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James R. Davis
Research Extension Center, University of Idaho Aberdeen, ID 83210
Short- and Long-term Effects
With solarization, vast possibilities for disease control are possible. Pathogen-populations reduced or suppressed by this procedure include fungi, bacteria, and nematodes, and with pathogen-suppression a wide range of weeds may also be controlled (24).
Although the majority of solarization studies have been made in areas with high soil temperatures (37-50 C) near the soil-surface (upper 30 cm), evidence also suggests disease control at lower temperatures. Ashworth and Gaona (2) and Pullman et al. (18) found solarization to effectively reduce Verticillium dahliae populations at depths of 70-120 cm, suggesting that more may be involved with solarization than an increase of temperature. Phytophthora cinnamomi populations have been shown to be reduced in soil at a 70 cm depth (173 and the nematodes Paratrichodorus porosus, ParatyIenchus hamatus, and Paratylenchus vulnus have been reduced at depths of 46, 46, and 91 cm respectively (23).
Investigations with the potato (5) show that the practice of solarization may also have positive effects under conditions of temperate climate. The potato is the number one vegetable crop in the world and is produced primarily within regions of moderate temperature. The use of solarization with the potato introduces a wide spectrum of Possibilities for future food production. The potato is plagued by numerous soilborne pathogens that include fungi, bacteria, and nematodes. Since soilborne diseases of potato may produce heavy losses at all stages of growth, this crop is an excellent candidate for investigating the effects and principles of solarization.
In the arid regions of the western United States, Verticillium wilt caused by Verticillium dahliae is one of the most significant potato pathogens. Our studies (5) show that Verticillium wilt of potato may be effectively controlled by the solarization technique even with temperature changes considered to be marginal (<37 C at 30 cm depth). Our studies showed the mean maximum temperature at the 15 cm depth to be 41°C, while the mean maximum at 30 cm to be only 33°C in contrast to 26° and 23°C in the non-solarized plots. At the 15 cm soil-depth, V. dahliae populations were shown to be reduced from 9.7 to 0.3 cfu/g of soil while Pratylenchus sp was reduced from 29 to 9 nematodes/250 cm³. In contrast, populations of both V. dahliae and PratyIenchus were not found to differ significantly between the solarized and non-solarized treatments at the 15-30 cm depth. Yet, in-spite of this lack of population change, differences were Still evident (Figs 1 and 2) when several parameters of Verticillium wilt were measured one year following solarization (incidence of symptoms, vascular discoloration, and cfu of V. dahliae in potato stems), and with these differences of disease incidence there also occurred significant increases of both potato yield and quality (Fig. 3). The colonization of V. dahliae in potato stem tissue was found to be highly correlated with both wilt severity (r=0.816, p=.001) and potato yield (r=-0.735, p=.001).
Because potatoes were planted within the 10-15 cm region below the soil surface and the roots were believed to grow within the 15 to 30 cm infested region of the soil profile, both the levels of disease control and increased yields were not anticipated. Although the explanation for this is not known, the relatively greater reduction of disease incidence compared with inoculum density (ID) may have been due to either or both of two possibilities: the initiation of soil suppressiveness (6, 11), and/or an adverse effect on inoculum potential of propagules by sublethal heating, thereby lowering the capacity to produce disease (14).
Interestingly, yield responses were also shown to occur with a highly resistant potato clone (A68113-4) that remained nearly symptomless throughout the growing season. Although symptom development of Verticillium was negligible with this clone, significant yield increases with this potato occurred by 18 percent and yields of US #I potatoes were found to increase by 25 percent (Fig. 3).
With the more susceptible Russet Burbank clone, yield increases as high as 46 percent occurred one year following treatments and yields of US #1 potatoes were found to increase by 118 percent (Fig. 3). These benefits of increased US #1 yield continued on into the second year of continuous potato cropping following solarization with a significant 16 percent increase of US #1 potatoes. Again l want to emphasize that all of these benefits occurred with conditions that were considered marginal with solarization.
Although additional benefits were also found to occur with solarization (e.g. suppression of common scab caused by Streptomyces scabies, a suppression of stem-lesions associated with both Rhizoctonia solani and Colletotrichum atramentarium, and an increase of specific gravity on the NDA 8694-3 potato clone), increases of both yield and quality were most closely associated with the control of Verticillium wilt caused by V. dahliae.
