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By Carlene A. Chase Mulching with clear polyethylene film during the hottest months of the year to achieve soil disinfestation is known as soil solarization. Soil solarization utilizes the sun's energy to heat moist soil. Transparent polyethylene film allows the solar radiation to be transmitted directly to the soil and also reduces moisture loss from the soil through evaporation. Higher soil temperatures may be obtained with dark-colored soils since they absorb more solar radiation than light-colored soils (Stapleton and DeVay, 1986). Moisture stimulates metabolic activity in resting structures, which increases pest and pathogen susceptibility to high temperature. Increases in maximum temperature of soil as soil moisture increased have been reported (Mahrer et al., 1984) that are apparently due to improved heat conduction within the soil at higher moisture levels. Soil solarization enhances the diurnal heating and cooling cycle of soil, resulting in daytime temperatures that are lethal to nematodes, fungal pathogens and weed propagules. Temperatures are highest near the soil surface and decrease with depth. Consequently, disinfestation is greatest near the soil surface and is less effective at increasing depth. As a result, reinfestation of solarized soil may occur by migration of organisms (such as nematodes) from unaffected zones of soil or due to field operations such as digging of planting holes for transplants. Because of the daily fluctuation in soil temperature and lower temperatures at deeper soil layers, solarization should be conducted over period durations of 4 to 6 weeks to ensure effective control. While off-season, preplant soil solarization is more common, postplant soil solarization is also practiced. However, this is more suitable for tree crops than row crops, since the latter are more susceptible to injury from high soil temperatures. The use of soil solarization to elevate soil temperatures for soilborne pathogen control was first reported in 1976 (Katan et al., 1976). By mulching soil with transparent polyethylene film during July or August, species of Fusariurn and Verticillium were suppressed, there were fewer diseased plants, weeds were controlled, plant stand was improved, and plant growth and yield were also increased (Katan et al., 1976). Soil solarization is a nonchemical control measure that can contribute to the reduction of pesticide use and environmental impact. A resurgence in interest was stimulated by the declaration of methyl bromide as a stratospheric ozone depleter and its scheduled phaseout. Entire fields may be solarized by bonding polyethylene sheets together to form continuous tarps across the field. Alternatively, individual beds can be solarized. In 1985, for a subsequent double crop of pepper and muskmelon in southern Texas, Hartz et al. reported on the application of solarization film to individual beds and retention of the film as a production mulch. Reflective pigment was applied to the film to cool the soil at the end of the solarization period. This may reduce the cost of implementing soil solarization by allowing the use of conventional mulching equipment and reducing the amount of film that is needed. However, it does require the additional cost and effort of painting the film, row orientation must be north-south for best results, and alternative weed control methods are required for between-row weed control. Soil solarization has been widely utilized in arid, cloud free climates where irrigation is available, in countries such as Egypt, India, Iran, Israel, Jordan, and Syria. In the U.S., and conditions in California and Texas have been amenable to utilizing solarization for soilborne pest control. The more temperate the climate, the shorter the period during which solarization can be conducted. Only the warmest months of the year are useful for solarization. In Florida, air temperatures are adequate for effective soil solarization from late spring and through summer. Despite Florida's subtropical climate, when cool-season solarization was initiated in mid-December and late January in Bradenton and Gainesville, respectively, lethal soil temperatures were not obtained (Chase et al., 1997b). Although Florida's hottest air temperatures occur during the summer, heavy rainfall and cloudiness can also be common at this time of year, and were initially considered to be fairly significant impediments to effective solarization. SOLARIZATION IN FLORIDA The initial studies on soil solarization in Florida were conducted in the Bradenton and Homestead areas for the subsequent