Solarization for greenhouse crops in Japan

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Seuzi Horiuchi

National Research Institute of Vegetables, Ornamental Plants and Tea, Mie, Japan

Japanese farm management is characterized by the greenhouse industry. In 1985, 30 000 ha and 3 000 ha of plastic greenhouse are utilized for vegetables and flowers, respectively. Particularly in fruit vegetables, the plastic house is quite important; large portions of both planted area and production are developed in the plastic houses. Under the structures, intensive cropping results in a severe outbreak of soilborne diseases. A simple technique, considered to be a form of solarization was found effective for the control of the diseases. Up to this time, the technique has been widely accepted among growers in Japan. In the present report, the author describes the solarization technique for the plastic greenhouse, as well as the current status of its implementation in Japan.

Development of Solarization Technique

In Japan, the technique for plastic greenhouses was developed by a collaboration of growers and scientists. They had studied the technique since 1970 or earlier, though it was 1979 when Kodama and Fukui (3) released the first official report. The idea of the new disease-controlling technique was based on the finding that air temperatures became unbearably high in the closed plastic house during the summer season (3).

Figure 1 represents a simplified notion of the technique, which was recommended by Kodama and Fukui (3). The procedure of the technique is as follows: a field plot within a plastic house is rotary-tilled after applying 10-20 t of rice straw and 1 000-1 500 kg of calcium cyanamide per hectare, and small ridges are made on it. The soil surface is covered with clear plastic film, and the soil is thoroughly moistened by temporary irrigation through the furrows. Then the greenhouse is kept closed for about a month during the summer season.

By the procedure described above, Kodama and Fukui (3) obtained maximum soil temperatures ranging from 50 to 60 C at a depth of 10 cm.

Then Kodama and his colleagues (4, 6) reported a successful control of several diseases, particularly Fusarium wilt of strawberry (F.oxysporum f. sp. fragariae). After this, many scientists have tried solarization for various objects. Up to the present, almost all important soilborne pathogens have been tested. Although the results obtained were not always sufficient, the technique in general has proved acceptable in the greenhouse industry for many areas.

Effect and Implication of the Procedure

Soil Temperature. - Soil temperature is a fundamental factor in the solarization effect. Maximum temperatures at a soil depth of 10 cm were 8 degrees higher and 20 degrees higher in a vinyl-mulched field in a closed plastic house, when compared with those in the mulched open field and bare open field, respectively (3). In the greenhouse treatment, the temperature range of 40 to 45 C was obtainable at least for several hours a day at the soil depth of 20 cm (3,4), within which most of soilborne pathogens arc inhibited. No appreciable difference was found in soil temperatures between sheeting film of vinyl-chloride and polyethlene (8). Used film was also applicable for covering material of the soil (8). Ridges on the soil were believed to raise soil temperatures, since they received much solar radiation (8).

Soil Moisture and Organic Amendment. - Laboratory experiments (2, 3) indicated that the disease-suppressing effect of heat treatment was higher in wet soils than in dry soils, both of which were infested with soilborne pathogens. It is generally understandable that wet heat causes much more serious damage on micro-organisms than dry heat. In addition, higher water contents in soil may give rise to soil reduction or soil anaerobiosis, which is enhanced by air-tight and high temperature conditions in solarization. Such conditions in soil possibly affect viability of pathogen propagules. Organic materials also accelerated both disease suppression and soil reduction, when soils were heat-treated in laboratory experiments (2, 3, 4). Similarly, organic amendments employed in solarization may have caused an increased reductive condition in the soil, although those materials were used to improve soil fertility and soil physical properties (3). In the practical scale of solarization, however, no clear relationship has been found between soil reduction and the disease-suppressing effect (3).

