CHAPTER I: LATIN AMERICA

ALTERNATIVES FOR THE REPLACEMENT OF METHYL BROMIDE IN ARGENTINA

L. A. Salles*, D. A. Sosa** and A. Valeiro**

* EMBRAPA - CPACT, Brazil, ** INTA, Argentina

Summary. The cultivation of strawberry, tomato, cut flowers and tobacco was considered. Strawberry is an economically important crop in Argentina. The amount of land devoted to the cultivation of strawberry varies depending on the region. The use of Methyl Bromide (MeBr) allows to protect the crop from the attack of several soil-borne pests. Several experiences have been carried out to validate already tested alternatives and to adapt them to local conditions for the replacement of MeBr. For this purpose, different chemical fumigants were compared. Dazomet and metam sodium were the fumigants compared for the control of soil-borne fungi, nematodes, insects and weeds. Both products came out as viable alternatives of MeBr. In addition, two other methods, soil solarization and steam, were also validated. Steam is a bit difficult to apply and the initial overall costs for its application may prevent its use. Some problems related to its application are not easily overcome under current conditions. Soil solarization, although effective under certain conditions, cannot be applied everywhere. The area of La Plata is suitable for the production of strawberry, especially for fresh consumption and in that area all these control technologies can be well applied in strawberry fields. Tomato is also an important horticultural crop in Argentina. Also in this case dazomet and metam sodium were effectively used as soil fumigants. Cropping practices of tomato vary in Argentina, depending on the geographical area, and this may determine the choice of the fumigant, besides the economical feasibility of the treatment. Carnation and lisianthus, as cut flowers, are the most important ornamentals in Argentina, being cultivated especially in the green belt of great Buenos Aires. The areas occupied by these crops increase every year. Dazomet and metam sodium showed the same effectiveness as MeBr, as mentioned above for strawberry and tomatoes. These fumigants are potential alternatives to replace MeBr, that is currently used. For tobacco, which is another economically important crop in Argentina, the alternatives for the replacement of MeBr were evaluated in two agricultural systems: (a) the conventional system using chemical fumigants, as metam sodium and dazomet, and (b) the soilless system using floating trays and supported trays. The results of this validation showed that both methods may satisfactorily replace the use of MeBr as soil fumigant for the control of soil-borne pests in tobacco seedbeds. The soilless system has also the advantage that it provides uniform and vigorous crop seedlings and requires a smaller area for the production of tobacco seedlings. Considering the results obtained, it is concluded that the replacement of MeBr in strawberry, tomatoes, tobacco and ornamentals is perfectly possible and feasible, either using a chemical alternative for seedbed desinfection, or soilless systems in trays. The choice of any of these methods should necessarily take into account aspects related to cultivation techniques in that specific zone of the country, economical feasibility and environmental safety.

Key words: disinfection, substratum, steam, dazomet, metam sodium, soiless system, solarization

I. Introduction

The Project Ozone (MP/ARG/97/186 for horticulture and ARG/98/G63/INTA-PNUD for tobacco) was funded by UNDP and executed by INTA (National Institute of Agricultural Technology). It started in 1999, and its main objective was to evaluate various alternatives to methyl bromide (MeBr) for the control of soil-borne pests.

One of the main constraints to the agricultural production is the soil degradation, which may take place under monocropping conditions, or short crop rotations, characteristics of the intensive production systems of vegetables, cut flowers and tobacco. The incidence of several soil-borne pests, particularly pathogens, is also a constraint to the production of the above-mentioned crops. To avoid damage and losses caused by these organisms, soil disinfection has been constantly required for these crops.

MeBr has had wide diffusion in the world and in Argentina it has been the most used soil fumigant. In recent years MeBr has been used largely in the country, e.g. horticulture consumes up to 280 tons annually, basically for tomato production, 230 tons in tobacco, 70 and 60 tons in cut flowers and strawberry, respectively.

MeBr is one of the main substances depleting stratospheric ozone and its replacement is necessary to protect the environment. The Government of Argentina has implemented a Program for the protection of the layer of ozone. Within the framework of this programme the projects MP/ARG/97/186 for horticulture and MP/ARG/98/G63/ INTA-PNUD for tobacco have been implemented in order to evaluate different alternatives, such as steam, soil solarization, soilless systems and other chemical fumigants.

