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Solarization for Controlling Soilborne Pests and Pathogens in Field Crop Cultivation
Solarization is a method in which clear plastic is laid on the soil surface to trap solar radiation and heat the soil. Solarization as a preplant soil treatment to control soilborne pathogens and pests can be a viable alternative to methyl bromide for shallow-rooted, short-season crops (Katan and DeVay 1991, Stapleton 1996). Solarization is a hydrothermal process that can be used in moist soils covered with clear plastic tarps and exposed to direct sunlight in tropical climates or during warm summer months in more temperate regions. Solarization traps solar radiation, and thereby heat, in the soil in order to raise temperatures sufficiently to suppress or eliminate soil-borne pests and pathogens (Katan 1981 and 1991). It can be effective against a broad spectrum of soil diseases, fungi, weeds, nematodes, insect pests and most soilborne bacteria. Solarization also causes complex changes in the biological, physical, and chemical properties of the soil that improve plant development, growth, quality, and yield for up to several years (Stapleton 1994, Katan 1981 and 1991, DeVay et al. 1990). In areas with a suitable climate, solarization can be used alone, or in combination with lethal or sublethal fumigation or biological control, to provide an effective substitute to methyl bromide (Hartz et al. 1993).
In addition to disinfecting the soil while reducing or eliminating the need for fumigants, solarization leaves no toxic residues, increases the levels of available mineral nutrients in soils by breaking down soluble organic matter and making it more bioavailable, changes the soil microflora to favor beneficial organisms, conserves water, and can serve as a mulch when maintained as a row cover during the growing season (Stapleton 1994, Katan and DeVay 1991). However, solarization appears only to be effective in warm climates and requires that cultivated land be left fallow for short periods of time (Katan and DeVay 1991).
Research
Solarization alone, or in combination with other pest control technologies, could be adopted as a preplant soil pest control measure for a variety of different pathogens and pests in a wide range of climates and different cropping systems. Since its inception in 1976, soil solarization has been tested and modified under local conditions in more than 50 countries, including the United States (Florida, California, North Carolina and other states), Israel, Greece, Morocco, and Japan, for the control of nematodes, weeds, and disease organisms affecting a variety of vegetable and fruit crops (Hartz et al. 1993, Katan 1984 and 1981, Ristaino et al. 1996 and 1991, Stapleton 1994, Wu 1996). Studies have shown that 1) solarization reduces or eliminates pathogens and pests prior to planting, 2) crop yields can be significantly increased, and 3) the effects of solarization can extend through several growing seasons (Afek et al. 1991). Solarization has already proven to be an effective pest control tool for tomato, pepper, and eggplant production in the northern part of Florida and North Carolina, strawberry and lettuce production in California (Gamliel and Stapleton 1993, Hartz et al. 1993, Ristaino et al. 1991), tree nursery production in the southeastern U.S., and orchard crops in California (Chellemi 1994, DeVay et al. 1990, Littke 1994a, Littke 1994b, Stapleton 1994). Solarization also has proven effective in controlling pests (pathogens) for a variety of other crops including pistachios, almonds, carrots, garlic, peanuts, potatoes, watermelon, onions, artichokes, and beans (Katan and DeVay 1991). It must be noted that the effectiveness of this pest control tool is directly linked to climate - that is, the amount of sunlight received during the solarization process. In addition, as with other pest control tools, the effectiveness of solarization is related to how well it is applied and the experience of the persons involved in the process.
