Control of soilborne pathogens with soil solarization in the southern region of Libyan Jamahiriya

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Ali M. Zaid, Waseem Ismail, Adel Khader and Muftah Mayof
Agricultural Research Centre, Tripoli, Libya

Abstract

Soil solarization for 30 and 45 days reduced the population of Meloidogyne sp. by 66.6 percent - 100 percent, Tylenchorhynchus sp. by 50 percent - 80 percent and Trichodorus sp. by 83.5 percent - 87.5 percent, respectively. Solarizing soil for 45 days reduced soilborne fungi by 76.6 percent. Actinomycetes were reduced in both solarized and non-solarized soil during the first 30 days; after 45 days the population was increased by 68.4 percent and 46.8 percent in solarized and non-solarized soil, respectively. The bacterial population was increased during the first 30 days in both soil treatments, but after 45 days the population decreased to 71.1 percent level in solarized soil and 79.1 percent level in non-solarized soil.

Introduction

Phytoparasitic nematodes and soilborne pathogens are common agents that cause serious damage to agricultural crops in Libya. Production of vegetable crops such as tomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativus L.) and pepper (Capsicum annum L.) is increasing in Libya. Soil sterilization by methyl bromide is now being used to control soilborne pathogens and weeds. This method is very effective, but at the same time it is expensive and risky. Therefore, the search for new inexpensive and non-hazardous techniques are needed to control soilborne diseases.

Soil solarization, which is a new method, has proven to be effective for controlling plant parasitic nematodes (7, 9, 14, 16), soilborne pathogens and weeds (1, 8, 9, 11, 12, 15), and to increase plant growth and crop yields (3, 8, 9, 10, 17). It has already been reported that micro-organisms beneficial to plant growth were stimulated (Rhizobium spp. and Trichoderma spp.) or were less affected (Bacillus spp. and Actinomycetes) by soil solarization as compared to pathogenic organisms (6, 12, 15). Therefore, this study was conducted to find out the effectiveness of soil solarization on soilborne pathogens in the southern region of Libya.

Materials and Methods

Experimental Design and Soil Sampling. - The study was conducted at a private farm in the southern region of Libya during July and August 1986. The field selected for the study had been planted with cantaloupe and watermelon previously. Both crops showed severe Fusarium wilt and infection by root knot nematode, Meloidogyne sp. The old plants were removed, the soil was cleaned, fertilized with organic manure, ploughed, levelled and irrigated two days before covering with polyethylene tarps. The field was divided into two plots, and each plot was subdivided into five replicates (5 m x 7 m). Five replicates were covered with polyethylene tarps of l mm thickness and five were left uncovered and were considered as control. Composite soil samples (1 - 2 kg) were collected from six to eight sites from each replicate at a depth of 5 - 25 cm. in plastic bags and kept refrigerated until processed (four to seven days later). All soil samples were collected before solarization and 30 and 45 days after the solarization period.

Culture Media. - Three culture media were used to estimate and compare the soil microflora population. Potato-dextrose agar medium amended with 30 ppm rose bengal was used for total fungi. Glucose-asparagine medium (5) was used for total Actinomycetes and nutrient agar was used for total bacteria.

Assay Procedures for Counts of Soil Microflora and Phytoparasitic Nematodes. - Composite soil samples were mixed well and from each, three subsamples of 2 gm were taken; 100 ml sterilized distilled water was then added to each subsample, shaken for one minute, allowed to settle and diluted into water blanks. At the proper dilution for colony counting, 0.1 ml of soil suspension was transferred to the various media in Petri plates and spread with an L--shaped rod. Aliquots from each subsample were introduced into four plates of each medium. The plates were incubated at 28°C. Bacterial colonies were counted after 48 to 72 hours, fungal colonies after three to five days and colonies of Actinomycetes after seven to nine days of incubation. The population of phytoparasitic nematodes was estimated in 250 gm soil by using Cobb's sieving and decanting method (4).

