Drip Application of Alternative Fumigants to Methyl Bromide for Strawberry Production

Strawberry (Fragaria ×ananassa Duchesne) is a high-value cash crop that benefi ts from preplant soil fumigation with methyl bromide (MB) and chloropicrin (CP). Methyl bromide will be banned in the U.S. and other developed countries by 2005 for most uses. Potential alternative chemicals to replace methyl bromide for soil fumigation include CP, 1,3-dichloropropene (1,3-D), and methyl isothiocyanate (MITC) generators such as metam sodium (MS). Commercial formulations of these fumigants applied singly and in combination through drip irrigation systems were evaluated at two sites for three consecutive growing seasons as alternatives to MB:CP fumigation for strawberry production. A mixture of 1,3-D and CP was shank injected as Telone C35 (62% 1,3-D and 35% CP) at 374 kg·ha–1. An emulsifi able concentrate (EC) formulation of 1,3-D and CP was applied as InLine (60% 1,3-D and 32% CP) at 236 and 393 L·ha–1 through drip irrigation systems in three amounts of irrigation water (26, 43, and 61 L·m–2). Chloropicrin (CP EC, 96%) was drip applied singly at 130 or 200 L·ha–1. Metam sodium was applied singly as Vapam HL in three amounts of water and in combination with InLine and CP EC. Strawberry growth, fruit yields, disease pressure, and weed biomass were compared to an untreated control and shank injection with MB:CP mixture (67:33) at 425 kg·ha–1. For soils high in pathogen populations, fruit yield from the untreated plots was 34% to 50% relative to the MB:CP treatment. The greatest (95% to 110%) yields relative to MB:CP were in the high rates of the InLine treatments. Yields from simultaneous drip fumigation with a combination of Vapam HL and InLine or CP EC were less (67% to 79%) than yields from shank fumigation with MB:CP due to 1,3-D and CP hydrolysis reactions with Vapam HL or the generated MITC in the irrigation water that reduced the effi cacy of these combinations to control soilborne pathogens. Application of reduced rates of InLine or CP EC followed 6 days later with reduced rates of Vapam HL controlled soil borne pathogens and weeds and produced the greatest fruit yield relative to all treatments. Chemical names used: 1,3-dichloropropene (1,3-D); methyl bromide (MB); trichloronitromethane (chloropicrin, CP); sodium methyldithiocarbamate (metam sodium); methyl isothiocyanate (MITC). resistance (Wilhelm and Paulus, 1980). All of the current California strawberry cultivars are highly susceptible to verticillium wilt (Shaw et al., 1996). The available chemical alternatives to MB are CP, 1,3-dichloropropene (1,3-D), and metam sodium (sodium methyldithiocarbamate). None of these fumigants alone controls soilborne pathogens and weeds to the same degree as MB:CP mixtures. Metam sodium, a methyl isothiocyanate (MITC) generator, has activity against weeds, nematodes, and some fungi (Baines et al., 1957; Csinos et al., 1997; Jaworski et al., 1978; Johnson et al., 1979; McCarter et al., 1976). For the past few years, metam sodium was ranked fi rst among the most heavily used fumigants in California (California EPA, 2001). Chloropicrin has a high activity against insects and fungi, but has less activity against nematodes and weeds than MB (Johnson and Feldmesser, 1987; Johnson et al., 1979; Wilhelm and Paulus, 1980). The fumigant 1,3-D has a high activity against nematodes and some soil arthropods, but a low to fair activity against fungi and weeds (Noling and Becker, 1994). Until 1990, 1,3-D was the most commonly applied soil fumigant in California at an annual rate of more than 7 million kg (McKenry, 1999). The fumigant 1,3-D, singly (Telone II) or in combination with chloropicrin (Telone C17 and Telone C35), has been used as a preplant soil treatment to protect a variety of high value vegetable crops and for replanting fruit and citrus trees (Lembright, 1990; Locascio et al., 1997). Methyl bromide is typically applied to soil by injection through hollow shanks that are pulled through the soil either at shallow depths (20 to 30 cm) followed by plastic mulch fi lm