II. Evaluation of the Alternatives

2. 1. Strawberry is a commercially important crop in Argentina. The technology used for the production of strawberry in Argentina varies according to the region. In general the soil is treated with MeBr before planting. This practice eliminates several soil-borne pests, which otherwise cause important losses to strawberry. Research conducted in the past showed the benefits of soil fumigation for strawberry production. Serious reduction of yields was observed in non fumigated areas, due to the severe incidence of soil-borne fungi.

Various experiments have been carried out in order to validate some alternatives to MeBr. They mainly consisted in comparisons of some chemical fumigants.

Dazomet is a granular product used at a rate of up to 70 g / m2 or 700 kg / ha. It is normally applied 30 days before planting. Granules are distributed uniformly on the soil surface then incorporated in the soil at a depth of 30 - 40 cm. After its incorporation the soil is irrigated and covered with a polyethylene sheet. For good effectiveness of the fumigant the soil has to be well prepared.

Metam sodium is a fumigant, applied 30 days before planting. Soil is irrigated at 100 % of field capacity immediately after the application. Its rates are 125 cm3 / m2 or 1,250 litres / ha.

In both cases two different polyethylene films (40 or 80 microns thickness) were also evaluated.

The results showed that both fumigants were effective to control soil-borne fungi, nematodes, insects and weeds. They did not affect negatively plant stand, yields and quality of the fruit. In addition, both polyethylene films gave similar results of efficacy.

It was concluded that either dazomet or metam sodium are effective alternatives to MeBr for strawberry in Argentina.

Two other methods tested were soil solarization and steam. The latter was a bit difficult to apply properly.

Soil solarization consists of using solar energy to heat the soil previously wetted and covered with a polyethylene film. This method, although interesting, cannot be applied every year. Its effectiveness largely depends on the environmental conditions, such as the air temperature, rainfall and others. One of the requirements for its effective application is to prepare the soil with good levelling and free of clods. The soil is then irrigated to field capacity and covered with a transparent polyethylene film (30 to 40 microns thickness). Solarization works well when air temperatures and sun radiation are high. The soil should remain covered with the film up to six-eight weeks. Soil solarization is an environmentally safe method, which under appropriate conditions eliminates harmful soil-borne organisms.

The area of La Plata is an important one for the production of strawberry, especially for fresh consumption. This is in fact the region where these new technologies can be well introduced and adapted.

2.2. Tomato is another important horticultural crop in Argentina. The techniques for its cultivation vary from one region to another. Most of MeBr in the country is used to protect tomato from soilborne pathogens. Soil disinfection using MeBr in tomatoes already started in the 40s.

Metam sodium and dazomet seem to be the main soil fumigants which can possibly replace the present use of MeBr. Although they are well known products, it is necessary to adjust their rates and ways of application to make sure their success in tomatoes.

Formol is a solution, which contains 40 % of formaldehyde, which has been evaluated for its action against main bacterial diseases in tomatoes. This chemical, however, shows problems of human toxicity.

In field trials metam sodium was evaluated at the rate of 125 cm3 / m2, while dazomet was tested at 70 g / m2 and formaldehyde 40 % (Formol) at 250 cm3 / m2.

After all these trials it was recommended to use these chemicals only when soil moisture is 40-70 % of the field capacity and temperatures between 18 and 24°C. Soil, as usual, must also be well prepared. The treated soil surface should remain moisted from 7 to 10 days before the application. After the application the soil has to be irrigated with 5-10 litres of water / m2 then covered with a polyethylene transparent film of 100 microns during the whole period of exposure. Before planting, the polyethylene film is removed and the soil is slightly moved for releasing any remaining gas to avoid any possible phytoxicity.

The results obtained in these trials did not show significant differences among the treatments tested, i.e. dazomet at 70g / m2, Vapam at 125 cm3 / m2 and formaldehyde at 250 cm3 / m2. Such results are similar to others obtained in strawberry in different parts of the world. These treatments are easy to apply either in strawberry or in tomato. In any case, the application of any of these four chemicals in tomato should be adapted to the characteristics of the soil and cropping practices of that particular part of the country. The economical feasibility of these chemicals has to be avaluated on a case-by-case basis.

