Introduction
Stevia is used to produce a zero-calorie sweetener, containing steviol glycosides, which are 200 to 400 times sweeter than sucrose (FDA 2018; Lester Reference Lester1999). As a result, it serves as an excellent sugar substitute, especially for diabetics (Mishra et al. Reference Mishra, Soni, Silawat, Mehta, Mehta and Jain2011). Stevia has been consumed as a sweetener for hundreds of years (PCSI 2017). With the authorization of stevia as a food additive, several companies have released stevia products including Coca-Cola (Truvia) and Pepsi (PureVia) (Cavaliere Reference Cavaliere2009).
In production, stevia is commonly grown from seed in tobacco float trays and then transplanted into the field 8 to 12 wk later (Koehler Reference Koehler2018). Stevia is a perennial, allowing multiple harvests each season, and has a field life of 3 to 5 yr; however, it is typically only harvested once during the first year (Koehler Reference Koehler2018). Diseases, insects, and weeds are important pests in stevia (Stevia Technology 2022; Taak et al. Reference Taak, Koul, Chopra and Sharma2021). Stevia’s poor competitive ability with weeds can reduce yield up to 25%, and weed control can increase production costs (Taak et al. Reference Taak, Koul, Chopra and Sharma2021). Stevia is particularly vulnerable to weed competition early in the season (Azimah et al. Reference Azimah, Ismail and Juraimi2018; Chriest Reference Chriest2019). Azimah et al. (Reference Azimah, Ismail and Juraimi2018) reported that the critical period for weed control for stevia in the greenhouse was 1 to 4 wk after planting for a mixture of dicotyledonous and monocotyledonous weeds. Few herbicides are registered for use in stevia (Chriest Reference Chriest2019; Harrington et al. Reference Harrington, Southward, Kitchen and He2011). Ethalfluralin may be applied pre-transplant incorporated for residual weed control; however, S-metolachlor and clethodim are the only conventional herbicides registered for use in postemergence-transplanting over-the-top of stevia (Chriest Reference Chriest2019). As a result, postemergence weed control options are limited in stevia. Nonsynthetic herbicides may be applied in stevia; however, these herbicides have not been evaluated to determine if injury from these herbicides will significantly affect stevia yield.
In organic production systems, chemical weed control options are limited to biological or botanical (nonsynthetic) herbicides for food crops and herbicidal soaps (synthetic) that can be used only for maintenance of noncrop areas of the farm and for fields used only for ornamental crops. In organic production systems, a biological or botanical substance (acetic acid) has been reported to provide control of annual ryegrass (Lolium multiflorum Lam.), goosegrass (Eleusine indica Gaertn.), and redroot pigweed (Amaranthus retroflexus L.) (Abouziena et al. Reference Abouziena, Omar, Sharma and Singh2009). In addition, citric acid has been reported to provide control of velvetleaf (Abutilon theophrasti Medik.), stranglervine (Morrenia odorata Lindle), and black nightshade (Solanum nigrum Linn.) (Abouziena et al. Reference Abouziena, Omar, Sharma and Singh2009). Cinnamon oil plus clove oil provided as much as 89% control when applied in studies containing redroot pigweed, common lambsquarters (Chenopodium album L.), and large crabgrass (Digitaria sanguinalis L. Scop) (O’Sullivan et al. Reference O’Sullivan, Acker, Grohs and Riddle2015). The herbicides that are permitted for use in organic production are nonselective and provide no residual weed control (Evans et al. Reference Evans, Bellinder and Hahn2011; Liu et al. Reference Liu, Zhan, Li, Li, Feng, Bagavathiannan, Zhang, Qu and Yu2021). As a result, over-the-top applications can cause significant crop injury (Evans et al. Reference Evans, Bellinder and Hahn2011; Liu et al. Reference Liu, Zhan, Li, Li, Feng, Bagavathiannan, Zhang, Qu and Yu2021). Additionally, organic herbicides are more efficacious when applied to small weeds and may require sequential applications to achieve effective control (Abouziena et al. Reference Abouziena, Omar, Sharma and Singh2009; Liu et al. Reference Liu, Zhan, Li, Li, Feng, Bagavathiannan, Zhang, Qu and Yu2021). However, directed applications can require less herbicide, which can reduce the cost of applying nonsynthetic herbicides. Prior research has shown that directed applications within the crop canopy of nonsynthetic herbicides provided effective weed control in bell pepper and broccoli (Brassica oleracea L. var. italica) (Evans et al. Reference Evans, Bellinder and Hahn2011).