production of tomato and strawberry. McSorley and Parrado (1986) found that although there was rainfall on 60% of the days during solarization, the reniform nematode and nutsedge populations were significantly decreased by solarization below levels obtained with a nontreated control. Subsequent fumigation of the solarized plots with methyl bromide: chloropicrin (67:33) did not increase tomato yield over that of nonfumigated solarized treatments. When off-season management schemes were compared, Overman (1985) found that soil solarization was most effective at reducing total number of nematodes and incidence of Verticillium wilt than native weed cover, herbicide fallow, or sorghum/sudangrass cover crop. In a more recent study of soil solarization as an alternative to methyl bromide for tomato production, season-long suppression of stubby root, ring and reniform nematodes was reported (Chellemi et al., 1993). Unlike McSorley and Parrado (1986), Overman (1985) found that fumigation after soil solarization resulted in improved tomato yield. Overman (1985) rotary tilled the soil to 20 cm (7.9 in) three times at the end of 31 days of solarization. Beds were formed and fumigated with Telone 11, Tetone-C17 or Vorlex. There were more opportunities for reinfestation from soil lower in the profile with Overman's methods, and this may explain the beneficial effect of post-solarization fumigant application. In a subsequent study by Overman and Jones (1986) of the persistence of control, soil solarization reduced root-knot nematode damage, incidence of Verticillium wilt, and yellow nutsedge plant density in a fall tomato crop, but the level of rootknot nematode control did not persist for the spring double crop. Fall tomato yields with post-solarization fumigation [Vorlex or methylbromide:chloropicrin (98:2)] did not exceed yield with solarization alone. However, yields for the spring double crop were higher with solarization + fumigation than with solarization alone. Although neither solarization nor fumigation was effective against bacterial wilt, one species of Phytophthora was controlled to 10-cm (3.9-in) depth and another to 25cm (9.8-in) depth, two species of Fusarium pathogenic to tomato were controlled in only the upper 5 cm (2 in) of soil (Chellemi et al., 1994). It is likely that because of local soil and environmental factors, very high pest and pathogen pressure, or innate tolerance of pathogens, soil solarization may not give the desired level of control. Chellemi et al. (1997) has reported improved control of root-knot nematode with solarization with a clear gas-impermeable film + fumigation with Telone-C 17. Preplant soil solarization for strawberry production resulted in suppression of the sting nematode (Overman et al., 1987). More recently, summer solarization for the production of strawberry was evaluated by Albregts et al. (1996) as a methyl bromide alternative, with and without broiler manure incorporated as an organic soil amendment. Pest pressure was low and the yield of nontreated control was not significantly different from the yields with solarization or methyl. The higher rate of manure depressed plant vigor, caused some plant death, and reduced berry yields. Locascio et al. (1999) have evaluated soil solarization for strawberry production under much higher pest pressure. However, solarization was conducted in late summer and early fall. Daily maximum soil temperatures declined as the solarization period progressed. Post-solarization nutsedge regrowth was suppressed under solarization mulches that were painted black at the end of the solarization period, but regrew profusely under unpainted clear film. Strawberry yields with solarization (film painted after solarization) were statistically similar to yields with alternative chemical control methods such as Telone-C I 7/Devrinol at both Gainesville and Quincy. However, where film was left unpainted, there was no yield at Gainesville and yield at Quincy was less than with the control. The use of an alternative fumigant together with solarization such as Vapam + chloropicrin + solarization + painted at Gainesville, and Basamid + solarization + painted at Quincy resulted in yields that were statistically equivalent of those with methyl bromide: chloropicrin. Despite the fact that Florida's hottest period of the year is prone to rainfall and cloudiness, soil solarization has been shown to suppress or control weeds, nematodes and other plant pathogens. The dependence on weather and the variable and short-term effect against some pathogens such as rootknot nematode support the application of soil solarization as