Previous - Next
Use of solarization in marginally suitable climates
Angelo Garibaldi and M. Lodovica Gullino
Istituto di Patologia vegetale, via Giuria 15, 10126 Torino Italy
Introduction
It has been estimated that over a third of the losses caused by plant diseases are due to soilborne pathogens of protected crops that cover an area of about 20 000 ha in Italy. Repealed planting of the same crop, particularly common with valuable crops, results in increased inoculum potential and disease. The aggressiveness and rate of accumulation of different pathogens in soils depend upon factors such as host susceptibility to pathogen, cropping history, chemical, physical and biological properties of the soil, cultural practices, climate, control measures and field hygiene (21).
Soil disinfestation is an effective method for managing soilborne plant pathogens as well as weeds and insects. In the last 50 years, soil disinfestation has been carried out with steam and the use of chemical fumigants. Some of the problems encountered when soil is disinfected by these measures include high Costs, creation of a microbiological vacuum, difficulties in reaching deep inoculum, and potential toxicological and environmental risks of fumigant chemicals. The high cost of treatment, calculated in Italy as 1.5 US$/m² for steaming and 0.4 to 1 US$/m² for fumigants, limits the use of these traditional means to high-value crops also in greenhouses. The non-selectivity of both steam and chemical treatments sometimes results in the so called "microbiological" vacuum", where beneficial organisms are eliminated. As a consequence, reinfestation by previously present pathogens can occur, i.e. Fusarium roseum on carnation (27) or new ones can become important, as in the case of Fusarium oxysporum f. sp. radicis-lycopersici on tomato (18).
The development of solarization for soil disinfestation by Katan et al. (22, 23) opened new perspectives in soil disinfestation, also in marginally suitable climates, especially on protected crops, that are discussed in this paper.
Principles of solarization
Solarization is based on solar heating of the soil by covering it with transparent plastic material during the hot season. The plastic sheets, laid on the soil, capture solar energy. The increased temperature kills pests, including a variety of crop pathogens (19, 20).
Solarization is based on heating soil to relatively mild levels, generally ranging from 36° to 50°C in the upper 30 cm. Although heat is the major killing agent, there is evidence that biological processes may contribute to the good control frequently achieved by solarization, especially when the temperatures attained were not sufficiently high to justify such control (11). The reduction of the pathogen population observed in solarized soils may be attributed to biological control mechanisms in addition to thermal killing. Biological control can occur by enhancement of antagonistic micro-organisms and/or through a shift in the equilibrium in favour of micro-organisms which prevent reinfestation of pathogens. If such a beneficial shift occurs, disease control would be expected to show a long-term effect (21) as has been observed in Northern Italy (14,15).
Reduction in disease incidence from solarization, results from modifications of the living components involved in disease (host, pathogen and surrounding micro-organisms). Changes in the physical and chemical environment affect the activity and interrelationships of these organisms. Although these processes occur primarily during solarization, they may continue after the removal of the plastic sheets and planting.
Increased plant growth response is frequently observed following soil solarization. The mechanisms for this phenomenon may be the release of mineral nutrients or growth factors, the elimination of toxins and pests, and the stimulation of beneficial organisms (20).
Improved results from solarization in marginally suitable areas, such as Northern Italy, may be achieved under the following conditions:
- cover soil during the period of high temperatures and intense solar radiation July-August in northern hemisphere);
- keep soil moist to increase the thermal sensitivity of resting structures and improve heat conduction;
- use the thinnest transparent plastic possible (25-30 nm) to increase effectiveness;
- use double layers of plastic materials inglobing air bubbles or two layers of polyethylene;
- extend mulching period (6 weeks or longer) to achieve control at desired depths. Longer mulching periods improve pathogen killing at deeper levels as has been shown by Berninger et al. (3);
- combine solar heating with biological control agents, if available.