Because of its passive nature, solarization alone is limited to growing areas in primarily hot, generally cloudless climates. However, it has also shown considerable promise for vegetable production in more humid areas, such as the southeastern United States where average soil temperatures at 5 cm depth are 50 C in solarized plots and 36 C in bare ground. Variations of the principle have also been used in other locations. For example, in the Rio Grade Valley in Texas and in Northern Florida, transparent film used to solarize fields during the hot summer off-season is then painted white and left in place to serve as a mulch for the fall melon and tomato crops. In the Jordan Valley in Israel, black plastic has been used to solarize soil during the extremely hot summer months, and is then left in place to serve as a mulch for the fall vegetable crop. These modifications represent excellent adaptations of the technique for maximizing cost effectiveness (Stapleton 1996). Additionally, while not currently commercially feasible in most cases, the use of recycled/old plastics has been shown to be more effective at heating the soils than new plastics because the photometric properties of transparent polyethylene sheets change significantly during the aging process (Katan and DeVay 1991).
Solarization Techniques
Generally, a layer of clear plastic film is applied to the soil prior to planting and is left in place for 4 to 6 weeks during the hot season in the appropriate climatic region. Optimal use, however, may require a longer period of time and adjustments in scheduling for other production practices. However, this time period may be reduced by combining solarization with chemical or non-chemical pesticides. Proper soil preparation also is essential to provide a smooth, even surface for the tarp and allow water to penetrate evenly and deeply into the soils (Stapleton 1996). To maintain proper soil moisture, irrigation using sprinklers is typically performed 1 to 4 days prior to applying the plastic tarp. Alternatively, drip irrigation lines can be installed underneath the tarp and utilized as necessary (Katan 1981). Plastics may be applied either in strips (usually 2 feet wide) over the planting beds or as continuous sheeting glued, heat fused, or held in place by soil. Because pathogens that survive heat treatment within or at the periphery of treated soils tend to multiply (a phenomenon known as the "edge or boarder effect"), continuous sheeting is thought to be more effective than strip applications, although it is more expensive (Katan and DeVay 1991, CEUC, 1984). However, soil temperatures under bed solarization in the southeastern United States are higher than temperatures achieved under full-field solarization. In addition, the border-effect associated with the lack of pest suppression along the edges of full-field solarization is eliminated by bed solarization (Chellemi, 1996). Currently the most common film types used for solarization are UV-resistant clear polyethylene or polyvinyl chloride film (Katan and DeVay 1991). Double layers of plastic, which simulate solarization of soil under glasshouse conditions, result in even greater temperature increases in soils (i.e., 3 to 10 C higher that achieved under a single layer of plastic) ( DeVay et al. 1990).
After solarization, the plastic is either removed or left in place to serve as mulch during the growing season (Katan and DeVay 1991). The physical, biological, and chemical changes that occur during solarization may persist for up to 2 years (Katan and DeVay 1991, DeVay et al. 1990). Because soil temperatures are the highest within the uppermost layers, cultivation after solarization should be kept to a minimum to avoid reinfestation from pests deep in the soils. To achieve lethal soil temperatures at greater depths, solarization must be maintained at higher temperatures for an extended period of time (two months or longer) (Katan 1987). However, the use of double layered plastic may reduce time necessary for this procedure to control pests.
Solarization causes physical, chemical, and biological changes in the soil by raising soil temperatures from 2-15 C above the temperatures of untreated soil. For example, temperatures achieved with solarization in Israel during July and August, at levels between 5 and 20 cm below the soil surface were 45-55 C and 39-45 C, respectively. In California, at a depth of 5 cm, the temperature of tarped soil was recorded at 60 C (Katan 1981). In Florida, at depths of 5, 15, and 25 cm, temperatures of 49.5, 46.0, and 41.5, respectively, were recorded in solarized soil (Chellemi 1994) The success of soil solarization is based on the fact that most plant pathogens and pests are mesophilic or unable to survive for long periods at temperatures above 37 C. The heat sensitivity of these organisms is related to an upper limit in the fluidity of cell membranes, which lose their ability to function at high temperatures. Other causes of death of organisms at high temperatures involve the sustained inactivation of enzyme systems, especially respiratory enzymes (DeVay et al. 1990). Pathogens may be killed either directly by the heat or are weakened by sublethal heat to the extent that they are unable to damage crops (DeVay 1996).