application, or at deep depths (>45 cm) followed by surface soil compaction. Methyl bromide has an extremely low boiling point (4 °C) and high vapor pressure (1420 mm Hg at 20 °C) that plays a predominant role in its penetration throughout the soil profi le and in controlling soilborne pathogens and pests. Although alternative soil fumigants such as CP or mixtures of 1,3-D and CP can be applied by shank injection (Locascio et al., 1997), their volatilization and distribution in soil may be limited by their lower vapor pressure and higher boiling point compared to MB (EXTOXNET–CP, 2001; EXTOXNET–MB 2001; Lakes Environmental Software, 2000). The registered alternative fumigants (metam sodium, chloropicrin, and 1,3-D) have much higher boiling points (105 to 118 °C) and lower vapor pressure (21 to 34 mm Hg at 20 °C) than MB. Therefore, the effi cacy of alternative fumigants to control soil pathogens and weeds is infl uenced largely by the delivery method, soil type and conditions, and meteorological conditions (Ajwa et al., 2002; Ben-Yephet and Frank, 1985; Lembright, 1990; McGovern et al., 1998). For example, metam sodium applied by shank injection was found to move only a short distance from the points of injection, resulting in inadequate lateral and downward distribution for effective pathogen control (Smelt et al., 1974). Applications of metam sodium generally have been more effective California produces >80% of total U.S. strawberry (Fragaria ×ananassa Duchesne) with 770,000 Mg of strawberries produced annually, valued at $1 billion (USDA, 2003). The California Strawberry Commission (2003) estimated that more than 11,430 hectares were planted to strawberry in 2003. Preplant soil fumigation with MB in combination with chloropicrin (CP), plastic mulching, integrated pest management, drip irrigation, and fertigation have played important roles in the stability of strawberry production in California. Over 95% of the strawberry land is fumigated with MB:CP mixtures (MB plus 2% to 45% CP) at 300 to 450 kg·ha (California EPA, 2002). The strawberry industry has favored the use of MB in combination with CP because of the synergistic effects of these two chemicals to control verticillium wilt (Verticillium dahliae Kleb.) and other soilborne pathogens and weeds (Wilhelm et al., 1961; Wilhelm and Paulus, 1980; Wilhelm and Storkan, 1990; Wilhelm and Westerlund, 1994). Methyl bromide (both natural and manmade) contributes to the depletion of the ozone layer in the stratosphere (Watson et al., 1992). In accord with the Montreal Protocol, the import and manufacture of MB in the U.S. and other developed countries will be banned by 2005, after stepwise reductions in 1999, 2001, and 2003 (USEPA, 1993; UNEP, 1997). The loss of MB as a soil fumigant will greatly impact the strawberry industry. Modern high yielding varieties were selected in fumigated soils for fruit quality, transport, and time of production characteristics rather than for disease HORTSCIENCE 39(7):1707–1715. 2004. Received for publication 8 Dec. 2003. Accepted for publication 26 Jan. 2004. The authors thank Steve Fennimore for weed control evaluation and Frank Martin, John Duniway, and Krishna Subbarao for their help with soilborne pathogen evaluation. Thanks to Frank Westerlund and Christopher Winterbottom, California Strawberry Commission, for their assistance in conducting this research. This project was funded by the USDA–ARS, Water Management Research Laboratory, Parlier, Calif. Mention of trade names or proprietary products is for the convenience of the reader only, and does not constitute endorsement or preferential treatment by USDA–ARS or the Univ. of California. Corresponding author; e-mail haajwa@ucdavis. edu. 154-Soil.indd 1707 10/14/04 5:04:51 PM HORTSCIENCE VOL. 39(7) DECEMBER 2004 1708 when applied with a large quantity of water (Baines et al., 1957; Noling and Becker, 1994; Roberts et al., 1988; Smelt et al., 1974). Raised beds covered with plastic mulch and drip irrigation systems are used in strawberry culture by most growers in California (Kasperbauer, 2000; McNiesh et al., 