Introduction
Purple and yellow nutsedge (Cyperus esculentus L.) are some of the most problematic weed species in fruiting vegetable crops in Florida (Webster Reference Webster2010). These two weed species are often found in mixed stands, but purple nutsedge is more likely to establish on well-drained soils, whereas yellow nutsedge is more likely to establish on wet soils (Holm et al. Reference Holm, Plucknett, Pancho and Herberger1977; Wills Reference Wills1987). The use of polyethylene mulch can effectively suppress broadleaf and grass weeds but not Cyperus spp. due to their pointed leaf tip and sturdy midrib that allow them to pierce the plastic film (Igbokwe Reference Igbokwe1996; Patterson Reference Patterson1998). Previous studies indicate that competition from Cyperus spp. can result in 73%, 85%, 51%, and 94% yield loss in bell pepper (Capsicum annum L.; Gilreath et al. Reference Gilreath, Motis and Santos2005), cucumber (Cucumis sativus L.; Johnson and Mullinix Reference Johnson and Mullinix1999), tomato (Solanum lycopersicum L; Morales-Payan et al. Reference Morales-Payan, Santos, Stall and Bewick1997), and watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai; Buker et al. Reference Buker, Stall, Olson and Schilling2003], respectively.
Weed management in small fruiting vegetable crops is difficult because a limited number of registered herbicides are available to control them (Besançon et al. Reference Besançon, Wasacz and Carr2020; Castro et al. Reference Castro, Pucci, Duarte, Burgos and Tseng2020; Randell et al. Reference Randell, Vance and Culpepper2020; Yu and Boyd Reference Yu and Boyd2017). For example, only halosulfuron is registered for postemergence control of Cyperus spp. in tomato and no herbicides are registered for control of Cyperus spp. in strawberry (Fragaria × ananassa Duch.). Growers traditionally relied on the use of plastic mulch and soil fumigation with methyl bromide (MB) for preplant control of soilborne pathogens and weeds. However, the use of MB in agriculture has been discontinued because it was listed as an ozone-depleting substance (USEPA 2019). At present, 1,3-dichloropropene (1,3-D), chloropicrin (Pic), dimethyl disulfide (DMDS), and isothiocyanate generators such metam sodium are used as MB alternatives (Eure and Culpepper Reference Eure and Culpepper2017; Guo et al. Reference Guo, Zhao, Rosskopf, Di Gioia, Hong and McNear2018; Hutchinson et al. Reference Hutchinson, McGiffen, Sims and Becker2003; McDonald et al. Reference McDonald, Collins, duToit and Adusei-Fosu2021; Ren et al. Reference Ren, Li, Fang, Yan, Huang, Zhu, Wang, Wang, Wang, Guo and Cao2018; Yu et al. Reference Yu, Land, Vallad and Boyd2018; Yu and Boyd Reference Yu and Boyd2021). Unfortunately, it is evident that none of these alternatives provide as broad-spectrum pest control as MB (Eure and Culpepper Reference Eure and Culpepper2017; Hanson and Shrestha Reference Hanson and Shrestha2006; Yu et al. Reference Yu, Land, Vallad and Boyd2018, Reference Yu, Sharpe and Boyd2019). For this reason, a weed management system comprising two or more weed management tactics, such as fumigants, herbicides, and fallow cover crops, has been recommended (Creamer et al. Reference Creamer, Bennett and Stinner1997; Eure and Culpepper Reference Eure and Culpepper2017; Snapp et al. Reference Snapp, Swinton, Labarta, Mutch, Black and Leep2005; Yu et al. Reference Yu, Sharpe and Boyd2019).