2.3. Cut flowers such as carnation and lisianthus, are the main ornamentals flowers in Argentina. The area of these crops has increased during the recent past. These ornamentals can be used for internal consumption and for export.

One of the major constraints to this type of production is the negative incidence of diseases caused by Fusarium fungi. The pathogen is usually present in the soil and it is normally resistant to adverse conditions, so that it remains for long periods in the soil. It is also able to survive conditions of stress, as water excess and drought. These fungi are also tolerant to various pesticides. A combination of factors in soil, still to be precisely determined, brings about the infection of the crop at different growth stages, causing high plant mortality and huge reduction of the production.

Results of recent survey in greenhouses of ornamental production report that disease caused by Fusarium is present in 100 % of the areas. This problem with others related to ornamental production compels to use MeBr as soil fumigant in greenhouses.

Research conducted recently clearly showed the possibility to use successfully other control methods for soil disinfection, such as the dazomet, metam sodium and steam. Soil solarization is an useful method, but with limitations if environmental conditions of the season change, i.e. low air temperatures and heavy rainfall.

Due the increased use of MeBr and the problems caused by diseases, some alternatives were evaluated, such as dazomet, applied at 70g / m2, 36 days before planting. The soil was covered with a polyethylene transparent film of 50 microns during 21 days after the application. The film was then removed to allow the release of gases still remaining in the soil. Metam sodium at rate of 125 cm3 in 5 litres of water / m2 was applied 36 days before planting. Then the same steps as described for dazomet were followed.

Water vapour was applied 15 days before planting with a machine for sterilisation Sterilter 50, endowed with a tank of 50 litres of capacity, burner, tank of fuel and a dosificator of vapour. Soil temperature in the first 15 cm, immediately after the application, oscillated between 50 and 90°C.

After the application of these treatments, tests were made to evaluate their control of Fusarium. There was no significant differences among the treatments.

Crop productivity, i.e. flower production, was also similar among the treatments. Dazomet and metam sodium showed nearly the same effect, which was better than steam. Both products can be considered as alternatives to MeBr in ornamentals.

The green belt of Great Buenos Aires is the main area of production of cut flowers in Argentina. Although these are relatively recent, areas have increased, so at this point, new alternatives will undoubtedly used in the near future to effectively replace MeBr.

Steam is an old method for soil disinfection. It consists in passing a flow of steam through soil pores, or any other substratum, that when taking contact with the cold particles it condenses, becoming liquid and releasing heat, thus allowing the elimination of several noxious living organisms. It is well known that the tolerance and / or susceptibility of these organisms depends on their physiological state at the time of the treatment. One aspect is also the temperature to be provided by the steam in the soil. Temperatures of 70-75°C are lethal to many harmful organisms, but undesirable if to preserve useful flora and fauna in soil. Temperatures of 60°C can be sufficient to control most of soil-borne pathogens, nematodes and weed seeds.

The effectiveness of the use of vapour of water depends on several factors, as temperature, the uniformity of its distribution, soil depth reached by the steam, etc. Good effectiveness also depends on the time of application and the quality of the preparation of the land. In order to be effective, the steam should also reach a depth of 10-16 cm.

The system generally consists of achieving the exchange of heat among the hot gases released from the burner impelled through the body of the boiler. The commercial boilers are operated through electricity, or with fuels. The mobile boilers of the Project MP/ARG/97/186 used gas-oil because of its availability in rural areas. After analysing the offer of various boilers in the market, it was decided to use TX-40 of 3 bar of pressure and 400 kg per hour of vapour generation for application with badge.

For soil application two methods were tested: a) low carp of plastic, or canvas, which distribute the vapour by means of tube-diffusers placed in soil; b) with mobile badge mounted to the tractor.

The system with mobile plate offers advantages in productivity, since it is able to treat large areas at reduced cost than the system of application using a carp.

Although the cost of vapour application with mobile plates is more expensive than the application of MeBr, it is a viable alternative. It controls a wide spectrum of soil-borne pests and it effectively protects crop of tomatoes and strawberry.

2.4. Tobacco is grown in seven counties in Argentina. In the last two years national annual production was about 113.000 tons and the value of the production is US$220-230 million. Two systems were evaluated as alternatives for the replacement of MeBr in tobacco production: a) in the conventional system, which includes use of either soil fumigant metam sodium and dazomet, and b) the soilless systems, which includes the evaluation of floating trays and supported trays.

