Fomesafen is a diphenylether that inhibits the protoporphyrinogen oxidase (PROTOX) enzyme and is primarily used for broadleaf weed control. PROTOX catalyzes the conversion of protoporphyrinogen IX to protoporphyrin IX as part of the biosynthesis of tetrapyrroles such as chlorophyll (Duke et al. Reference Duke, Lydon, Becerril, Sherman, Lehnen and Matsumoto1991; Scalla and Matringe Reference Scalla and Matringe1994). The herbicidal effects of PROTOX inhibitors are relatively quick due to the fast buildup of substrates rather than from the depletion of chlorophyll (Becerril and Duke Reference Becerril and Duke1989).
Fomesafen has the potential to be used as an alternative mechanism of action for PRE nutsedge (Cyperus spp.) control in Florida plasticulture production of small fruit and vegetables. Drip-applied fomesafen at 0.28 kg ai ha−1 reduced yellow nutsedge (Cyperus esculentus L.) punctures of mulch by 89% at 56 d after treatment (Monday et al. Reference Monday, Foshee, Blythe, Wehtje and Gilliam2015). In Florida, Miller and Dittmar (Reference Miller and Dittmar2014) stated that PRE applications of fomesafen at 0.42 kg ai ha−1 controlled 52% of a mix of purple and yellow nutsedge 11 wk after treatment. In a greenhouse study, Reed et al. (Reference Reed, Boyd and Dittmar2016) reported fomesafen had the ability to suppress short-term purple and yellow nutsedge growth with applications made at or before tuber sprouting, thereby maximizing efficacy. However, Boyd (Reference Boyd2015) and Boyd and Reed (Reference Boyd and Reed2016) found that fomesafen application alone did not reduce purple nutsedge density in strawberry (Fragaria × ananessa Duchesne) and tomato (Solanum lycopersicum L.) production systems in Florida.
Fomesafen is a weak acid with increased sorption at low pH. Bioavailability and solubility of fomesafen in soil is affected by pH with a logarithmic acid dissociation constant (pKa) equal to 2.7 with solubility of 50 mg L−1 at pH 7 and less than 1 mg L−1 at pH 1 (Shaner Reference Shaner2014; Weber Reference Weber1993a, Reference Weber1993b). Previous research indicates fomesafen dissipation varies under different soil and environmental conditions. Fomesafen photodecomposes readily under relatively low sunlight conditions and degrades rapidly under anaerobic conditions with a half-life less than 3 wk (Shaner Reference Shaner2014; Wauchope et al. Reference Wauchope, Buttler, Hornsby, Augustijn-Beckers and Burt1992). Fomesafen half-life was reported to be from 28 to 66 d following applications of 0.18 kg ai ha−1 alone or 0.09 followed by 0.18 kg ai ha−1 in a Madalin silty clay loam (Rauch et al. Reference Rauch, Bellinder, Brainard, Lane and Thies2007). Mueller et al. (Reference Mueller, Boswell, Mueller and Steckel2014) reported fomesafen half-life over three experimental years on a loam soil averaged 46 d. Li (Reference Li2014) reported the half-life of fomesafen applied at 0.28 and 0.56 kg ai ha−1 was 6 and 4 d, respectively, for a Cecil sandy loam and a Tifton loamy sand and was not detectable past 28 d.
Florida producers apply PRE herbicides on top of a formed bed after fumigation and before laying plastic mulch. Bond and Walker (Reference Bond and Walker1989) showed that dissipation of linuron, pendimethalin, chlorobromuron, and flurochloridone was reduced when applied to soil under perforated polyethylene covers compared with bare soil. It took twice the amount of time for S-metolachlor to dissipate 50% under low-density polyethylene mulch compared with bare ground (Grey et al. Reference Grey, Vencill, Mantripagada and Culpepper2007). Given these results, it is likely that fomesafen persistence under plastic mulch would also increase compared with bare ground, and this may affect plant-back dates or the possibility of double-cropping for vegetable growers who utilize the plasticulture production system.
