Use of black plastic for soil solarization and post-plant mulching.

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W. I. Abu-Gharbieh, H. Saleh, and H. Abu-Blan

Professor, Assistant Professor, and Associate Professor, respectively.
Plant Protection Department, Faculty of Agriculture, University of Jordan, Amman, Jordan

The authors wish to thank the Deanship of Research al the University of Jordan for sponsoring this research

Abstract

The effect of 10-week soil solarization, by row coverage with transparent or black plastic soil tarping, was tested in field experiments in the Jordan Valley. The black plastic was used for solarization and subsequently as soil mulch after planting, whereas the transparent film was later covered with perforated black mulch. Both tarps significantly reduce Fusarium oxysporum, F. solani, and Meloidogyne javanica, with the transparent being slightly more effective. Numbers of Sclerotinia, Pythium, Rhizoctonia, and Verticillium spp. from tarped soils were not significantly different from the control. Higher populations of Aspergillus, and to a lesser extent Penicillium, were found in solarized soils, while populations of free-living nematodes were generally reduced. Both tarps equally reduced incidence of Sclerotinia blight on eggplant by nearly 44 percent, compared to the control. The respective increase of yield as a result of solarization with transparent and black plastic tarps was 79 and 65 percent in tomato, 500 and 498 percent in eggplant, but only 21 and and 10 percent in cucumber. Use of black plastic for solarization then left in place as a postplant mulch, is economical and practical since black plastic mulching is normally used, the climate is suitable and relatively long periods of non-cropping are permissible for solarization in the Jordan Valley.

Introduction

As early as 1939, Grooshevoy (7) presented convincing evidence for control of soilborne plant diseases by solar heating in glass hot beds. Since Katan et al. (12) in 1976 and Pullman and DeVay (15) in 1977 used clear polyethylene for soil solarization, scientists from many countries reported successful control of many plant pathogens, weeds and mites (11). Use of transparent polyethylene for effective soil solarization has been frequently recommended (11, 12, 18). Black plastic, however, was either not recommended (10, 16) or generally ineffective (2, 13). In the Jordan Valley, Al-Asa'd in 1982 (1), found that solarization as preplant row-coverage with black plastic was effective in reducing soil fungi and increasing yields of tomato and eggplant.

In the Jordan Valley, black plastic soil mulching and drip irrigation are widely used in vegetable production in plastic houses and in open fields. These lands are mostly left fallow between July and October, due to shortage of irrigation water. These factors, in addition to intense solar healing in the summer months, contribute favourably to allow relatively long periods of soil solarization.

The objectives of these experiments were to lest effectiveness of solarization with black polyethylene tarping, as compared to transparent film, on three fall-season vegetable crops in the Jordan Valley; and hence, to minimize production costs by keeping the same tarp in place and subsequently using it as a postplant mulch. Another objective was to test this tarping procedure against pathogens not considered earlier in the Jordan Valley.

Material and Methods

Three field experiments were conducted at the Jordan University Farm in the Central Jordan Valley to test the effect of soil solarization with transparent vs. black polyethylene film on three fall-season crops. The land is of clay soil (16 percent sand, 39 percent silt and 45 percent clay) with EC = 1.27 and pH = 7.7; and naturally infested with several soilborne pathogens including the root-knot nematode Meloidogyne javanica. The soil was fertilized with poultry manure (1 kg/m²), flood irrigated, ploughed twice, listed lo form raised beds (1 m wide and 1 m apart) and fitted with drip irrigation lines laid in the middle of the beds.

The three experiments were arranged to accommodate three test crops. Each experiment included four blocks of three randomized treatments: 1) no solarization (dry summer fallow - control), 2) solarization with 60 mm thick transparent polyethylene film, and 3) solarization with 60 m m - thick black polyethylene film. Each experimental plot consisted of IWO rows (beds) 9 m long and 2 m between centres of beds. Preplant tarping with transparent or black polyethylene films was initiated on July 6, 1987. Strips (160 cm width) were centered over the drip lines and edges were anchored in 15-20 cm deep furrows dug on either side, then compacted with soil. Covered plots were dripirrigated for 3 hrs every week throughout the three-month solarization period. Maximum and minimum soil temperature, al 10 cm depth, were recorded every five minutes through a T type thermocouple connected to an "Orion" monitoring computer. Thermocouples were placed in representative plots of the three treatments.

