compared with pendimethalin at 2130 and 4259 g·ha−1 and indaziflam at 52 and 73 g·ha−1. Studies were conducted in Fall 2020 (study 1), Fall 2021 (study 2), and Spring 2021 (study 3) (Table 1). In this region, most annual precipitation occurs during late fall to early spring; study 1 received 102 mm of rain and study 2 received 191 mm of rain during the first 30 DAT (California Irrigation Management Information System 2023). The spring study (3) was sprinkler irrigated due to complete lack of rainfall; study 3 received 51 mm of water 21 DAT. In study 2, a spray with glufosinate (Rely 280®; BASF Corporation) at 1143 g·ha−1 was conducted on 12 Jan 2022, at 30 DAT to control a heavy population of swinecress (Lepidium coronopus).
Application details with pyroxasulfone, indaziflam, and pendimethalin in experiments near Davis and Arbuckle, CA, USA.
An irrigation incorporation experiment (study 4) was conducted at the UCD Plant Sciences Field Facility (38.53, −121.78) in Summer 2021 to evaluate performance differences of pyroxasulfone (146, 219, and 293 g·ha−1), pendimethalin (2130 and 4259 g·ha−1), and indaziflam (52 and 73 g·ha−1) applications made relative to two irrigation incorporation timings. Each main plot was divided into two subplots; the subplots received the same herbicide treatment but at different application timings relative to the first sprinkler irrigation. Applications “A” and “B” were conducted 18 and 5 d before initial irrigation, respectively. Approximately 12.7 mm of water was applied weekly via sprinkler irrigation up to 120 DAT-B (days after treatment B). Because of an abundance of field bindweed (Convolvulus arvensis), the entire experiment was treated on 3 Jul 2021 (45 DAT-B) with glyphosate (Roundup Powermax®, Bayer Crop Science LP, Research Triangle Park, NC) at 1548 g a.e./ha . Additional spot spraying with glyphosate at 8.78 g a.e./L for control of field bindweed was conducted twice a month up until 120 DAT-B.
An orchard experiment was conducted in a 2-year-old almond orchard (study 5) at the University of California Nickels Soil Laboratory near Arbuckle, CA (38.95, −122.07) and in a walnut orchard (study 6) at the UCD Plant Sciences Field Facility (38.54, −121.79) in Spring 2022. The weed efficacy and crop safety of tank mixes of pyroxasulfone (219 and 293 g·ha−1), pendimethalin (4259 and 6389 g·ha−1), and indaziflam (29, 39, 49, and 73 g·ha−1) mixed with glufosinate (984, 1334, or 1704 g·ha−1) were evaluated. Irrigation was based on crop need as determined by the local orchard manager.
Crop safety experiment.
A series of crop safety studies were conducted in a young (<2-yr-old) mixed species orchard that included almond (study 7), pistachio (study 8), and walnut (study 9) trees at the UCD Plant Sciences Field Facility (38.54, −121.79) (Table 2). The orchard was planted in March of 2020, studies were initiated in February of 2021 and continued for a second application to the same plots during Spring 2022. Pyroxasulfone at 1199 g·ha−1 and S-metolachlor (Dual II Magnum®; Syngenta Crop Protection, LLC, Greensboro, NC, USA) at 14,010 g·ha−1 were evaluated for crop safety. Applications were made during spring either before (timing “A”) or after (timing “B”) blooming and leafing of trees. Visual tree injury assessments were conducted in reference to nontreated plots. Assessments were conducted every 7 d up to 45 DAT-A and -B, followed by assessments every 15 d between 30 and 120 DAT-B. Trunk diameter measurements were used to evaluate treatment effects; measurements were taken before treatment initiation, 1 year after treatment (2022), and 2 years after the initial treatment (2023). Trunk diameter measurements have often been used to observe orchard growth (Hernandez-Santana et al. 2017; Martín-Palomo et al. 2019). The orchard was irrigated with a single-line drip irrigation system based on crop need as determined by the orchard manager.
Application details for a crop safety experiment in young almond, pistachio, and walnut trees evaluating high rates of pyroxasulfone and S-metolachlor near Davis, CA, USA, in Spring 2021 and 2022.
Study application methods.
Treatments were applied using a compressed carbon dioxide backpack sprayer. For control of existing weeds, POST herbicide treatments were added to the mixes; various rates of glufosinate (984 to 1704 g·ha−1) and/or glyphosate (948 to 3083 g·ha−1) were applied in accordance with the size and density of weeds present.
Soil analysis.
Soil samples from each field site were collected and oven dried at 40 °C. The soil samples were sieved with a 2-mm mesh screen and 500-g subsamples were sent to the UCD Analytical Laboratory for characterization.
Statistical analysis.