The occurrence of these benefits in moderate temperature conditions provide hope for the future. In light of increased reductions of pesticides, threats of ground water pollution, and the everlasting need to control a wide range of soilborne pathogens and pests, the science of solarization provides many possibilities for improved yield.
Katan (12) summarized the effect of soil solarization on yield. In certain instances these yield benefits were infinitive since the crops in unsolarized soil were totally destroyed by the pathogens (10, 21). Increases in yield by solarization may depend upon a variety of factors: damage resulting from the disease being controlled, inoculum potential, efficacy of control, compensation by neighbouring plants, and the phenomenon of increased growth response (11). Perhaps more important, solarization may also improve quality with increases of yield (5, 7, 9, 19).
In addition to long-term effects on Verticillium wilt of potato (5), extended benefits of solarization for more than one season have also been documented for a variety of diseases including both Fusarium and Verticillium wilts of cotton (11, 13, 18). Pullman et al. (18) showed the effectiveness of solarization against Verticillium wilt in successive cultivations of safflower and cotton. Katan et al. (13) reported a long-term effect of soil solarization against Fusarium oxysporum f. sp. vasinfectum in cotton trials for over three years. Abdel-Rahim et al.(1) reported yield increases of 25432 percent in broad beans, onion, tomatoes, and clover in various soil types and reported long-term effects for 2-3 seasons in both disease control and yield increases. Tjamos and Paplomatas (27) found soil solarization to control Verticillium wilt of globe artichokes for three successive cropping seasons.
Integrated Control
The possibility of introducing a variety of combined treatments (biological, chemical, and cultural) with solarization may provide for an even wider spectrum of disease suppression. The possibility exists that with an integrated approach shorter periods of solarization may be required, and perhaps most importantly, the use of solarization with integrated pest management procedures may further open the door for the successful use of solarization in cooler regions of this planet that are considered to be marginal for solarization.
The use of biological control with solarization provides many possibilities. The use of Trichoderma harzianum with solarizaton in fields infested with Rhizoctonia solani has been shown to improve disease control while delaying the build-up of inoculum (4, 6). Similarly, Sztejnberg et al. (26) showed that when T. harzianum was combined with sublethal heating that the control of Rosellinia necatrix was improved when compared with either treatment applied separately.
Greenberger et al. (8) concluded that solarized soils are frequently more suppressive and less conducive to certain soilborne pathogens than non-solarized soils. Stevens et al. (25) showed the density of green florescent pseudomonads to increase in the rhizosphere of sweet potato roots with solarization. These responses to solarization with increased populations of green florescent Pseudomonas spp. have also included increases of both yield and quality (increased U.S. #I roots). Similarly, Stapleton and DeVay (22) showed an increase in populations of green fluorescent pseudomonads, along with an increase of Penicillium and Aspergillus spp. following solarization. Triolo et al. (28) found sclerotia of Sclerotinia minor to be more heavily colonized by bacteria and fungi in solarized soils than in non-solarized soils. With solarization, the prevalence of Aspergillus, Fusarium, Penicillium, and Trichoderma spp. was also found to increase. The work of Martyn and Hartz (16) showed a shift in populations with solarization from a pathogenic Fusarium (Fusarium oxysporum f. sp. niveum) to higher populations of saprophytic Fusarium spp., suggesting the possibility for increased pathogen competition with solarization.
Further intriguing possibilities for disease control are provided by Tjamos and Paplomatas (27). Their work shows an increase in populations of Talaromyces flavus in solarized soils, and with this increase, the potential involvement of biological control by T. flavus may contribute to the effectiveness of solarization. Since these workers were able to achieve a long-term effect by solarization on the control of V. dahliae in an artichoke field, it might be claimed that T. flavus was at least partially responsible for this long-term benefit. Marois et al. (15) have previously reported Verticillium wilt of eggplant to be controlled by suspensions of conidia and/or ascospores of T. flavus. Based on this study, the number of T. flavus propagules obtained from the roots of healthy artichoke plants by Tjamos and Paplomatas in solarized fields approximated those considered sufficient for the suppression of Verticillium wilt.