Results and Discussion

The results of the study showed that mulching of soil with polyethylene tarps during July and August significantly reduced soilborne fungi and phytoparasitic nematodes. As shown in Figure l, solarizing the soil for 45 days significantly reduced soilborne fungi, but 30 days of solarization had no effect on the population of soilborne fungi. Fusarium spp., causing cantaloupe wilt, Penicillium spp., Rhizopus spp. and Aspergillus spp. were identified. The bacterial population was significantly increased during the first 30 days in solarized and nonsolarized soil with 51.8 percent and 31.3 percent, respectively, as compared to the initial population; after 45 days the population decreased with 71 percent and 79 percent in solarized and non-solarized soil, respectively (Figure 2). Conversely, the Actinomycetes population was reduced during the first 30 days of solarization and increased after 45 days with 68 percent and 46.8 percent in solarized and non-solarized soil, respectively (Figure 3). Other studies showed that Bacillus spp. and many Actinomycetes and thermotolerant/ thermophilic fungi and other high temperature soilborne micro-organisms are prominent among the primary organisms recolonizing solarized soil.

Solarizing the soil for 30 days and 45 days significantly reduced the population of Meloidogyne spp. with 66.6 percent and 100 percent; Trichodorus spp. with 83.5 percent and 87 percent; and TyIenchorhynchus spp. with 50 percent and 80 percent, respectively, as compared to non-solarized soil (Table 1). These results indicate that solar heating of soil can give an effective control of soilborne fungi and plant parasitic nematodes. The effect of soil solarization on reduction of soilborne pathogens at 0 - 25 cm depths could be the result of the many factors involved. It has been reported that mulching with polyethylene tarps increases the soil temperature by 7 - 14°C compared with non-solarized soil (8, 12); it influences the chemical and physical characteristics of soil (2, 9, 13); it induces the production of volatile compounds (9, 16) and antibiotics (13, 15). All the above factors may play a possible role in reducing soilborne diseases through the effect of physical and chemical characteristics and through the changes in biological compounds of soil in favour of antagonists. More studies should continue to examine the effect of solarization on the role of soil microflora.

References

1. Al-Raddad, A.M.M. 1979. Soil Disinfestation by Plastic Tarping. M.Sc. Thesis, University of Jordan, 95 pp.

2. Chen, Y. and J. Katan. 1980. Effect of solar heating of soils by transparent polyethylene mulching on their chemical properties. Soil Sci. 130:271277.

3. Chen, Y., T. Solowitch, J. Navrot and J. Katan. 1981. The effect of solar heating of soils on their chemical characteristics and plant growth stimulation. Phytoparasitica 9:236-237.

4. Cobb, N.A. 1981. Estimating the nema population of soil. U.S. Department of Agriculture Bull. Plant Ind. Agric. Tech. Cir. 1:1-48.

5. Dhingra, O.D. and J.B. Sinclair. 1985. Basic Plant Pathology Methods. CR.C. Press p. 295.

6. Grinstein, A., J. Katan, A. Abdul Razik, O. Zeydan, and Y. Elad. 1979. Control of Sclerotium rolfsii and weeds in peanuts by solar heating of the soil. Plant Dis. Reptr. 63:1056-1059.

7. Grinstein, A., D. Orion, A. Greenberger and J. Katan. 1979. Solar heating of the soil for the control of Verticillium dahliae and Pratylenchus thornei in potatoes. pp.431 -438. In: Soil Borne Pathogens. B. Schippers and M. Gams, eds. Academic Press, New York.

8. Hartz, T.K., C.R. Bogie and B. Villalon. 1985. Response of pepper and muskmelon to row solarization. Hort. Sci. 20:699-701.

9. Katan, J. 1981. Solar heating (solarization) of soil for control of soil-borne pests. Ann. Rev. Phytopath. 19:211-236.

10. Porter, IJ. and P.R. Merriman. 1985. Evaluation of soil solarization for control of root diseases of row crops in Victoria. Plant Pathology 34:108-118.

11. Pullman, G.S. 1979. Effectiveness of soil solarization and soil flooding for control of soil-borne diseases of Gossypium hirsutum L. in relation to population dynamics of pathogens and mechanisms of propagule death. Ph.D. Thesis, University of California, Davis. 95 pp.

12. Pullman, G.S., J.E. DeVay, R.H. Garber and A.R. Weinhold. 1981. Soil solarization effect on Verticillium wilt of cotton and soil-borne populations of Verticillium dahliae Pythium spp. Rhizoctonia solani and Thielaviopsis basicola. Phytopathology 71 :954-959.

13. Rubin, G. and A. Benjamin. 1984. Solar heating of soil involvements of environmental factors on the weed control process. Weed Sci. 32: 138142.

14. Siti, E., E. Cohn, J. Katan, and M. Mordechai. 1982. Control of Ditylenchus dipsaci in garlic by soil treatments. Phytoparasitica 10:93100.