Solarization also promotes increases in plant growth and development and crop quality and yield by increasing the availability of plant nutrients and the relative populations of beneficial organisms such as rhizosphere bacteria (Bacillus spp.) and pseudomones species (Ristaino et al. 1991). Heating causes the release of soluble mineral nutrients from soil organic matter and heat killed soil biota and induces the upward movement of mineral nutrients in the soil profile. Reductions in populations of soil borne pathogens also constitute the basis for biological control of plant pathogens and in some cases the development of disease suppressive soils (Katan and DeVay 1991, Stapleton 1996, Ristaino et al. 1991).
The effectiveness of solarization and the heat dosages achieved for disinfesting soil depend on soil moisture and texture; air temperature (maxima, minima, and duration); season; length of day; intensity of sunlight; wind speed and duration; and the type, color, and thickness of the plastic (DeVay et al. 1990, Katan and DeVay 1991). The greater the temperature, the less time is needed to reach a lethal heat dosage. For example, at soil temperatures of 37 C (the lower threshold temperature for lethal and sublethal damage for many mesophilic fungi) exposure may require from 2 to 4 weeks, however at 47 C, 1 to 6 hours of exposure is a lethal dose (Katan and DeVay 1991). Because solarization is a hydrothermal process, its success also depends on moisture for maximum heat transfer to soilborne organisms (Chellemi 1995). However, recently a soil temperature model that predicts temperature under plastic mulch based on above ground meteorological data has been developed (Wu et al. 1996).
Reducing Chemical Usage and Costs
Solarization can be a cost-effective technique for controlling soil-borne pests of fruits, vegetables, nursery, and orchard crops, making it a viable alternative to methyl bromide in many warm climates. This is supported by an economic analysis of 30 single-crop, single-season experiments which suggested that solarization was effective and profitable for numerous shallow-rooted, short season crops (Katan and DeVay 1991). Furthermore, additional benefits of solarization, such as an increased growth response and its long-term effects, strengthen its economic profitability (Katan and DeVay 1991, Stapleton 1994). In addition, solarization can be combined with other chemical, physical, and biological methods (e.g., fertilizers, soil amendments, integrated pest management strategies, limited pesticide use, and biological control agents) for enhanced management of soil and root pests and diseases (DeVay 1996, Katan and DeVay 1991). Cost estimates for solarization compared to methyl bromide fumigation are provided in the Table 1 below. As shown, reduced chemical usage and cost savings can be achieved by using solarization for controlling soil-borne pests. It should be noted however that with strip and bed solarization, costs can be reduced as the application techniques are virtually identical to the standard polyethylene mulch culture without the additional labor costs listed here for solarization (Chellemi, 1996).
| Cost Factor | Solarization | Methyl Bromide |
|---|---|---|
| Tarp | 175-180 | 225-300 |
| Labor | 100-160 | 50-80 |
| Other | 25 - 100 (post-treatment white paint or tarp removal) | 200-350 (fumigant costs) |
| Total | 300-440 | 475-730 (broadcast up to $1,500) |
Source: Chellemi 1995, DeVay 1996, Olson 1996, Hartz 1996, Katan and DeVay 1991.
References
- Afek et al. 1991. "Interaction among mycorrhizzae, soil solarization, metalaxyl, and plants in the field." U. Afek, J.A. Menge, and E.L.V Johnson. Plant Disease. The American Phytopathological Society. Volume 75, No. 7, pp. 665-672.
- CEUC 1984. Soil solarization: a nonchemical method for controlling diseases and pests. Cooperative Extension University of California. Division of Agriculture and Natural Resources. Leaflet #21377.
- Chellemi, D.O., 1996 (November). Personal Communication. Dr. D. Chellemi. University of Florida. Gainesville, FL.
- Chellemi, D.O., 1995 (April). Personal Communication. Dr. D. Chellemi. University of Florida. Gainesville, FL.