Solarization in cooler regions
Although developed in warm regions such as Israel, Egypt and California, solarization soon extended to cooler regions. In these areas, where soil covered with plastic film does not reach temperatures high enough to inhibit pathogens in open fields, solarization has been applied in closed plastic houses or glasshouses. This technique has been developed and used in Belgium, France, Greece, Italy, Japan, Portugal, Spain (20). This technique is now practically applied at least in Japan, Italy and Greece (13, 17).
In Japan, control of Fusarium wilt of strawberry is achieved, under plastic house, by attaining 40°C at 20 cm depth. In strawberry plantations solarized for several years, the population density of F. oxysporum decreased, and increased yield and plant growth were observed. The addition to soil, before mulching, of soluble starch (25-50 g/kg of dry soil) improved the effect of solarization (24). Solarization under greenhouses is largely used in Japan: over 2 200 ha of strawberry, eggplant, tomato and cucumber soils, at present, are disinfested by means of this technique (17).
In Greece, where Verticillium wilt represents a limiting factor for several crops, solarization effectively reduced soil infestation by V. dahliae propagules. Solarization carried out in plastic houses controlled Verticillium wilt on tomato and increased fruit yields up to 300 percent (33). Solarization under greenhouse conditions effectively controlled F. oxysporum f. sp. radicis-lycopersici, causal agent of crown and root rot of tomato (4). Isolations from the rhizosphere of artichoke and tomato plants grown in solarized soil showed a higher occurrence and survival of Talaromyces flavus, an antagonist of V. dahliae. In plastic houses of Northern Greece, solarization was successful also in controlling corky root of tomato, caused by P. Iycopersici (32); and even to a greater extent in Crete, where solarized soil reaches higher temperatures (25).
Also in France, under greenhouse conditions, it is possible to achieve results similar to those obtained in warmer regions for the control of corky root of tomato. In 1982, during solar heating treatment, carried out from July 15 to August 15, the maximum temperatures reached were 47 C at 10 cm depth, and 38°C at 35 cm in the area of eastern Pyrenecs. These temperatures are similar to those obtained in Lot et Garonne (16). In the Frejus area, the conditions necessary for good solarization are encountered four years out of five, and solarization effectively reduced the pathogenic soil mycoflora and nematofauna up to a depth of 40 cm and controlled weeds. On the contrary, soil bacterial populations were unaffected (3).
In Spain, in the Murcia region, solarization in open fields and under greenhouses with two plastic sheets gave good soil temperature increases and controlled Meloidogyne javanica (7, 8). Soil solarization, carried out for a two month period under plastic house, eradicated F. oxysporum f. sp. niveum on watermelon in Southern Spain (26). Solarization also effectively controlled P. Iycopersici and Meloidogyne spp. on tomato grown under greenhouse conditions and eliminated weeds in Portugal (28).
In Northern Belgium, solar heating was effective in reducing microbial populations in the soil, only when applied for a three month period under glasshouses. In open fields, on the contrary, solarization was effective only when applied in combination with fumigants. Solarization caused significant increases in Trichoderma and bacterial populations and decreases in pathogenic Fusarium and Verticillium spp. (38). In Northern Europe, temperature increases reached through solarization might not be sufficient for effective control of soilborne pathogens. When control of Plasmodiophora brassicae on cabbage by means of solar heating was attempted in the United Kingdom, increases in soil temperature in polyethylene-covered fields sufficient to reduce clubfoot incidence on cabbage seedlings were reached only in the first 10 cm of soil. Only small temperature increases were recorded below the first 10 cm of soil and there was not effect on clubfoot incidence (39). This temperature increase reached under UK conditions seems not sufficient for practical application of this method of soil disinfestation.
Under Northern Italy climatic conditions, solarization must be carried out exclusively under greenhouse conditions, but in Central and Southern Italy, solar radiation is sufficient for control of several soilborne pathogens even in open field. Reductions of corky root (P. Iycopersici) and of Verticillium wilt (V. dahliae) severity on tomato have been obtained in Northern Italy under greenhouse conditions. The control of corky root was still significant 7-8 months after treatment. Under plastic house conditions, the influence of solarization on corky root was sometimes less marked, probably as a consequence of the lower soil temperatures in plastic houses than in glasshouses (14). Solarization in greenhouses was effective also against Sclerotium cepivorum on leek (31), Rhizoctonia solani on bean and basil (30), Phytophthora nicotianae var. parasitica and F. oxysporum f. sp. dianthi on carnation (Garibaldi, unpublished). Solarization showed good activity also against weeds (12,30).