1985). Strawberry beds are irrigated with one or two drip tapes (collapsible, thin-wall polyethylene tubing) placed a few centimeters below the soil surface (Trout and Ajwa, 1999). Growers use drip tapes with low (0.7 L·h) or high (1.6 L·h) fl ow emitters spaced 20 or 30 cm apart. Emulsifi ed formulations of alternative fumigants, singly or in combination, can be applied at preplant with irrigation water through these irrigation systems (Ajwa et al., 2002). An advantage of drip fumigation is that a more uniform distribution of chemicals can be achieved by spreading them in the liquid phase (Ajwa et al., 2002; Gan et al., 1998). Application of soluble formulations through drip irrigation systems may be economical and more environmentally-friendly, reduce worker exposure, and may reduce the amount of chemicals required. However, the amount of irrigation water used to deliver the fumigant, soil hydraulic properties, and fumigant application rate determine the success of drip-applied fumigants in controlling soil pathogens and weeds (Ajwa et. al, 2002). The objectives of this study were to 1) evaluate strawberry growth and yield, disease pressure, and weed control in response to preplant application of metam sodium (Vapam HL formulation), InLine, and CP EC through drip irrigation systems; and 2) determine the optimum amount of irrigation water needed for drip fumigation of raised soil beds. Variables evaluated were fumigant application rate (maximum label rate and 60% of that rate), application of combination of fumigants, and amount of water used to apply the fumigants (26, 43, and 61 L·m). Strawberry yields from drip applied treatments were compared to yields from standard MB:CP and Telone C35 shank injection treatments and nonfumigated plots. Materials and Methods Site description and fi eld preparation. The research was conducted in two major commercial strawberry production areas in California (Watsonville and Salinas) for three consecutive growing seasons between September 1997 and August 2000; nearly one-half of California strawberry production is located in the central coastal area, around Watsonville and Salinas (California Strawberry Commission, 2003). The soil in both locations had not been fumigated for more than 15 years prior to these studies. The soil in Salinas was classifi ed as Chualar loam (Fine-loamy, mixed, thermic, Typic Argixerolls) with a pH of 6.5 and organic matter content of 0.7%. The soil in Watsonville was classifi ed as an Elder sandy loam (Coarse-loamy, mixed, thermic, Cumulic Haploxerolls) with a pH of 6.1 and organic matter content of 0.6%. The nonfumigated soil in Watsonville was heavily infested with Verticillium dahliae Kleb. and Pythium spp., but weed pressure was medium to low. The pathogen infestation in Salinas was medium (mainly black root rot pathogens such as Pythium ultimum and binucleate Rhizoctonia spp.), but weed pressure was very high. In both locations, phytoparasitic nematodes were not detected in the soil. At both locations, the soil was cultivated and beds (76 cm wide, 132 cm center to center, 30 cm high) were formed every year following commercial cultural practices. Slow release fertilizer (27N–10P–12K) was band applied to beds at 400 kg·ha, and the drip irrigation system was installed and tested. At the time of fumigant application, the average daily soil temperature within the raised soil bed ranged between 16 and 19 °C in Watsonville and between 18 and 20 °C in Salinas. The 3-year average gravimetric soil water percentage (determined before fumigation) within 30 cm depth was 12% ± 2% in Watsonville and 10% ± 2% in Salinas. Fumigant and treatment description. The fumigants used in this study were commercial grade formulations. Metam sodium (Vapam HL formulation, 42% sodium methyldithiocarbamate) was provided by AMVAC (Newport Beach, Calif.). Chloropicrin (trichloronitromethane) (CP, 99%) and an emulsifi ed concentrate (EC) formulation (CP, 96%) were provided by Niklor Chemical Co. (Long Beach, Calif.). Telone C35, a mixture of 1,3-D and CP (61% 1,3-D and 35% CP) and an emulsifi ed