Role of Soil Fumigants in Florida Agriculture - American Chemical

Chapter 2

Role of Soil Fumigants in Florida Agriculture

Downloaded by PENNSYLVANIA STATE UNIV on September 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1997-0652.ch002

J. W. Noling Institute of Food and Agricultural Sciences, Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850

Historically, preplant soil fumigants have had a very global, profound, and stabilizing influence on production agriculture, and catalyzed the development of several high value, multiple cropping systems, In some cases, fumigants have been adapted almost to the exclusion of all other soil pest management strategies because of their superior broad-spectrum control efficacy and consistent enhancement of crop growth, development, yield, and quality. In general, soil fumigation has allowed growers to repeatedly use the same fields for crop production each year, 2) to specialize in a few crops and integrate crop production cycles with market requirements, 3) to take increased advantage of financial investments in property and land improvements, and 4) to minimize capital investments in farm machinery and labor requirements. Certain agricultural industries will be adversely affected by the removal of soil fumigants from commercial use unless new, economically and environmentally acceptable integrated pest management (IPM) strategies are developed and implemented.

Prior to 1950, Florida vegetable culture can best be described as nomadic. One or two vegetable crops were produced in sequence on rented land after expensive clearing operations had been performed or after long pasture rotations to minimize soil borne pest and disease problems (7,2). For example, tomato farmers found it profitable to clear, ditch, dike, install pumps and wells and construct graded roads to virgin land each season in order to escape soil borne pest and disease problems on previously cropped soils (3). Once a problem developed, Florida truck farmers (as they were called at the time) were then forced to migrate from one field or area to another, opening new land and abandoning the old to avoid the crop pests which, perforce, became more severe with re-use of the same fields. As urban growth

0097-6156/96/0652-0014$15.00/0 © 1996 American Chemical Society

In Fumigants; Seiber, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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increased, suitable land became more difficult to locate as well as prohibitively expensive, both in terms of purchase or leasing, and land preparation (2,4). Due to these constraints, Florida vegetable farmers increasingly adopted the use of soil fumigants to managed established weed, nematode, and disease pests within their fields(5-72). Reverting back to such a system would no longer be considered a viable option due to widespread urbanization and environmental and water management regulatory policies (13,14).


Historical Development and Use of Fumigants Discovered in 1869, the first fumigant material to become available for agricultural use in Florida was carbon bisulfide (75). Because of its flammability, high rates of application, cost, and pest control inconsistency, carbon disulfide was seldom recommended or extensively used within Florida (15-17). Only recently, has interest been renewed for the use of carbon bisulfide (18). In 1920, Russell (19) reported excellent control of the root-knot nematode with chloropicrin which was later confirmed by other workers (16,20). It was demonstrated that treating soil with chloropicrin cured the problems of "soil sickness" and restored the soil to a higher productivity than could be obtained from years of crop rotation or fallow. In the United States, the war surplus of chlorpicrin was shipped to Hawaii, where beginning in 1927, it was applied as a preplant fumigant to pineapple (6,15). Its use in Hawaii continued until the supply was exhausted, after which very little chloropicrin was used for agricultural purposes. In 1935, Neller and Allision (27) discussed the use of a machine for subsurface treatments of nematode infested soil with chloropicrin and carbon bisulfide. After considerable delay since its discovery, chloropicrin was commercially introduced as a soil fumigant to Florida in 1937 (22). It wasfirstused on a limited basis by nurserymen and greenhouse operators as a substitute for soil steaming. In 1941, Taylor and McBeth (23) proposed the spot treatment method of fumigation wherein chloropicrin was used to fumigate small seedbed areas within fields. Because of the high material cost, very little development of large scale application equipment, other than hand gun type injectors, had been undertaken in Florida (24). It was not until D-D (1,3-dichloropropene, 1,2,-dichloropropane and related C3 hydrocarbons) was developed in 1943 (25) and repeatedly demonstrated to be an effective nematicide, was any concentrated effort made on developing specialized field application equipment. By 1945, Shell Chemical Corporation had developed a tractor drawn cart with applicator, shanks, pumps, and tank for soil fumigation and offered thefirstcustom application service. As a result, D-D, the first really effective and inexpensive nematicide for general field use, was commercially introduced into Florida in 1945 (22,26). In 1945, Christie (27) reported that ethylene dibromide (EDB) gave excellent control of root-knot nematode (Meloidogyne spp.) in preliminary tests. The Dow