Starting in 1988, solarization has been applied under practical conditions on tomatoes grown under plastic houses, in the Liguria region (Northern Italy): this soil disinfestation technique is now successfully applied every year on about 10 hectares in this area (13). The results obtained by growers under practical conditions are very promising: solarization does reduce the incidence of corky root and Verticillium wilt of tomato, R. solani and Sclerotinia sclerotiorum on lettuce, Fusarium wilt (F. oxysporum f. sp. basilicum) of basil and of weeds on many crops. On the contrary, the results obtained against crown rot of tomato, caused by F. oxysporum f. sp. radicis-lycopersici, in this area are very poor (Garibaldi, unpublished). Moreover the cost of the treatment is very low in comparison to other methods for controlling soilborne pathogens.
In Central Italy, solarization significantly reduced the incidence of lettuce drop (Sclerotinia minor) (35) and R. solani on radish (36). The treatment reduced the inoculum density of S. minor in the soil. Sclerotia from solarized soils were frequently colonized by Penicillium, Aspergillus, and Fusarium spp. (37).
In Sicily (Southern Italy), solarization controlled P. Iycopersici and V. dahliae on tomato and eggplant, Phoma Iycopersici spp. on carrot. The treatment also reduced stem base necrosis of pepper (causal agent unknown) and damage caused by M. incognita and M. javanica. In closed glasshouses, the high temperatures reached (>60°C), strongly reduced survival of foliar pathogens such as Botrytis cinerea, Cladosporium fulvum and powdery mildew. In solarized soils, a strong increase of Trichoderma populations was observed (5, 6).
Use of different plastic materials
The choice of plastic material for mulching is crucial in countries such as Italy, France and Spain, placed at the limit for use of solarization, since less efficient plastics may not result in sufficient temperature increases for giving control of many soilborne pathogens. The effect of different types of plastic has been evaluated under different conditions by several authors in Europe. Thin, transparent polyethylene has thermal properties and a relatively low cost. UV-absorbing polyethylene has the advantage of being more resistant to degradation under intense sunshine. In France, Berninger et al. (3) found, under open field and greenhouse conditions, ethylvinylacetate (EVA) (150 nm) and polyethylene inglobing air bubbles (5 nm) to be superior to plain polyethylene (150 nary). The use of double plastic sheets inside a closed greenhouse provided longer periods at higher temperatures, compared to mulching with single plastic sheet in Spain (7). In Northern Italy under greenhouse conditions soil covered for 50 days with PVC (polyvinylchloride) reached higher temperatures for longer periods compared with polyethylene of the same thickness. The number of hours with temperatures above 40°C at 15 and 20 cm depth were 347 and 156 for PVC and 308 and 36 for polyethylene (14). The use of a double polyethylene film, inglobing small air bubbles (Tristar) raised soil temperatures to an average 42.5°C at 24 cm depth. This was 2-2.5°C higher than those obtained by using single polyethylene (15). Also the use of two polyethylene sheets, kept separate, achieves results similar to those obtained with the use of Tristar, at a lower cost. Although both single and double polyethylene sheet mulching in greenhouses significantly reduced R. solani attacks and weed emergence under northern Italy conditions, the use of double polyethylene sheets might be more effective at higher latitudes where reaching high soil temperatures is problematical (15).
