Introduction
Palmer amaranth is the most problematic weed in sweetpotato (Smith and Moore unpublished data; Webster Reference Webster2010). Palmer amaranth can reduce marketable sweetpotato yield 80% to 95% if left uncontrolled (Barkley et al. Reference Barkley, Chaudhari, Jennings, Schultheis, Meyers and Monks2016; Basinger et al. Reference Basinger, Jennings, Monks, Jordan, Everman, Hestir, Waldschmidt, Smith and Brownie2019; Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010a; Smith et al. Reference Smith, Jennings, Monks, Chaudhari, Schultheis and Reberg-Horton2020). These yield reductions are factors of vigorous growth of Palmer amaranth (Horak and Loughin Reference Horak and Loughin2000; Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010a; Sellers et al. Reference Sellers, Smeda, Johnson, Kendig and Ellersieck2003), high fecundity (Keeley et al. Reference Keeley, Carter and Thullen1987; Sellers et al. Reference Sellers, Smeda, Johnson, Kendig and Ellersieck2003; Sosnoskie et al. Reference Sosnoskie, Webster, Grey and Culpepper2014), and resistance to many previously efficacious herbicidal modes of action (Heap Reference Heap2020). The current herbicide program for Palmer amaranth control in sweetpotato is heavily reliant on PRE herbicides including flumioxazin or fomesafen preplant, and S-metolachlor posttransplant (KM Jennings, personal communication; SC Smith and LD Moore, unpublished data). The evolution of protoporphyrinogen oxidase (PPO)-inhibiting herbicide-resistant Palmer amaranth biotypes have been reported in the United States (Heap Reference Heap2020), including North Carolina (Mahoney et al. Reference Mahoney, Jordan, Roma-Burgos, Jennings, Leon, Vann, Everman and Cahoon2020), which produces more sweetpotato than any other state (USDA-NASS 2020). Therefore, an urgent need exists for alternatives to flumioxazin and fomesafen to control Palmer amaranth in sweetpotato.
Previous research demonstrates sweetpotato tolerance to linuron, a photosystem II-inhibiting herbicide (Batts Reference Batts2019; Beam et al. Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018; Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). However, increasing the rate of linuron, delaying application later than 7 d after planting (DAP), and combining it with S-metolachlor, a very long-chain fatty acid–inhibiting herbicide, can increase sweetpotato injury (Beam et al. Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018; Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). S-metolachlor applied PRE can provide 84% or greater Palmer amaranth control 10 wk after planting (WAP) sweetpotato (Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010b, Reference Meyers, Jennings and Monks2013a) and is a key mode of action for resistance management weed control in sweetpotato. However, when applications were delayed to 2 WAP, control was variable because emerged weeds are not controlled (Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010b, Reference Meyers, Jennings and Monks2013a). Therefore, with the high populations of Palmer amaranth in many sweetpotato fields, the use of linuron with S-metolachlor would likely be beneficial for weed control.
Linuron applied PRE (335 to 2,240 g ai ha−1) provided 58% to 100% Palmer amaranth control until at least 3 wk after treatment (WAT; Grichar et al. Reference Grichar, Dotray and Trostle2015; Whitaker et al. Reference Whitaker, York, Jordan, Culpepper and Sosnoskie2011). However, Moore et al. (Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020) reported 26% or less PRE Palmer amaranth control in North Carolina sweetpotato with linuron (280 to 840 g ai ha−1) use. Because of increased foliar injury and subsequent yield reduction from tank mixing linuron with S-metolachlor, linuron was not recommended in a weed control program that included flumioxazin followed by (fb) S-metolachlor (Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). However, linuron is novel for sweetpotato weed control because it has POST efficacy on broadleaf weeds (Anonymous 2013). Moore et al. (Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020) did not observe a POST weed control benefit from adding linuron to a herbicide program that included flumioxazin fb S-metolachlor because protoporphyrinogen oxidase (PPO)-susceptible Palmer amaranth populations were present in these studies; therefore, weeds did not emerge prior to S-metolachlor application. In addition, the experiment did not account for planting or S-metolachlor application delays from weather or time constraints, which could allow weeds to emerge prior to S-metolachlor application.
POST Palmer amaranth control from linuron application rates low enough for tolerance by ‘Covington’ sweetpotato, the primary sweetpotato cultivar planted in North Carolina (NCDACS 2015), has yet to be demonstrated. Furthermore, the effects on sweetpotato injury and POST Palmer amaranth control from the addition of a surfactant, which is recommended by the product label to increase POST control (Anonymous 2013), or S-metolachlor is unknown. Thus, field studies were conducted without a preplant PPO-inhibiting herbicide to determine the efficacy of linuron applied POST with or without an adjuvant and/or S-metolachlor for Palmer amaranth control in sweetpotato.