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FUMIGANTS: ENVIRONMENTAL FATE, EXPOSURE, AND ANALYSIS

Chemical Company was also testing EDB and soon introduced it as a low cost fumigant under the trade name Dowfume W. Other companies were soon marketing EDB under various trade names. Within a few years of 1945, use of D-D and EDB increased significantly because of the oftentimes 'astonishing' improvements in plant growth and yield which resulted following their use (22,26). The most significant outcome from these early soil fumigation trials was the demonstration of the very great importance of plant-parasitic nematodes in regulating crop productivity (28). Because use of these soil fumigants made the difference between an excellent yield and no yield, many growers began adopting soil fumigation as a general practice (1,7,12,22,29). Methyl bromide was first reported to be effective as a soil fumigant in 1940 (30) and 1942 (16). As a soil fumigant, it was applied under a gas tight cover (57), with the gas introduced by special applicator into the space between the cover and soil surface (raised tarp method). At the time however, the use of methyl bromide was only considered warranted for small plots such as greenhouse beds or benches in which it was identified as a replacement for steam sterilization. In 1953, Wilhelm et al. (32) reported the synergistic effect of methyl bromide-chloropicrin mixtures on control of Verticillium wilt of strawberries. Extensive field use of methyl bromide in Florida would not come until some 10 years later. Much of the experimental soil fumigation work in Florida dates back to the spring of 1945 (33). In these studies, chloropicrin and D-D significantly increased crop yields while seedbed studies showed that plots fumigated with methyl bromide yielded significantly more plants suitable for transplanting than nonfumigated plots. The specialized equipment required for field application was expensive, and because of its high cost, was only used by large Florida growers. By 1949 soil fumigation with D-D and EDB had expanded to such an extent in many areas of the west and southeast, that the first mass production of field application equipment was undertaken (24). At the time however, it was also apparent from field observations, that plant stunting and inconsistent responses in crop growth and yield could be attributed to physical and edaphic factors as well as to the detrimental impacts of some of these fumigants on soil nitrifying organisms (33). Soil fumigation efficacy was shown to be affected by physical soil factors such as soil texture, pH, organic matter content, temperature, and moisture (34-37), formulation (9) and application methods (11,15,16,38-40). It was also shown that use of EBD did not interfere with conversion of complex organic and ammoniacal forms of nitrogen, but methyl bromide did significantly depress nitrification (41). It was soon discovered however, that the low nitrate-nitrogen values and high ammoniacal nitrogen levels following soil fumigation could be alleviated without phytotoxic plant responses if nitrate forms of fertilizer were used (2). It was also common knowledge within these early years of fumigant use, that no fumigant, regardless of method or rate of application, could be effective in preventing the development of root-knot nematode on a susceptible crop that



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remained in the field for an extended period, i.e. 3 months (33). The general rule was: The longer the life of the plant, the greater the need for protection against infestation (42). Because of these shortcomings, and even though extensively used in some cases, none of the fumigants were vigorous encouraged by cooperative extension personnel for field use except as a last resort (33). Fumigation for vegetable production began in earnest during the early 1950's with in-furrow applications of nematicides to control root-knot nematode and other nematodes which, as indicated, quickly became an economic problem in re-cropped fields. In 1954, DBCP (l,2-dibromo-3-chloropropane; Nemagon) was determined to be a highly effective nematicide (43). Sodium methyldithiocarbamate (Vapam) was introduced circa 1956 (44) and methyl isothiocyanate mixed with D-D and released as DD-MENCS (Vorlex) about 1960 (45). This in-the-row, open bed (without plastic mulch covering) soil fumigation practice was at the time superior to no treatment but proved to be inadequate for protecting late maturing, long-term crops or for crops grown in fields heavily infested with disease organisms (22). Because of this, these early broad spectrum fumigants were used primarily in seedbed production until the appearance of vascular wilt diseases occurred (IE. Fusarium wilt race 2 & 3) in the sandy soils of the vegetable producing areas of the state. As of 1953, the majority of Florida tomatoes were still grown on newly cleared sandy land, the tomato grower cooperating "with the cattleman to the benefit of both" (42). The list of yield limiting soil-borne tomato diseases which became important after 1 or 2 crop cycles included Fusarium wilt, Rhizoctonia, Southern blight, black-spot, early blight, buckeye-rot, bacterial wilt, root-knot, and other diseases due to nematodes. At the time and still relevant today is the fact that there are no commercially available crop cultivars resistant to all races or variants of these diseases. By 1960, the scarcity of 'new land' for vegetable crop production had encouraged growers to returned to fields converted in the past to pastures when diseases, weeds, and nematodes complicated vegetable production (46). Broadcast soil fumigation had been too expensive on a field scale to be practical for wide row crops such as tomatoes (41). However, in-the-row soil fumigation (29), as practiced since 1949 on short-term crops for nematode control, had proved inadequate for the long-term trellised tomatoes gown for pink harvest and for crops grown in fields heavily infested with disease organisms (41). In 1962, Geraldson (2) reported that a plastic mulch bed covering proved effective for weed control, significantly reduced fertilizer leaching losses, and restricted soil moisture fluctuations in vegetable crops. This study was the first to justify the benefits with the high cost of mulch and the cost of removal. By the mid 1960's, soil fumigation had become a common practice for controlling nematodes in some crops. For example, the use of nematicides in strawberry production was such an established practice in Florida that 97 percent of strawberry acreage was treated with a soil fumigant in 1964 (47). Within the previous seven years (1957-64), the use of polyethylene mulch and herbicides in