15. Stapleton, J.J. and J.E. DeVay. 1982. Effect of soil solarization on population of selected soil-borne micro-organisms and growth of deciduous fruit tree seedlings. Phytopathology 72:323-326.

16. Stapleton, J.J. and J.E. DeVay. 1983. Response of phytoparasitic and free living nematodes to solarization and 1,3,-dichloropropene in California. Phytopathology 73: 1323- 1326.

17. Stapleton, J.J. and J.E. DeVay. 1984. Thermal components of soil solarization as related to changes in soil and root microflora and increased plant growth response. Phytopathology 74:255-259.

Table 1. Effect of soil solarization on phytoparasitic nematodes*

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* Number of nematodes/250 gm soil.

Figure 1. (A) Effect of soil solarization on soil fungi; (B) on soil bacteria; and (C) on soil Actinomycetes.

Soil solarization in tropical agriculture for pre- and post-plant applications

James J. Stapleton
Statewide Integrated Pest Management Project, University of California, 733 County Center III, Modesto, California 95355 USA

Abstract

Soil solarization was developed primarily in temperate climates, and information on its effects and potential uses in tropical agriculture is incomplete. Several experiments done in Mexico, India, and South America indicated that the effects of solarization are similar to those found after solarization in temperate zones. In Colima, Mexico, maximum soil temperatures beneath transparent, polyethylene tarps were increased 6-13°C over non-treated control soil. Soilborne fungal and bacterial population densities were reduced 62-100 percent in pre-plant experiments, and weed emergence was reduced 97-100 percent. Although crop damage due to charcoal rot (Macrophomina phaseolina) of sesame (Sesamum indicum) was not significantly reduced, seed yield was sometimes increased. Beneficial effects were more pronounced in sandy, nutrient-poor soil. Post-plant application of solarization with transparent polyethylene film around young Mexican lime (Citrus aurantifolia) or mango (Mangifera indica) trees for 4-10 weeks did not significantly aid in tree growth. Similar treatment around herbaceous papaya (Carica papaya) and banana (Muse cv.) resulted in stunting or death. In most eases, mulching of moist soil with black polyethylene film was as effective as solarization with transparent film in controlling diseases and weeds, and sometimes was superior in promoting plant growth. Both solarization and black film mulching allowed as much as a tenfold reduction in dry season irrigation requirement. Solarization may be ideally accomplished in semi-arid tropical climates when applied during the dry season, provided water is available to irrigate soil when the film is applied. Greater intensity of ultraviolet rays in tropical climates may necessitate more durable films to prevent premature polymer degradation.

Introduction

Soil solarization is a non-chemical method of soil disinfestation which uses hydrothermal energy for its biocidal and plant growth promoting activity. In its present state, solarization is effected by covering moist soil with polyethylene film mulches during the hottest months of the year. The subsequent heating process causes complex changes in soil which are deleterious to most pathogens and pests, while stimulating antagonistic microorganisms and crop growth (6, 9). Transparent films are most often used for solarization, since they permit passage of most of the incoming solar radiation into the soil. In some cases, mulching with black film may be as effective as the transparent material. This is especially true with post-plant mulching of perennial plants, since high temperatures accumulated during solarization with transparent films may result in excessive stunting of the crop (10, 11). Soil moisture assists the solarization process by conducting heat energy to target pathogens and pests, which when moist arc often actively metabolizing and thus more susceptible to lethal dosages of heat (1, 6, 9). In addition, surviving organisms may be weakened by sub-lethal heating, making them more susceptible to attack by heat-tolerant antagonistic organisms. Soil heating may result in the release of soil volatiles, many of which are biocidal. Among the volatiles known to increase after solarization is ammonia (NH3), generated from ammonium-nitrogen (NH₄-N) under conditions of reduced soil oxygen or alkaline pH. In addition to contributing to soil disinfestation, increased levels of NH₄-N and nitrate-nitrogen (NO₃-N) are available for plant nutrition after solarization. The increased soluble nitrogen (and often other mineral nutrients such as phosphorus, calcium, and magnesium) available after solarization may reduce or eliminate the need for pre-plant fertilizer application for the following crop (6, 9).

Soil solarization was developed primarily in temperate climates, and information on its effects and potential uses in tropical agriculture is incomplete. Nevertheless, a number of studies have been done which indicate that solarization can be of significant value in tropical climates. Results of these studies will be discussed, with reference to both pre-planting and post-planting applications.