Calcium Cyanamide. - Calcium cyanamide was al first introduced to the solarization technique for the purpose of rapid decomposition of organic amendments, thereby improving soil fertility. Calcium cyanamide, however, proved to have a marked effect for suppressing Plasmodiophora (2, 9) and Aphanomyces (10), when combined with heat treatment or solarization of the soil. Nevertheless, the fertilizer was ineffective to the other pathogens, such as Fusarium. (1, 3, 5,) and Pyrenochaeta (73. In solarization for greenhouses, therefore, calcium cyanamide may principally play a role in soil management.

Implementation of Solarization in Japan

The author distributed a questionnaire in the autumn of 1988, to understand the precise status of the implementation of solarization. The following topics are all based on the answers obtained from most of the regions throughout Japan.

Geographical Consideration. - The answers were obtained from 40 regions surveyed out of 47 regions. The scale of implementation was quite diversified depending on the regions. A clear distribution was noticeable when plotted on the territory of Japan; the treatment was executed in the southern half and not in the northern half. The distribution roughly agreed with a boundary of average air temperatures exceeding 26 C in August. However, fairly big differences were occasionally found in the solarized area between neighboring regions.

Target Crops. - Table 1 shows the important target crops for solarization listed in the answers. Strawberry is the biggest in both the treated area and the number of regions. The solarized area for cucumber and sweet pepper are second and fourth biggest, respectively. However, most of these areas are treated for Thrips palmi. As for soilborne pathogens, therefore, strawberry, eggplant, and tomato are the most important crops.

Pea and fuki (Japanese buterbur) are rather unique products of Wakayama and Osaka, respectively, and solarized areas are localized to these regions. In addition to vegetables, solarization has been adopted to flowers such as gypsophila and stock in Wakayama.

Table 2 shows the acceptance of solarization among strawberry growers, to cite one example. A fairly big portion in the area of plastic houses is solarized in some major producing regions. Table 2 suggests that many growers have already considered solarization to be their accustomed work.

The practical use of solarization was on record in 1973 (not confirmed) and 1974 for eggplant and strawberry, respectively. For tomato, the earliest record appeared in 1975. The treatment for strawberry rapidly spread to major producing regions until 1981, while the pace was relatively slow in tomato and eggplant. For cucumber cropping, solarization was first adopted in 1978 and then expanded to many regions within several years.

Target Pests and Diseases and Evaluation of the Treatment. - Table 3 indicates the important targets of solarization and the evaluation of the treatment. In listing target pests and diseases, plural answers were obtained in many cases. The importance of an individual pest or disease was surmised by using the number of regions listing its name. The effect of the treatment was evaluated from the respondents' subjective views. Based on the answers, degree of satisfaction was classified into three categories; (a) effective, good, adequate, sufficient, and other affirmative answers, (b) effective to a certain degree, fairly good, and other neutral answers, and (c) insufficient, imperfect, unsteady, and other negative answers. The degree was expressed by a "satisfaction index," where 3, 2, and I were given for affirmative, neutral, and negative answers, respectively. Mean index among regions was used for evaluating total efficacy of solarization for the individual target.

In the solanaceous crops, bacterial wilt, Pseudomonas solanacearum, was frequently nominated as the target. Fusarium and Fusarium Verticillium were second in tomato and eggplant, respectively. For strawberry, almost all regions practicing the treatment aim at Fusarium wilt. As described previously, strawberry is the biggest crop for solarization. Fusarium wilt of strawberry is accordingly the most important target in Japanese solarization. Nematodes, particularly root-knot nematodes, were nominated by many regions for cucurbits. Nematodes are a common target in solanaceous crops and strawberry, as well as in cucurbits. TMV is also targeted in sweet pepper.

The treatment also aims at diseases such as gummy stem blight of cucurbits and crown rot of strawberry, which are not usually considered to be soilborne. The pathogens possibly infect all kind of materials inside plastic houses, and may be secondarily controlled in the process of solarization. Thrips palmi is similarly targeted in sweet pepper and cucumber.

Besides the targets indicated, the following diseases appeared to be important; stem necrosis disease of pea (pea stem necrosis virus, transmitted by Olpidium sp.), Verticillium wilt and southern blight of fuki and damping-off of spinach. In flowers in Wakayama, the target is unspecified damping-off diseases.