Combining solarization with other control measures
The combination of pesticides, biocontrol agents or cultural practices with soil solarization could improve disease control in marginally suitable climates. In Italy, the combination of solarization with Di trapex (methyl isothiocianate + shell DD), applied at half of the normal recommended dosage (50 g/m²) did not improve the control of P. Iycopersici and V. dahliae on tomato compared to solarization alone (14). On the contrary, the combination of solarization with metham-sodium (12.5 or 25 m1/m²) was more effective in controlling V. dahliae and F. oxysporum f. sp. vasinfectum on cotton as compared with solarization or metham-sodium alone (3). The combination of a low dosage of methyl bromide (30 g/m²) with solarization under glasshouse conditions, did control corky root (P. Iycopersici) and root-knot nematodes (M. incognita and M. javanica) on pepper. The combination of the two soil disinfection methods controlled nematodes for two years, while solarization alone must be carried out every year (6). Generally, however, the combination of reduced dosages of fumigants with solarization against soilborne pathogens did not lead to an improved efficacy. In Greece, reduced dosages of methyl bromide (34 g/m²) combined with solarization were effective against V. dahliae on artichoke for at least three years. The survival and increase of beneficial T. flavus populations in treated artichoke fields may be, al least partially responsible for the long- term effect. Propagules of T. flavus increased and survived better in solarized soil than in solarized and fumigated soil; solarization alone kept the pathogen population at low levels throughout the three-year experimental period, while the combination was effective against V. dahlias for only two cropping seasons. This could be due to an adverse effect of the fumigant against a broad spectrum of antagonists (34).
The combination of different fumigants with solarization have not improved the control of P. Iycopersici on tomato in crete (25). The application of calcium cyanamide in combination with solarization did not increase the control of Fusarium will on strawberry (24). The combination of solarization with biocontrol agents is a promising concept. Biocontrol agents added to treated soil to reduce reinfestation of soil by pathogens may extend the period of disease control. Combining the antagonistic Trichoderma harzianum with solarization in R. solani and V. dahliae infested soils improved disease control and delayed inoculum build-up (10). The combination of solar heating with T. harzianum did result in a better control of Sclerotium rolfsii on beans (9). In R. solani infested soil, soil application of Trichoderma sp. after solarization, did not improve the activity of solar heating on bean (29, 30). The application of antagonistic Fusarium spp. after solarization improved the control of Fusarium wilt of carnation over solar heating alone (Garibaldi, unpublished).
Conclusions
Soil solarization has potential advantages over other soil disinfestation methods especially in protected crops. It is less expensive, non-chemical, with low hazards to the user, and does not involve the use of toxic substances. These advantages are of special interest in many European countries, where the arsenal of chemicals available is restricted, especially for protected vegetable crops. Furthermore, the cost of solarization can still be reduced by eliminating the attachment of the plastic sheets to the soil in protected structures and by reusing the plastic sheets (33). Avissar et al. (1) showed that recycling the plastic insures even better results. This is attributed to changes in the photometric properties of the aged mulching plastic (2).
Solarization appears as a simple component of integrated pest control, based on easily manipulated physical factors. It can be easily adopted by farmers, and is viewed as part of the soil preparation before planting. To date, there are only economic and timing constraints for the use of solarization before any crop. In estimating its economic value, solarization should be considered not only as a possible replacement for herbicide, fumigant and other pesticide treatments, but also as a solution for situations in which no efficient and safe control method is available for weeds and soilborne pathogens.
All the advantages of solarization cannot be achieved in every situation, and in certain cases advantages can even be counterbalanced by disadvantages. Actually, solarization, as well as most control measures, has limitations, problems, and potential negative side effects. First of all, its use is restricted in Europe and in other marginally suitable climates to regions with intense sunshine or to protected structures. Soil must be crop-free for at least one month at the time suitable for mulching, and this may be during the growing season for some crops. For example, this control measure cannot be adopted against soilborne pathogens of carnation in Italy and in other Southern Europe countries, because this crop is planted in May-June, before solarization can be applied in this area. Moreover, solarization is ineffective or not completely effective against certain pathogens. In trials carried out in Northern Italy under glasshouses, F. oxysporum f. sp. radicis Iycopersici was not controlled by solarization. It is possible that heat sensitive pathogens will develop heat resistant strains after repeated applications of solarization. Another possible negative side effect could be an increase in pathogen populations due to the suppression of antagonists. But, at present, none of these negative effects has been observed under greenhouses in a marginally suitable area such as the Riviera coast in Northern Italy.