an integrated pest management toot rather than a stand alone substitute for methyl bromide fumigation. Since cool-season soil solarization is not possible, the use of solarization for spring-planted crops will require a system of soilborne pest management that persists through a double crop. The majority of studies have been directed at disinfestation for tomato or strawberry and emphasis is now appropriately being refocused on crops such as pepper for which there are no registered chemical alternatives to methyl bromide for weed control in the bed. ENHANCING THE EFFICACY OF SOIL SOLARIZATION Gamliel and Stapleton (1997) have suggested the incorporation of organic amendments as a nonchemical approach to improving the efficacy and predictability of pathogen control by soil solarization. They have attributed the improved control to enhanced production of volatile substances from the amendments. Evidence for organic amendments such as cabbage residue has been conflicting. Recently Coelho et al. (1999) reported that incorporation of cabbage residue did not enhance the control of two species of Phytophthora by soil solarization. However, Lodha et al. (1997) reported that populations of Macrophomina phaseolina, the dry root rot pathogen that is normally not well controlled with solarization, were reduced by a combination of soil solarization and incorporated mustard cake and cauliflower residues. In addition to the action of volatiles, biological control may have also enhanced control since densities of lyric bacteria were also increased. While additions of organic matter often stimulate activity of microorganisms that are antagonistic to plant pathogens, amendments with high carbon:nitrogen ratios such as some yard-waste composts may adversely affect the crop if supplemental nitrogen fertilizer is not applied. Such a situation may explain a lack of uniformity in a pepper plant stand where solarization was combined with incorporation of compost (Ted Winsberg, personal communication). Weed control was excellent, disease incidence was low, and plants responded well to supplemental nitrogen fertilization. Another approach has been to utilize improved mulching materials to reduce energy losses from the soil. Standard clear low density polyethylene (LDPE) transmits sun, light very efficiently. However, the soil loses energy to the atmosphere primarily through long-wave infrared (IR) radiation, which is also readily transmitted through LDPE film. A film that was specifically formulated to reduce IR radiational losses from the soil resulted in higher soil temperatures than a clear LDPE film (Chase et al., 1999a). In addition, purple nutsedge control was more effective with infrared-absorbing film than with LDPE film (Chase et al., 1999b). A double-layered bubble film was also evalu, ated with the intention of reducing energy losses that are due to convection. However, the bubble film did not consistently produce higher soil temperatures than the standard low density polyethylene and the bulkiness of the film will not allow its use with conventional mulching equipment. The efficacy and range of pests controlled can also be improved by combining soil solarization with chemical or biological methods of control. LETHAL TEMPERATURE AND DURATION OF SOLARIZATION Two important factors affecting the efficacy of soil solarization are the soil temperature and the duration of exposure. Pullman et al., (1981) showed that four pathogenic fungi were controlled at constant temperatures ranging from 37 to 50 %C (99 to 122 %F). The time required for pathogen mortality decreased as temperature increased. Rhizoctonia solani, for example, was killed in 10 min at 50 %C but required 14 days at 39 %C (102 %F). At Davis and Shafter, California, maximum soil temperatures for six experiments ranged from 43 to 48 %C (109 to 118 %F) at 15-cm (5.9-in) depth and from 35 to 41 %C (95 to 106 %F) at 30 cm (11.8 in), and resulted in the reduction of Verticillium dahliae infestation. V. dahliae was undetectable at soil depths of 30 and 46 cm (11.8 and 18.1 in) after a 4-week solarization period with maximum soil temperatures of 39 and 37 %C (102 and 99 %F), respectively. In Florida, Cheltemi et al. (1993) conducted solarization for 32 days with a green photoselective film and obtained maximum soil temperatures of 49.5, 46.0, and 40.5 %C (121, 115, and 105 %F) at soil depths of 5, 15, and 25 cm (2, 3.9, and 5.9 in), respectively. They demonstrated season-long control of phytoparasitic nematodes when sampled at 20-cm (7.9-in) depth. Horowitz et al. (1983) found that soil solarization effectively controlled weeds when