James Stapleton

UC Plant Protection Quaterly, 7(3):1-5

Reprinted with permission

Editor's Note: The following is excerpted from an article that appeared in the July 1997 issue of the UC Plant Protection Quarterly, a publication of the University of California Kearney Plant Protection Group and Statewide IPM Project. For the full text of the article, or for more information about soil solarization contact the author at the UC Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648, Tel. (209) 646-6015.

Solarization is a natural, hydrothermal soil disinfestation process which is accomplished through passive capture of solar radiation in moist soil. Soil solarization occurs through a combined physical, chemical, and biological mode of action, and is compatible with other disinfestation materials, such as organic amendments, biological control organisms, or pesticides. It is currently used on a relatively small scale worldwide as a substitute for synthetic chemical toxicants. The use of solarization is expected to increase as methyl bromide is phased out. Solarization, as any other soil disinfestation method, has both benefits and limitations. It is simple, safe, effective within its use limitations, and can be readily combined with biological and chemical control measures. On the other hand, solarization is dependent upon local meteorological conditions, is most effective near the soil surface, does not consistently control certain heat-tolerant pathogens such as Macrophomina phaseolina and Meloidogyne spp., should be done during the hottest part of the year, and requires disposal of plastic film.

The practical value of soil solarization, as of any pest management strategy, must be assessed by several factors, including pesticidal efficacy, effect on crop growth and yield, economic cost/benefit, and user acceptance (Stapleton, 1995; Stapleton and DeVay, 1995). Its routine use as a viable alternative to chemical fumigants in several areas of the world indicates that solarization has already achieved limited user acceptance. There is now a substantial body of literature describing organisms which are controlled or partially controlled by solarization, including in excess of 40 fungal plant pathogens, more than 25 species of nematodes, numerous weeds, and a few bacterial pathogens (Katan, 1987; Stapleton and DeVay, 1995; Stapleton, 1996; Elmore et al., 1997). In addition to the major pathogens that are reduced by solarization, a number of minor pathogens also are reduced. This is one of the reasons that an "increased growth response" (IGR) is often observed after solarization, similar to that commonly found after chemical fumigation. Solarization has been frequently documented to stimulate IGR in plants even when no major pathogens can be isolated, and reductions in the overall number of soil microorganisms have been significantly correlated with increased plant growth following treatment (Katan, 1987; Chen et al., 1991; Stapleton and DeVay, 1995).

Current Use

The principal use of solarization, on a total acreage basis, is probably in conjunction with greenhouse grown crops. Another application for which solarization has come into common use, particularly in developing countries, is for disinfestation of seedbeds, containerized planting media, and cold-frames (Stapleton and Ferguson, 1996). As with use in greenhouses, these are natural niches for solarization, since individual areas to be treated are small, soil temperature can be greatly increased, the cost of application is low, the value of the plants produced is high, and the production of disease-free planting stock is critical for producing healthy crops. Solarization of containerized soil can be accomplished in less than a week during periods of hot weather. For example, moist soil in black polyethylene nursery sleeves covered by a single layer of clear plastic film reached 69°C, and in sleeves covered by a double plastic layer temperatures reached 72°C in the San Joaquin Valley of California (Stapleton and Ferguson, 1996). These temperatures are lethal to most soilborne pests within hours, and approach the heat levels produced during soil disinfestation using aerated steam.

On a global scale, solarization for disinfesting soil in open fields is being implemented at a relatively slow but increasing rate. It has been used commercially in areas such as the central and southern desert valleys of California and Israel where air temperatures are very high during the summer and much of the cropland is out of production at this time due to excessive heat (Bell and Laemmlen, 1991; Becker and Wrona, 1995; Grinstein and Ausher, 1991). This system is also a natural window of advantage for using solarization, since the summer fallow provides a time period of several weeks for rotating into solarization. Most growers in California who are now using solarization in production fields are those that have some aversion to the use of methyl bromide or other chemical soil disinfestants, either because of their close proximity to urban or residential areas, personal preference, or because they are growing for organic markets.