In the conventional system two fumigants were applied in conventional seedbeds, that is to say, seedbeds built on the level of the floor whose borders were built with plywood and stakes. A layer of forest organic soil was deposit on the seedbed surface, previously sieved to separate the sticks and roots of trees.

Seven days previous to the application of metam sodium the seedbeds were abundantly watered and covered with a carp of transparent plastic to stimulate biological activity in the soil. Metam sodium was applied at rate of 100 cm3 / m2. After its application, it was incorporated a little more water and the seedbed again was covered with the plastic during the next 28 days. After this period the seedbed was uncovered and left for aeration during other 12 days.

Dazomet was also applied similarly to metam sodium. Its rate of application was 50 g / m2. Just after its application on the soil surface it was incorporated using a hoe and followed by another abundant watering. Later the seedbed was covered with a plastic film for 28 days, then uncovered and left for aeration during 12 days.

Planting density was the same one used in the region (0,15 g of raw seeds / m2). After planting, a light irrigation was made and the seedbed was covered with a transparent plastic of 80 microns.

The evaluation of germinated plants and crop stand and development showed that MeBr effect is far better than the one given by both new fumigants. In any case, metam sodium and dazomet showed some degree of soil disinfection, but not equal to MeBr.

The advantage here is that metam sodium and dazomet are less toxic than MeBr, which makes them easier to handle and much safer for humans. Metam sodium application is 40 % much cheaper than dazomet. However, both fumigants, probably due to some level phytoxicity, presented reduced crop seedling stand and low effect against soil-borne fungi. The fumigants were effective against nematodes and weeds.

The soilless system allows the production of tobacco seedlings without the need of use MeBr, since the substrate is guaranteed pest and disease-free and contains the main nutrients required for optimal plant growth.

The soiless system developed for tobacco is an open hydroponics, where the solution with nutrients is not always recycled.

Seedling production through this system is carried out in plastic pools of 10 m long, 1,20 m wide and 10-15 cm height. It is important that the land in the pools be well levelled covering the surface with a layer of 2 cm of sand to avoid punctures in the plastic. The borders can be built with diverse materials (wires, plywood, bricks, others). The covering of the internal part of the pool is carried out with black plastic of no less of 200 microns. The pool is full with clean water until it reaches 3-5 cm of the superior border. The pool is protected by a micro-tunnel of transparent plastic UV of 150 microns, which is kept by iron arches of 6 mm diameter. The plastic was fixed to both ends by stakes placed one meter of the head of the pool. The pools has to be opened often. The dimensions of the pools can be changed according to the size of the greenhouse.

Tobacco planting is carried out in styrofoam trays with 288 small cells per tray, previously filled in with a sterilised substratum. Pelletized seeds are used, which favours better distribution at the time of planting. Trays are placed in the pool and left floating during 60-80 days until the seedlings are well developed. During this period the seedlings are regularly checked, especially the whole pool system ventilation (open during the day on the lateral up to 15 cm of height and closed at night), maintaining constant the level of water, fertilising the water (15N - 10P - 15K, or 20N - 10P - 20K), using foliar treatments for pest control (imidacloprid, carbendazim, iprodione, kasugamycin, streptomycin and others) treating water with copper hydroxide, and pruning the plants three times.

The pools of supported trays system are smaller (4 x 1 x 0,5 m) than those used in common floating trays. Its walls are built with bricks. In this system plastic tray of 50 x 33,5 x 0,5 cm, with 150 cells each one, is used. The pool is filled in with water and due to the fact that plastic trays do not float, it is supported by the plastic, immerses in a volume of water that arrives to 2 cm height. The reposition of water to keep the required level is constant.

The handling and managing procedures of the supported tray and floating tray systems are basically the same thing, with very few variations, such as in the supported trays it is carries out only two prunings.

Among soilless systems, the system using supported plastic trays offers additional advantages, such as that the cycle of development seedlings is shorter (10 days), the seedlings develop a stronger root system, it is easy to remove them from the trays and handle the trays, a smaller storage space is required and bigger durability of the material. The disadvantage is the higher initial cost of acquisition and more care is needed to reposition of the water in the pool.

The validation of these alternatives demonstrated that they can replace the present use of MeBr in tobacco seedling production.