Materials and Methods
Field studies were initiated at the Horticultural Crops Research Station near Clinton, NC, in June 2019 (35.023°N, 78.280°W) and 2020 (35.024°N, 78.279°W) in fields with historically high Palmer amaranth populations (50 to 100 plants m−2). Soil at the study sites were an Orangeburg loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudult), pH 5.4, and 0.9% organic matter content in 2019; and a Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudult), pH 6.3, and 0.7% organic matter content in 2020. Nonrooted ‘Covington’ sweetpotato cuttings (slips) were mechanically transplanted at a 30 cm in-row spacing on 1.07-m-wide beds. Commercial sweetpotato growing recommendations were followed (Kemble et al. Reference Kemble, Meadows, Jennings and Walgenbach2019), and overhead irrigation was applied as needed.
The experimental design for each study was a randomized complete block with treatments replicated four times. Plots consisted of two 6.1-m-long rows; the first a border row and the second row received a treatment and was used for data collection. Treatments were arranged in a two by four factorial in which the first factor consisted of two rates of linuron (420 and 700 g ai ha−1; Linex 4L; Tessenderlo Kerley, Inc., Phoenix, AZ), and the second factor consisted of linuron applied alone or in combinations of linuron plus a nonionic surfactant (NIS; 0.5% vol/vol; Scanner; Loveland Products, Inc., Loveland, CO), linuron plus S-metolachlor (800 g ai ha−1; Dual Magnum; Syngenta Crop Protection, LLC, Greensboro, NC), or linuron plus NIS plus S-metolachlor. In addition, S-metolachlor alone and nontreated weedy and weed-free checks were included for comparison. Weeds in the weed-free checks were hand-removed weekly. Treatments were applied 8 DAP using a CO2-pressurized backpack sprayer calibrated to apply 187 L ha−1 at 150 kPa through a boom equipped with two flat-fan XR 8003VS nozzles (TeeJet 8003; TeeJet® Technologies, Springfield, IL) spaced 50 cm apart. Interrow weeds were controlled using cultivation, and clethodim 135 g ai ha−1 (Select Max; Valent U.S.A. Corporation, Walnut Creek, CA) was applied to the whole study area for annual grass control.
Effects of treatments on visible sweetpotato foliar injury, stunting, and Palmer amaranth control were evaluated 1, 2, 3, 4, and 8 WAT using a scale of 0% (no treatment effect) to 100% (plant death; Frans et al. Reference Frans, Talbert, Marx, Crowley and Camper1986). Sweetpotato storage roots were harvested using a chain-digger 112 and 104 DAP in 2019 and 2020, respectively; hand sorted into canner (>2.5 to 4.4 cm diam), number (No.) 1 (>4.4 to 8.9 cm), and jumbo (>8.9 cm) grades (USDA 2005); and weighed. Marketable yield was calculated as the sum of jumbo and No. 1 grades and total yield was calculated as the sum of all grades.
Data were assessed for homogeneity of variance by examining residual plots. Arcsine square root transformations were required for injury and Palmer amaranth control data, and square root transformations were required for yield data to normalize the distribution of residuals. Back-transformed data were presented. ANOVA was conducted using the MIXED procedure in SAS, version 9.4 (SAS Institute, Cary, NC). Fixed effects included year, herbicide combination, linuron rate, and their interactions, whereas replication nested within year was considered a random effect. The weedy and weed-free checks were not included in crop injury or Palmer amaranth control analyses because all observations equaled 0%, and similarly, the S-metolachlor alone treatment was not included in the injury analyses. If all interactions between the years, herbicide combinations, and linuron rates were nonsignificant (P > 0.05), then the main effect least square means were presented. Least square means were separated according to Fisher’s protected LSD at a significance level of α = 0.05.
Results and Discussion
Sweetpotato Tolerance
Injury from linuron appeared as interveinal chlorosis and necrosis (foliar injury) on older leaves fb stunting, similar to observations for ‘Covington’ sweetpotato grown in North Carolina reported by Beam et al. (Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018) and Moore et al. (Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). Treatments with S-metolachlor applied alone resulted in no visible injury symptoms. Significant year by herbicide combination (P = 0.043) and year by linuron rate (P = 0.005) interactions were present 2 WAT. However, graphs of the interaction means were assessed and adjudged biologically unimportant and uninformative (Vargas et al. Reference Vargas, Glaz, Alvarado, Pietragalla, Morgounov and Zelenskiy2015); therefore, data were pooled across years. All other foliar injury interactions were not significant (P > 0.05). Significant foliar injury effects for herbicide combinations and linuron rates were observed at 1 and 2 WAT (P ≤ 0.0001). Significant stunting effects were present for herbicide combinations 2 WAT (P = 0.036) and linuron rates 2 (P = 0.0034) and 3 (P = 0.0033) WAT.