Fumigation in combination with the use of plastic mulch may also increase herbicide persistence, as microbial populations and enzymatic activity in the soil can be altered with fumigant use (Klose et al. Reference Klose, Acosta-Martinez and Ajwa2006; Ladd et al. Reference Ladd, Brisbane, Butler and Amato1976; Yamamoto et al. Reference Yamamoto, Ultra, Tanaka, Sakurai and Iwaski2008). EPTC half-life was 9 d, but when applied in conjunction with metam sodium, half-life increased to 22 d (Stiles et al. Reference Stiles, Sams, Robinson, Coffey and Mueller2000). Feng et al. (Reference Feng, Li, Zhang, Zhang, Huang, Lu and Li2012) demonstrated fomesafen degradation by microbial activity. It is possible that microbes play a role in fomesafen dissipation under plastic mulch, given that photodecomposition is reduced, because limited light penetrates the mulch. Investigation into herbicide dissipation in plasticulture systems is needed to determine best management practices for future production. The objective of this research was to evaluate the effect of fumigation on fomesafen dissipation, eggplant tolerance to fomesafen for possible label expansion, and fomesafen control of purple nutsedge.
Materials and Methods
Field experiments were conducted at the University of Florida Gulf Coast Research and Education Center (GCREC), Balm, FL (27.76°N, 82.23°W), in 2015 and 2016. Soil was a Myakka series fine sand (sandy, siliceous, hyperthermic Aeric Alaquods) with 1.0% organic matter, a pH of 6.8, and a sand, silt, and clay content of 95%, 4%, and 1%, respectively, at the 2015 experiment site and 1.5% organic matter, a pH of 6.0, and a sand, silt, and clay content of 98%, 1%, and 1%, respectively at the 2016 site. Treatments were fomesafen (Reflex® 2L Liquid Herbicide, Syngenta Crop Protection, Greensboro, NC) at 0.42 kg ai ha−1, an industry standard for nutsedge control, S-metolachlor (Dual Magnum®, Syngenta Crop Protection) at 1.06 kg ai ha−1, and a nontreated control in either a fumigated or nonfumigated bed. The rate for fomesafen is currently labeled for use in Florida tomato and pepper (Capsicum annuum L.) production. Treatments that included fumigation received Pic-Clor® 60 (Soil Chemicals, Hollister, CA), a combination of 39% 1,3-dichloropropene and 59.6% chloropicrin at 336 kg ha−1, injected into the bed. Fumigants were injected into the bed with a three-shank fumigation rig (Kennco Manufacturing, Ruskin, FL) at a 0.2-m depth. Herbicide applications were made with a CO2-pressurized backpack sprayer (Bellspray, Opelousa, LA) calibrated to deliver 187 L ha−1 with a single 8002VS flat-fan nozzle (TeeJet®, Spraying Systems, Wheaton, IL) to the top of preformed beds with 1.2-m centers, a height of 0.3 m, and bed top width of 0.7 m. Treatments were applied August 11, 2015, and January 25, 2016. Immediately after bed fumigation and herbicide application each bed was covered with 0.03-mm-thick virtually impermeable film (Blockade™, Berry Plastics, Evansville, IN). Fertigation was applied through two rows of drip tape separated by 0.2 m buried just beneath the surface of the bed with emitters every 0.3 m and a flow rate of 0.95 L min−1 per 30.0 m (Jain Irrigation, Haines City, FL). Growing season rainfall and drip-irrigation amounts are listed in Table 1. Rainfall data were acquired from University of Florida Institute of Food and Agricultural Sciences Florida Automated Weather Network from a weather station located at GCREC.
Table 1 Rainfall and drip irrigation between soil sampling dates in Balm, FL, in 2015 and 2016.
a Application, transplant, and harvest were 0, 22, and 119 d after treatment, respectively (2015), and 0, 44, and 128 d after treatment, respectively (2016).