Pre-Plant Applications

Several studies on pre-plant soil solarization in tropical areas have been reported. Most of them deal with effects on pests and pathogens of annual crops. Applications of solarization have mostly been made during the dry season in lowlands of the semi-arid tropics. A few studies have been done in highland areas, where effects of solarization may be expected to diminish somewhat due to cooler climatic conditions and more cloud cover. After studying annual temperature records in the Tecoman Valley of Colima, Mexico (19°N 104 W. 50 m elevation), Stapleton and Garza-Lopez (10) found that the hottest months not normally receiving rain (May to mid-June) occurred just prior to the onset of the rainy season. They solarized two soils during that period, then listed up seed beds. Sesame (Sesamum indicum) seed was planted after the first rain and the crop was naturally irrigated by precipitation during the growing season. The results of their experiments with pre-plant solarization were similar to those found in the San Joaquin Valley of California (9, 11). Soil temperatures in solarized soil were 6-11°C higher than those in non-treated soil at 23 cm depth. Weed emergence was suppressed 97-100 percent by solarization shortly after planting the sesame seed and remained good throughout the crop season, except for nutsedge (Cyperus) spp. which encroached into the plots from the non-treated periphery. Monitored soilborne fungal and bacterial population densities were reduced 62-100 percent, although control of charcoal rot caused by the primary target pathogen, Macrophomina phaseolina, was ineffective as often found in previous studies (6, 9). Nevertheless, dry sesame seed yield was increased 14-72 percent in solarized plots (Table 1). Differences in crop vegetative growth and seed yield were more pronounced in a mineral nutrient-poor, loamy sand soil than in a richer clay loam soil type. Greater charcoal rot disease pressure in the loamy sand soil also may have contributed to this effect. Pre-solarization irrigation was done in these experiments, but water sources may not be available during the dry season in other tropical areas. Although moist or wet soil is preferable for disinfestation by heat, some studies have shown that solarization of dry soil may be effective (3, 4). Additional work is needed in this area to define agro-ecosystems where dry soil may be solarized with acceptable efficacy.

Another intensive study on the use of solarization in semi-arid tropical regions was conducted by Chauhan et al. (3) in Andhra Pradesh, India (17°N 78°E, 545 m elevation). They also found that solarization at their location gave results very similar to those in temperate regions (6, 9). Working with a pigeonpea (Cajanus cajan)/chickpea (Cicer arietinum)/Fusarium wilt system, and solarizing for six weeks in April-May, they found that soil population densities of Fusarium, nematodes, and weeds were significantly reduced, control of Fusarium wilt in susceptible varieties was achieved, and increased plant growth and yield was obtained. They reported nodulation of legume roots by Rhizobium spp. to be decreased after solarization, but found no adverse effect on crop growth. They attributed this finding to increased nitrate-nitrogen availability in solarized soil which compensated for reduced nodulation.

Several reports from Mexico, in addition to the Colima study (10), have been published. Vidales et al. (12) found that the incidence of Fusarium wilt (Fusarium oxysporum f. sp. melonis) of field-grown melon (Cucumis melo) in the coastal Apatzingan Valley, Michoacan, was reduced from 86 percent in the non-treated control to as low as 8.5 percent after 50 days solarization with pre-irrigation. Solarization of 30 or 40 days duration, with or without pre-irrigation, was statistically as good as the longer treatment. Davalos and Castro (5) reported that fresh-market strawberry yields in two soils in the highlands of Irapuato, Guanajuato with a history of "secadera" disease were increased after solarization of two or five months to a level equivalent to a commercial methyl bromide/chloropicrin application of 600 kg/ha. Salgado et al. (8) reported that solarization of soil infested with root knot nematode (Meloidogyne incognita) resulted in a reduction of galling of bean roots of 85 percent, which was equivalent to control given by aldicarb (3 kg/ha) or ethoprop (3 kg/ha). An increase in bean yield of 58 percent also was observed over the control treatment. Combining solarization with chicken manure (10 ton/ha) did not improve control of galling, but bean yield was lower (25 percent greater than the control) than after solarization alone.

The benefits of irrigating soil prior to solarization also was investigated by Daelemans (4) in the highlands of Dschang, Cameroon (5°N 10°E, 1400 m elevation). Under less than optimal climatic conditions during the experimental period, he reported that irrigation or incorporation of crop residue had no effect on control of weeds, soil chemical properties, numbers of soil fungi, or primary infection of groundnut by soilborne propagules of Cercospora, and imparted no additional efficacy to solarization Solarization, however, was effective in controlling weeds and primary lesions of Cercospora leaf spot of groundout.