Linuron alone resulted in 17% foliar injury 1 WAT (Table 1). Injury from linuron alone was similar to that observed by Moore et al. (Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020), though previous reported foliar injury from linuron applied at similar rates is variable, ranging from 13% to 54% 1 WAT (Beam et al. Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018; Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). At 2 WAT, foliar injury and stunting from linuron applied alone was 3% and 9%, respectively, in our experiment. The addition of an NIS increased linuron foliar injury 1 WAT by 10% and the addition of S-metolachlor to linuron increased foliar injury by 19% 1 WAT, relative to linuron alone. Total foliar injury from linuron plus S-metolachlor was higher (19%) than that reported by Moore et al. (Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020) but lower (62%) than that reported by Beam et al. (Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018). Adding an NIS to linuron plus S-metolachlor did not exacerbate injury. Increasing the linuron rate from 420 to 700 g ai ha−1 resulted in a 13% increase in foliar injury 1 WAT, and 9% increase in stunting 2 WAT. Sweetpotato injury was transient, thus not observed 4 WAT.
a Treatments applied 8 d after transplanting.
b Data pooled across years.
c Means within a column for dependent variables followed by the same letter are not significantly different according to Fisher’s protected LSD, α = 0.05. Means within a column not followed by a letter are not significantly different according to a nonsignificant F statistic (P > 0.05).
d Foliar injury was observed as chlorosis and necrosis.
e Abbreviations: WAT, weeks after treatment application; NIS, nonionic surfactant.
f Rating scale: 0%, no treatment effect; 100%, plant death.
g NIS (0.5% vol/vol); S-metolachlor (800 g ai ha−1).
Palmer Amaranth Control
Palmer amaranth emerged soon after planting both years, resulting in weeds at the cotyledon to three-leaf growth stage at the time of applications. In 2020, Palmer amaranth plants were present in the field before beds had been prepared and no burn-down herbicide was applied. Preplant cultivation and bed preparation did not control many of these weeds, which resulted in plants up to 15 cm tall at treatment application. However, this difference in weed size did not result in significant year by herbicide combination (P > 0.37) or linuron rate (P > 0.08) interactions.
Herbicide combination had a significant (P > 0.0001) effect on Palmer amaranth control for each week assessed. S-metolachlor applied alone provided unacceptable Palmer amaranth control because weeds emerged prior to application (Table 2). All treatments that included linuron resulted in greater than 90% Palmer amaranth control 1 and 2 WAT. All treatments that included linuron provided 84% to 92% Palmer amaranth control at 4 WAT, and the addition of S-metolachlor increased the extent of control even more. Adding an NIS to linuron or linuron plus S-metolachlor did not increase efficacy. Increasing the linuron rate increased control by 5% and 9% at 4 and 8 WAT, respectively. At 8 WAT, Palmer amaranth control was 69% or less for all treatments.
a Treatments applied 8 d after transplanting.
b Data pooled across years.
c Means within a column for dependent variables followed by the same letter are not significantly different according to Fisher’s protected LSD, α = 0.05. Means within a column not followed by a letter are not significantly different according to a nonsignificant F statistic (P > 0.05).
d Abbreviations: WAT, weeks after treatment application; NIS, nonionic surfactant.
e Rating scale: 0%, no treatment effect; 100%, plant death.
f NIS (0.5% vol/vol); S-metolachlor (800 g ai ha−1).
Yield
Herbicide combination had an effect on No. 1 (P = 0.001), marketable (P = 0.0005), and total (0.003) yield grades. Linuron rate by year interactions were present for No. 1 (P = 0.001), jumbo (P = 0.0084), marketable (P = 0.0004), and total (P = 0.002) yield grades. Graphs of interaction means were assessed and deemed biologically important; thus, the effect of linuron rate was analyzed separately by year. Linuron rate had an effect on No. 1 (P = 0.009), jumbo (P = 0.024), marketable (P = 0.005), and total (P = 0.012) yield grades in 2019, but not in 2020 (P > 0.05).