Among the systems evaluated, the soilless systems is technically the most successful and innovative, since besides the elimination of MeBr, it represents a technological progress for producing uniform seedlings for their quick establishment in the field and to establish more homogeneous plantations. Moreover, it decreases the large amount of space normally needed for seedling production (75 % less surface).

Any of these systems of seedling production in trays can be adopted for tobacco seedling production in Argentina.

BIBLIOGRAPHY

Fernández, R., Wolcan, S., Lori, G., Ronco, L., Rolleri, J., Kitagawa, A., Mittideri, A. Alternativas al Uso de Broumuro de Metilo en el Control de la Podredumbre Basal en el cultivo de Lisianthus. Seminário de Cierre. Proyecto MP/ARG/97/186 (Alternativas al Uso de Bromuro de Metilo en Frutilla, Tomate y Flores de Corte. Buenos Aires, 4 y 5 de Mayo de 2000), pp. 23- 30.

Kryvenki, M.A., Mayol, R.M., Sosa, D.A., Ohashi, D.V. Alternativas para la Substitución del Bromuro de Metilo en el cultivo de Tabaco. INTA/SERNDS/PNUD Project. EEA Cerro Azul-INTA. 1999-2000.

Mezquiriz, N.B.. Alternativas Químicas al Uso de Broumuro de Metilo en el Cultivo de Tomate. Seminário de Cierre. Proyecto MP/ARG/97/186 (Alternativas al Uso de Bromuro de Metilo en Frutilla, Tomate y Flores de Corte. Buenos Aires, 4 y 5 de Mayo de 2000), pp. 18-22.

Sangiacomo, M.A., Gamboa, S., Aprea, A., López, M.C., Mitidieri, A. Zembo, J.C. Evaluación de Alternativas Químicas al Broumuro de Metilo en el Cultivo de Frutilla. Seminario de Cierre. Proyecto MP/ARG/97/186 (Alternativas al Uso de Bromuro de Metilo en Frutilla, Tomate y Flores de Corte. Buenos Aires, 4 y 5 de Mayo de 2000), pp. 13-17.

Zembo, J.C., Ramirez, M., Mesquirez, N., Fernandez, R., Sangiacomo, M.A., Giaccio, J.J. Substituición del Bromuro de Metilo con Vapor en la Desinfección de Suelos en el Gran La Plata (R.A.). Seminário de Cierre. Proyecto MP/ARG/97/186 (Alternativas al Uso de Bromuro de Metilo en Frutilla, Tomate y Flores de Corte. Buenos Aires, 4 y 5 de Mayo de 2000), pp. 99- 110.

EFFECTIVE ALTERNATIVES TO METHYL BROMIDE IN BRAZIL

L. A. Salles

Brazilian Agricultural Research Corporation - Agricultural Research Center for Temperate Climate (EMBRAPA - CPACT), Pelotas, Brazil

Summary. Three validated alternatives to the use of MeBr for soil fumigation are presented: (1) solarization of conventional seedbeds; (2) the use of metam sodium in conventional seedbeds; and (3) the "floating trays" system. Solarization proved to be a technically feasible and cost-effective non-chemical alternative to MeBr to produce healthy and adequate seedlings. Metam sodium is a very good alternative, too. It is very easy to apply, practically odorless, and it is safer and easier to use than MeBr. The float production system produces tobacco seedlings for transplant that are of greater uniformity, with a much stronger root system and at reduced labor costs.

Key Words: methyl bromide, alternatives, solarization, metam sodium, soilless cultivation, floating trays

I. INTRODUCTION

Crops grown in soil, including tobacco, are exposed to soilborne pathogens (fungi, viruses and bacteria), nematodes, arthropods and weeds. As a method of treatment against a wide range of pests, MeBr is used in many geographical regions of the world. MeBr appeared on the market in the 1930s and has been used in Brazil for more than five decades. Its largest use is in tobacco seedbeds, accounting for more than 95 % of Brazilian use of this pesticide. The widespread use of this fumigant has been encouraged by its simple mode of application and technology needed.

In the early 1980s the connection between halogenated hydrocarbons and the destruction of the ozone layer in the stratosphere was made and later it was confirmed that MeBr was also implicated in this phenomenon. Considerable evidences have been accumulated that MeBr is a potent ozone depletor and the possibility of eliminating this fumigant from agricultural use was strongly considered since then.