A randomized complete block design (RCBD) was used for most studies, except study 4, which was conducted as split plot design (SPD). All data were analyzed using a two-way analysis of variance and means separated using Fisher’s Protected least significant difference test with alpha 0.05, where applicable. For study 4, data were first analyzed as an SPD; however, statistical calculations demonstrated no significant differences between the two incorporation timings. Therefore, the weed control data within each plot were averaged over the two incorporation timings and reanalyzed as a RCBD with a factorial arrangement of herbicide treatments (N = 8).
where Y is the trunk diameter measurement, A is the y intercept, B is the slope of the line, and X is year of measurement (Bevans 2022). All analyses were conducted using R version 4.2.2 (Posit Team 2022).
Results and Discussion
Weed control experiments
Fallow field experiments.
In study 1 (Fall 2020), the average control at 75 DAT for the two dominant weed species, filaree (Erodium spp.) and shepherd’s purse (Capsella bursa-pastoris), was 65% and 72% control, respectively (Table 3). Pyroxasulfone at 146 g·ha−1 provided 38% control of filaree and shepherd’s purse, whereas indaziflam at 73 g·ha−1 provided total control for both weeds. The other prominent weed species was annual bluegrass; pendimethalin provided 53% to 64% control of annual bluegrass and all other treatments provided 94% to 100% control.
Weed control 75 d after treatment in a fallow field experiment using preemergence herbicides in Fall 2020 and 2021 near Davis, CA, USA.
In study 2 (Fall 2021), 191 mm of rainfall was received during the first 10 DAT leading to a significant flush of swinecress (Table 3). To allow rating of other weed species, a maintenance treatment was applied 30 DAT to suppress the swinecress. At 75 DAT swinecress had begun to regrow with an average control of 77%. Pendimethalin treatments provided 44% to 58% control of swinecress and all other treatments provided 68% to 98% control.
In study 3 (Spring 2021), the two most prominent weeds in the study were redroot pigweed (Amaranthus retroflexus) and common lambsquarters (Chenopodium album). At 30 DAT pyroxasulfone provided 75% to 88% control of redroot pigweed and indaziflam and pendimethalin treatments provided less than 63% control (Table 4). The average control for common lambsquarters was 71%; pyroxasulfone provided 50% to 100% control. By 60 DAT no treatment provided control of redroot pigweed with an average control of 13%. Pyroxasulfone at 293 g·ha−1 provided the highest control of common lambsquarters with 88%, and all other treatments provided less than 63% control.
Weed control in a fallow field experiment (study 3) using preemergence herbicides in Spring 2021 near Davis, CA, USA.
Our results agree with an experiment conducted by Nurse et al. (2011) in which less than 80% control of common lambsquarters was provided with rates of pyroxasulfone lower than 250 g·ha−1. For redroot pigweed, Nurse et al. (2011) observed that pyroxasulfone at 93 g·ha−1 provided 90% control at 56 DAT; in contrast to our results where pyroxasulfone at 134 and 268 g·ha−1 provided 0% to 13% control of redroot pigweed at 60 DAT. Pyroxasulfone has been evaluated for control of other pigweed species. Meyer et al. (2016) observed pyroxasulfone at 179 g·ha−1 provided 98% control of common waterhemp (Amaranthus tuberculatus) at 21 DAT and provided 96% control of Palmer amaranth (Amaranthus palmeri) at 30 DAT. Results from Houston et al. (2019) demonstrated that pyroxasulfone at 368 g·ha−1 provided up to 79% control of Palmer amaranth at 35 DAT. Our results agree with Meyer et al. (2016) and Houston et al. (2019) that during the first 30 DAT pyroxasulfone can suppress pigweed species.
Irrigation incorporation experiment.
In study 4, the weed control efficacy of pyroxasulfone, pendimethalin, and indaziflam was measured as a stability response to two incorporation timings. Overall weed control 90 DAT-B averaged 93% and decreased to 88% by 150 DAT-B (Table 5). The most widespread weed in this location was yellow nutsedge (Cyperus esculentus). Pyroxasulfone at 219 and 293 g·ha−1 provided 73% control of yellow nutsedge and all other treatments provided less than 65% control. The irrigation incorporation study demonstrated no differences in the tested PRE herbicide residual activity when incorporated 5 or 18 d after treatment application, which suggests that all three herbicides have similar stability performance.
Overall weed and yellow nutsedge control results in a fallow field experiment (study 4)i near Davis, CA, USA, in Summer 2021.