formulation of Telone C35 (InLine, 58% 1,3-D and 33% CP) were provided by Dow AgroSciences (Redeck, N.C.). Methyl bromide-CP (67% MB:33% CP) formulation was provided by Tri-Cal Inc. (Hollister, Calif.). Fumigant treatments (Table 1) were applied to the same beds each year within the last week of September or the fi rst week of October, at least 4 weeks before planting. Fewer treatments were possible in Watsonville than in Salinas due to limited space. In addition to the untreated control, 12 treatments in Watsonville and 16 treatments in Salinas were randomized in a complete-block design with four replications in Watsonville (10-m-long beds) and six replications in Salinas (33 m long beds). At both locations, MB:CP (67:33) at 365 to 425 kg·ha and Telone C35 at 453 kg·ha (374 L·ha) were shank injected at 25 to 30 cm depth with two chisels spaced 35 cm apart into soil beds that were immediately covered with green, embossed polyethylene mulch (0.03 mm thickness). Vapam HL, InLine, and CP EC were applied into soil beds covered with polyethylene mulch through two drip tapes (Netafi m Streamline 60, Netafi m, Fresno, Calif.), with emitters spaced 30 cm apart and emitter fl ow Table 1. Fumigant application rate and method. Irrigation Rate water Treatment (L·ha) Method (L·m) Concn in water (mg·L) 1) MB:CP (67:33) 365-425 kg·ha Shank ---2) Telone C35 374 kg·ha Shank ---3) InLine 393 Drip 26 1060 mg 1,3-D and 603 mg CP 4) InLine 393 Drip 43 641 mg 1,3-D and 365 mg CP 5) InLine 393 Drip 61 452 mg 1,3-D and 257 mg CP 6) InLine 236 Drip 43 385 mg 1,3-D and 219 mg CP 7) InLine/4 lines 236 Drip 43 385 mg 1,3-D and 219 mg CP 8) InLine/preirrig 236 Drip 43 385 mg 1,3-D and 219 mg CP 9) InLine+Vapam 236+420 Drip 43 Combination 10) InLine+Vapam 393+700 Drip 61 Combination 11) Vapam 700 Drip 43 772 mg metam sodium 12) Vapam 700 Drip 61 570 mg metam sodium 13) Vapam 420 Drip 43 408 mg metam sodium 14) Vapam 700 Drip 26 1357 mg metam sodium 15) CP EC+Vapam 130+420 Drip 43 Combination 16) CP EC 130 or 200 Drip 43 365–560 mg CP 17) Untreated control 0 ------MB:CP (67:33) was injected at 425 and 365 kg·ha-1 in Watsonville and Salinas, respectively. InLine (60% 1,3-D and 32% CP) was drip applied at equivalent rate to shank injected Telone C35 (62% 1,3-D and 30% CP). Vapam HL formulation contained 42% metam sodium. Chloropicrin EC (treatment 16) was applied at 130 and 200 L·ha in year 1 and year 2, respectively. Except for treatment 7, all drip treatments were applied through two drip tubes per bed. Chemical rate and volume of irrigation water are presented per bed treated area (58% of total land area). Treatment 9 is a combination of treatments 6 and 13, treatment 10 is a combination of treatments 5 and 12, and treatment 15 is a combination of treatments 13 and 16. The combination treatments were applied simultaneously in year 1 and year 2, but sequentially in year 3 when Vapam HL was applied 6 d after InLine or CP EC. 154-Soil.indd 1708 10/14/04 5:04:53 PM 1709 HORTSCIENCE VOL. 39(7) DECEMBER 2004 rate of 0.87 L·h at 69 kPa. The drip tapes were placed 8 cm (in Watsonville) or 13 cm (in Salinas) from the bed center and buried at a soil depth ranging from 2 to 5 cm. Variables evaluated (Table 1) were application rate (labeled commercial rate and 60% of that rate), application of combination of fumigants, and amount of water used to apply the fumigants (26, 43, and 61 L·m of bed area). In year 1 (1997–98 growing season) and year 2 (1998–99 growing season), the combination treatments (InLine plus metam sodium or CP plus metam sodium) were injected simultaneously into the irrigation system. In year 3 (1999–2000 growing season), InLine and CP EC were applied in 43 L·m of irrigation water, but Vapam HL was applied 6 d later in an additional 43 L·m of water. Fumigants were injected throughout the water application period into the drip irrigation system from either a nitrogen-pressurized cylinder containing the fumigant or with a positive Fig. 1. The concentration of 1,3-dichloropropene (mg·L air) in the gaseous phase of the Watsonville sandy loam soil (top) and the Salinas loam soil (Bottom) 24 h after application of InLine at 393 L·ha (58% 1,3-D) through the drip irrigation systems. The concentration of 1,3-D in 26, 43, and 61 mm of irrigation water were 1060, 641, and 452 mg·L water, respectively. Data are pooled across years 1 and 2. 