References
1. Avissar, R., Y. Mahrer, L. Margulies, and J. Katan. 1986. Field aging of transparent polyethylene mulches: I. Photometric properties. Soil Soc. Am. J. 50:202.
2. Avissar, R., O. Naot, Y. Mahrer, and J. Katan. Field aging of transparent polyethylene mulches. 11. Influence on the effectiveness of soil heating. Soil Sci. Am. J. 50:205.
3. Ben-Yephet, Y., J. M. Melero-Vera, and J. E. DeVay. 1988. Interaction of soil solarization and metham-sodium in the destruction of Verticillium dahliae and Fusarium oxysporum f. sp. vasinfectum. Crop Protection 7:327.
4. Bourbos, V. A. and M. T. Skoudrikakis. 1989. La solarisation du sol contre la pourriture des rapines et du collet de tomate en serre. Proc. EEC Expert's Meeting: on "Practical application of integrated control in protected crops, Antibes (abstract).
5. Cartia, G. La solarizzazione del terreno: esperienze maturate in Sicilia. Informatore Fitopatologico 39:49.
6. Cartia, G., T. Cipriano, and N. Greco. 1989. Effect of solarization and fumigants on soil-borne pathogens of pepper in greenhouse. Acta Horticulturae 255:111.
7. Cenis, J. L. 1987. Double plastic sheet for improving soil solarization efficiency. Proc. VII Congress Mediterranean Phytopathological Union. p. 73.
8. Cenis, J. L. 1984. Control of the nematode Meloidogyne javanica by soil solarization. Proc. Vlth Congr. Mediter. Phytopathol. Union. p. 132.
9. Chet, I. 1987. Trichoderm - Application, mode of action, and potential as a biocontrol agent of soilborne plant pathogenic fungi. In: Innovative approaches to plant disease control. I. Chet (ed.). John Wiley & Sons. N.Y.
10. Chet, I., Y. Elad, A. Kalfon, Y. Hadar, and J. Katan. 1982. Integrated control of soilborne and bulb-borne pathogens in iris. Phytoparasitica 10:229.
11. Davis, J. R. and L. H. Sorenson. 1984. Effect of soil solarization with moderate air temperatures on potato production. Am. Potato J. 61:520 (Abstract).
12. Duranti, A., and L. Cuocolo. 1988. Solarization in weed control for onion (Allium cepa L.). Adv. Hort. Sci. 2:104.
13. Garibaldi, A., G. Bozzano, and U. Longoni. 1989. Prime applicazioni nella pratica della solarizzazione nelle serre dell'albenganese. L'Informatore agrario 45:61.
14. Garibaldi, A., and G. Tamietti. 1983. Attempts to use soil solarization in closed glasshouses in Northern Italy for controlling corky root of tomato. Acta Horticulturae 152:237.
15. Garibaldi, A. and G. Tamielli. 1989. Solar heating: recent results obtained in Northern Italy. Acta Horticulturae 255:125.
16. Goisque, M. J., H. Lormet, C. Martin, J. Lagier, P. Davet, Y. Courteadier, and J. Lormet. 1984. La désinfection solaire du sol. Rev. Hon. 247:1.
17. Horiuchi, S. 1990. Solarization for greenhouse crops in Japan. First International Conference on Soil Solarization, Amman, p. 6 (Abstract).
18. Jarvis, W.R. 1988. Fusarium crown and foot rot of tomatoes. Phytoprotection 69:49-64.
19. Katan, J. 1983. Soil solarization. Acta Horticulturae 152:227.
20. Katan, J. 1987. Soil solarization. p. 77. In: Innovative approaches to plant disease control 1. Chet (ed.) John Wiley & Sons. N.Y.