At the end of solarization period, black plastic mulch(120 cm wide, 60 mm thick perforated in two rows al 20 cm distances) was laid on the non-solarized fallow plots and over the transparent tarps which were retained and manually perforated. The black tarp (used for solarization) was also retained in place and merely perforated al distances similar to those of the black mulch to facilitate transplanting.

Seedlings of tomato (Lycopersicon esculentum cv. Claudia Raf), eggplant (Solanum melongena cv. Black Beauty), and cucumber (Cucumis sativus cv. Beit Alfa F1), were established in peatmoss placed in plastic trays. Transplanting of tomato and eggplant seedlings was performed on Oct. 7, 1987 while cucumber seedlings were transplanted on Oct. 7, 1987. On Oct. 24, 1987, cucumber beds were covered with low plastic tunnels (1.2 m wide and 50 cm high), as required for this planting dale. Irrigation, chemical fertilization, and pest management practices were followed as normally required for the test crops.

Soil samples were taken from the experimental plots of each crop (5-15 cm depth) before tarping (July 6); at the end of solarization period (=planting date), nearly IWO months after planting; and at termination of each crop. Composite samples (three subsamples from each plot) were used for determination of fungal propagules (8) and number of second stage juveniles (J2) of M. javanica and other free-living nematodes (17).

Plant samples were taken from all plots for assessment of plant growth response (weight of foliage and roots), and roof galling and rolling. A root rolling index of (0-5) was used, with ()= no rolling and 5= maximum rolling. Four and eight plants were randomly taken, ca two months after planting and at the end of the growing season, respectively. Also, a plant growth index (0-5) was made on Nov. 25, 1987 for determination of general plant size and health in all plots. On March 25, 1988, foliage of eggplant and tomato plants was inspected in all plots for infection with Sclerotinia sclerotiorum.

The number of fruits and weight of yield per plot was periodically recorded until the end of harvesting of tomato (March 27,1988), eggplant (June 11, 1988) and cucumber June 3 1987).

Results

Soil temperature. - In the hottest period of soil solarization the absolute maximum soil temperature reached 50.0°, 47.8 and 46.5°C, at 10 cm depth, in soils tarped with transparent and black polyethylene, and non-tarped, respectively. The weekly average of maximum soil temperatures in the respective treatments ranged between 42.0-49.1°, 41.0-47.0° and 37.9-45.0°C.

Effect of soil solarization on soil fungi. - Before initiation of soil solarization, no significant differences were found in number of propagules of various fungi isolated from the soils of all respective treatments (Table 1; A, B. and C). At the end of the soil solarization period and approximately two months after planting, however, number of propagules of Fusarium oxysporum and F. solani was significantly reduced in both tarping treatments in almost all cases (Table 1: A, B. and C). Numbers of Alternaria solani propagules were significantly reduced in black and transparent solarization treatments in the tomato plots seven weeks after planting (Table 1A). Population density levels of Sclerotinia, Pythium, Rhizoctonia, and Verticillium spp. isolated from tarped plots were not different from those of the control. At termination of all crops, differences in population densities of monitored fungi were insignificant among the respective treatments except for F. solani in the eggplant experiment, where numbers in the control were significantly higher than those found in soils of both tarped treatments.

Two months after solarization, a significant increase in the number of Aspergillus spp. was found in soil of tarped plots planted to tomato and eggplant (Table 1; A and B). However, soil solarization did not significantly affect numbers of Penicillium spp., although numerical tendency indicated a population increase in the soils of both solarization treatments.

Incidence of eggplant plants infected with S. sclerotiorum in March 1988 showed a significant reduction of nearly 44 percent in both solarized treatments. However, no differences were found in the tomato experiment. Also, no significant differences were found in disease severity among the various treatments in both crops.

Effect of soil solarization on nematodes. - Soil samples assayed for numbers of J2 larvae of M. javanica before, and at the end of soil solarization, showed virtually no nematode recovery. However, juveniles began to appear in isolations made 7-8 weeks following planting of tomato, eggplant and cucumber (Table 2; A, B. and C). In the tomato experiment, results indicated a reduction of nearly 84 percent and 65 percent in the solarized soil with transparent and black plastic, respectively, as compared with the control.

Respective reductions were 83 and 37 percent in the cucumber. While numbers decreased by 80 percent after transparent tarping in the eggplant experiment, there was an increase of about 73 percent in the black tarping treatment compared to the non-solarized control. At the termination of the tomato experiment, larval counts were significantly less (87 percent) in the transparent tarping treatment compared to the non-tarped; while such effect was not evident in the black tarping treatment. Although larval populations became greater in the eggplant and cucumber experiments, significant differences among treatments did not exist.