The improvement of disease control through a synergistic combination of treatments, preferably at reduced pesticide dosages, is one of the major goals for integrated pest management. Solarization may provide a means to accomplish this. Significant increases of disease control have been achieved when metham sodium has been combined with solarization (3, 7).
Frank et al. (7) compared the effects of metham sodium at 900 L/ha, solarization, and the two treatments together and found yields of peanut pods without necrotic spots to increase by 114 percent, 440 percent, and 893 percent respectively. These workers suggested that pre-wetting the soil with metham solution instead of water when preparing the soil for solarization may be useful in controlling additional soilborne disease agents. Ben-Yephet et al. (3) found that the fumigant dose of metham sodium required to kill 50 percent of either V. dahliae or F. oxysporum f. sp. vasinfectum populations was four times higher at 25°C than at 35°C. Field experiments also supported this. Two successive field experiments with both solarization and metham sodium treatments demonstrated that when these treatments were combined, the effectiveness against V. dahliae and F. oxysporum f. sp. vasinfectum was enhanced.
Combining solarization with a suitable crop sequence of resistant varieties may also influence the long range effects of solarization. Katan et al. (13) compared the cropping of a cotton variety that was resistant to Fusarium wilt (Acala) with a susceptible variety (Pima) during the first cropping season following solarization. When the susceptible variety (Pima) was grown over all plot sites during the second cropping season following solarization, the disease was lower and yields were higher in plots where the resistant variety had been grown during the preceding year. The beneficial effect of solarization was prolonged by this cropping sequence into the third year. It was not known, however, whether disease reduction following the Acala crop was due to a reduction of inoculum density or to the lower frequency of incorporating tissues into the soil, thus slowing the build-up of inoculum.
Similarly, Davis and Sorensen (5) grew a Verticillium-resistant potato clone (A68113-4) in soil following solarization, and then followed this in the second season with the more susceptible cv Russet Burbank. Potatoes grown in plots previously cropped with the resistant clone were colonized less severely by V. dahliae. The use of a resistant clone following solarization was found to delay the increase of V. dahliae. Similarly, when a potato clone with moderate Verticillium resistance (Russet Burbank) was used in place of a highly susceptible clone (NDA 8694-3) immediately after solarization the degree of Verticillium control and yield was enhanced. These relationships exemplify an approach to integrated pest management with solarization and resistant varieties.
Katan (11) has suggested that adding suitable organic residues to the soil may enhance the benefits of solarization. Studies by Ramirez-Villapudua and Munnecke (20) support this hypothesis. They showed that both solar heating alone and cabbage amendments reduced soilborne populations of Fusarium oxysporum f. sp. conglutinans, but these treatments were not as effective as the combination of the two treatments. They indicated that a tarp is necessary not only to increase the temperature of soil to critical levels, but also to trap fungitoxic gases coming from cabbage amendments.
Katan et al. (13) has suggested that suppressiveness in solarized soils may result from a shift in microbial populations in favour of heat-resistant antagonists. This factor and the possibility of chemical breakdown products from green manure residues may provide an even wider variety of additional interactions leading ultimately to the control of many soilborne pathogens under conditions that are currently considered to be marginal for the effective practice of solarization.
It appears that we have only touched the tip of the iceberg with soil solarization. The possibilities for pathogen and disease control are numerous with this technique. Within the next decade, solarization should provide for an infinite range of new approaches to disease control. It is the way of the future.
References
1. Abdel-Rahim, M. F., M. M. Satour, K. Y. Mickail, S. A. El-Eraki, A. Grinstein, Y. Chen, and J. Katan. 1988. Effectiveness of soil solarization in furrow-irrigated Egyptian soils. Plant Disease 72:143.
2. Ashworth, L. J., Jr., and S. A. Gaona. 1982. Evaluation of clear polyethylene mulch for controlling Verticillium wilt in established pistachio nut groves. Phytopathology 72:243.
3. Ben-Yephet, Y., I. 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. Chet, I., Y. Elad, A. Kalfon, Y. Hadar, and J. Katan. 1982. Integrated control of soilborne and bulbborne pathogens in iris. Phytoparasitica 10:229.
5. Davis, J. R. and L. H. Sorensen. 1986. Influence of soil solarization at moderate temperatures on potato genotypes with differing resistance to Verticillium dahliae. Phytopathology 76:1021.