The California Central Valley typically receives rain during the winter from November to March. Without rainfall, irrigation incorporation may be required (Jordan et al. 1963; Knake et al. 1967; Smith et al. 2016). The longer a PRE herbicide is left on the soil surface without incorporation, the higher the probability of dissipation, especially during the summer months when temperatures can reach up to 38 °C (Savage and Barrentine 1969). Our study did not directly evaluate dissipation of any treatment; however, adequate residual control was observed throughout its entirety when the average air temperature was 33 °C regardless of whether it was sprinkler-incorporated 5 or 18 d after treatment.
Orchard experiment.
Study 5 was conducted in an almond orchard; the most prominent weeds were common knotweed (Polygonum arenastrum), annual sowthistle (Sonchus oleraceus), and Italian ryegrass. At 75 DAT, pendimethalin at 4259 and 6389 g·ha−1 provided complete control of common knotweed whereas all other treatments provided less than 53% control (Table 6). Control for annual sowthistle and Italian ryegrass averaged 33% and 65%, respectively.
Weed control in an orchard experiment conducted in almond and walnut in Spring 2022 near Arbuckle and Davis, CA, USA.
Study 6 was conducted in a walnut orchard; the prominent weeds were foxtail barley (Hordeum jubatum) and bermudagrass (Cynodon dactylon). By 60 DAT the average control for bermudagrass and foxtail barley was 28% and 64%, respectively (Table 6). Pendimethalin at 4259 and 6389 g·ha−1 provided 93% and 96% control of foxtail barley, respectively, whereas all other treatments provided less than 77% control.
The orchard experiment had an additional evaluation on different rates of indaziflam and glufosinate. Many PRE herbicides have limited effects on emerged plants, requiring appropriate burndown treatments to control existing weeds. This experiment evaluated the residual efficacy of pendimethalin (4259 and 6389 g·ha−1) and pyroxasulfone (219 and 293 g·ha−1) each mixed with a standard rate (1334 g·ha−1) of glufosinate in comparison with indaziflam (29, 39, 49, and 73 g·ha−1) when mixed with glufosinate (984, 1334, or 1704 g·ha−1). The different rates of glufosinate provided no differences in burndown control of existing weeds in either study (data not shown). However, incomplete burndown in study 6 led to regrowth of foxtail barley.
Crop safety experiment.
After treatments in the spring, the first and second years after transplanting, all treated almond, pistachio, and walnut trees blossomed and leafed out similarly to the untreated trees in the subsequent season (data not shown). Growth was unaffected by herbicide treatments of pyroxasulfone at 1199 g·ha−1 and S-metolachlor at 14,010 g·ha−1 (Fig. 1). Almond and walnut trees had an ∼40-mm and 25-mm increase in diameter each season, respectively. Pistachios had an increase of ∼30-mm at the end of the study; however, these results were affected by significant ground squirrel damage in the young pistachio trees.
Trunk diameter measurements of almond (A), walnut (B), and pistachio (C) before initiation of pyroxasulfone or S-metolachlor treatments, 1 year after initial treatment (2022), and 2 years after initial treatment (2023). Application rates were pyroxasulfone at 1199 g·ha−1 and S-metolachlor at 14,010 g·ha−1. Timing “A” was before flowering and leafing, and timing “B” was after flowering and leafing in both 2021 and 2022.
Citation: HortScience 59, 9; 10.21273/HORTSCI17963-24
These crop safety results were similar to those reported by Pedroso and Moretti (2022) on a study conducted in transplanted hazelnuts. Pedroso and Moretti (2022) found that pyroxasulfone at 240 to 950 g·ha−1 and S-metolachlor at 1390 to 4160 g·ha−1 did not affect trunk cross-sectional areas and caused negligible (<3%) node injury.
Pyroxasulfone has demonstrated potential for use in California orchard systems, with weed control and crop safety performance similar to commercially used herbicides. No treatment-related injury was documented on any of the established (≥4 years old) or young trees (≤2 years old) tested, even when used at an extremely high pyroxasulfone rate of 1199 g·ha−1. In the fall fallow field experiment, pyroxasulfone provided similar broadleaf weed control compared with indaziflam and pendimethalin; pyroxasulfone and indaziflam provided similar control of annual bluegrass. In the spring fallow field experiment, pyroxasulfone (293 g·ha−1) was the only herbicide to suppress (>70%) common lambsquarters at 60 DAT, this indicates possible differences to the different chemistries tested. However, in the irrigation incorporation experiment, all three herbicides provided similar weed control and similar stability on the soil surface. Pyroxasulfone provides an additional site of action herbicide for weed management programs in orchard crops.
Future experiments should evaluate different incorporation methods including drip irrigation vs. sprinkler irrigation and how this can affect PRE herbicide weed control performance, soil dissipation, as well as tree crop reactions to these methods. An analytical component should be used to evaluate herbicide stability with the parent molecule and metabolites to determine dissipation rates under different soil type, organic matter content, and water status conditions common in California orchard production systems.
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