Quality and Yield of Broccoli Hybrids Treated Postemergence with Tank Mixtures of Chelated Iron and Bentazon

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Marcellus Washington Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Mark Farnham USDA United States Vegetable Laboratory, 2700 Savannah Highway, Charleston, SC 29414

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David Couillard USDA United States Vegetable Laboratory, 2700 Savannah Highway, Charleston, SC 29414

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H. Tyler Campbell Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Brian K. Ward Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Matthew Cutulle Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Abstract

Increased broccoli production in the eastern United States necessitates the exploration of novel concepts to improve weed management in this region. Currently, there are minimal selective postemergent herbicide options available for broccoli growers in the southeastern United States. Research was conducted to determine if bentazon, an effective nutsedge herbicide, could be used safely for broccoli when tank-mixed with chelated iron in both greenhouse and field settings. Initial greenhouse screens in Charleston, SC, demonstrated that when 224 g⋅ha−1 active ingredient of chelated iron was tank-mixed with bentazon, a reduction in injury occurred in most of the cultivars that were evaluated. However, based on injury ratings, yield parameters, and broccoli quality observed in the field, it appears that the applications of chelated iron yielded no positive effects. Furthermore, for some of the broccoli cultivars it appeared to exacerbate bentazon injury in the field.

Currently, the overwhelming majority of broccoli produced in the United States is grown on the west coast, with ≈80% to 85% of the crop grown in California and Arizona (Cutulle et al., 2019; USDA-NASS, 2017). Certain challenges arise with this relatively small region of the country producing for consumers nationwide, including the possibility of not producing enough to meet all local market demands. Additionally, fuel and other carbon costs associated with transporting produce around the country and the difficulty of maintaining freshness from the field are challenging from a sustainability perspective. To mitigate some of these challenges, a collaborative initiative called the East Coast Broccoli Project involving breeders, producers, distributors, and research scientists across multiple disciplines has been working to shift some United States production to the East Coast for almost a decade (Stansell et al., 2017). However, with a shift in the geographic region comes a change in the environment and new, specific agricultural obstacles posed by that environment, especially with respect to the Southeast. Greater annual rainfall in this region coupled with generally high year-round temperatures create an environment where weeds and soilborne pathogens can flourish for nearly the entire year. Currently, substantial acreage devoted to broccoli is found in Maine during the summer months and in northern Florida during the winter. Additionally, vegetable producers along the eastern seaboard in other states, including Georgia, the Carolinas, Virginia, New Jersey, and New York, have successfully grown broccoli during fall and spring months, thus supplying the commodity to regional and local markets. This increased production on the East Coast was spurred by the increased popularity of the crop, interest by consumers in purchasing and eating a relatively locally grown crop, and the lower shipping costs associated with growing and distributing within one region (Atallah et al., 2014).

Broccoli can be a difficult crop to grow in the Southeast because of environmental conditions that are less than ideal, strong disease and weed pressure, and a limited number of approved herbicide options for controlling the most problematic weeds. One of these problematic weeds is yellow nutsedge (Cyperus esculentus L.). This sedge species is hardy because of the numerous subterranean tubers and sharp, fast-growing leaves, and a single mature plant is capable of producing more than 1000 seeds during a single growing season. This means a field of several hundred thousand flowering plants can introduce hundreds of millions of seeds into the weed seed bank (Thullen and Keeley, 1975). Yellow nutsedge can be one of the more problematic weeds during the fall broccoli establishment period in the coastal Carolinas. The warmer temperatures from late summer can extend into midfall, providing ample opportunity for young broccoli to rapidly increase mass before the arrival of cooler temperatures in late fall to midwinter, which are needed for vernalization and flower bud setting. Yellow nutsedge competition with broccoli in early fall can limit the broccoli yield in coastal Carolina (Cutulle et al., 2019). One effective postemergent herbicide for controlling yellow nutsedge is bentazon. This herbicide would be attractive to use for broccoli grown on diversified vegetable farms because there is limited availability of bentazon in the soil after application for uptake by a rotational crop (Wagner et al., 1996). However, the post-transplant application of bentazon injures most broccoli cultivars (Umeda, 1996). Potential strategies to increase the safety of bentazon for broccoli would be to screen experimental germplasm for tolerance, evaluate compounds that increase the safety or reduce phytotoxicity for broccoli treated with bentazon, or a combination of these two strategies.