Prior studies have examined the effects of directed applications of nonsynthetic herbicides in other crops (Evans et al. Reference Evans, Bellinder and Hahn2011); however, to our knowledge no peer-reviewed research has evaluated nonsynthetic or reduced-risk synthetic herbicides in stevia. In addition, Although polyethylene mulch can reduce weed pressure, weeds within the planting holes may affect crop yield; characterization of weed control from reduced-risk synthetic and nonsynthetic herbicides would assist organic growers in deciding whether or not to apply reduced-risk synthetic and nonsynthetic herbicides. Therefore, greenhouse and field studies were conducted to determine the effect of reduced-risk synthetic and nonsynthetic herbicides applied over-the-top and postemergence-directed to transplanted stevia in a polyethylene mulch production system, respectively.
Materials and Methods
Greenhouse Study
Greenhouse trials were conducted at the Marye Anne Fox Science Teaching Laboratory (35.787°N, 78.674°W) at North Carolina State University, Raleigh, in 2021. Stevia was transplanted in 3-L (14 cm tall, 20 cm diam) round pots containing Fafard 4P potting mix (Conrad Fafard Inc., Agawam, MA). Stevia did not receive supplemental light; greenhouse temperature ranged from 18 C to 24 C. The plants were hand-watered twice daily to maintain consistent soil moisture. Treatments consisted of reduced-risk synthetic and nonsynthetic herbicides (Table 1) applied over-the-top of stevia 4 WAT with a CO2-pressurized backpack sprayer calibrated to deliver 700 L ha–1 spray solution at 200 kPa utilizing a DG 8003VS nozzle (TeeJet Technologies, Wheaton, IL), with the exception of eugenol, which was applied at 280 L ha–1 to meet label instructions (Agro Research International 2022). The study was arranged in a randomized complete block design with six replications, and the study was repeated twice with two experimental runs that were separated in time. Data collected included visible stevia injury at 3 and 28 DAT, with 0% representing no injury and 100% representing plant death (Frans et al. Reference Frans, Talbert, Marx, Crowley and Camper1986). Yield was determined for each treatment by cutting plants 1 cm above the soil surface 28 DAT, drying them at 70 C for 3 d, and then measuring dry weights.
Table 1. Herbicide treatments in stevia studies in the Marye Ann Fox greenhouse and in the field at Clinton and Castle Hayne, NC, in 2021
a Nonionic surfactant (Kinetic; Helena Agri-Enterprises, LLC, Collierville TN) was included at 0.25% v/v.
b Ablaze does not list percent active ingredient in the formulated product on the label.
c Pelargonic acid is not permitted in organic production and thus not OMRI certified. Axxe and FinalSan are OMRI listed but only for use as herbicides for farmstead maintenance and fields used only for ornamental crops.
Field Study
Field trials were conducted under conventional production practices at the Horticultural Crops Research Station in Clinton (35.023°N, 78.280°W) and Castle Hayne (34.321°N, 77.9217°W), NC, in 2021. Soils in Clinton and Castle Hayne were a Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) with 2.4% silt and pH 6.7, and Stallings fine sand (coarse-loamy, siliceous, semiactive, thermic Aeric Paleaquults) with 13.6% silt and pH 6.2, respectively. Stevia seeds (Johnny’s Selected Seeds, Winslow, ME) were seeded into 50-cell (110 mL) trays containing potting mix (Fafard 4P, Conrad Fafard Inc., Agawam, MA) and then allowed to germinate and grow in a greenhouse for 2 mo. To establish stevia in the field, raised 1.5-m beds spaced 3.02 m apart were formed, polyethylene drip irrigation lines were installed, and the beds covered in 0.25 mm thick white on black polyethylene mulch (TriEast Ag Group, Greenville, SC). Stevia plugs were transplanted by hand May 10 in Clinton and Castle Hayne, NC, at a density of 0.3 plants m–1 of row. Plots consisted of one row 12.2 m longer, in which the first 6.1 m consisted of stevia maintained weed free and the second 6.1 m consisted of holes punched into the plastic to allow weeds to emerge. Weedy sections were seeded at stevia transplanting with Palmer amaranth at a rate of 5 to 10 seeds per hole. Due to their proximity to the stevia, weeds in the weedy section of each plot were terminated 4 WAT to prevent confounding competition with the stevia.