The experiment was a split-plot design organized as a randomized complete block with four replications. Each main plot was 21.3 m of a single bed with 1.5-m buffers between plots. The main plot factors were the presence or absence of fumigation. Each main plot consisted of three 6.1-m subplots of herbicide application with 1.5-m buffers. Three 0.1-m-depth core samples were collected in each fomesafen-applied subplot on the day of treatment (August 11, 2015, and January 25, 2016), transplant (September 2, 2015, and March 9, 2016), and at the end of the trial (December 8, 2015, and May 31, 2016). The soil-corer diameter was 2.0 cm with each sample having a volume of 31.4 cm3. For each fomesafen subplot, the three samples were combined and homogenized for analysis. Samples were kept frozen at −20 C until analysis. Sample extractions, high-performance liquid chromatography (HPLC), and mass spectrometry analyses were conducted similar to protocols for determining residues of fomesafen in soil (Leung Reference Leung1997; Lin Reference Lin2009). Extraction solution consisted of 10 ml HPLC water, 0.1 ml glacial acetic acid, and 10 ml methylene chloride for 10 g of air-dried soil sample. Preparation of samples for analysis included hand shaking solution; determining pH level ≤4.5; 60 min of machine shaking; 10 min of centrifugation at 5,000 rpm to separate the phases; collection of methylene chloride layer with fomesafen residue; addition of 0.1 g of sodium sulfate to remove moisture; vacuum extraction of fomesafen residue with 2 ml of methylene chloride through 3-ml silica tubes (SampliQ Solid Phase Extraction, Agilent Technologies, Santa Clara, CA), rinsing out with 10 ml of ethyl acetate, and collecting; placement in a 60 C water bath with nitrogen gas to dry; and addition of 1 ml of methanol placed on vortex for 1 min alternating with 15 min of sonication for three repetitions before transfer to vials for analysis.
Analysis was performed using the Surveyor HPLC System (Thermo Fisher Scientific, Waltham, MA). Each 25-µl sample was injected directly onto a Zorbax Eclipse plus-C18 (3.5 µm×2.1mm×100mm) analytical column (Agilent Technologies, Santa Clara, CA) at 25 C. A gradient liquid chromatography method used mobile phases A (0.1% formic acid in water) and B (0.1% formic acid in methanol) at a flow rate of 0.3 ml min−1. For mass spectrometry detection, the electrospray ionization source was operated in the negative ion mode and run with Xcalibur v. 2.0 software (Thermo Fisher Scientific, Waltham, MA). High-purity nitrogen was used as the sheath (41 arb) and auxiliary (25 arb) gas and high-purity argon was used as the collision gas (1 mTorr). The parameters were as follows: spray voltage, 4.6 kV; capillary temperature, 280 C; scan width for selected reaction monitoring (SRM), 0.1 m/z; scan time, 0.2 s. The peak width settings for both Q1 and Q3 were 0.7 m/z. The SRM ion-pair transitions and collision-energy (CE) levels of each component were: parent 437 m/z; quantifier product, 195 m/z, 40 CE; qualifier product 222 m/z, 33 CE. Recoveries from fortified nontreated soil samples indicated that recovery was 92±3%. The limit of detection for fomesafen was approximately 2.5 μg kg−1.
Hybrid ‘Night Shadow’ eggplant (Siegers Seed, Holland, MI) was transplanted September 2, 2015, and March 9, 2016, in the beds in a single row with 0.6-m spacing with 10 plants subplot−1. Production and pest management practices were in accordance with industry standards and University of Florida Institute of Food and Agricultural Sciences recommendations (McAvoy et al. Reference McAvoy, Boyd, Ozores-Hampton, Roberts and Smith2015). Crop visual injury ratings on a percent scale, where 0 equaled no visible chlorosis or stunting and 100 equaled complete desiccation, were taken at 2, 4, and 6 wk after transplant (WATr). Eggplant heights were taken at 6 WATr on 5 plants subplot−1. Counts and weights per subplot of eggplant marketable yield were taken weekly on six and five occasions in 2015 and 2016, respectively. Purple nutsedge counts per subplot were recorded 1 and 2 mo after treatment.
