desinfecting soil

8/14/2019 Desinfecting Soil

1/15

TECHNIQUES IN PLANT VIROLOGYCIP Training Manual2.2 DETECTION/Indicator Plants

Section 2.2.2Disinfecting Soil

Basic potato seed and virus indicator plants can be produced in thegreenhouse using a substrate mix consisting of soil, peat moss, andsand. Disinfest the mix to prevent tuber and plant contamination by soil-borne pests and pathogens.

Disinfesting through biological, chemical, or physical methods is usuallyexpensive and requires careful handling, especially when using highlytoxic chemicals.

In the greenhouse, pathogen-free in vitro plantlets, seeds, cuttings ortuberlets are planted in a disinfested substrate to ensure that tuberproduction is free from fungi, bacteria, viruses, or nematodes.Disinfestation should also include the elimination of insects and weed

seed that affect the production of high-quality seed and vigorous plants.

Disinfestation with Methyl Bromide

Several commercial products are available for disinfesting substrate, buttheir application may require several days. However, in spite of its hightoxicity, methyl bromide gas can be easily applied in a few days and ishighly effective.


2/15

P.V. Sec 2.2.2 99 Page 2 -INTERNATIONAL POTATO CENTER

1. Treatment chamber

Figures 1 and 2 show the materials needed and their proportions. Wallsshould be built on a concrete base using concrete covered bricks.Chamber capacity must be 3 m3, with loading and unloading facilities forthe mix or substrate. The chamber must be 3.60 m long, 1.20 m wide,and 0.70 m deep.


3/15


Along the upper edge of the walls of the chamber, make a ditch 5 cmwide and 2 cm deep around the chamber (A).

Use clear plastic sheets (0.81 mm thick) as a cover (B). This sheetingshould be between A and B, so that it may be used more than once.

Construct a 1/4" x 1 and 3/4" laminated iron frame (C) that fits into theditch of the container. Include handles on this frame to ease lifting andlowering it on and off the treatment chamber. The function of this frame isto hold the plastic sheet in place. The sheet, in turn, seals the containerbefore treatment, thereby preventing any gas escape.

Insert a tube (D) capped with a nut through one of the walls at 65 cm(Figure 2). The tube should be slightly above the level of the soil to allowthe gas to flow into the chamber freely. Connect the methyl bromide gasto the chamber. There are two ways of doing this:

a) With a nut (E) that connects a large (92-kg or 200-lb) cylinder tothe tube by means of a duct. Gas is released by means of acontrol tap on the cylinder.

b) With an applicator tube, specially designed for 460-g cylindersconnected directly to the tube in the chamber. This type ofapplicator has a clamp that, when tightened, causes the gascylinder top to be perforated and a hole to be formed, allowing thegas to escape down the tube and into the chamber.

Make holes (Figure 3) in the damp soil (F) to facilitate the flow of the gasinto the chamber. The holes should have a 2-cm diameter made by awooden stick of that size. Make sure they reach the bottom of thechamber and are 1520 cm apart.

To seal the container, cover it with the thick plastic, and place the metal

frame on top, ensuring that it sits in the rim of the chamber and effectivelyseals it. To make the container totally airtight, spread damp soil or sandon top of the frame.

Note: During application and when opening the chamber, use gloves, gasmask, etc. Close the disinfestation chamber tightly to avoid gas leaks.

2. Dosage

The amount of gas (m3) used will vary from 11.5 lbs, depending on thetype of soil mixture. Heavy or highly infested soils will need higher doses.After the first application of a given dose, it is advisable to verify if thereare still fungi, bacteria, or nematodes at the applied dose.

3. Treatment period

The treatment period for methyl bromide is 72 hours. After this treatmentperiod, uncover the chamber, turn the soil, and let it ventilate for 72hours. After this, the soil may be used.

As a control, to ensure that there are no toxic residues, put some of thesoil and a lettuce plant into a pot: if the plant suffers no changes, the soilis safe to use.


4/15


4. Sealing the container

An alternative method of sealing the chamber (rather than making a rim)is to attach 10 bolts to the top edge of the chamber, construct the frame,and cover as shown in Figure 4.

This cover is a wooden frame lined with 4-mm plywood (G). For the

frame, use 3/4" lathes for a final depth of 2 cm (1" rough = approx. 3/4"planed). Perforate holes along the edge of the cover. These holes mustcoincide with the positions of the bolts around the chamber. Between thewooden cover and the chamber, place a sheet of rubber or canvas as aseal, remembering to make 10 holes in this as well, for the bolts.