Sulfuryl fluoride (SF, SO2F2) is presently used as a fumigant for control of drywood termites, weed-infesting beetles, and other insects in building structures, vehicles, and wood products (Derrick et al. Reference Derrick, Burgess, Baker and Binnie1990; Kenaga Reference Kenaga1957). It is also used as a fumigant gas for pest control in post-harvest commodities and food processing facilities (Aung et al. Reference Aung, Leesch, Jenner and Grafton-Cardwell2001; Derrick et al. Reference Derrick, Burgess, Baker and Binnie1990; Rajendran et al. Reference Rajendran, Kumar and Sriranjini2008; Zettler and Arthur Reference Zettler and Arthur2000). At present, all soil fumigants registered in the United States have higher boiling points and lower vapor pressures than MB. As a result, they move less readily through the soil profile, which can contribute to reduced efficacy or inconsistent pest control. SF (boiling point = −55 °C at 760 mm Hg; vapor pressure = 1.16 × 104 mm Hg at 20 C) has a lower boiling point, but higher vapor pressure compared to MB (boiling point = 3.5 C; vapor pressure = 1.42 × 103 mm Hg at 20 C). The lower boiling point of SF indicates that it can be applied during cool seasons (Meister Reference Meister1992), whereas the higher vapor pressure of SF indicates that it can be quickly distributed in the soil profile. In addition, during the fumigation, SF has lower emissions compared to MB (Cao et al. Reference Cao, Guo, Yan, Mao, Wang and Li2014). These physical properties give SF significant advantages compared to the existing MB alternatives as a preplant soil fumigant (Cao et al. Reference Cao, Guo, Yan, Mao, Wang and Li2014). Previous research showed that SF exhibited control activities against Fusarium spp., root-knot nematodes (Meloidogyne spp.), and weeds including large crabgrass [Digitaria sanguinalis (L.) Scop.] and velvetleaf (Abutilon theophrasti Medik.; Cao et al. Reference Cao, Guo, Yan, Mao, Wang and Li2014). The use of SF as a preplant soil fumigant for control of Cyperus spp. has not been previously studied. There remains a need to understand its efficacy on weeds in plastic-mulched vegetables.
Previous studies have shown that Pic alone is inadequate for control of Cyperus spp. (Santos et al. Reference Santos, Gilreath and Motis2006); however, adding Pic to 1,3-D, DMDS, propargyl bromide, MB, methyl iodide, or metam-sodium significantly enhanced the control of Cyperus spp. (Hutchinson et al., Reference Hutchinson, McGiffen, Sims and Becker2003; Yu et al. Reference Yu, Land, Vallad and Boyd2018; Yu and Boyd Reference Yu and Boyd2021). The efficacy of Pic used in conjunction with SF for weed control is unknown. Therefore, the objectives of this research were to 1) determine the efficacy of SF used alone or in conjunction with Pic against purple nutsedge and their impact on tomato growth and yield, and 2) compare the efficacy of SF and several other soil fumigant treatments (1,3-D + Pic and 1,3-D + Pic + metam potassium) on purple nutsedge in tomato.
Materials and Methods
Experiment Description
Field experiments were carried out in Balm, FL (Trial I, fall 2017; Trial II, spring 2018; GPS coordinates 27.76°N, 82.22°W) and Quincy, FL (Trial III, spring 2018; Trial IV, fall 2018; GPS coordinates 30.58°N, 84.59°W). In Balm, FL, the field trials were established at the Gulf Coast Research and Education Center on a Myakka fine sand, pH 6.8 and 1% organic matter. In Quincy, FL, the field trials were established at the North Florida Research and Education Center on a Dothan-Fuquay complex sandy loam, pH 6.4 and 1% organic matter.
At both sites, raised beds (20 cm tall, 81 cm tall, and 71 cm wide at the bed top) were shaped using a bed-forming machine (Kennco Manufacturing, Ruskin, FL). Plot size was 27 m of a single raised bed with between-row spacing of 1.5 m. Two separate drip tapes (Jain Irrigation Inc., Haines City, FL) with emitters spacing at 30 cm and a flow rate of 0.95 L min−1 30 m−1 were installed 2.5 cm underneath the bed top surface and positioned 20 cm apart.