From the value of "satisfaction index," the following features of solarization are roughly surmised. (a) It is rather difficult to control bacterial wilt in solanaceous crops. (b) For Fusarium disease, the treatment effect is lower in solanaceous crops, and higher is strawberry and cucurbits. Particularly, a feeling of satisfaction seems high in Fusarium wilt of strawberry, which is the most popular target. (c) The efficacy of the treatment is considered low in solanaceous crops, while it is high in strawberry and cucurbits. Accordingly, solarization appears to cause a limited disease-controlling effect when a crop has a deeper root system and a pathogen inhabits deeper soil layers.

Current Technique among Growers. The basic procedure for solarization in plastic greenhouses is shown in Figure 1. Growers are trying to adapt the technique of solarization to their way of crop production.

Most growers apply various crude amendments to soils before solarization Rice straw is the most popular material, and is used principally at 10-30 t per hectare in quantity. Besides rice straw, they use compost or compost with animal manure, wheat straw, or bark compost. Soilage crops such as some species of Sorghum are also utilized especially in strawberry. Some growers use no organic amendment due to sufficient soil fertility. In addition, calcium cyanamide is employed by many growers, at a rate of 1000 kg per hectare, frequently combined with the organic materials.

In Japan, we have the most intense heat in the period from the middle of July to the middle of August. Solarization is in most cases practiced for about 30 days in this period. However, the time and period of the treatment are restricted by both meteorological conditions and cropping system.

Examples for short-term treatment were also reported. Solarization for seven to ten days suppressed Pythium and Rhizoctonia in spinach, probably because these pathogens inhabit the surface layer of the soil. A longer period of treatment was required to control Fusarium wilt of spinach.

A technique peculiar to Thrips palmi, an important insect pest of fruit vegetables, has been developed among growers particularly in Miyazaki. Plastic houses are planted with seedlings of cucumber or sweet pepper in autumn, and the insect invades them at that time. When the production ceases the next spring or early summer, plant residues heavily infested with the insect are pulled from the soil and laid in the greenhouse. Further work such as tillage, film tarping, and watering also cease. Then the greenhouse is completely closed, and the treatment usually lasts for seven days. The treatment drastically reduces the insect density in the greenhouse. It is not clear, however, whether or not the treatment can suppress the parasitizm in the next cropping season, since the insect is reproducible on the weed hosts through the summer. Nevertheless, large areas of plastic houses are treated in this way in Miyazaki.

Limitations of Solarization. - Soil solarization is not always sufficiently acceptable for farm management. One limitation is the inevitable problem due to meterorological restriction. The effectiveness of solarization is greatly influenced by the weather during the treating period and locality of the field.

Solarization requires high soil moisture content, and the treatment is believed to be less effective in a field with poor water retentivity. Soil temperatures are lower in soil near the sides and near the pillars of the greenhouse. The effect is relatively low for deep-rooted crops and for pathogens inhabiting deeper soil layers.

Some growers regard solarization as too laborious in balance with its beneficial effects. They eventually have to employ other measures of disease control, when solarization is not fully successful. Growers prefer short-term soil disinfestation such as fumigation. Also, solarization is not attractive when diseases are satisfactorily controlled by other means, such as solanaceous rootstocks resistant to bacterial wilt. Moreover, some growers are averse to solarization due to the difficulty in regulating nutrient level in soil, and consequent plant growth. Other problems involve obvious damage due to the treatment on the material and structure of plastic houses, such as cover film and iron pipe frames.

Prospects. - In the plastic greenhouse culture, soil degradation is a serious problem as well as soilborne diseases. Solarization is positively evaluated in improving soil fertility and removing accumulated salts, owing to decomposition of organic amendments and temporary submerging of the soil, respectively. Growers are accustomed to the work of solarization in areas where the large-scale treatment has already been carried out. In such areas the use of solarization will continue. On the other hand, solarization will probably decline in areas where few growers have adopted the treatment since those areas have less suitable conditions for soil solarization.