Sweetpotato yield from herbicide combinations that included linuron were similar regardless of the addition of S-metolachlor or S-metolachlor plus NIS (Table 3). Adding an NIS to linuron reduced No. 1, marketable, and total yield 31%, 32%, and 25%, respectively. This yield reduction was likely the result of increased foliar injury with no benefit for weed control. S-metolachlor applied alone yielded similar results as those of the weedy check because weeds had emerged before application, resulting in poor weed control. Because Palmer amaranth escaped all treatments, it is not clear how much yield loss was due to crop injury or interference from Palmer amaranth alone. Palmer amaranth densities as low as 0.5 plants m−1 can reduce marketable sweetpotato yield by 36% (Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010a). All treatments yielded at least 30%, 48%, and 39% less than the weed-free check for No. 1, marketable, and total yield grades, respectively. Increasing the rate of linuron increased No. 1, jumbo, marketable, and total yield in 2019 but not 2020 (Table 4).
a Treatments applied 8 d after transplanting.
b Data pooled across years and linuron rates (420 and 700 g ai ha−1).
c Means within a column followed by the same letter are not significantly different according to Fisher’s protected LSD, α = 0.05. Means within a column not followed by a letter are not significantly different according to a nonsignificant F statistic (P > 0.05).
d Sweetpotato storage roots were hand graded into canner (>2.5 to 4.4 cm diam), No. 1 (>4.4 to 8.9 cm), and jumbo (>8.9 cm). Marketable yield is the sum of No. 1 and jumbo storage-root grades; total yield is the sum of all grades.
e Nonionic surfactant (NIS; 0.5% vol/vol); S-metolachlor (800 g ai ha−1).
a Treatments applied 8 d after transplanting.
b Data pooled across years and herbicide combinations including linuron, linuron plus nonionic surfactant (0.5% vol/vol), linuron plus S-metolachlor (800 g ai ha−1), and linuron plus nonionic surfactant plus S-metolachlor.
c Means within a column for main effects followed by the same letter are not significantly different according to Fisher’s protected LSD, α = 0.05. Means within a column not followed by a letter are not significantly different according to a nonsignificant F statistic (P > 0.05).
d Sweetpotato storage roots were hand graded into canner (>2.5 to 4.4 cm diam), No. 1 (>4.4 to 8.9 cm), and jumbo (>8.9 cm). Marketable yield is the sum of No. 1 and jumbo storage-root grades; total yield is the sum of all grades.
Combining linuron with S-metolachlor is too phytotoxic for use with ‘Covington’ sweetpotato (Beam et al. Reference Beam, Jennings, Chaudhari, Monks, Schultheis and Waldschmidt2018; Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020), which was confirmed in our present experiment. Linuron applied at either rate provided excellent (≥98%) control of Palmer amaranth 1 WAT, and the 420 g ai ha−1 rate caused minimal sweetpotato injury; however, residual Palmer amaranth control was poor 8 WAT. The typical recommendation for S-metolachlor application is a window between 7 and 14 DAP to increase sweetpotato tolerance while minimizing the risk that weeds emerge prior to application (Beam et al. 2019; KM Jennings, personal communication; Moore et al. Reference Moore, Jennings, Monks, Boyette, Jordan and Leon2020). S-metolachlor is safe when applied 2 WAP in sweetpotato (Meyers et al. Reference Meyers, Jennings, Schultheis and Monks2010b, Reference Meyers, Jennings and Monks2012, Reference Meyers, Jennings and Monks2013b). Therefore, a system that includes 420 g ai ha−1 linuron applied 1 WAP fb S-metolachlor applied 2 WAP could supplement PRE activity such that weeds are better controlled during the critical period of weed control while minimizing the risk that weeds will be present at S-metolachlor application. Although linuron has POST efficacy, the herbicide will likely not replace a form of late-season weed control in sweetpotato required to reduce the amount of weed seeds added into the soil seedbank, as is commonly practiced by sweetpotato growers in North Carolina using hand roguing (SC Smith and LD Moore, unpublished data).
Acknowledgments
We thank the North Carolina Agricultural Foundation, North Carolina Department of Agriculture and Consumer Services, North Carolina College of Agriculture and Life Sciences at North Carolina State University, and North Carolina SweetPotato Commission for funding these studies and Jim Jones for providing sweetpotato slips. We also thank Colton Blankenship, Stephen Ippolito, Kira Sims, Cole Smith, Chitra, and the staff at the Horticultural Crops Research Station at Clinton, NC, for aiding in the management of this experiment. No conflicts of interest have been declared.