All experiments were established as a randomized complete block design with four replications. Fumigant treatments consisted of 1) nonfumigated control, 2) 224 kg ha−1 SF (ProFume®), 3) 336 kg ha−1 SF, 4) 448 kg ha−1 SF, 5) 224 kg ha−1 SF +168 kg ha−1 Pic, 6) 336 kg ha−1 SF + 168 kg ha−1 Pic, 7) 448 kg ha−1 SF + 168 kg ha−1 Pic, 8) 159 kg ha−1 1,3-D + 121 kg ha−1 Pic (Pic-Clor 60), 9) 159 kg ha−1 1,3-D + 121 kg ha−1 Pic (Pic-Clor 60) + 260 kg ha−1 metam potassium (K-PAM® HL), 10) 168 kg ha−1 Pic (Tri-Pic 100). Pic alone at 168 kg ha−1 was not included in Trial I, whereas 1,3-D + Pic + metam potassium was not included in Trials III and IV. All fumigants, except SF, were applied during the bed-forming process. Pic or 1,3-D + Pic was applied 20 cm below the soil surface using a fumigation rig with three equally distributed fumigant shanks. Metam potassium was applied 10 cm beneath the soil surface with a fumigation rig with six shanks. Immediately following the shank fumigation, TIF plastic film (Berry Plastic Corp., Evansville, IN) was used to cover the raised beds. All SF treatments were injected through drip tapes within 24 h following the shank injection of Pic. The dates of bed formation and fumigation are presented in Table 1. The fumigant product and application method information are presented in Tables 2 and 3, respectively.
Table 1. Dates of bed formation, fumigation, tomato transplant, and harvest.
Table 2. Fumigant product and manufacturer information.
Table 3. Fumigant application rate and method.
a Abbreviations: SF, sulfuryl fluoride; 1,3-D,1,3-dichloropropene; Pic, chloropicrin.
Tomato (c.v. Winter Heaven; approximately 10 cm tall) was transplanted on August 31, 2017, in Trial I and March 7, 2018, in Trial II, whereas tomatoes (c.v. Quincy) were transplanted on April 2, 2018, in Trial III and August 6, 2018, in Trial IV. Tomato plants were transplanted in the center of the bed with 60-cm spacing between plants. Tomato was fertilized, irrigated, and managed for foliar pest control as per industry standards in the region.
Control of Purple Nutsedge
Purple nutsedge shoots that penetrated through the TIF plastic film in the entire plot was recorded within each plot at 4, 13, and 17 wk after fumigation (WAF) in Trial I, 4, 11; at 13 WAF in Trial II; and 4, 9, and 13 WAF in Trials III and IV. These timings roughly correspond to crop transplant, mid-season, and at harvest.
Tomato Growth and Yield
Tomato damage, where 0 represents no injury and 100 represents entire foliage desiccation, was visually determined at 1, 2, and 3 wk after transplanting (WATP) in all trials. To evaluate the potential adverse effect of fumigant treatments on tomato growth, tomato height was measured at 7 WATP in Trial I and 3, 9, and 12 WATP in Trial II. Tomato vigor, where 0 represents plant death and 10 represents a highly vigorous plant, was visually evaluated at 1, 2, and 4 WATP in Trial III and 2 and 4 WATP in Trial IV. Fruits from 10 tomato plants were harvested in each plot by season end and each fruit was graded before weighing as medium (5.5 cm < diam < 6.5 cm), large (6.5 cm < diam < 7 cm), or extra-large (diam > 7 cm; USDA-AMS 1991).
Statistical Analysis
Data were subjected to one-way ANOVA using SAS software (version 9.4; SAS Institute, Cary, NC). All experiments were analyzed separately because they were performed as separate trials, and weather conditions and weed pressures varied between trials. Data were examined for homogeneity of equal variance. Data were square root transformed when needed to normalize the data. Back-transformed data are presented. Treatment means were separated using the Fisher’s protected LSD test at P = 0.05.
Results and Discussion
Purple Nutsedge Control
Purple nutsedge shoot densities varied across Trials I, II, III, and IV with an average of 32, 0.61, 13, and 14 shoots m−2 at 4 WAF in the nonfumigated control treatment (Table 4). In Trials III and IV, purple nutsedge shoots increased to 41 and 35 shoots m−2 at 13 WAF in the nonfumigated control treatment.
Table 4. Cyperus rotundus density following various fumigant programs in field experiments. a
a Values followed by the same letter in the same column do not differ according to the Fisher’s protected LSD test at P = 0.05.
b Pic at 168 kg ha−1 was not included in Trial I; 1,3-D + Pic + metam potassium was not included in Trials III and IV.
c Trials I and II were conducted in Balm, FL; Trials III and IV were conducted in Quincy, FL.
d Abbreviations: SF, sulfuryl fluoride; 1,3-D, 1,3-dichloropropene; Pic, chloropicrin; WAF, weeks after fumigation.