A galling index taken nearly two months following planting showed significant reductions in plots solarized with transparent plastic, with black tarp (Table 2; A, B. and C). Black plastic tarping reduced galling index, but only in the cucumber experiment. Similar results were also found at the end of the growing seasons of tomato and cucumber, but galling was equally heavy on the roots of eggplant in all treatments. At the end of the experiment, it appeared that soils solarized with transparent plastic resulted in significantly less rotting of tomato roots than in the control, while not significantly different from black tarping. Rotting of eggplant roots was relatively heavy and not statistically different among treatments.

Densities of free-living nematodes were generally not significantly different among treatments before, and at the end of soil solarization (Table 2; A, B. and C). During and near the end of the growing season of all crops, however, numbers tended to be less in solarized than non-solarized soils, and in black than in transparent polyethlene-tarped soils, but without significant difference between the two tarping treatments.

Effect of soil solarization on plant growth and yield. - Growth index of plants made in Nov 11, 1987, indicated that plants of all tested crops grown in solarized soil, whether transparent or black, were always significantly larger and more thrifty-looking than those planted in non-solarized plots (Table 3). Fresh weight of foliage taken nearly two months after planting was significantly greater only in tomato plants tarped with transparent film. Root fresh weight of the same plants was not significantly different among treatments, except for the roots of eggplant which were heavier (P= 0.05) in both solarized treatments than in the non-solarized. At the end of the growing season, fresh weight of tomato foliage was significally higher in both solarized treatments but this was not reflected in root weights. In eggplant, however, significant differences existed among treatments in foliage weight, with the transparent film being highest, followed by black tarping. Roots of plants were heaviest when grown in transparent solarized plots. Weights of shoots and roots of cucumber were not significantly different among treatments.

Number of fruit and yield of tomato and eggplant were significantly higher in solarized plots than non-solarized, but with no differences between transparent and black tarping. Yield increase in plots solarized with transparent and black plastic over the non-solarized was 79 and 65 percent in tomato, and 501 and 498 percent in eggplant. In cucumber plots, however, number of fruits and yield were significantly higher in the transparent treatment only. Yield of cucumber in plots solarized with transparent and black tarping was 21 and 10 percent over the non-tarped.

Discussion

Our results indicated that soil solarization with balck polyethylene cover was only slightly inferior to the transparent in reducing populations of soilborne fungi and nematodes, but equally effective in improving plant growth and productivity. The weekly average maximum soil temperatures recorded at 10 cm depth in soils tarped with black plastic, in this experiment, and others (4; Saleh, Abu-Gharbieh and Abu-Blan, unpublished) conducted in the Jordan Valley revealed an average increase of only 2-3°C ( 1, 10). However, the weekly average of maximum temperatures during solarization with black tarping ranged between 41-47°C in these experiments. Such temperatures are considered sufficient for effective soil solarization. Katan (11) indicated that exposing pathogens to elevated temperatures is an important, but not exclusive, mechanism for control of soilborne pests by solarization. Other biological, chemical and physical factors are also involved, and may explain the surprisingly effective control of Verticillium wilt on potato by solarization at marginal conditions of temperatures not sufficiently high to justify such control (6). Baker (3) stated that mild soil heating was less detrimental to many saprophytes, thus reducing chances of reinfestation by parasitic microorganisms through induced suppressiveness. In the present experiments, numbers of Aspergillus and Penicillium spp., increased in solarized soils.

Plant growth and yield of the respective crops in these trials showed that black plastic tarping was effective for solarization. Such effects, however, were not always reflected in the populations of measured biological components. Other non-measured agents might have also contributed to these results (5).

Results of these experiments confirmed earlier findings in microplots by Al-Asa'd (1), and further demonstrated effectiveness against M. javanica and S. sclerotiorum, two major pathogens of vegetable crops. Positive results were previously obtained when clear polyethylene tarps used for soil solarization were laid over a black soil surface made of powdered charcoal (14) or treated after solarization with reflective pigment around transplanting holes (9).

Results of the cucumber experiment were not considered complete, for the crop was terminated prematurely due to severe damage from downy mildew.

In conclusion, use of black plastic tarping was considered quite satisfactory, due to the dual function of the same cover for soil solarization, and later as a post-plant mulch. This is particularly feasible, economically and practically, since black mulching is already much in use in the Jordan Valley, the climate is suitable, and periods of two months or longer of non-cropping are permissible for solarization purposes.