One of the most interesting findings in the Colima study (10) was that black polyethylene film mulching worked as well as mulching with transparent film. Disinfestation of soil under black film is probably most effective when target pathogens and pests are concentrated or limited to the upper few centimetres of soil or when crop roots are shallowly distributed, since transmission of solar heat through black pigment is greatly restricted. Nevertheless, confinement of wet soil under any polyethylene film may result in micro-aerobic or anaerobic conditions deleterious to pest organisms (9). In a report from Sudan, however, investigators found that mulching soil with black film was inferior to solarization with transparent film in controlling weeds including broomrape (Orobanche ramosa), and galling of roots by Meloidogyne sp. (2). Nevertheless, while solarization for 30 or 60 days completely eradicated broomrape in eggplant, black film mulching for 60 days reduced the incidence by 97 percent. No crop yield data were included in the report, so comparison of effect of film colour on plant growth cannot be made. An additional consideration for use of coloured film is that pigment in polyethylene films may inhibit absorption of damaging ultraviolet (UV) radiation, thus extending the useful life of the material. Transparent films for prolonged use in tropical areas where UV radiation is relatively intense must be manufactured with considerable UV inhibition in order to prevent premature polymer degradation. Such films are more expensive to produce than those without UV inhibition.

In Peru, Raymundo and Alcazar (7) reported that solarization with two layers of transparent polyethylene film for 30 days was as successful in reducing potato root galling by M. incognita as was bromoethane, and was better than treatment with dazomet. Solarization with one layer of film was not as effective as with two layers, but gave control as good as dazomet. Both methods of solarization increased potato yield over the non-treated control. These results are encouraging, although the cost of using two layers of film must be weighed against additional benefit received.

Post-Plant Applications

Several experiments have been done to test post-plant applications of soil solarization for beneficial effects on disease and pest control, crop growth, and irrigation water conservation (6, 9, 10, 11). In most eases, post-plant solarization has been done with perennial crops. Post-plant solarization was effective in controlling Verticillium wilt (V. dahliae) in pistachio groves in California, and in reducing the necessity for irrigation of peach (Prunus persica) transplants (9). These findings were followed up by studies in Colima, Mexico, with several perennial fruit crops (10). In one experiment, nursery trees of Mexican lime (Citrus aurantifolia) and papaya (Carica papaya), and sprouted corms of banana (Muse) sp. were planted into moist clay loam soil during the rainy season. Maximum soil temperature at 23 cm depth in solarized soil was 44°C (6°C higher than control soil). After one to two months of mulching, the herbaceous perennial papaya and banana plants were observed to be growing poorly in comparison to those which were not mulched. Poor growth or death of papaya and banana plants also was observed in conjunction with a black film mulching treatment, where soil temperature was raised only 2°C above that of control soil. This finding suggested that these cultivars were sensitive to even small increases in soil temperature, or to effects of reduced soil aeration after post-plant mulching. In a second experiment, Mexican lime and mango (Mangifera indica) transplants were planted into pre-irrigated and mulched, loamy sand soil for ten weeks during the dry season. Non-mulched control trees were irrigated on a weekly basis, but those covered by the transparent or black polyethylene films required no irrigation. Although no significant differences in mango growth were observed during the five-month observation period, Mexican lime trees mulched with black film showed a four-fold increase in fresh weight compared to those in solarized or control soil.

Perhaps the most useful information from these experiments was the confirmation that woody perennials could be grown in pre-irrigated soil under film mulching for relatively long periods without supplemental irrigation. These findings were later rested in California with pecan (Carya illinoensis) trees in California (11). In this study, transplants were grown in pre-irrigated soil under transparent or black film mulches in two pre-irrigated soils for an entire season (May-October) without supplemental irrigation. No significant differences in tree vegetative fresh weight were found, although non-mulched control trees each received cat 1 475 litres more water during the season than the mulched trees. Control of pathogenic fungi Pythium ultimum and Verticillium dahliae, and of winter weed cover was nearly as good with black film as with transparent, probably due to the long treatment period.

It was noted during the course of all the post-plant studies that use of robust transplants was necessary for them to thrive in mulched soil. Small or weak plants may not survive the treatment. Furthermore, planting during relatively cool periods of the year allowed them to acclimate to the mulches prior to the onset of hot conditions. For this reason, mulching with black film may be advantageous to inhibit weed growth, since weeds may flourish under transparent film when soil temperature is not limiting.