Most transparent polyethylene films are suitable for conducting solarization. However, use of lower quality films may be problematic since the plastic may break down prematurely, leaving a myriad of fragments which are difficult to dispose of. Higher quality film more resistant to degradation by ultraviolet light is worth the extra price. The thickness (gauge) of the film is relatively unimportant, except for cost; film strength does not directly correlate with thickness. Plastic is priced based on the cost of petroleum, so thicker plastic weighs more and costs more than thinner film. Certain plastics manufacturers produce films specially designed for solarization. Most farm supply outlets and many nurseries stock or can order suitable films.

The cost-benefit ratio of solarization compared to other soil disinfestation practices must be calculated on a case-by-case basis. Few economic analyses have been done to compare solarization with conventional disinfestation practices (Elmore, 1991; Yaron et al., 1991). As a rough estimate, the cost of solarization, including film, application, and removal, is one-third to one-half that of tarped, methyl bromide fumigation ($400-600 per treated acre vs. $1,100). The yield, quality, and value of the following crops will determine the relative benefit of the soil disinfestation treatments. In organic production without the use of chemical disinfestants, crop yield and quality are often lower than in conventional production, but the unit value of produce is often higher. In this case, only small increases in yield following solarization are needed to pay for the treatment, and large increases in yield often occur (Elmore, 1991).

How Can Solarization be Improved?

With both benefits and limitations considered, solarization is an effective soil disinfestant in numerous geographic areas for certain agricultural and horticultural applications. Nevertheless, there are many situations where it may be desirable to increase the efficacy and/or predictability of solarization through combination with other methods of soil disinfestation. Since solarization is a passive process with biocidal activity dependent to a great extent upon local climate and weather, there are occasions when even during optimal periods of the year, local atmospheric conditions (i.e., cool air temperatures, extensive cloud cover, frequent or persistent precipitation events) may not permit effective solarization. This uncertainty must be overcome if widespread implementation of solarization is to occur, since commercial users cannot tolerate soil disinfestation treatments which are not consistently effective. Integration of solarization with other disinfestation methods may be essential in order to increase treatment predictability, and thus, commercial acceptability (Stapleton, 1995).

Previous studies have shown that solarization may be productively combined with other chemical and biological control methods (Katan, 1987; Chellemi et al., 1994; Stapleton and DeVay, 1995; Tjamos and Fravel, 1995). Recently, considerable interest has been generated regarding the use of organic amendments in combination with solarization to achieve biofumigation (Gamliel and Stapleton, 1993a, b). A wide range of organic amendments, including plant residues, by themselves have some degree of soil disinfestation activity. Addition of biocidal soil amendments or crop residues as part of a crop rotation scheme may in certain cases be useful for managing population levels of soilborne pests. However, for routine use in high value, intensively-farmed horticultural crops, it is unlikely that periodic rotations into bioactive plants alone will provide sufficient efficacy, predictability, or economic return to be of consistent value. Combining a variety of soil amendments with solarization to accomplish biofumigation is an improved option.

One promising combination of organic amendments with solarization involves residues of cruciferous plants, which release a number of biotoxic volatile compounds into soil during the decomposition process (Ramirez-Villapudua and Munnecke, 1987). Production and release of these compounds was demonstrated to be greatly increased, both qualitatively and quantitatively, when cabbage (Brassica campestris var. capitata) amendment was combined with soil heating. The aldehydes and isothiocyanates produced by the decomposing cabbage were positively correlated with fungicidal activity in treated soil (Gamliel and Stapleton, 1993a). Release of these compounds was a function of the decomposition process. Various products and intermediaries were produced and dissipated in a chemical cascade. In conjunction with soil heating, the formation and release of these biotoxic volatile compounds occurred mainly during the first three weeks of solarization. After that time, concentrations of most compounds dropped to low or undetectable levels.