References

1. Al-Asa'd, M. A. 1983. Effect of solarization on soilborne fungi and nematodes in the central Jordan Valley. M. Sc. Thesis. Faculty of Agriculture, University of Jordan. 74 pp.

2. Al-Raddad, A. M. 1979. Soil disinfestation by plastic tarping. M. Sc. Thesis. Faculty of Agriculture, University of Jordan. 95 pp.

3. Baker, K. F. 1983. The future of biological and cultural control of plant disease. pp. 422-430. In: Challenging Problems in Plant Health. J. Kommedahl and P. H. Williams (eds.). APS, St. Paul.

4. Barakat, R. 1987. Comparative effect of different colors of polyethylene tarping on soilborne pathogens. M. Sc. Thesis. Faculty of Agriculture, University of Jordan. 82 pp.

5. Barbercheck, M. E. and S. L. Von Broembsen. 1986. Effect of soil solarization on plant-parasitic nematodes and Phytophthora cinnamomi in South Africa. Plant Disease 70:945-950.

6. Davis, J. R and L. H. Sorenson. 1984. Effect of soil solarization with moderate air tempertures on potato production. Am. Potato J. 61:520 (Abstract).

7. Grooshevoy, S. E. 1939. Disinfestation of seed-bed soil in cold frames by solar energy. Rev. Appl. Mycol. 18:635-636.

8. Grossman, D. F. 1967. Selective isolation of soil microorganisms by means of differential media. pp. 19-21. Sourcebook of Laboratory Exercises in Plant Pathology. Ameican Phytopathological Society. San Francisco and London.

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

10. Katan, J. 1981. Solar heating (solarization) of soil for control of soilborne pests. Ann. Rev. Phytopathol. 19:211-236.

11. Katan,J. Soil solarization. pp. 372. In: Innovative Approaches to Plans Disease Control. I. Chet (ed.). John Wiley & Sons., N.Y.

12. Katan. J. A., 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.

13. LaMondia, J. A. and B. B. Brodie. 1984. Control of Globodera rostochiensis by solar heat. Plant Disease 68:474-476.

14. LaMondia, J. A., B. B. Brodie, and R. Carter. 1984. The effect of solarization on decline of Globodera rostochiensis in naturally infested soil, Journal of Nematology 17:503 (Abstract).

15. Pullman, G. S. and J. E. DeVay. 1977. Control of Verticillium dahliae by soil tarping. Proc. Am. Phytopathol. Soc. 4:210 (Abstract).

16. Pullman, G. S. and J. E. DeVay. 1984. Soil solarization: a nonchemical method for controlling diseases and pests. Cooperative Extension, Davis, University of California, Division of Agriculture and Natural Resources. (Leaflet 21377).

17. Schindler, A. F. 1961. A simple substitute for a Baermann Funnel. Plant Dis. Reptr. 45:747-748.

18. Stapleton, J. J. and J. E. DeVay . 1986. Soil solarization a non-chemical approach for management of plant pathogens and pests. Crop Protection 5:190-198.

Table 1. Effect of soil solarization with transparent or black polyethylene sheets on populations of certain fungi recovered at different intervals from field plots

(cont'd)

Table 1 (cont'd)

1 1=Before solarization, 2 = End of solarization, 3 = Two months after planting, 4 = End of growing season.

2 Numbers followed by the same letter within each column are not significantly different (P = 0.05) according to Duncan's multiple range test. No significant differences were found among treatments in number of propagules of Sclerotinia sclerotiorum, Phytophthora spp., Venicillium spp., Rhizoctonia spp., Penicillium spp. at the four sampling dates.

Table 2. Nematode population densities, and indices of root galling and root rolling in field plots solarized with transparent or black sheets

(cont'd)

Table 2 (cont'd)

1 No J2 of M javanica were detected in isolations made June 6 or Oct. 7, 1987.

2 (0-4) galling index: 0 = galling, 1 = 1-25 %, 2 = 26-50 %, 3 = 51 - 75 % and 4 = 76-100 % root galling.

3 (0-5) Rotting index: 0 = no rolling, 5 = maximum root rolling.

4 Numbers followed by the same letter within each column are not significantly different (P = 0.05) according to Duncan's multiple range test.

Table 3. Plant growth and yield of three vegetable crops grown in field plots solarized with transparent or black polyethylene sheets

1 Low yield of cucumber is due to short life span caused by early of cowing mildew.

2 Numbers followed by the same letter within each column are not significantly different (P = 0.05) according to Duncan's multiple-range test.

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