Fomesafen dissipation data were subjected to ANOVA (P ≤ 0.05) in SAS (v. 9.4, SAS Institute, Cary, NC) using PROC MIXED with block as random factor. Eggplant tolerance and nutsedge density data were subjected to ANOVA at the 0.05 probability level in SAS using the PROC MIXED with block and block by main plot as the random factors. Data were checked for normality and constant variance before analysis. Means were compared using the least-squares means statement with the Tukey adjustment at P=0.05. Fomesafen concentration, eggplant injury, counts, weights, and purple nutsedge counts after transplant were collected on multiple dates and were analyzed using the repeated statement. Fomesafen dissipation data are presented by growing season. Growing season by fumigant and herbicide interactions were not detected, and results were combined across seasons. Harvest date and nutsedge counts by fumigant and herbicide interactions were not significant, and total yields and average nutsedge density are presented.
Results and Discussion
Fomesafen Dissipation
Fomesafen dissipated throughout the growing season for each treatment. Fomesafen concentrations across treatments at transplant were similar to concentrations at application (Table 2). Fomesafen concentration in soil decreased 83% and 96% from application to harvest in 2015 and 2016, respectively. Fumigation did not affect fomesafen dissipation on any sampling date. This is similar to laboratory experiments that found soil microorganisms had minimal effect on fomesafen degradation (Li Reference Li2014). At the depth sampled, fomesafen concentration observed over time was greater than previous research conducted on sandy soils. In open field production, Li (Reference Li2014) reported the half-life of fomesafen applied at 0.28 and 0.56 kg ai ha−1 was 6 and 4 d, respectively, for a Cecil sandy loam and a Tifton loamy sand and was not detectable past 28 d. Weissler and Poole (Reference Weissler and Poole1982) reported 47% to 67% of fomesafen at 0.30 kg ai ha−1 applied with 0.66 L of water remained at 0.0- to 0.1-m depth in a loam, loamy sand, and silty loam after 9 wk. Mobility was greater in a coarse sand, with 18% of fomesafen remaining at a 0.0- to 0.1-m depth. Small fruit and vegetable production in Florida is largely conducted on soils composed primarily of sand, which would suggest fomesafen has potential to leach. However, fomesafen dissipation was similar to silt or clay-based soils in open-field production that were exposed to rainfall (Cobucci et al. Reference Cobucci, Prates, Falcão and Rezende1998; Mueller et al. Reference Mueller, Boswell, Mueller and Steckel2014; Rauch et al. Reference Rauch, Bellinder, Brainard, Lane and Thies2007). In plasticulture systems, mobility may be reduced, as solute movement is influenced by localized, drip irrigation that is optimized to prevent leaching below the crop root zone. This could decrease fomesafen dissipation more than what would be expected on a sandy soil from leaching.
Table 2 Fomesafen concentrations in soil averaged across nontreated and fumigated fomesafen treatments in field experiments, 2015 and 2016, Balm, FL.Footnote a
a Treatments included fomesafen (Reflex® 2L Liquid Herbicide, Syngenta Crop Protection, Greensboro, NC) at 0.42 kg ai ha−1 and treatments with fumigation had Pic-Clor® 60 (Soil Chemicals, Hollister, CA), a combination of 39% 1,3-dichloropropene and 59.6% chloropicrin at 336 kg ha−1.
b Soil cores were taken at application, transplant, and harvest at 0, 22, and 119 d after treatment, respectively (2015), and 0, 44, and 128 d after treatment, respectively (2016).
c Standard error of the mean indicated by ± symbol. Means within columns followed by different letters are significantly different at P<0.05 using Tukey adjusted-means comparisons.