There is still general consensus that, because of its versatility, there is no single alternative chemical treatment, or combination of treatments, that at present can fully substitute MeBr. However, alternatives to its use are currently available for specific problems and additional alternatives may be developed using non-chemical methods, new organic amendments, biological control, cultural practices, and physical and chemical methods. Many of the alternatives to be used should be part of an overall integrated pest management system and should also be combined with other pest control tactics to achieve an economically sustainable method of management.

To find new technologies in order to avoid the use of MeBr in tobacco seedbeds was the objective of the demonstration UNIDO project developed with EMBRAPA, EPAGRI and SINDIFUMO during the years of 1998, 1999 and 2000. This project was joined by the effort of the Brazilian tobacco sector to find economical and technically feasible alternatives to the use of MeBr for production of tobacco seedlings. The field work was carried out in the tobacco production regions in Rio Grande do Sul and Santa Catarina. These two states are located in the southern part of Brazil, between latitudes of 23 and 32ºS.

This document describes, in addition to the conventional seedbeds, three validated and most promising alternatives developed by the Brazilian tobacco companies and by the UNIDO project:

1. solarization of conventional seedbeds;
2. use of metam sodium in conventional seedbeds; and
3. "floating trays"system.

II. CONVENTIONAL TOBACCO SEEDBED

The traditional tobacco nursery, or seedbed, in southern Brazil is made directly on the soil, measuring 25 m long by 2 m wide. The soil is risen of 10 - 15 cm above the ground level.

Due to the fact that weeds, soilborne pests and diseases are common in the tobacco areas, it is necessary to suppress those potential pest problems in order to produce an acceptable quantity and quality of seedlings. For this purpose, the fumigant MeBr has been used for fumigation of the tobacco seedbeds.

Seedbed installation should be leveled and the soil well prepared free from clods, undecomposed roots and stalks, and with enough moisture. If the soil is excessively wet, it is necessary to wait until it reaches ideal moisture or, if very dry, it is necessary to water uniformly; to build a contour segment (barrier) on the surface of the seedbeds, to avoid flash flooding.

Prior to punching bromide cans (680 g per can), transparent plastic anti-UV 100-micra sheet should be kept stretched for at least three days. It is not desirable to start seedbed treatment if room temperature is below 10°C. It is convenient to allow the seedbed to be fumigated for at least three days. Sowing (3.5 g of raw seeds per seedbed) should be done only after 2 days elapsed from removal of plastic sheet.

A plastic film is constantly used on the seedbed to protect the emerging and young seedlings from sunburn, adverse temperatures, wind, rain, etc.

Control of foliar diseases, such as alternariose, yellow stunt, sore shine, damping off, sclerotinia rot, is currently done with fungicides (thiabendazole, mancozeb, iprodione, maneb) sprayed every week or just after a rainy day. Control of insect pests is not normally required in the seedbed. Snails are locally potential problems.

Despite of potential problems pointed out due to the use of MeBr, labor intense use, quality of seedling, etc., conventional tobacco seedbeds are still used and preferred by a large proportion of tobacco growers in the south of Brazil. It is considered a very cheap process and with an adequate cost-benefit ratio (Table 1).

Table 1. Cost of production (in US$) of seedlings for one hectare of tobacco with conventional seedbeds with methyl bromide (source: UNIDO project, 1998 - 2000)

III. CONVENTIONAL SOLARIZED SEEDBED

In the conventional seedbed, the soil is irrigated until its saturation, to improve heat transportation in the soil profile, and covered with anti - UV 100mm transparent plastic, during a period of at least 60 days. Normally, the plastic is left in its place during the summer, laid in February and removed just before the sowing period, i.e. by May. After the period of solarization, the plastic is removed and the area is sowed. Before sowing the soil is revolved only superficially. Control of disease, insect pests and snails are done as explained for the conventional seedbeds.

Solarization is a process which, due to the diversity of conditions, requires more research and development of the methodology, the type of plastic film, the period of application, etc. In southern Brazil, which has a temperate climate, temperatures reach up to 60ºC at 10 cm of depth, which is high enough to kill the most common weeds and soilborn pests.