Most commercial safeners on the market target monocot crops and complimentary herbicides (Cutulle et al., 2018). Numerous studies of monocot species reported that safeners increase the activity of cytochrome P450s, resulting in increased tolerance to multiple herbicide modes of actions through the conjugation and metabolism of the herbicide molecule (Hatzios, 1991). Interestingly, the application of melatonin to several broadleaf vegetables has been shown to increase cytochrome P450 activity and act as an antioxidant (Mandal et al., 2018). Furthermore, melatonin has been shown in vitro to reduce bentazon injury to sweetpotato (Caputo et al., 2020). However, to use melatonin as a safener in commercial production would be cost-prohibitive at this time.

Chelated iron (Fe) safener was chosen for this experiment because it has been patented for use with transgenic soybeans to safeguard against 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides by Syngenta in 2014, and it is relatively inexpensive. Furthermore, the Clemson University Coastal Research and Education Center vegetable weed science laboratory demonstrated that chelated Fe can reduce injury from metribuzin in tomato (unpublished data). The reduction in injury to a photosystem II-inhibiting herbicide is promising and provides some incentive to evaluate the impact of chelated Fe on bentazon safety for broccoli. These experiments aimed to determine whether chelated Fe influences herbicide injury, broccoli quality, and yield loss associated with the post-transplant application of bentazon on broccoli.

Materials and Methods

Greenhouse trial

Plant propagation and greenhouse layout.

Seeds from commercial broccoli hybrids Emerald Crown (Johnny’s Selected Seeds, Fairfield, ME), Eastern Crown (Seedway, Hall, NY), Ironman (Stokes Seeds, Thorold, Ontario, Canada), and Belstar (Johnny’s Selected Seeds, Fairfield, ME), in addition to the experimental hybrid BH026 developed at the Agricultural Research Service U.S. Vegetable Laboratory, were planted in 10-cm square pots filled with 50% promix and 50% native soil composed of Yonges loamy sand (fine loamy mixed, thermic Albaqualf). Each pot contained one plant. The pots were seeded on 4 Mar. 2019, for first run of the experiment, whereas pots were seeded for the second run on 15 Apr. 2019. The greenhouse was maintained at ≈26 °C with 35% relative humidity. The trial was designed as a randomized complete block with four replications. The treatments were constructed as a factorial with the five rates of bentazon (0, 10, 100, 1000, 10,000 g⋅ha−1 a.i.) by two rates of chelated Fe (0 and 112 g⋅ha−1 a.i.). Pot spacing was ≈15 cm between rows and 5 cm between pots in a row.

Herbicide and safener application.

Bentazon and chelated Fe were applied when the plants reached the stage of six true leaves. Both were applied simultaneously using a mixed tank CO2-powered backpack sprayer equipped with XR8002 spray nozzles (Teejet Technologies, Glendale, IL). Treatments were applied at 200 L/ha water carrier volume. Applications were made on 4 Apr. 2019 and 15 May 2019 for the first and second experimental runs, respectively.

Data collection.

Injury ratings were determined 10 d after the application date using a scale from 0 to 100 relative to the untreated check for each repetition (the untreated check was 0 on this scale). The injury ratings focused on chlorosis and necrosis. For example, if 25% of the treated broccoli plant was chlorotic, 25% of the plant was necrotic, and 50% of the plant did not have symptoms, then the overall injury would be given a rating of 50% injury.

Field trials

Plant propagation and field layout.