6. Elad, Y., J. Katan, and I. Chet. 1980. Physical, biological, and chemical control integrated for soilborne diseases of potato. Phytopathology 70:418.
7. Frank, Z. R., Y. Ben-Yephet, and J. Katan. 1986. Synergistic effect of metham and solarization in controlling delimited shell spots of peanut pods. Crop Protection 5:199.
8. Greenberger, A., A. Yogev, and J. Katan. 1987. Induced suppressiveness in solarized soils. Phytopathology 77: 1663.
9. Grinstein, A., J. Katan, A. Abdul Razik, O. Zeidan, and Y. Elad. 1979. Control of Sclerotium rolfsii and weeds in peanuts by solar heating of soil. Plant Dis. Rep., 63:1056.
10. Jacobsohn, R., A. Greenberger, J. Katan, M. Levi, and H. Alon. 1980. Control of Egyptian broomrape (Orobanche aegyptiaca) and other weeds by means of solar heating of the soil by polyethylene mulching. Weed Science 28: 312.
11. J.Katan. 1981. Solar heating (solarization) of soil for control of soilborne pests. Annual Review of Phytopathology 19: 211.
12. J. Katan. 1987. Soil solarization. In: Innovative approaches to plant disease control. 1. Chet, Ed., John Wiley & Sons. Chapter 4.
13. J. Katan, G. Fishler, and A. Grinstein. 1983. Short- and long-term effects of soil solarization and crop sequence on Fusarium wilt and yield of cotton in Israel. Phytopathology. 73:1215.
14. Lifshitz, R., M. Tabachnik, J. Katan, and 1. Chet. 1983. The effect of sublethal heating on sclerotia of Sclerotium rolfsii, Can. J. Microbiol. 29:1607.
15. Marois, J. J., S.A. Johnston, M. T. Dunn, and G.C. Papavizas. 1982. Biological control of Verticillium wilt of eggplant in the field. Plant Disease 66:1166.
16. Martyn, R. D. and T. K. Hartz. 1986. Use of soil solarization to control Fusarium wilt of watermelon. Plant Disease 70:762.
17. Pinkas, Y., A. Kariv, and J. Katan. 1984. Soil solarization for the control of Phyrophthora cinnamomi: thermal and biological effects. Phytopathology (abstr), 74:796.
18. Pullman, G. S., J. E. DeVay, R. H. Garber, and A. R. Weinhold. 1981. Soil solarization: Effects of Verticillium wilt of cotton and soilborne populations of Verticillium dahliae Pythium spp., Rhizoctonia solani, and Thielaviopsis basicola. Phytopathology 71:954.
19. Rabinowitch, H. D., J. Katan, B. Ben David, l. Rotem,and U. Zig. 1985. Soil solarization in onion: Effects in successive years, Hassadeh, 65,1792.
20. Ramirez-Villapudua, R.J. and D. E. Munnecke. 1985. Effects of solarization of soil amended with cabbage residues on Fusarium oxysporum f. sp. congIutinans race-5. Phytopathology (abstr), 75:1291.
21. Siti, E., E. Cohen, J. Katan, and M. Mordechai. 1982. Control of Ditylenchus dipsaci in garlic by bulb and soil treatments. Phytoparasitica 10:93.
22. Stapleton, J. J. and J. E. DeVay. 1982. Effect of soil solarization on populations of selected soilborne microorganisms and growth of deciduous fruit tree seedlings. Phytopathology 72:323.
23. 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.
24. Stapleton, l. J. and J. E. DeVay. 1986. Soil solarization: A non-chemical approach for management of plant pathogens and pests. Crop Protection 3:190.
25. Stevens, C., V. Khan, A. Y. Tang, and C. Bonsi. 1988. The effect of soil solarization on growth response and root knot damage of sweet potato. Hort Science (abstr), 5:827.
26. Sztejnberg, A., S. Freeman, 1. Chet, and J. Katan. 1987. Control of Rosellinia necatrix in soil and in apple orchard by solarization and Trichoderma harzianum. Plant Disease 71:365.
27. Tjamos 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
28. Triolo, E., G. Vannacci, and A. Materazzi. 1988. La solarizzazione del terreno in orticoltura, 2. Alcune indagini sui possibili meccanismi d'azione. Colture Protette 17:59.