It should be mentioned that solarization could be a complicated process for use on a large scale. The effectiveness of this method is directly linked to climate, that is, the amount of sunlight received during the solarization process. It should be an ideal method for tropical climates. It is considered a very cheap process and with an adequate cost-benefit ratio. Solarization showed the lowest cost among alternatives studied in southern Brazil (Table 2).

Overall, solarization proved to be a technically feasible and cost-effective non-chemical alternative to MeBr, to produce healthy and adequate tobacco seedlings.

Table 2. Cost of production (in US $) of seedlings for one hectare of tobacco with conventional solarized seedbeds (source: UNIDO project, 1998 - 2000)

IV. CONVENTIONAL SEEDBED USING METAM SODIUM

Metam sodium is a broad spectrum soil fumigant that is used to control nematodes, weeds, and fungi affecting a variety of economically important fruit and vegetable crops. In general, metam sodium is considered a technically sound and cost-effective alternative to MeBr to control pests in the soil which affect high value fruit and vegetable crops. This fumigant is readily available, moderately toxic and versatile and has been used in a variety of commercial applications to treat soils prior to planting for the control of annual weeds, nematodes and soilborne pathogens.

Metam sodium is a water-soluble liquid that after having been applied to the soil becomes a gas. It is applied in the conventional seedbeds at a rate of 75-80 ml / m2, and the soil is covered with a plastic film during 4 to 5 days, to improve the fumigation effect. Prior to the application, soil moisture is increased by irrigation. It could be applied with ordinary back sprayers, or watering cans.

When the plastic is removed, the soil must be revolved (up to 15 - 20 cm of depth) to release possible gases that remained in the soil. A safe interval for waiting is from 7 to 21 days, after the plastic is removed, before planting, depending on the amount of organic material and the temperature of the soil.

Metam sodium is a very good alternative to MeBr, especially for farmers who want to continue to use chemicals for soil desinfection, or sterilization. It is a very easy product to apply, practically odorless. Metam sodium is safer and easier to use than MeBr.

One of the greatest advantages attributed to the use of metam sodium is the low cost. However, the cost of seedlings produced with metam sodium in southern Brazil was practically equal to that of seedlings obtained with MeBr (Table 3). It is possible to speculate that metam sodium still has a higher price due to its small scale of commercial use.

V. FLOATING TRAYS ("FLOAT") SYSTEM

Currently, in the state of Rio Grande do Sul, south of Brazil, 60 percent of tobacco seedlings are produced with the float system. This system also prevails in Santa Catarina, the second largest tobacco-producing state.

The tobacco sector is rapidly shifting from the outdoor seedbed method, which requires fumigating the soil (with MeBr), to on-farm plastic houses, which use floating trays and the soilless system. The shift is occurring primarily because seedlings grown in plastic houses are less labor intensive and therefore can be cheaper over the long run. It is estimated that 75 % and 100 % of tobacco seedlings will be produced in plastic houses in southern Brazil by the season of 2000 / 01 and 2003 / 04, respectively. A big advantage of this system is the fact that the production of tobacco seedlings requires 50 to 60 days until they reach a height of 15 - 20 cm. In the conventional seedbed three months are necessary for seedlings to complete their development.

Table 3. Cost of production (in US $) of seedlings for one hectare of tobacco with metam sodium in conventional seedbeds (source: UNIDO project, 1998 - 2000)

There are two types of plastic house production systems; the direct-seeded float system in low and in the high tunnel. However, usually the high tunnel is not being adopted due to the currently very high costs for its construction. The float system in low tunnel is the most common system and is used by approximately 60 % of tobacco growers in southern Brazil. The float system uses commercially prepared and sanitized media. The most commonly used media preparation contains fermented pine barks, expanded vermiculite and perlite. In brief, the float is a way to grow seedlings developed in styrofoam trays, with special media, where the seedlings are grown in a pool with water.