Seeds from the commercial hybrids Emerald Crown, Ironman, and Belstar and the experimental BH026 were seeded in 10 × 20 Styrofoam plug trays filled with Metromix 360 (Sun Gro Horticulture, Agawam, MA) soil and allowed to grow until they reached the four-leaf stage before being transplanting into the field. Seedlings were hand-transplanted on 15 Sept. 2018 and 18 Sept. 2019 in two separate field trials. Plants were placed in a single row on raised beds spaced 91 cm apart with 15-cm spacing between each plant and 1-m spacing between each plot. Each plot was 1.8 m × 3 m, with one plot across two rows. Plots were split into four quadrants in which 10 plants from each hybrid were planted. Oxyflurofen was applied pre-emergence at 560 g⋅ha−1 a.i. so the plots would not be inundated with weed pressure (Dow AgroSciences., LLC., Indianapolis, IN). The experimental design of this trial was a randomized complete block with four replications. The treatment design of this trial was a factorial with three rates of bentazon (0, 514, and 1030 g⋅ha−1 a.i.) by two rates of chelated Fe (0 and 224 g⋅ha−1 a.i.) and two rates of nonionic surfactant (0 and 0.25% volume/volume). Plants were watered as needed using drip tape irrigation. Then, 168 kg⋅ha−1 of calcium nitrate was side-dressed along each bed in the field on 17 Oct. 2018 and 23 Oct. 2019 for each experimental field run.

Herbicide and chelated Fe application.

Bentazon (BASF Corporation, Wyandotte, MI) and chelated Fe (BASF Corporation, Wyandotte, MI) were applied when the plants reached the stage with six to seven true leaves. The treatments were applied at 200 L⋅ha−1 with a backpack sprayer (Bellspray Inc., Opelousa, LA) equipped with 8002VS nozzles pressurized to 275 kPa (Teejet Technologies, Wheaton, IL). Applications were performed on 30 Sept. 2018 and 4 Oct. 2019. The average temperature during application for the two runs was 31.6 °C, with 67% relative humidity, no cloud cover, and an average wind speed of 2.9 km⋅h−1.

Data collection.

Injury ratings were determined 1 week after treatment in the same manner as that used for the greenhouse trials. When flower head onset began and head diameters began to reach the chosen marketable diameter of 12 to 14 cm, the broccoli harvest was started. Harvests occurred 2 to 3 d per week depending on whether viable heads were present in the field. The first harvest was performed on 7 Nov. 2018, during the first year; the last harvest was performed on 30 Dec. 2018. During year two, the first harvest was performed on 13 Nov. 2019; the last harvest was performed on 3 Jan. 2020. Qualitative and quantitative characteristics of heads were recorded for the first five heads harvested per hybrid subplot. The qualitative characteristics including head extension, color, smoothness, firmness, bracting, bead size, bead uniformity, and overall quality value were rated using a scale of 1 to 5 scale, with a value of 1 being the worst and 5 being the best as described by Stansell et al. (2017). Head weight (g) and stem diameter (mm) of each head were also measured. After the first five heads were characterized, the remaining heads were taken for yield purposes only.

Data analysis.

Data from the greenhouse trial were analyzed using the MIXED procedure in JMP (JMP Pro 14; SAS Institute, Cary, NC). For this procedure, replication was designated as a random effect, and cultivar, chelated Fe, and bentazon were considered fixed effects. Data regarding the percent injury were fit to a dose-response curve using a five-parameter log-logistic model (Gottschalk and Dunn, 2005; Seefeldt et al., 1995):
y=c + d + c1 + ex[a(Herbicideb)].

The model was used to calculate the effective dose (ED) of bentazon to elicit injury by 30% (ED30), where y is the herbicide dose necessary to cause the predicted injury, c is the asymptote for high doses, d is the asymptote for low doses, a is the slope parameter, b is the inflection point, and ƒ is the symmetrical power. The upper asymptote is the point of the growth curve that represents the maximum of the parameter measured. The lower asymptote is the point of the growth curve that represents the minimum of the parameter measured (Paine et al., 2012). Data from the field trials were also analyzed using a MIXED procedure test with the same random and fixed effect designations. The trial run did not have a significant effect on any variables; therefore, they are pooled across both trial years.

Results

Greenhouse trials

For most of the hybrids tested, samples treated with chelated Fe and bentazon exhibited a lower rate of injury than those treated with bentazon alone. The chelated Fe-treated treatments reduced injury from bentazon by half for Belstar, Eastern Crown, and Emerald Crown. A dosage-response curve was generated and used to estimate the ED30 for each cultivar with and without chelated Fe added (Table 1). Interestingly for BH026, greater bentazon injury was observed in the chelated Fe-treated groups when compared with plants that did not receive chelated Fe, which is noted by the ED30 values in Table 1. Differential tolerance to the herbicide may be the result of a combination between anatomical characteristics and metabolism of the different cultivars.