Treatments consisted of the herbicides (Table 1) used in the greenhouse study directed to the lower third of the stevia (two passes, one to each side). Weeds were less than 7 cm tall at application and thus were fully covered by the treatment application, as the boom height was held constant for both halves of the plot. In addition, a nontreated check was included for comparison. All treatments were applied 2 wk after planting with a CO2-pressurized backpack sprayer calibrated to deliver 700 L ha–1 spray solution at 200 kPa utilizing a DG 8003VS nozzle (TeeJet 8003; TeeJet Technologies, Wheaton, IL), with the exception of eugenol, which was applied at 280 L ha–1 (Agro Research International 2022). Stevia was 25.4 to 30.5 cm tall at application. Treatments were arranged in a randomized complete block with four replications. Data collection included visible stevia injury (2 and 6 WAT) and weed control (1, 2, and 4 WAT) on a scale of 0 to 100% with 0% being no injury and 100% being plant death (Frans et al. Reference Frans, Talbert, Marx, Crowley and Camper1986). Stevia was harvested on August 8 and September 10, 2021 in Castle Hayne and Clinton, respectively. Yield was collected by cutting plants 1 cm above the soil surface, drying them at 71 C for 3 d, and then measuring dry weight.
Statistical Analysis
For both the greenhouse and field studies, data were subjected to ANOVA using the MIXED procedure in SAS version 9.4 (SAS Institute Inc., Cary, NC). Residuals were plotted to inspect homogeneity of variance. Herbicide treatment and experimental run were treated as fixed effects, whereas replication nested within experimental run was considered a random effect. Least squared means were separated using Fishers protected LSD (α = 0.05). Injury and weed control data from the field study were transformed using arcsine square root transformations, and back-transformed for presentation.
Results and Discussion
Greenhouse Study
As a significant interaction between experimental runs was not observed, data were pooled across experimental runs. Injury was observed as necrosis. At 3 DAT, caprylic acid plus capric acid, pelargonic acid, acetic acid, and ammonium nonanoate caused >45% injury, with caprylic acid plus capric acid and ammonium nonanoate causing the greatest crop injury (>60%) (Table 2). Although stevia regrowth occurred, injury from these herbicide treatments was still substantial by 28 DAT, with little change from 3 DAT for the majority of the treatments. Eugenol was a notable exception, resulting in a 22% increase in stevia injury from 3 to 28 DAT. Citric acid, ammoniated soap of fatty acids, and clove oil plus cinnamon oil caused no more than 18% stevia injury at 3 and 28 DAT. Eugenol and d-limonene caused no more than 30% injury at 3 and 28 DAT.
Table 2. Stevia injury and yield (dry above ground biomass) from reduced risk synthetic and nonsynthetic herbicide treatments applied over-the-top of stevia at the Marye Anne Fox Science greenhouse, Raleigh, NC in 2021. a
a Least squared means within a column followed by the same letter are not significantly different according to Fishers protected LSD (α = 0.05).
b Data were pooled across experimental runs. The nontreated check was not included in the crop injury analysis because crop injury was 0% and therefore had a variance of 0.
c Stevia injury was assessed at 3 and 28 d after transplanting (DAT). Injury is a sum of chlorosis and necrosis.
d Rating scale: 0 being no injury and 100% being plant death.
Stevia yield was reduced by all herbicide treatments when compared to the nontreated check (Table 2). Consistent with the observed injury, caprylic acid plus capric acid, pelargonic acid, acetic acid, eugenol, and ammonium nonanoate reduced yield >40% compared to the nontreated check. Citric acid and clove oil plus cinnamon oil were the least injurious and reduced yield 16% to 20%, respectively. These results suggest that all products evaluated are too injurious to be applied over-the-top of stevia.
Field Study
Weed Control
d-Limonene, pelargonic acid, ammonium nonanoate, and ammoniated soap of fatty acids all controlled Palmer amaranth (two- to four-leaf) >90% 1 WAT (Table 3). In addition, the application of d-limonene and pelargonic acid resulted in >90% control of annual sedge (Cyperus compressus L.). However, citric acid, acetic acid, and eugenol did not provide adequate control of Palmer amaranth and annual sedge (<65%). These results are similar to those of Abouziena et al. (Reference Abouziena, Omar, Sharma and Singh2009), who reported that citric acid provided ≤25% control of sedges. Treatment with either d-limonene or pelargonic acid resulted in ≥94% control of annual sedge 1 WAT. At 2 WAT acetic acid resulted in Palmer amaranth control (70%) similar to the broadleaf weed control reported by Abouziena et al. (Reference Abouziena, Omar, Sharma and Singh2009). In prior research, clove oil applied alone resulted in minimal weed control for most broadleaf and grasses (Abouziena et al. Reference Abouziena, Omar, Sharma and Singh2009); however, in our studies clove oil plus cinnamon oil resulted in 98% and 75% Palmer amaranth and annual sedge control 1 WAT, respectively. Although none of the herbicide treatments have residual effects, by 4 WAT caprylic acid plus capric acid, clove oil plus cinnamon oil, pelargonic acid, ammonium nonanoate, ammoniated soap of fatty acids, and d-limonene still provided ≥75% Palmer amaranth control.