Eggplant Tolerance
Similar injury was observed with herbicides regardless of fumigant application (Table 3). Fomesafen injured eggplant 11% to 14% at 2 and 4 WATr, but damage decreased over time. S-metolachlor caused less than 5% injury on all evaluation dates. Eggplant recovered from initial injury, and heights were similar across treatments (P=0.8683), averaging 53±1 cm at 6 WATr (unpublished data). Eggplant fruit counts (P=0.7543) and total yield (P=0.6854) were similar across treatments, averaging 121,095±5,382 fruit ha−1 and 53,290 ± 2,722 kg ha−1, respectively (unpublished data). Chaudhari et al. (Reference Chaudhari, Jennings, Monks, Jordan, Gunter, Basinger and Louws2016) reported ‘Santana’ eggplant tolerance to fomesafen with or without tomato rootstocks. Fomesafen applied at 0.42 kg ai ha−1 to the bed top 1 d before transplant caused no eggplant stand loss or reduction in yield, height, or biomass. Injury was observed in one growing season with 12% and 3% injury at 1 and 4 WATr, respectively. Fomesafen applications appear to be safe for use in eggplant even if early-season damage is observed.
Table 3 Eggplant injury after transplant from applications of fumigant and herbicide combinations in combined field experiments in Balm, FL in 2015 and 2016.
a WATr, weeks after transplant, no injury observed at 8 WATr in either year. Means within columns followed by different letters are significantly different at P<0.05 using Tukey adjusted-means comparisons.
b Treatments with fumigation had Pic-Clor® 60 (Soil Chemicals, Hollister, CA), a combination of 39% 1,3-dichloropropene and 59.6% chloropicrin at 336 kg ha−1.
c Herbicide treatments were fomesafen (Reflex® 2L Liquid Herbicide, Syngenta Crop Protection, Greensboro, NC) at 0.42 kg ai ha−1 and S-metolachlor (Dual Magnum®, Syngenta Crop Protection) at 1.06 kg ai ha−1.
Purple Nutsedge Control
Fumigation did not affect purple nutsedge density (Table 4). S-metolachlor reduced purple nutsedge density 48% compared with treatments with no herbicide. Nutsedge density where fomesafen was applied was intermediate between S-metolachlor and no herbicide treatments. Boyd (Reference Boyd2015) and Boyd and Reed (Reference Boyd and Reed2016) noted that fomesafen did not consistently control purple nutsedge in tomato and strawberry production. Fomesafen may not be a viable tool for purple nutsedge control in Florida plasticulture production systems.
Table 4 Purple nutsedge density averaged across evaluation dates that were 1 and 2 mo after treatment with fumigant and herbicide combinations in field experiments in Balm, FL, in 2015 and 2016.
a Treatments with fumigation had Pic-Clor® 60 (Soil Chemicals, Hollister, CA), a combination of 39% 1,3-dichloropropene and 59.6% chloropicrin at 336 kg ha−1.
b Standard error of the mean indicated by ± symbol. Means followed by different letters are significantly different at P<0.05 using Tukey adjusted-means comparisons.
c Herbicide treatments were fomesafen (Reflex® 2L Liquid Herbicide, Syngenta Crop Protection, Greensboro, NC) at 0.42 kg ai ha−1 and S-metolachlor (Dual Magnum®, Syngenta Crop Protection) at 1.06 kg ai ha−1.
The majority of fomesafen dissipated throughout the growing season from the top 0.1 m of the bed regardless of fumigation in the plasticulture system. Fomesafen PRE applications do not adequately control purple nutsedge, but further evaluation for broadleaf control is warranted. Eggplant yields where fomesafen was applied were comparable to the industry standard, which suggests that fomesafen may be of use as a management tool for broadleaf weeds. However, early-season injury is a concern. Further research is warranted to determine fomesafen efficacy on broadleaf weeds and to identify the means of fomesafen dissipation.