In Trial I, SF alone at 448 kg ha−1 reduced purple nutsedge shoot density compared to the nonfumigated control at 4 WAF, whereas lower rates at 224 and 336 kg ha−1 were ineffective on any date (Table 4). In Trial II, all SF-alone treatments did not differ in purple nutsedge control but reduced purple nutsedge shoot density compared to the nonfumigated control. In Trial III, all SF-alone treatments significantly reduced purple nutsedge shoot density compared to the nonfumigated control at 4, 9, and 13 WAF. The highest rate of SF at 448 kg ha−1 was more effective and resulted in significantly lower purple nutsedge shoot density compared to the lowest rate at 224 kg ha−1. In Trial IV, none of the SF-alone treatments reduced purple nutsedge shoot density compared to the nonfumigated control. In Trial IV, SF applied at a rate of 336 or 448 kg ha−1 applied in conjunction with Pic significantly reduced purple nutsedge shoot density compared to the nonfumigated control at 4 WAF. However, soils fumigated with Pic alone had approximately 2-fold of purple nutsedge shoot density compared to the nonfumigated control in all rating dates. SF + Pic provided greater reduction of purple nutsedge shoot density than the nonfumigated control in three of four trials, whereas SF alone provided greater reduction of purple nutsedge shoot density than the nonfumigated control in two of four trials.
Although 1,3-D + Pic reduced purple nutsedge shoot density at all rating dates in Trials I and III, this fumigant treatment was ineffective and did not reduce purple nutsedge shoot density at any date in Trials II and IV (Table 4). 1,3-D + Pic + metam potassium reduced purple nutsedge density at all dates in Trial I, and at 11 and 13 WAF in Trial II. In addition, soils fumigated with 1,3-D + Pic + metam potassium had significantly less purple nutsedge shoot density compared to 1,3-D + Pic at 11 and 13 WAF in Trial II.
In Trial II, Pic alone at 168 kg ha−1 did not reduce purple nutsedge shoot density compared to the nonfumigated control at any date (Table 4). In Trial III, Pic alone reduced purple nutsedge shoot density compared to the nonfumigated control at 4 WAF but not at 9 and 13 WAF. In Trial IV, soils fumigated with Pic alone displayed significantly higher purple nutsedge density than the nonfumigated control at all dates. In general, Pic alone at 168 kg ha−1 was ineffective for purple nutsedge control and in some cases increased purple nutsedge density.
The current MB alternatives, including 1,3-D, DMDS, Pic, and methyl isothiocyanate generators such as metam sodium and metam potassium, are used by vegetable growers for weed management in plasticulture vegetables (Hanson and Shrestha Reference Hanson and Shrestha2006; Snapp et al. Reference Snapp, Swinton, Labarta, Mutch, Black and Leep2005; Yu et al. Reference Yu, Land, Vallad and Boyd2018, Reference Yu, Sharpe and Boyd2019). Inconsistent control of Cyperus spp. in plastic-mulched vegetable crops with these fumigants has been documented (Boyd et al. Reference Boyd, Vallad, Wu, Noling and Guan2017; Hutchinson et al. Reference Hutchinson, McGiffen, Sims and Becker2003; Hanson and Shrestha Reference Hanson and Shrestha2006; Snapp et al. Reference Snapp, Swinton, Labarta, Mutch, Black and Leep2005; Stevens et al. Reference Stevens, Freeman and Dittmar2016, Reference Stevens, Freeman and Boyd2019). For example, in previous research, Yu et al. (Reference Yu, Sharpe and Boyd2019) reported that 131 kg ha−1 1,3-D + 205 kg ha−1 Pic did not effectively control purple nutsedge at 12 WAF in plastic-mulched bell pepper. In another study, however, Stevens et al. (Reference Stevens, Freeman and Boyd2019) reported that 109 kg ha−1 1,3-D + 171 kg ha−1 Pic was effective and significantly reduced purple nutsedge populations at 90 d after fumigation in plastic-mulched tomato. Yu et al. (Reference Yu, Land, Vallad and Boyd2018) noted that shank injection of 159 kg ha−1 DMDS + 379 kg ha−1 Pic effectively controlled purple nutsedge. However, Pic is currently not registered in many countries, which limits the use of this fumigant combination.