Feasible alternatives to chemical soil fumigants must provide effective, predictable, economical, and relatively rapid reductions of pest and disease organisms. Solarization has limitations which prevent it from universally replacing fumigants. However, in suitable climates and for compatible applications, solarization alone, or in combination with other agents, is ready for implementation.

References

Becker, J.O., and Wrona, A.F. 1995. Effect of solarization and soil fumigation on Pythium, nematodes, weeds and carrot yield, 1993/94. Biological and Cultural Tests 10:134. APS Press, St. Paul.

Bell, C.E., and Laemmlen, F.F. 1991. Soil solarization in the Imperial Valley of California. Pages 245-255 in: Soil Solarization, Katan, J., and DeVay, J.E., Eds., CRC Press, Boca Raton.

Chellemi, D.O., Olsen, 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.

Chen, Y., Gamliel, A., Stapleton, J.J., and Aviad, T. 1991. Chemical, physical, and microbial changes related to plant growth in disinfested soils. Pages 103-129 in: Soil Solarization. J. Katan and J.E. DeVay, eds. CRC Press, Boca Raton.

Elmore, C. L. 1991. Cost of soil solarization. Pages 351-360 in: Soil Solarization. J. E. DeVay, J. J. Stapleton, and C. L. Elmore, eds. Plant Prod. Prot. Pap. 109. FAO, Rome.

Elmore, C.L., Stapleton, J.J., Bell, C.E., and DeVay, J.E. 1997. Soil solarization: A nonpesticidal method for controlling diseases, nematodes, and weeds. Publication 21377, University of California Division of Agriculture and Natural Resources, Oakland. 14 pages.

Gamliel, A., and Stapleton, J.J. 1993a. Characterization of antifungal volatile compounds evolved from solarized soil amended with cabbage residues. Phytopathology 83:899-905.

Gamliel, A., and Stapleton, J.J. 1993b. Effect of soil amendment with chicken compost or ammonium phosphate and solarization on pathogen control, rhizosphere microorganisms, and lettuce growth. Plant Dis. 77:886-891.

Grinstein, A., and Ausher, R. 1991. Soil solarization in Israel. Pages 193-204 in: Soil Solarization. J. Katan and J.E. DeVay, eds. CRC Press, Boca Raton.

Katan, J. 1987. Soil solarization. Pages 77-105 in: Innovative Approaches to Plant Disease Control. I. Chet, ed. John Wiley & Sons, New York.

Ramirez-Villapudua, J., and Munnecke, D.M. 1987. Control of cabbage yellows (Fusarium oxysporum f. sp. conglutinans) by solar heating of field soils amended with dry cabbage residues. Plant Dis. 71:217-221.

Stapleton, J.J. 1997. Solarization: An implementable alternative for soil disinfestation. In: Biological and Cultural Tests for Control of Plant Diseases 12:1-6, APS Press, St. Paul.

Stapleton, J.J. 1996. Fumigation and solarization practice in plasticulture systems. HortTechnology 6(3):189-192.

Stapleton, J.J. 1995. Evolving expectations for integrated disease management: Advantage Mediterranea. Journal of Turkish Phytopathology 24:93-98.

Stapleton, J.J., and DeVay, J.E. 1995. Soil solarization: A natural mechanism of integrated pest management. Pages 309-322 in: Novel Approaches to Integrated Pest Management, R. Reuveni, ed. Lewis Publishers, Boca Raton.

Stapleton, J.J., and Ferguson, L. 1996. Solarization to disinfest soil for containerized plants in the inland valleys of California. Page 6 in: Proceedings of the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reduction, Orlando, Florida, November 4-6, 1996.

Tjamos, E.C., and Fravel, D.R. 1995. Detrimental effects of sublethal heating and Talaromyces flavus on microsclerotia of Verticillium dahliae. Phytopathology 85:388-392.

Yaron, D., Regev, A., and Spector, R. 1991. Economic evaluation of soil solarization and disinfestation. Pages 171-190 in: Soil Solarization. Katan, J., and DeVay, J.E., eds. CRC Press, Boca Raton.

(DEC. 549) Contributed by James Stapleton