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conjunction with the pre-plant fumigants had resulted in a four-fold increase in average strawberry yields (48). However, even at this time, these growing practices (use of these fumigants) were not considered sufficient for control of damping-off caused by Rhizoctonia solani. It was for this specific shortcoming that continued research with new fumigants such as methyl bromide in combination with chloropicrin were initiated (47). In 1965 an integrated systems approach to soil pest management for sandy soils was introduced to the Florida vegetable industry to solve the "old land" problems which developed in repeatedly cultivated fields. This new systems approach to pest management allowed and encouraged vegetable growers to use the same fields for vegetable crop production each year, taking advantage of their financial investment in drainage and irrigation systems and site location relative to climate. The system which was advocated combined broad spectrum soil fumigation using methyl bromide and chloropicrin with full-bed plastic film mulch, and a constant water table utilizing seep irrigation to provide a minimum stress environment in the root zone of the crop. By 1977, sixty-four percent of the tomato acreage in peninsular Florida was fumigated with formulations of methyl bromidechloropicrin, a 15% increase over the preceding 4 year period (/). Currently, soil fumigation with methyl bromide chloropicrin for tomatoes occurring on effectively 100% of the acreage, begins in early July and continues through February of each year. Two formulations, 98% methyl bromide and 2% chloropicrin and 67% methyl bromide and 33% chloropicrin, are most extensively used with in-row application rates of 168-224 kg/ha. It is delivered under a full-bed plastic mulch (i.e., a 75-80 cm wide raised bed covered the entire width with 0.0025 cm thick polyethylene plastic film). Although single stream fumigation resulted in an increase in yield (38) it has been the practice, especially with the development of Fusarium wilt, incited by Fusarium oxvsporum Schl. f.sp. lycopersici in Florida to use multiple streams, injected with 3 chisels 20-25 cm apart and 15-20 cm deep to protect the tomato cropfromsoil-borne pathogens, nematodes, and weeds. Fertilizers are placed on the top or shoulders of the bed. Subsequent development of disease resistant cultivars and containerized transplants has further refined the integrated system for added crop protection from soil pests. Development and use of the polyethylene mulch, high analysis soluble fertilizers, seep irrigation, and methyl bromide chloropicrin soil fumigation has allowed and assured high quality vegetable crop yields on lands of low natural fertility infested with nematodes, soil-borne disease organisms and numerous weed pests. Protection of plants from moisture and nutrient stress, competition by weeds and the root pruning associated with cultivation apparently contributed to increased tolerance of the tomato plants to root-knot nematode infection (49) and depressed expression of symptoms of southern blight of tomato (50). Polyethylene mulch has become an integral component of the system and has been advocated for numerous reasons: 1) to preserve and protect raised soil beds against erosion, 2) to retard leaching of nutrients, 3) to retard soil evaporation and to maintain consistent and