- Chellemi, D.O., Olson, S.M., and Mitchell, D.J. 1994. Effects of soil solarization and fumigation on survival of soilborne pathogens of tomato in northern Florida. Plant Dis. 78:1167-1172.
- DeVay 1996 (August). Personal Communication. J.E. DeVay. Professor, Department of Plant Pathology. University of California. Davis, California.
- DeVay et al. 1990. Soil Solarization. J.E. DeVay, J.J. Stapleton, and C.L. Elmore. Food and Agricultural Organization, United Nations. FAO Report #109. Rome, Italy.
- Gamliel and Stapleton 1993. "Effect of chicken compost or ammonium phosphate and solarization on pathogen control, rhizosphere microorganisms, and lettuce growth," A. Gamliel and J.J. Stapleton. Plant Disease. The American Phytopathological Society. Volume 77, No. 9, pp. 886-891.
- Hartz 1996 (August). Personal Communication. T. Hartz. Professor, Department of Plant Pathology. University of California. Davis, California.
- Hartz et al. 1993. "Solarization is an effective soil disinfestation technique for strawberry production." T. Hartz, J. DeVay and C. Elmore. HortScience. Volume 28, No. 2, pp. 104-106.
- Katan 1987. Soil Solarization. J. Katan. In: Innovative Approaches to Plant Disease Control. John Wiley & Sons, Inc. pp. 77-105.
- Katan 1984. "Soil solarization," presented at Second International Symposium on Soil Disinfestation. J. Katan. Leuven, Belgium. Convener C. Van Assche, Commission for Plant Protection.
- Katan 1981. "Solar heating (solarization) of soil for control of soilborne pests." J. Katan. Annual Review of Phytopathology. Volume 19, pp. 211-36.
- Katan and DeVay 1991. Soil Solarization. J. Katan and J.E. DeVay. CRC Press Inc. Boca Raton, Ann Arbor, Boston, London.
- Littke 1994a. "Methyl bromide loss: meeting resource management goals through sustainable forest seedling production using alternative seedling production." Dr. W. Littke. In Proceedings of the 1994 International Conference on Methyl Bromide Alternatives and Emissions Reductions. Kissimmee, FL.
- Littke 1994b (December). Personal communication. Dr. W. Littke, Weyerhaeuser Corporation.
- Olson 1996 (August). Personal Communication. Dr. S. Olson. University of Florida. Gainesville, FL.
- Ristaino, J.B. , K.B. Perry and R.D. Lumsden. 1996. Soil solarization and Gliocladium virens reduce the incidence of souther blight (Sclerotium rolfsii) in bell pepper in the field. Biocontrol Science and Technology 6/4: 583-593.
- Ristaino, J.B. , K.B. Perry and R.D. Lumsden. 1991. Effect of soil solarization and Gliocladium virens on Sclerotia of Sclerotium rolfsii, soil microbiota, and the incidence of southern blight in tomato. Phytopathology 81:1117-1124.
- Stapleton 1996. "Fumigation and solarization practice in plasticulture systems." J.J. Stapleton. HortTechnology. Volume 6, No. 3, pp. 189-192.
- Stapleton 1994. "Solarization as a framework for alternative soil disinfestation strategies in the interior valleys of California." J.J. Stapleton, In Proceedings of the 1994 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. Kissimmee, FL.
- Wu, Y. B.P. Perry and J.B. Ristaino. 1996. Estimating temperature of mulched and bare soil form meteorological data. Agricultural and Forest Meteorology 81:299-323.
Please note that this publication discusses specific proprietary products and pest control methods. Some of these alternatives are now commercially available, while others are in an advanced stage of development. In all cases, the information presented does not constitute a recommendation or an endorsement of these products or methods by the Environmental Protection Agency (EPA) or other involved parties. Neither should the absence of an item or pest control method necessarily be interpreted as EPA disapproval.
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