maximum soil temperatures were greater than or equal to 45 %C (113 %F), and that control improved as the number of days with temperatures above that threshold increased. Perennial weeds were more difficult to control than annual weeds and this was thought to be due to the occurrence of propagules at soil depths not exposed to lethal temperature. Purple and yellow nutsedge were killed after 2-week exposure to daily soil temperature cycles of maximum of 50 %C (122 %F) and minimum 26 %C (79 %F). Cycles with maxima of 40 and 45 %C (104 and 113 %F) did not cause tuber death. In the field, temperatures lethal to nutsedge tubers were not observed at 25-cm (9.8-in) depth but were obtained in the upper 10 cm (3.9 in) of soil (Chase et al., 1999a, 1999b). Although the potential for direct tuber kill is limited to the upper 10 cm, it is likely to contribute considerably to nutsedge control since 70 to 90 % of nutsedge tubers occur in within 10 cm of the soil surface. In addition to propagule death, emerged weeds are trapped under the solarization film and are also killed by foliar scorching. Nutsedges are particularly amenable to control by foliar scorching since they do not penetrate clear films as readily as opaque films such as black and white-on-black. Foliar scorching is an important mechanism for controlling nutsedge shoots that emerged from tubers that were not exposed to lethal temperature. Under clear mulches rhizomes emerging through the soil surface detect light and begin producing leaves that are trapped under the mulch (Chase et al., 1998). With opaque mulches no light is detected and rhizome growth continues and rhizome tips puncture the mulch. Improved nutsedge control was obtained with IR film compared with conventional clear film (Chase et al., 1998, 1999b). With high nutsedge infestations, the duration of soil solarization should be extended to promote depletion of tuber buds or food reserves through a series of alternating sprouting and foliar scorching. ADVANTAGES AND DISADVANTAGES The primary advantage of soil solarization is that it is a nonchemical method of soil disinfestation so worker and environmental exposure to chemicals are reduced. It can be readily integrated into the existing system of vegetable plasticulture and utilize existing equipment. It may reduce the cost of soilborne pest management by eliminating or reducing the amount of pesticide used. In locations with no known major pathogens, soil solarization often results in improved plant growth, a phenomenon called increased growth response. It is not related to improved root growth but may be partially due to delayed leaf senescence (Gruenzweig et al., 1993). The limitations of soil solarization include its climatic restrictions and its seasonality. It requires a fallow period of at least four weeks, and is not effective against many pests and pathogens. Row orientation must be north-south for best results. With strip solarization of individual beds, weed control is required between rows. Some strawberry growers contend that polyethylene film in the field during summer and early fall prior to transplant will increase runoff after heavy summer rains and film can be blown off during tropical storms and hurricanes. There is additional cost and effort for painting film that is retained as a production mulch. Thus far, the gauge of infrared-absorbing films has been thicker than those of conventional opaque mulches and will, therefore, cost somewhat more. Film manufacturers are not yet committed to producing solarization film, so that it not yet readily available to farmers. Soil solarization is certainly not the "magic bullet" for which farmers have been hoping. However, it is a useful tool that can be integrated into the pest management program. In recent studies (Chellemi et al.; Locascio et al., unpublished) have found that tomato and strawberry yields with soil solarization + Telone-C17 were either higher or not significantly different from those with methyl bromide:chloropicrin. In Gainesville, studies are planned in which a broadcast application of TeloneC17 will be made prior to bed formation. Clear infrared polyethylene film will then be applied to the beds for solarization during July and August. This is a good example of an effort to maximize the performance of alternative methods that have shown promise. With the increasing environmental awareness of the public and the encroachment of development into agricultural areas of Florida and California, adoption of pest management systems that reduce pesticide usage is to everyone's benefit. Carlene A. Chase, postdoctoral associate, Gulf Coast Research and Education Center, University of Florida, Institute of Food & Agricultural Sciences, Bradenton. LITERATURE CITED Albregts, E.E., J.P. Gilreath, and C.K. Chandler. 