References

1. Fukui, T., T. Kodama, and Y. Nakanishi. 1981. Solar heating sterilization in the closed vinyl house against soil-borne diseases. IV. Solar heating sterilization by polyethylene mulching in the open-field. Bull. Nara Pref. Agric. Exp. Sta. 12:109-119 (Jap., Eng. sum.).

2. Horiuchi, S., M. Hori, S. Takashi, and K. Shimizu. 1982. Factors responsible for the development of clubroot-suppressing effect in soil solarization. Bull. Chugoku Nat. Agric. Exp. Sta. E 20:2548.

3. Kodama, T. and T. Fukui. 1979. Solar heating sterilization in the closed vinyl house against soil-borne diseases. I. The movements of soil temperature and determination of thermal lethal conditions for some soil-borne pathogens. Bull Nara Pref. Agric. Exp. Sta. 10:71-82. pap., Eng. sum.).

4. Kodama, T. and T. Fukui. 1982. Solar heating in closed plastic house for control of soilborne diseases. V. Application for control of Fusarium wilt of strawberry. Ann. Phytopath. Soc. Japan 48:570-577. (Jap., Eng. sum.).

5. Kodama, T. and T. Fukui. 1982. Application of solar heating with plastic film mulching in the outdoor field for control of Fusarium wilt of strawberry. Ann. Phytopath. Soc. Japan 48:699-701. (Jap., Eng. sum.).

6. Kodama, T., T. Fukui, and Y. Nakanishi. 1979. Solar heating sterilization in the closed vinyl house against soil-borne diseases. 11. Effects of solar heating sterilization in commercial vinyl house and basic methods for estimating the effects obtained by the treatment. Bull. Nar Pref. Agric. Exp. Sta. 10:83-92. Jap., Eng. sum.).

7. Morita,H., H. Nakamura, and T. Suzuki. 1979. Control of wilting symptom of tomato plants grown semi-forcingly. IX. Control of tomato corky-root disease by closure of vinyl house in summer season. Bull. Shizuoka Agric. Exp. Sta. 24:42-47. Jap.).

8. Sakamoto, 1., K. Jinnoh, M. Aino, J. Yoshikura, and K. Shiwaku. 1986. Control of soilborne diseases of Chinese cabbage by solar heating with plastic-film mulching in the out-door field. 1. On effective soil temperature for sterilization. Bull. Hyogo Pref. Agric. Center 34:63-68. (Jap.).

9. Shimizu, K., Y. Suzuki, S. Takashi, and H. Kawata. 1987. Study of solar heating with plastice-film mulching in the out-door field for control of soilborne disease of vegetables. 2. Effect of mulching with plastic film on clubfoot control. Bull. Shiga Pref. Agric. Exp. Sta. 28:7-21 Jap.).

10. Yoshikura, J., K. Futami, Y. Aoyama, K. Jinnoh, and 1. Sakamoto. 1986. Control of soilborne diseases of Chinese cabbage by solar heating with plastic-film mulching in the out-door field. 11. Effects of calcium cyanamide and organic matter for soilborne disease. Bull. Hyogo Pref. Agric. Center 34:69-74. (Jap., Eng. sum.).

Table 1. Important target crops for greenhouse solarization treatment

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a Number of regions nominating each crop.
b Japanese butterbur (Petasites japonicus Miq.).
c Almost all of the treatment target Trips palmi.

Table 2. Soil solarization in strawberry cropping in major regions

Table 3. Pest and disease targets of solarization and evaluation of the effect

a The indexes of 3, 2, and 1 are given for affirmative, neutral, and negative answers, respectively. The value is expressed by the mean index among regions.
b Cucumber, melon and watermelon

Figure 1. Soil solarization technique for plastic greenhouses (Kodama & Fukui, 1979).

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