Table 1.

Dose-response of four broccoli cultivars treated with bentazon and chelated iron (Fe) in the greenhouse averaged across two trials.

Table 1.

Field trials

Generally, Fe reduced injury from bentazon (Table 2). The only exception was Belstar, which was injured more when Fe was tank-mixed with bentazon. At two-times the rate of bentazon, Ironman was injured more when treated with chelated Fe, whereas little difference was noted in the other cultivars when chelated Fe was applied. Although not directly compared, the higher maximum injury observed in the field is interesting because, usually, the injury in the field is less than that observed in the greenhouse. Environmental stresses and increased light levels in the field may have been the cause of the increased injury of the field broccoli because bentazon is a PS2-inhibting light-dependent herbicide.

Table 2.

Percentage of injury ratings 1 week after treatment averaged across the 2018 and 2019 field trials.

Table 2.

Neither herbicide nor chelated Fe treatments had a significant effect on the mean overall quality of the broccoli heads collected across most of the four hybrids (F-value >0.05). The one exception to this was that herbicide had a significant effect on the mean overall quality of heads collected from Belstar (Table 3). Regarding the individual qualitative characteristics that were factored into the overall quality ratings, there was no consistent significant effect of herbicide or chelated Fe observed across all four hybrids. For Belstar, nearly every qualitative characteristic measured was affected by herbicide application, but only the head extension was affected by chelated Fe, whereas bead size exhibited a bentazon × chelated Fe interaction effect. Head extension and smoothness of Emerald Crown were significantly affected by herbicide, whereas chelated Fe appeared to influence head extension and color.

Table 3.

Impact of different bentazon and chelated iron (Fe) treatments on qualitative broccoli head characteristics averaged across the 2018 and 2019 field trials. Ratings are based on a scale of 1 to 5, with 1 being the worst and 5 being the best with regard to marketability.

Table 3.

The mean number of market-size heads collected, the stem diameters of harvested heads, and the mean weights of individual heads were the yield components measured in this study. The major trend observed was fewer heads and higher head weights in the groups treated with chelated Fe. Across all cultivars, the average number of heads was lower when treated with tank mixtures containing chelated Fe than when no chelated Fe was applied regardless of herbicide concentration, with the one exception being the chelated Fe-treated/herbicide untreated group of BH026, for which the opposite effect was observed (Table 4). The number of market-size heads was significantly different between treatments containing chelated Fe and treatments with no chelated Fe for Emerald Crown and Ironman (Table 4). The average stem diameters of the heads collected did not seem to have a consistent trend between treatments or across cultivars. Furthermore, the weights of the heads collected from plants treated with a 1× herbicide rate plus chelated Fe tended to be higher than the heads collected from plants treated with a 1× herbicide rate and no chelated Fe; however, the differences were not statistically significant. The exception to this observation was again seen for BH026; the mean head weights collected from plants receiving a 1× herbicide rate and no chelated Fe were higher than those of heads collected from plants receiving the same 1× herbicide rate plus chelated Fe.

Table 4.

Impact of bentazon and chelated iron (Fe) treatments on quantitative broccoli head characteristics averaged across the 2018 and 2019 field trials.

Table 4.

Discussion

Certain situational factors may affect the degree to which chelated Fe increases the safety of nontarget crop plants. The results of this study suggest that although some of the phytotoxic effects of bentazon are prevented during the early stages of broccoli growth, chelated Fe had no significant positive effect in terms of preventing yield loss or quality decline caused by bentazon application and, in some instances, may have exacerbated the damage. Similar results with the use of chelated Fe in tandem with bispyribac-sodium to reduce injury in turfgrass have been previously reported; for example, the presence of chelated Fe correlated with higher quality ratings the first year but with lower quality ratings the following year (Cutulle et al., 2012). The inverse correlation between the average head weight and number of heads harvested is likely caused by competition between adjacent plants, which would be lessened if one plant dies because of a treatment or environmental effect.