Substantial effort has been made to determine herbicides that can be applied on raised beds following soil fumigation for control of Cyperus in plastic-mulched vegetable crops. Alves et al. (Reference Alves, MacRae, Hunnicut, Jacoby, MacDonald and Dittmar2013) documented that fumigation with 1,3-D + Pic followed by postemergence halosulfluron effectively reduced purple nutsedge tuber count in plasticulture tomato. Eure and Culpepper (Reference Eure and Culpepper2017) noted that DMDS + Pic plus the herbicide napropamide applied as a preemergence herbicide prior to transplanting followed by S-metolachlor applied as postemergence herbicide is required to replace MB in plasticulture bell pepper. Recently, Yu et al. (Reference Yu, Sharpe and Boyd2019) documented that S-metolachlor applied preemergence following the fumigation of 1,3-D + Pic or DMDS + Pic improved the control of purple nutsedge and various broadleaf and grass weeds. Our results show that SF + Pic treatments are highly efficacious for purple nutsedge control. The highest rate of SF at 448 kg ha−1 applied in conjunction with Pic reduced purple nutsedge to ≤2.1 shoots m−2 by season end, suggesting that the supplemental herbicides for Cyperus spp. control may not be needed when this fumigation program is used.
In the present study, soils fumigated with Pic alone exhibited an approximately 2-fold increase in purple nutsedge shoot density than the nonfumigated control in Trial IV. Santos et al. (Reference Santos, Gilreath and Motis2006) performed a multiseason field research trial from 1999 to 2003 in Florida and found that Pic rates ranging from 119 to 206 kg ha−1 significantly stimulated Cyperus spp. sprouting. Based on these findings, we postulated that SF might be more efficacious on sprouted purple nutsedge than dormant tubers. As a result, fumigation with Pic at 168 kg ha−1 prior to injecting SF through drip tapes provided equivalent or better control of purple nutsedge than SF alone. SF was injected through drip tapes a few hours after shank injection of Pic. Additional study is needed to identify the best application interval between SF and Pic applications.
In this study, a significant rate response was observed in some cases when SF was applied alone. At some of the rating dates, the highest rate of SF alone (448 kg ha−1) provided statistically better control of purple nutsedge compared to the lowest rate at 224 kg ha−1. However, there was no rate response when SF was applied in conjunction with Pic because all SF + Pic treatments were highly effective. In addition, results revealed that the SF + Pic treatments were statistically equally effective compared to 1,3-D + Pic + metam potassium, but generally provided greater control of purple nutsedge than 1,3-D + Pic. In previous research, Cao et al. (Reference Cao, Guo, Yan, Mao, Wang and Li2014) compared the efficacy of SF and MB against Abutilon theophrasti and Digitaria sanguinalis and found that SF alone was generally less effective than MB on these weed species. Further study is needed to assess SF + Pic for control of other weed species.
Tomato Growth and Yield
All fumigants were safe to tomato (data not shown). In Trial II, fumigant treatments containing Pic, including SF + Pic, 1,3-D + Pic, 1,3-D + Pic + metam potassium, and Pic alone, displayed a positive effect on tomato growth and resulted in taller tomato plants than the nonfumigated control at 9 WATP (Table 5). Tomato plants growing in plots treated with 336 kg ha−1 SF were also taller than the nonfumigated control at 9 WATP in Trial II. None of the fumigants negatively affected tomato height in Trial I or tomato vigor in Trials III and IV.
Table 5. Tomato height and vigor following various fumigant programs in field experiments in Florida. a
a Values followed by the same letter in the same column do not differ according to the Fisher’s protected LSD test at P = 0.05.
b Tomato vigor visually evaluated on a scale from 0 to 10 with 0 being plant death and 10 being highly vigorous plants.
c Pic at 168 kg ha−1 was not included in Trial I; 1,3-D + Pic + metam potassium was not included in Trial III and IV.
d Trials I and II were conducted in Balm, FL; Trials III and IV were conducted in Quincy, FL.
e Abbreviations: SF, sulfuryl fluoride; 1,3-D, 1,3-dichloropropene; Pic, chloropicrin; WATP, weeks after transplanting.