Table 3. Effect of reduced risk synthetic and nonsynthetic herbicides applied post transplanted directed to stevia on annual sedge and Palmer amaranth control in Clinton and Castle Hayne, NC in 2021. a, b
a Data were pooled across locations. The nontreated check was not included in analysis because control was 0% and therefore had a variance of 0.
b Least squared means within a column followed by the same letter are not significantly different according to Fishers protected LSD (α = 0.05).
c Rating scale: 0 = no control and 100% = control.
d Herbicides were applied over-the-top of the weeds.
Crop Injury
There was not an interaction between experimental run and herbicide; therefore, data were pooled across experimental runs. Injury was primarily characterized by contact necrosis. However, eugenol caused slight chlorosis. Similar to injury reported in bell pepper by Evans et al. (Reference Evans, Bellinder and Hahn2011), more injurious chemicals such as pelargonic and acetic acid caused necrosis at the plant stem, which resulted in some stem girdling.
At 2 WAT, caprylic acid plus capric acid, pelargonic acid, and ammonium nonanoate caused >30% injury. In contrast, clove oil plus cinnamon oil and eugenol caused <10% injury. d-Limonene, citric acid, acetic acid, and ammoniated soap of fatty acids caused 18% to 25% injury by 2 WAT. By 6 WAT substantial stevia regrowth and recovery occurred, resulting in <20% injury for all treatments. In particular, clove oil plus cinnamon oil, citric acid, and eugenol all caused <5% injury. However, substantial stunting was observed, with caprylic acid plus capric acid, pelargonic acid, d-limonene, and ammonium nonanoate all causing 25% to 54% stunting. All other treatments caused ≤18% stunting (Table 3).
Crop Yield
The treatment-by-location interaction was not significant for stevia yield; therefore, data from both locations were combined for analysis. Despite being injurious, organic herbicides did not cause a reduction in yield relative to the nontreated check (Table 4). This is likely a result of harvesting later in the season. Stevia is able to regrow within the same season and can be harvested more than once within a year. Based upon yield data, these herbicides have potential for use in stevia; however, when applied to established stevia, these products could delay harvest. Caution should be taken before applying the majority of these organic herbicides on established stevia if an early harvest date is desired. In addition, sequential applications of these herbicides may be required for continued weed suppression, which could increase injury as well as add to the cost of production.
Table 4. Effect of reduced risk synthetic and nonsynthetic herbicides applied post transplanted direct to stevia on crop injury, stunting, and yield in Clinton and Castle Hayne, NC in 2021. a, b
a Data were pooled across locations. The nontreated check was not included in crop injury and stunting analysis because injury or stunting was 0% and therefore had a variance of 0.
b Least squared means within a column followed by the same letter are not significantly different according to Fishers protected LSD (α = 0.05).
c Rating scale: 0 being no injury and 100% being plant death. Injury is the sum of chlorosis and necrosis.
Injury to stevia from clove oil plus cinnamon oil was similar to that reported by O’Sullivan et al. (Reference O’Sullivan, Acker, Grohs and Riddle2015) in tomato (Solanum lycopersicum L), corn (Zea mays L.), and bell pepper. The application of clove oil plus cinnamon oil over-the-top resulted in <10% injury 28 DAT in the greenhouse and 3% injury 6 WAT postemergence-directed in the field. In addition, it provided excellent control of Palmer amaranth (two- to four-leaf) (Table 3). Further evaluation of weed control from these herbicides on other weed species common in stevia is needed. Clove oil plus cinnamon oil may be potentially useful for ear-ly-season weed management for organic production systems; however, because this study was conducted in a conventional production system, additional research is needed to evaluate the effect of these herbicides when applied in an organic production system. Future research is needed to explore the application of clove oil plus cinnamon oil applied at later growth stages of stevia than this study’s treatment timing followed by stevia harvest at various maturities. In addition, stevia tolerance to sequential application of organic herbicides should be evaluated.
Practical Implications
At present, there are few options available for weed management in organically grown stevia. Based on the results from this study, several nonsynthetic herbicides could potentially be used to supplement current weed management practices in stevia. In particular, directed applications such as the method used in this study target weeds within the planting holes that are often competitive and difficult to control with current practices.
Acknowledgments
The authors would like to thank the NC Tobacco Trust Fund Commission for funding these studies. The authors would also like to thank Kira Sims, Stephen Smith, Chitra, Patrick Chang, Andrew Ippolito, Rebecca Middleton, Rebecca Cooper, and the staff at the Horticultural Crops Research Stations in Clinton and Castle Hayne, NC, for technical assistance with the trials.
Competing Interests
No conflicts of interest have been declared.
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