In addition to this somewhat inconsistent effect of increasing the safety, the possibility of antagonism occurring as a result of using safeners with herbicides must also be considered when evaluating how effective chelated Fe can be as a safener. Massey et al. (2006) observed that chelated Fe, when mixed with MSMA, provided 55% to 60% control of southern crabgrass, thus demonstrating a lower propensity for antagonism compared with the FeSO4 safener that was also tested during the trial. However, the exact mechanism by which antagonism occurs, or if it occurs at all, may vary from herbicide to herbicide as a result of the specific mode of action of each herbicide and the possible disruption of these modes of action by the safener. Although not explicitly examined during this study, the capacity for chelated Fe to act in an antagonistic way with bentazon when the two are tank-mixed requires further study because antagonistic activity, coupled with possible yield loss, could render chelated Fe an ineffective safener option. This is because the mode of action of bentazon involves the disruption of photosystem II activity, which is a mode of action that generally involves blocking the exchange of electrons between plastoquinone and the D1 protein, and preventing this exchange causes lipid peroxidation and general oxidative stress that can disrupt chloroplasts (Cutulle et al., 2014). However, the presence of chelated Fe could provide an iron source to be used for the synthesis of more chloroplasts to replace the ones disrupted by bentazon, thereby lessening the effectiveness of the bentazon application in terms of weed control.

Ultimately, chelated Fe does not act as an herbicide safener for the multiple broccoli cultivars evaluated in this study. Compared with other methods of weed control, a chelated Fe–bentazon application is probably not a means of effectively controlling weeds if, as is implied by the results of this study, it has a detrimental effect on the yield of the broccoli harvested. If such a yield loss proves to be a consistent effect of the use of chelated Fe on broccoli, then other methods of effective weed control that do not disrupt production require further testing.

Pretransplant herbicide applications coupled with cultivation have been shown to be an effective means of early-stage weed control. Pairing cultivation with napropamide, S-metolachlor, or oxyfluorfen resulted in relatively low early-stage injury of two cultivars tested (Emerald Crown and Lieutenant), significant control of a number of problematic weeds, including yellow foxtail and common purslane, and no significant effect on the yield or quality of broccoli collected across both years of the study (Cutulle et al., 2019). Future research should evaluate whether post-transplant directed application of bentazon coupled with cultivation can effectively control yellow nutsedge and other problematic weeds and whether this treatment will have an effect on harvest yield and quality.

Although the use of chelated Fe was not effective as an herbicide safener in the experiments reported here, it should not prevent investments in safener research involving broccoli or even research of chelated Fe with other herbicide ingredients. More research of other potential safener compounds is needed to improve the postemergent control of yellow nutsedge with broccoli.

Literature Cited

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  • Cutulle, M.A., Derr, J., Nichols, A.D., McCall, D. & Horvath, B. 2012 Impact of bispyribac sodium on annual bluegrass control and brown patch severity in tall fescue J. Environ. Hortic. 30 195 200

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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
  • Thullen, R.J. & Keeley, P.E. 1975 Yellow nutsedge sprouting and re-sprouting potential Weed Sci. 23 333 337

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    • Search Google Scholar
    • Export Citation
  • Atallah, S.S., Gomez, M.I. & Bjorkman, T. 2014 Localization effects for a fresh vegetable product supply chain: Broccoli in the eastern United States Food Policy 49 151 159

    • Search Google Scholar
    • Export Citation
  • Cutulle, M.A., Derr, J., Nichols, A.D., McCall, D. & Horvath, B. 2012 Impact of bispyribac sodium on annual bluegrass control and brown patch severity in tall fescue J. Environ. Hortic. 30 195 200

    • Search Google Scholar
    • Export Citation
  • Cutulle, M.A., Armel, G.R., Brosnan, J.T., Best, M.D., Kopsell, D.A., Bruce, B.D., Bostic, H.E. & Layton, D.S. 2014 Synthesis and evaluation of heterocyclic analogues of bromoxynil J. Agr. Food Chem. 62 329 336