Seal the container securely, capping the bolts with

butterfly clasps


5/15



6/15


5. Methods for Applying the Gas

The alternative methods for applying the methyl bromide gas are shownin Figures 5 and 6.

a) Gas under high pressure from a 92-kg tank

Build the disinfestation chamber (Figure 5, H). Place a tube through thewall and connect it by using a conducting hose to the 92-kg (200 lb)methyl bromide cylinder. Inside the chamber, attach the tube to anotherplastic tube with small holes at equal distances from each other along itslength. Lay this along the bottom of the chamber to allow for equaldistribution of the gas. The gas flows because it is under pressure in thecylinder. Before treatment, make the holes in the damp soil, as describedin Figure 3.

Because it is heavier than air, methyl bromide gas rises quickly, thenflows down through the holes into the soil. When a large gas cylinder isused, place it on a scale to determine, by checking weight loss, thequantity of gas used in relation to the volume of soil being treated.

b) Gas in a small cylinder

The same type of disinfection chamber is used (Figure 5, I). When a 460-g cylinder is used, apply the gas using the nozzle connected to the pipe,which passes through the chamber wall. The gas flows out over thesurface of the soil, then sinks into the holes made in the soil and spreads.

c) Without applicator nozzles


7/15


If there is no applicator nozzle on either the 460-g or 92-kg cylinder, usethe same type of chamber with a plastic cover (as shown in Figure 6, J).

A piece of wood pierced by a nail should then be attached to the top ofthe small 460-g cylinder. Place the cylinder in the chamber and cover withplastic, as described earlier. Put pressure on the wood through the plasticcover; this will perforate the cylinder and allow the gas to escape andmove through the soil as described.

d) When no chamber is available

If neither a chamber nor applicator nozzle are available (Figure 6, K), pilethe soil mixture to be treated on plastic sheeting. Place a small 460-g gascylinder and the wooden piece containing the piercing device on top ofthe pile. Cover completely with more plastic sheeting and seal the edgesby piling damp sand or soil around the edge of the plastic. Press thewood through the plastic so that the piercing device makes a hole in thecylinder and allows the gas to escape and spread through the soil.

Disinfection Using the Solarization Method

Pathogen eradication from both soil and substrate can be achieved usingphysical means such as dry heat, steam, hot water, solar radiation, or lowtemperatures.

Using solarization, it is possible to eliminate fungi, nematodes, and otherpathogens, as well as weed seeds in the substrate. In addition, thismethod permits the use of complementary fungicides, nematicides, etc.,in much lower-than-recommended doses.

Solarization involves the use of solar radiation to control insects andpathogens, using transparent plastic sheets to cover the substrate, whichintensifies the effects of sunlight.

The substrate to be disinfected is a mixture of 2 parts peat moss, 1 partsoil, and 1 part sand (2:1:1), with a moisture content equivalent to that offield capacity.

Solarization is performed outside the greenhouse in a yard or plot wherethe substrate can be exposed to the sun. A transparent plastic sheet0.050.50 cm thick is used. A thicker sheet is recommended, because itis more durable and maintains the temperature better. The transparentplastic sheet lets sunlight pass through but keeps it from refracting. Thistraps all solar radiation with higher temperatures during the hours ofgreater insolation.

Disinfection is based on the physical process of alternating high and lowtemperatures. The moisture in the substrate plays a major role due to theproduction of mist during the hottest hours of the day. This steamcondenses when the temperature goes down at night. Thus, apasteurization process takes place throughout the treatment. Fluctuationsbetween daytime and nighttime temperatures easily break the biologicalcycles of pathogens present in the substrate.

1. Making the outdoor treatment structure


8/15


In a cleared area, level a space 1-m wide and the required length, andcut a piece of plastic sheeting 1.5-m wide and 0.5-m longer than thespace. Take two boards (10" x 8" x 1") and arrange them perpendicularlywithin the cleared area (see Figure 7). Use bricks at each end to supportthe wooden boards. Place plastic inside the wooden bed and up over theedges.

Pile the substrate on the plastic sheet up to 2030 cm high (Figure 8).Using a rake, spread it uniformly over the entire prepared surface. Addwater until the normal field capacity is reached, and shape the pile byfolding in the four edges. The bed should now be about 1-m wide x 20-30cm x the chosen length (Figure 9). After the bed is formed and coveredwith plastic, remove the bricks and lumber pieces used to shape it.


9/15



10/15


Cover the top with a second plastic sheet (1.5 m x same length). Theedges should extend beyond the bed to facilitate the hermetic sealing(pile wet sand around the edge as shown in Figure 10). Insert athermometer through the plastic into the substrate to register thetemperature during solarization (Figure 10). This will record when thehighest temperature was reached, according to the incidence of solarradiation.