The construction of the float system should begin with the pool. The float should be built next to the farmer's house, on a well ventilated site, with good sunshine exposure and on a level surface. To build the pool, the first thing to do is to level the surface and to construct the four sides of the pool using 10 cm high bricks (Figures 1 and 2). A complete pool module has 10.55 m long by 1.45 m width and 10 cm high. A black plastic is put insight and over the pool edges and the pool is filled up with water. Water quality is extremely important and fundamental to prevent seedbed diseases. Therefore, only use drinking water treated with copper sulfate upon placement in the float bed. The outside edges of the black plastic are covered with soil (Figures 3 and 4). Eleven wire arches, 1.06 m apart and 0.90 m high at the center of the pool, are fixed just beside the lateral pool edges (Figure 4). Four wood stakes are fixed at both ends of the pool to support the wire arches and the plastic. The top of the stacks, as well as any sharp end, should be covered with ordinary plastic, or other material to protect the plastic covering (Figure 5). Anti - UV 100 mm transparent plastic is expanded along one side of the pool and then expanded over the arches. The plastic edges are fixed with small stacks at the end of the pool (Figure 6). Eleven rubber strips are used to tighten the plastic tunnel over the arcade. It is used one rubber strip on each wire arch site (Figure 7). Tobacco seedlings are produced in 200 cells styrofoam trays (34 x 68 x 6 cm) that are maintained floating in the pool. To prepare the trays, the first step is to wet the media as desired. The media is put on the trays and slightly compacted by shacking or knocking the tray. The media leftover is taken out of the tray. Seeding, with a special seeder tray, is done in half of the tray with only one coated seed per cell and inverting the tray position, repeating the operation again at another half of the tray but putting two seeds per cell to prevent enough seedlings for possible transplant. When the tray is seeded it is ready to be put in the pool (Figure 8). The water level in the pool should be maintained at 3-5 cm of depth. Fungicides and algaecides (e.g. thiabendazole, mancopper, iprodione, mancozeb) are periodically sprayed and added in

the water to prevent fungal diseases and development of algae (Figure 9). Aerial clipping is started when seedlings are 5 cm tall and repeated once, or twice. Clipping tools are disinfected with soap and water before use, or reuse. Clipping should be done outside of the pool area. Clipping makes the seedlings uniform, stronger and more resistant. For the success of the float, two aspects should be considered carefully: ventilation and fertilization. For a proper ventilation it is necessary to keep the sides of the plastic open during the day and closed only during the night, or in days with strong winds, or rain (Figure 10). Another aspect is the strict control of water fertilization. Five hundred grams of the fertilizer (20N, 10P, 20K) should be added to the pool every 10 - 15 days, as shown in Figure 9. In a complete pool module, 10.55 m long by 1.45 m wide, 60 styrofoam trays are housed and more than 10,000 usable seedlings could be produced (Figure 10).

The float production system produces tobacco seedlings for transplant that are of greater uniformity, with a much stronger root system and at reduced labor costs.

Switching from the conventional system to produce seedlings to the float system results in some difficulty to quantify benefits, such as more uniform transplants, much stronger and well developed root system, less replants and possible savings, that offset a slight increase in production costs (Table 4).

Table 4. Cost of production (in US$) of seedlings for one hectare of tobacco with float system in low tunnel (source: UNIDO project, 1998 - 2000)

Increased costs associated with float production include a greater capital investment than required by conventional growing practices. Float also requires more management than conventional growing systems to be successful. However, float production reduces labor needs, primarily for the person pulling up seedlings, boxing them, and transporting them to the field for transplantation.

Production costs for conventionally grown tobacco seedlings for one hectare were US$ 115.06 with MeBr, US$ 115,74 with metam sodium and US$ 98.55 with solarization. With the float system in low tunnel the production cost was US$ 147.65.

Adjustments in current float system, such as the size, materials and management may also result in additional savings. For example, as the size of the tunnel increases, the costs of producing tobacco seedlings decrease. Different management techniques can also reduce the difference in costs.

Fig. 1. Levelling the area for pool construction.

Fig. 2. Constructing the walls of the pool with bricks.

Fig. 3. Black plastic covering the pool area.

Fig. 4. Pool filled with water and outside plastic edges covered with soil, showing the wire arches.

Fig. 5. Protection of the edge of the wood stack.

Fig. 6. Plastic ends expanded and fixed at the pool end.

Fig. 7. Expanded plastic tunnel with rubber strips for holding it.

Fig. 8. Seeded tray being put in the pool.

Fig. 9. Fungicides, algaecides and fertilizers being added in the pool.

Fig. 10. Normal daylong opening of the plastic film of the tunnel. Set of trays floating.