21. Katan, J. 1984. The role of soil disinfestation in achieving high production in horticultural crops. Proc. Br. Crop Prot. Conf. p. 1189.
22. Katan, J., A. Greenberger, H. Alon, and A. Grinstein. 1976. Solar heating by polyethylene mulching for the control of diseases caused by soilborne pathogens. Phytopathology 66:683.
23. Katan, J., A. Grinstein, A. Greenberger, O. Yarden, and J. E. DeVay. 1987. The first decade (1976-1986) of soil solarization (solar heating): a chronological bibliography. Phytoparasitica 15:229.
24. Kodama, T. and T. Jukui. 1982. Application of solar heating with plastic-film mulching in the outdoor field for control of Fusarium wilt of strawberry. Ann. Phytopath. Soc. Japan 48:299.
25. Malathrakis, N. E., G. E. Kapetanakis, and D. C. Linardakis. 1983. Brown root rot of tomato and its control in Crete. Ann. Appl. Biol. 102:251.
26. Melero-Vera, J. M., J. Gomez-Vazquez, and R. Gonzalez-Torres. 1989. Eradicative control of Fusarium wilt of watermelon under plastic greenhouse conditions. Proc. EEC Expert's Meeting on Practical Application of Integrated Control in Protected crops. Antibes (Abstract).
27. Migheli, Q. and A. Garibaldi. 1985. Ricerca sull'effetto della carica microbica del terreno sulfa gravita' della fusariosi parenchimatica del garofano. Ann. Fac. Sci. Agr. Univ. (Torino) 14:33.
28. Silveira, H. L., and M. L. V. gorges. 1984. Soil solarization and weed control. p. 345. Proc. EWRS 3rd Symposium on Weed Problems in the Mediterranean Area.
29. Tamietti, G. and A. Garibaldi. 1987. Effectiveness of soil solarization against Rhizoctonia solani in Northern Italy. Proc. CEC/IOBC Group Meeting on Integrated Pest management in protected vegetable crone.
30. Tamietti, G. and A. Garibaldi. 1989. Impiego della pacciamatura riscaldante contro Rhizoctonia solani nelle condizioni di coltura protetta in Liguria, Informatore fitopatologico 39:43.
31. Tamietti, G., D. Valentino, and A. Garibaldi. 1987. Solar heating in Northern Italy to control soilborne pathogens on vegetable crops. p. 73. Proc. VII Congress Mediterranean Phytopathological Union.
32. Tjamos, E. C. and A. Faridis. 1980. Control of soilborne pathogens by solar heating in plastic houses. p. 82. Proc. Vth Congress of the Mediterranean Phytopathological Union, Patras.
33. Tjamos, E. C., V. Karapapa, and D. Bardas. Low cost application of soil solarization in covered plastic houses for the control of Verticillium wilt of tomatoes in Greece. Acta Horticulturae 255:139.
34. Tjmos, E. C. and E. J. Paplomatas. 1988. Long-term effect of soil solarization in controlling Verticillium wilt of globe artichokes in Greece. Plant Pathology 37:507.
35. Triolo, E., G. Vannacci, and G. Scaramuzzi. 1985. Possibilità di applicazione della solarizzazione del terreno in Italia: indagini sul binomio lattuga-Sclerotinia minor Jagger. La difesa delle piante 8:127.
36. Vannacci, G., A. Materazzi, and E. Triolo. 1987. Effects of solar heating on soil-borne Rhizoctonia solani in greenhouses. p. 55. Proc. VIIth Congr. Medit. Phytopath. Union.
37. Vannacci, G., E. Triolo, and A. Materazzi. 1988. Survival of Sclerotinia minor Jagger sclerotia in solarized soil. Plant and Soil 109:49.
38. Welvaert, W. and J. Poppe. 1986. Influence of plastic mulching and disinfection on the fungal flora of soils in Belgium. EPPO Bull. 16:311.
39. White, J. G. and S. T. Buczacki. 1979. Observations on suppression of clubroot by artificial or natural heating of soil. Trans. Br. Mycol. Soc. 73:271.