1996. Soil solarization and fumigant alternatives to methyl bromide for strawberry fruit production. Soil Crop Sci. Soc. Florida Proc. 55:16-20. Chase, C.A., T.R. Sinclair, D.O. Chellemi, S.M. Olson, J. Rich, S.J. Locascio, J. P. Gilreath, and J. P. Jones. I 997a. Soil solarization as an alternative to methyl bromide in vegetable production. Proc, South. Weed Sci. Soc. 50: 82-83. Chase, C.A., T.R. Sinclair, S.J. Locascio, J.P. Gilreath, J.P. Jones, and D.W. Dickson. 1997b. An evaluation of improved polyethylene films for cool-season soil solarization. Proc. Fla. State Hort. Soc. 110:326329. Chase, C.A., T.R. Sinclair, D.G. Shilling, J.P. Gilreath, and S.J. Locascio. 1998. Light effects on rhizome morphogenesis in nutsedges (Cyperus spp.): implications for control by soil solarization. Weed Sci. 46:575-580. Chase, C.A., T.R. Sinclair, D.O. Chellemi, J.P. Gilreath, ST Locascio, and S.M. Olson. 1999a. Heat-retentive films for increasing soil temperature in a humid, cloudy climate. HortScience (In press). Chase, C.A., T.R. Sinclair, and ST Locascio. 1999b. Effects of soil temperature and tuber depth on nutsedge (Cyperus spp.) control by soil solarization. Weed Sci. (In press). Chellemi, D.O., S.M. Olson, and D.J. Mitchell. 1994. Effects of soil solarization and fumigation on survival of soilborne pathogens of tomato 1 . n northern Florida. Plant Dis. 78:1167-1172. Chellemi, D.O., S.M. Olson, D.J. Mitchell, 1. Seeker, and R. McSorley. 1997. Adaptation of soil solarization to the integrated management of soilborne pests of tomato under humid conditions. Phytopathology 87:250-258. Chellemi, D.O., S.M. Olson, J.W. Scott, D.J. Mitchell and R. McSorley. 1993. Reduction of phytoparasitic nematodes on tomato by soil solarization and genotype. J. Nematol. 25 (SUPPI.):800-805. Coelho, L., D.O. Chellemi, and D.J. Mitchell. 1999. Efficacy of solarization and cabbage amendment for the control of Phytophthora spp. in north Florida. Plant Dis. 83:293-299. Gamliel, A. and J.J. Stapleton. 1997. Improvement of soil solarization with volatile compounds generated from organic amendments. Phytoparasitica 25 (Suppl.):3 I S-38S. Gruenzweig, J.M., H.D. Rabinowitch, and J. Katan. 1993. Physiological and developmental aspects of increased plant growth in solarised soils. Ann. Appl. Biol. 122:579:59 1. Hartz, T.K., C.R. Bogle, and B. Villalon. 1985. Response of pepper and muskmelon to row solarization. HortScience 20:699-701. Horowitz, M. Y. Regev, and G. Herzlinger. 1983. Solarization for weed control. Weed Sci. 31:170-179. Katan, J., A. Greenberger, H. alon, and A. Grinstein. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soil-borne pathogens. Phytopathology 66:683-688. Locascio, S.J., S.M. Olson, C.A. Chase T.R. Sinclair, D.W. Dickson, D.J. Mitchell and D.O. Chellemi. 1999, Strawberry pro duction with alternatives to methyl bromid fumigation. Proc. 28th National Agricultura Plastics Congress. Pp. 148-152. Lodha, S., S.K. Sharma, and R. K. Aggarwal. 1997. Solarization and natural heating of irrigated soil amended with cruciferous residues for improved control of Macrophomina phaseolina. Plant pathology 46:186-190. Mahrer, Y, 0. Naot, E. Rawaitz, and J. Katan. 1984. Temperature and moisture regimes in soils mulched with transparent polethylene. Soil Sci. Soc. Am. J. 48:362-367. McSorley, R. and J.L. Parrado. 1986. Application of soil solarization to Rockdale soils in a subtropical environment, Nematropica 16:125-140. Overman, A.J., C.M. Howard, and E.E. Albregts. 1987. Soil solarization for strawberries. Proc. Fla. State Hort. Soc. 100:236239. Overman, A J. and J.P. Jones. 1986. Soil solarization, reaction, and fumigation effects on double-cropped tomato under full-bed mulch. Proc. Fla. State Hort. Soc. 99: 315318. Overman, A.J. 1985. Off-season land management, soil solarization and fumigation for tomato. Soil Crop Sci. Soc. Florida Proc. 44:35-39. Pullman, G.S., J.E. DeVay, and R.H. Garber. 1981. Soil solarization and thermal death: a logarithmic relationship between time and temperature for four soilborne plant pathogens. Phytopathology 71:959-964. Stapleton, J.J. and J.E. DeVay. 1983. Response of phytoparasitic and free-living nematodes to soil solarization and 1,3-dichloropropene in California. Phytopathology 73:1429-1436. Stapleton, J.J. and J.E. DeVay. 1986. Soil solarization: a nonchemical approach for the management of plant pathogens. Crop protection 5: 190-198. |
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