In Trials I and IV, the evaluated fumigant treatments did not differ from the nonfumigated control for tomato fruit yield in various sizes (Table 6). In Trial II, tomato plants growing in plots treated with SF + Pic, 1,3-D + Pic, and 1,3-D + Pic + metam potassium resulted in higher total marketable yield than the nonfumigated control, whereas SF alone did not improve tomato yield in any size category except for extra-large harvested from plots fumigated with 336 kg ha−1 SF (Table 6). In addition, yield for large or extra-large tomato in plots fumigated with SF + Pic had higher yield than the nonfumigated control. Tomato plants grown in plots fumigated with 1,3-D + Pic + metam potassium or Pic alone produced significantly higher fruit yield for large and extra-large grading categories than the nonfumigated control.
Table 6. Tomato yield following various fumigant programs in field experiments. a
a Values followed by the same letter in the same column do not differ according to the Fisher’s Protected LSD test at P = 0.05.
b Pic at 168 kg ha−1 was not included in Trial I; 1,3-D + Pic + metam potassium was not included in Trial III and IV.
c Fruit was graded prior to weighing as medium (5.5 cm < diameter <6.5 cm), large (6.5 cm < diameter <7 cm) or extra-large (>7 cm).
d Trials I and II were conducted in Balm, FL, while Trials III and IV were conducted in Quincy, FL.
e Abbreviations: SF, sulfuryl fluoride; 1,3-D, 1,3-dichloropropene; Pic, chloropicrin.
In Trial III, plots treated with 224 kg ha−1 SF +168 kg ha−1 Pic and 448 kg ha−1 SF + 168 kg ha−1 Pic resulted in significantly greater large-size tomato fruit yield compared to the nonfumigated control (Table 6). However, we did not observe any significant difference among the fumigant treatments and the nonfumigated control in terms of total marketable fruit yield. In addition, none of the SF-alone treatments differed from the nonfumigated control in terms of tomato fruit yield in various size categories.
Large and extra-large tomato fruits often command a premium price. In a few cases in Trials II and III, SF + Pic resulted in significantly more large, extra-large, and total marketable fruit yields than the nonfumigated control and SF alone. Previous studies have shown an increase in tomato height and markable yield when Pic is applied in conjunction with other soil fumigants (Boyd et al. Reference Boyd, Vallad, Wu, Noling and Guan2017; Yu et al. Reference Yu, Land, Vallad and Boyd2018). Boyd et al. (Reference Boyd, Vallad, Wu, Noling and Guan2017) observed that tomato plants growing in soils fumigated with 1,3-D + Pic or DMDS + Pic were slightly taller than those fumigated with DMDS alone and nonfumigated control. Stevens et al. (Reference Stevens, Freeman and Boyd2019) reported that shank injection of 1,3-D + Pic resulted in taller tomato plants and better tomato vigor than the nonfumigated control. In other vegetables, Miller et al. (Reference Miller, Dittmar, Vallad and Ferrell2014) noted that bell pepper plants growing in plots treated with 1,3-D + Pic or DMDS + Pic were taller than the nonfumigated control.
In summary, at the rates tested, all SF + Pic treatments were efficacious and provided season-long control of purple nutsedge. However, the overall trend showed that SF alone was less effective than SF + Pic and provided inconsistent control of purple nutsedge. Results also revealed that 1,3-D + Pic + metam potassium was highly effective and provided comparable control of purple nutsedge when compared to SF + Pic, whereas 1,3-D + Pic provided inconsistent control of purple nutsedge. In addition, our results agree with those from previous research that found that Pic alone was ineffective for purple nutsedge control (Santos et al. Reference Santos, Gilreath and Motis2006). Moreover, none of the fumigant treatments exhibited an adverse effect on tomato growth and marketable yield. In one of the four tomato seasons, SF + Pic programs resulted in significantly greater tomato plant height and total marketable fruit yields than the nonfumigated control. Overall, this research demonstrates that fumigation with SF in conjunction with Pic is an efficacious solution for control of purple nutsedge in plasticulture vegetables. Additional studies are needed to determine the efficacy of SF + Pic on other weed species and soilborne pathogens, along with optimum time interval between Pic and SF injection.
Acknowledgments
We acknowledge financial and technical support from Douglas Products and Tri-Est Ag. We also thank the farm crews at the Gulf Coast Research and Education Center and North Florida Research and Education Center for assistance with fumigation and crop management. No conflicts of interest have been declared.