    • Search Google Scholar
    • Export Citation
  • Cutulle, M.A., Armel, G.R., Kopsell, D.A., Wilson, H.P., Brosnan, J.T., Vargas, J.J., Hines, T.E. & Koepke-Hill, R.M. 2018 Several pesticides influence the nutritional content of sweet corn J. Agr. Food Chem. 66 3086 3092

    • Search Google Scholar
    • Export Citation
  • Cutulle, M., Campbell, H., Couillard, D.M., Ward, B. & Farnham, M.W. 2019 Pre transplant herbicide application and cultivation to manage weeds in southeastern broccoli production Crop Prot. https://doi.org/10.1016/j.cropro.2019.104862

    • Search Google Scholar
    • Export Citation
  • Caputo, G.A., Wadl, P.A., McCarty, L., Adelberg, J., Jennings, K.M. & Cutulle, M.A. 2020 In vitro safening of bentazon by melatonin in sweetpotato (Ipomoe batatas) HortScience 55 1406 1410 https://doi.org/10.21273/HORTSCI15128-20

    • Search Google Scholar
    • Export Citation
  • Gottschalk, P.G. & Dunn, J.R. 2005 The five parameter logistic: A characterization and comparison with the four-parameter logistic Anal. Biochem. 343 54 65

    • Search Google Scholar
    • Export Citation
  • Hatzios, K.K. 1991 An overview of the mechanism of action of herbicide safeners Z Naturforsch 46c 819 827

  • Mandal, M.K., Suren, H., Ward, B., Boroujerdi, A. & Kousik, C. 2018 Differential roles of melatonin in plant-host resistance and pathogen suppression in cucurbits J. Pineal Res. 65 1 23

    • Search Google Scholar
    • Export Citation
  • Massey, J.H., Taylor, J.M., Binbuga, N., Coats, G.E. & Henry, W.P. 2006 Iron antagonism of MSMA herbicide applied to bermudagrass: Characterization of the Fe2+ -MAA complexation reaction Weed Sci. 54 23 30

    • Search Google Scholar
    • Export Citation
  • Paine, C., Marthews, T.R., Vogt, D.R., Purves, D., Rees, M., Hector, A. & Turnbull, L.A. 2012 How to fit nonlinear plant growth models and calculate growth rates: An update for ecologists Methods Ecol. Evol. 3 245 256

    • Search Google Scholar
    • Export Citation
  • Seefeldt, S.S., Jenson, J.E. & Fuerst, E.P. 1995 Log–logistic analysis of herbicide dose-response relationships Weed Technol. 9 218 227

  • Stansell, Z., Bjorkman, T., Branham, S., Couillard, D. & Farnham, M.W. 2017 Use of a quality trait index to increase the reliability of phenotypic evaluations in broccoli HortScience 52 1490 1495 https://doi.org/10.21273/HORTSCI12202-17

    • Search Google Scholar
    • Export Citation
  • Thullen, R.J. & Keeley, P.E. 1975 Yellow nutsedge sprouting and re-sprouting potential Weed Sci. 23 333 337

  • Umeda, K. 1996 Postemergent weed control in broccoli Vegetable Report http://hdl.handle.net/10150/214734

  • USDA-National Agricultural Statistics Service 2017 https://www.nass.usda.gov/. [accessed Dec. 2019]

  • Wagner, S.C., Zablotowicz, R.M., Gaston, L.A., Locke, M.A. & Kinsella, J. 1996 Bentazon degradation in soil: Influence of tillage and history of bentazon application J. Agr. Food Chem. 44 1593 1598

    • Search Google Scholar
    • Export Citation
Marcellus Washington Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Mark Farnham USDA United States Vegetable Laboratory, 2700 Savannah Highway, Charleston, SC 29414

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David Couillard USDA United States Vegetable Laboratory, 2700 Savannah Highway, Charleston, SC 29414

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H. Tyler Campbell Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Brian K. Ward Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Matthew Cutulle Clemson Coastal Research and Education Center, 2700 Savannah Highway, Charleston, SC 29414

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Contributor Notes

M.C. is the corresponding author. E-mail: mcutull@clemson.edu.

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