154-Soil.indd 1709 10/14/04 5:04:54 PM HORTSCIENCE VOL. 39(7) DECEMBER 2004 1710 displacement pump equipped with metering valves as described in detail by Ajwa et al. (2002). Water meters were used to determine the actual amount of water applied. Fumigant concentration in the soil profi le. To evaluate the effect of application water volume on the distribution of fumigants in soil, 1,3-D and CP concentrations were measured in the InLine treatments 3, 4, and 5 (393 L·ha applied in 26, 43, and 61 L·m). Monitoring was conducted in years 1 and 2 for one-half of the raised beds following procedures described by Ajwa et al. (2002). In brief, stainless steel soil-air sampling probes (1.0 mm i.d.) were placed 2 cm apart at the various depths (0, 5, 15, 30, 45, and 60 cm) from the soil surface in the center and at the edge (30 cm from the center) of the raised soil bed. Soil-air sampling started immediately following the end of each application and continued for 8 d. Soil-air 1,3-D and CP samples were taken by passing 50 mL of air through XAD-4 resin cartriges (Sigma-Aldrich, Milwaukee, Wis.) using a 50-mL air-tight syringe. The trapped fumigants were extracted from the cartridges with hexane and analyzed using an Agilent (Wilmington, Del.) 6890 series gas chromatograph (GC) system equipped with an Agilent 5973N mass selective detector (MS) and a Zebron ZB-624 column (30-m × 0.25 mm i.d. × 1.4 (m fi lm thickness). Plant vigor and fruit yield. At least four weeks after fumigation, plant holes were cut in the plastic mulch at a 30-cm spacing and strawberry (Fragaria ×ananassa Duchesne) cultivar ‘Selva’ was planted. When this research was initiated, ‘Selva’ was the dominant day neutral cultivar in California (personal communication, Frank Westerlund, California Strawberry Commission, Watsonville, California). Before planting, 25 L·m of water were applied through the drip irrigation systems to wash away any residual fumigants or breakdown products that could be present in the planting zone. After transplanting, overhead sprinklers were used for a few weeks before initiating drip irrigation. Fruit was harvested at least once every week (twice per week in Salinas) throughout the production season (early April to September) and graded into marketable (fresh market grade) and nonmarketable (culls) fruits. Plant diameter was determined for the center 40 living plants in each bed by taking two measurements (north–south and east–west) of the above ground foliage from each plant about 5 months after planting and before overlapping of foliage. Fruit size was estimated by weighing 20 randomly selected marketable fruits. The percentage of diseased plants (wilting and dead) was recorded from all plants in the bed fi ve and eight months after planting. Diseased plants were randomly selected and the surrounding soil was sampled to identify the dominant plant pathogens. Soil and plant tissue samples were assayed for Verticillium dahliae Kleb. as described by Nicot and Rouse (1987), and for Pythium spp. as described by Martin (1992). Weed control by the various treatments was assessed in February and April of years 2 and 3. Weeds were uprooted and shaken to remove residual soil from the roots, and weighed to measure fresh weed biomass. Statistical analysis. The general linear model (GLM) procedure and Fisher least signifi cant difference (LSD) at P = 0.05 tests were performed using the Statistical Analysis System (SAS) program (SAS Institute, Cary, N.C.) to determine signifi cant differences among treatments for strawberry marketable and total yields, plant diameter, disease incidence, and weed biomass. Statistical analyses were performed on the original data without transformation. SAS was also used to determine the existence of a signifi cant relationship between average fruit weight and marketable yield.