The ability of woody perennials to grow for several months under polyethylene mulch with only a pre-irrigation may be utilized with considerable advantage by horticulturists in tropical regions where irrigation water is scarce or unavailable during the dry season. Durable films could be placed as mulches around trees during periods of drought, removed when irrigation water or rain is plentiful, and reused later.

Conclusion

Soil solarization appears to be a practice that can be of significant value in tropical agriculture, dependent upon climate, crop selection, and cost of materials. It would appear that horticultural production on small farms would be ideally suited to this safe and effective method of non-chemical soil disinfestation. Another excellent use for solarization is in the production of nursery crops and increase of propagative material, and in disinfestation of soil for research plots (3, 6, 9). A great deal remains to be known, however, and additional information from tropical ecosystems certainly is needed.

References

1. Baker, K.F. 1962. Principles of disinfestation of heat-treated soil and planting material. Journal of the Australian Institute of Agricultural Science 28:118-126.

2. Braun, M., H. Burgstaller, and A.M.G.F. Eldin. 1985. Approaches for the control of the parasitic weed Orobanche ramosa L. with special references to the use of glyphosate spraying and solar heating techniques on eggplant under the conditions of Sudan. Acta Horticulturae 158:335345.

3. Chauhan, Y.S., Y.L. Nene, C. Johansen, M.P. Haware, N.P. Saxena, Sardar Singh, S.B. Sharma, K.L. Sahrawat, J.R. Burford, O.P. Rupela, J.V.D.K. Kumar Rao, and S. Sithanantham. 1988. Effects of soil solarization on pigeonpea and chickpea. Research Bulletin No. 11, ICRISAT, Patanccheru, A.P. 502 324, India.

4. Daelemans, A. 1989. Soil solarization in West-Cameroon: effect on weed control, some chemical properties and pathogens of the soil. Acta Horticulturae 255:169-175.

5. Davalos G., P.A., and F. J. Castro. 1987. La solarizacion como un medio de control pare la "secadera" de la fresa en Irapuato, Gto. p. 68. In: Memorias del XIV Congreso Nacional de Fitopatología Sociedad Mexicana de Fitopatologia, Morelia, Mich., Mexico.

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

7. Raymundo, S.A. and J. Alcazar. 1986. Effects of soil solarization, dazomet and bromoethane on root knot nematode and yield of potatoes. American Potato Journal 63:450 (Abstract).

8. Salgado S. M., M. N. Marban, and V. Zamudio. 1988. Comparacion de los efectos de agregados organicos, nematicidas y solarizacion en la incideneia de Meloidogyne incognita asociado al cultivo de frijol en Tecamachalco, Pue. p. 10. In: Memorias del XV Congreso Nacional de Fitopatologia, Sociedad Mexicana de Fitopatología, Xalapa, Veracruz, Mexico.

9. Stapleton, JJ. and J.E. DeVay. 1986. Soil solarization: a non-chemical approach to disease and pest management. Crop Protection 5:190-198.

10. Stapleton, J.J. and J.G. Garza-Lopez. 1988. Mulching of soils with transparent (solarization) and black polyethylene films to increase growth of annual and perennial crops in southwestern Mexico. Tropical Agriculture (Trinidad) 65:29-33.

11. Stapleton, J.J., W.K. Asai, and J.E. DeVay. 1989. Use of polymer mulches in integrated pest management programs for establishment of perennial fruit crops. Acta Horticulturae 255:161-166.

12. Vidales F. J.A., O. D. Munro, and R. J. J. Alcantar. 1987. Control de pathgenos del suelo mediante el uso de energia solar en el cultivo de melon (Cumumis melo L.), en el valle de Apatzingan, Mich. p. 69. In: Memorias del XIV Congreso Nacional de Fitopatología, Sociedad Mexicana de Fitopatología,, Morelia, Mich., Mexico.

Table 1. Effect of pre-plant transparent solarization and black polyethylene film mulching on growth of sesame (Sesamum indicum) and incidence of charcoal rot (Macrophomina phaseolina) in two field soils in Colima, Mexico (from Reference 10)

z Air-dried seed. Values within columns followed by different letters are different at P=0.10 according to Duncan's multiple range test.

y Values within columns followed by different letters are different at P=0.05, except where noted, according to Duncan's multiple range test (from Stapleton and Garza-Lopez, 1988).

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