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nearly optimum soil moisture, 4) to improve the efficacy of in-the-row fumigation with methyl bromide/ chloropicrin, 5) to retard re-infestation of fumigated soil, 6) to prolong weed control, 7) to reduce damage of crop plant roots associated with cultivation, and combined with the high water table, 8) to limit plant roots to the fumigated soil volume which protects the plant longer from root-infesting disease organisms (49). Since 1960, many different chemicals capable of killing some of the specific pests attacking vegetables in Florida have been field evaluated. Six soil fumigant have been used at one time or another for tomatoes: including methyl bromidechloropicrin; DD-MENCS [chlorinated C3 hydrocarbons (80% D-D) + 20% methyl isothiocyanate] (Vorlex); sodium methyl dithiocarbamate (Vapam); 1,3-D (1,3dichloropropenes (D-D, Telone II); EDB (ethylene dibromide) (Dowfume, Soilfume, W85); andDBCP (l,2-dibromo-3-chloropropane (Nemagon, Fumazone). In general methyl bromide chloropicrin (67/33) and DD-MENCS shared the major role in fumigation of about 70% of the tomato fields. During the period of 1974 to 1978, use of methyl bromide and chloropicrin increased from 30-43% of fields, while DDMENCS decreased from 39 to 30% (7). Of these fumigants, some have shown promise for certain pests (primarily nematode) under Florida conditions (51-54), but none have proven to be the ideal material, that is, one which possesses herbicidal, fungicidal, and nematicidal properties and, at the same time, did not leave toxic residues in the soil for an inconvenient length of time or pose potentially serious environmental risks to surface or groundwater supplies. For example, in May 1979, DBCP was discovered in groundwater in California, and shortly thereafter withdrawn from most of the global marketplace (75). In 1983, EDB was similarly banned for environmental contamination. The benefits of methyl bromide soil fumigation have not been confined exclusively to soilborne pest and disease control within Florida vegetable cropping systems. For example, a 38% corn and grain sorghum yield reduction which occurred during the period of 1977-1981 was reversed by soil fumigation with methyl bromide-chloropicrin (55). In some cases the large increase in crop yields associated with methyl bromide fumigation is attributed to weed control (56). None of the nonfumigant nematicides have proven to be consistently effective against root-knot nematodes as the fumigant type nematicides such as methyl bromide, while some have actually enhanced yellow nutsedge problems (54). Many of the nonfumigant nematicides have also been identified as chemical which are mobile in soil and have been found to contaminate groundwater as a result of agricultural use (6). Role of Nematodes in Florida Crop Production The primary nematode parasites of vegetable crops grown throughout the state of Florida include the root-knot nematodes (Meloidogvne spp.) and the sting nematode



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(Belonolaimus longicaudatus). either of which may cause extensive root damage and yield loss. Other species of plant-parasitic nematodes are found in fine sandy and calcareous Rockdale soil in the southeastern part of the state. The root-knot nematode is a serious and ubiquitous pest of vegetables on both soil types. On sandy soils, sting, awl and stubby root nematode can affect vegetable crops but these species are rare or unknown on Rockdale soil. On the heavier calcareous Rockdale soils of southeastern Florida the reniform nematode is common and known to damage some vegetable crops (54). With the exception of south Florida and small areas of muck land vegetable production, Florida vegetables are grown on fine sandy soils (95-97% sand, 3-4% silt, 0.2-0.5% clay) with low soil organic matter content (< 2%). In south Florida, vegetable soils are shallow, underlain by limestone, and contain 33-67% small limestone rock and gravel, mixed with fine sand or fine sandy loam soil. Much research data and field observations suggest that root-knot nematode causes greater damage on light sandy soils than on the heavier sandy loam or clay soils. Poor water and nutrient retention on the sandier soil undoubtedly contributes to poorer crop growth on the sandier soils. Due to the extensive use of methyl bromide chlropicrin formulations as a broadspectrum fumigant nematicide in Florida, nematode induced crop loss is probably less than 1% of total production. Plant damage, when it is observed, generally occurs in areas where fumigants are not applied. These areas often are ends of rows where fumigants delivery is discontinued prematurely or in areas where exhausted fumigant cylinders are changed. Soil fumigants are also not generally applied in small farming operations of less than 5 acres where nematode damage is consequently often severe. Small farm (

Role of Soil Fumigants in Florida Agriculture - American Chemical

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