Immediately after sealing, construct three arches to support a plasticcover for the bed. The arch (Figure 11) can be made of plastic, iron,branches, plastic pipe, or other materials. The different arch materials aresecured using stakes in the following manner (see Figure 12):

--plastic or iron rods inserted into a wooden stake (A),

--green bamboo branches tied to a wooden stake (B),

--green tree branches tied to a wooden stake (C),

--plastic pipes (those used for electric cables) placed over an iron stake(D).

Arrange the arches over the substrate bed, 1 m apart. The distancebetween the stakes at the base of the arches should be approximately1.4 m, depending on the thickness of the bed. Be sure to leave a spaceof at least 20 cm between the arches and the bed (Figure 12).

Place a third plastic sheet over the arches to cover the entire chamber.Use two plastic sheets, if one is not wide enough. Join the plastic usingtwo wooden sticks 1 cm x 2 cm the length of the bed. Place one stickunder the plastic sheets and the other on top, joining the two sheetsbetween the sticks (Figure 13). Nail together using 3/4" nails. Carefullyplace the plastic sheets over the anchored arches with the joined edgesof the plastic in the center (Figure 14).

Cover the edges of the plastic that are in contact with the ground withsand to hermetically seal the chamber (Figure 14).

2. Alternative methods using the solarization treatment

An alternate method is to place the substrate in transparent polyethylenesleeves or tubes. These can be made by using two wooden strips andnails, or by folding the opening of the tube inward to form a bag (Figure15).

Bags prepared in this way do not need to be covered with arches or extraplastic sheets. Trials using this system have demonstrated that the

substrate reached 45o

55o

C during daylight hours when the sun's raysfell perpendicularly (maximum radiation). However, when they were putunder arches and an extra plastic sheet (Figure 16), the temperature roseto 74oC under the solar radiation conditions (La Molina, Lima, Peru).


11/15



12/15



13/15


3. Duration of treatment

The solarization process takes 46 weeks. Tests should be done todetermine how long the process takes in a particular location.

Depending on the number of hours of exposure to sun and the intensityof solar radiation, a well-disinfested substrate can be obtained in a shorttime.

4. Potential range of plant pests and pathogens present in the

substrate subject to elimination

Substrate holds a wide range of pathogenic microorganisms, nematodes,and insects. They usually attack roots and stems near the soil line,producing considerable losses in greenhouse propagation beds.

Fungi


14/15


According to Pullman (1981), solarization reduces the populations ofseveral fungus species present in the planting substrate, such asVerticillium dahliae, Pythium sp., Rhizoctonia solani, and Thielaviopsisbasicola.

Bacteria

Many of the bacteria affecting the growth of potatoes are eliminated usingthis method. Some researchers have found that solarization-treated soilspresented fewer bacteria and fungus colonies on PDA (potato dextroseagar), thus confirming the efficiency of solarization treatment.

Nematodes

Solarization eliminates all nematodes present in treated substrate.Meloidogyne incognita is efficiently eliminated. Globodera spp.,Pratylenchus, Paratrichodorus, Criconemella, Xiphinema, and Pratylenchus

spp. are reduced between 42% and 100%.

Insects

Some insect larvae and pupae that attack potatoes can be eliminated.

5. Other effects of solarization

Nutrient availability

Solarized soil shows a definite increase in organic matter (OM) and mineralcontent. Concentrated NO3 increases. Other ions such as NH

4, K+, Ca+, Mg+,

and Cl+

also increase.

The substrate is usually prepared using a mixture of peat moss, sand, andsoil. Thus, it has a high organic matter content. When subject to variations intemperature and moisture content, the mineralization process of organicmatter is accelerated. However, in certain stages of plant development,symptoms of minor deficiencies may be present, but this may be due todestruction of beneficial soil flora. The deficiencies can be easily overcomeby applying chelated foliage fertilizer.

Weed seeds

Up to 100% of weed seeds and vegetatively-propagated organisms present

in planting substrate (i.e. rhizomes and bulbs) are eliminated with thistreatment.

The high temperatures obtained during solarization effectively destroy thosekinds of propagative organisms in substrates that are used to produce basicseed in greenhouses and virus indicator plants.


15/15


Recommended Literature

Aguilar, J.; Vitorelli, C. 1987. Desinfeste el substrato de siembra conbromuro de metilo para producir "semilla basica" de papa eninvernadero. INIPA/COTESU, Centro Internacional de la Papa (CIP),Lima, Peru.

Aguilar, J. et al. 1989. Desinfeste el substrato de siembra por el mtodode solarizacion. INIPA/COTESU/CIP, Centro Internacional de laPapa (CIP), Lima, Peru. Pullman (1981) 12pp.

desinfecting soil

Documents