Nematodes are one of the major pests that canseriously limit sugar beet production in Idahoand eastern Oregon. The severity of damagedepends on the nematode species present andpopulation densities in the soil at the time ofplanting. The most common sugar beet nema-todes in Idaho and eastern Oregon are the sugarbeet cyst nematode (Heterodera schachtii), rootknot nematode (Meloidogyne hapla), and stubbyroot nematode (Paratrichodorus or Trichodorusspecies). For more information about thesenematodes, refer to CIS 1072, “Sugar BeetNematodes in Idaho and Eastern Oregon,”http://info.ag.uidaho.edu/Resources/PDFs/CIS1072.pdf.

This publication deals only with the cystnematode (Heterodera schachtii). Over half of thesugar beet acreage in Idaho and eastern Oregonis infested with the sugar beet cyst nematode ata level that requires treatment.

Cyst nematodes generally have limited hostranges compared to root knot nematodes orstem nematodes. However, the sugar beet cystnematode can parasitize a number of vegetablecrops such as red table beet, broccoli, radish,Brussels sprouts, mustard, kohlrabi, and rape-seed, and weeds such as chickweed, nightshade,and goosefoot. Growers can reduce damage fromsugar beet cyst nematodes by using a variety of

crop management practices such as crop rota-tion, weed control, early planting, use of greenmanure, and proper fertilization. For more infor-mation on using green manure trap crops, seeCIS 1071, “Management of Sugar BeetNematode,”http://info.ag.uidaho.edu/Resources/PDFs/CIS1071.pdf.

Soil fumigation is one way to manage thesugar beet nematode. Soil fumigation is anexacting procedure that must be done properly ifsatisfactory results are to be obtained. To controlsugar beet cyst nematode and optimize sugarbeet yield, fields should be fumigated if the soilcontains 5 or more viable cysts per 500 cubiccentimeters of soil, and the period between sugarbeet crops is 3 years or less. These recommenda-tions are based on University of Idaho diagnosticnematology lab results. For more information onsoil sampling, refer to CIS 1056, “SamplingProcedure to Diagnose Nematode Infestations,”http://info.ag.uidaho.edu/pdf/CIS/CIS1056.pdf.

Fumigation is used in many of the majorsugar beet production areas to suppress and/orcontrol plant parasitic nematodes and disease-causing organisms; and to improve beet qualityand yield. Nematicides are generally used as apreventive measure. By the time nematode dam-age becomes apparent, the infested crops are

CIS 1163

University of Idaho • College of Agricultural and Life Sciences

Soil FumigationA Tool for Sugar Beet CystNematode Management

by Saad L. Hafez and P. Sundararaj

usually injured so severely that control measuresare ineffective in correcting the damage.Therefore, control procedures are normallybased on pre-plant application (mostly fumigantcompounds) or at-planting application (non-fumigant compounds).

Soil fumigants will not completely eradicatenematodes. The objective of such treatments isto reduce nematode populations to a level whereserious crop damage will not result.

Types of FumigantsTwo classes of soil fumigants are available. Thefirst class includes the true fumigants. True fumi-gants are volatile compounds that rapidly convertfrom liquid to a vapor or gas state. True fumi-gants move as a gas through the soil airspace bydiffusion, whereby molecules of gas spread uni-formly throughout available space. True fumi-gants include Telone II and Telone C-17.

The second class of fumigants includes thenon-true fumigants such as Vapam (metam-sodium). Non-true fumigants are extremelywater-soluble and move easily in the soil as a liq-uid. Metam-sodium will move from a region ofhigh concentration to one of lower concentrationand during the final stage of conversion, as gasin the soil air space, similar to true fumigants.

Factors Influencing theEffectiveness of Soil FumigationNematodes are aquatic animals that live in thewater films surrounding soil particles. To beeffective, fumigants must penetrate and diffuseinto soil pores to contact the nematodes and, inaddition, must penetrate the moisture film sur-rounding the nematodes. Several factors caninfluence the effectiveness of soil fumigation.

The grower must consult with the sugar fac-tory agronomist to select the correct fumigantnematicides. This depends on the availability ofapplication equipment, time of year applied,and the cost/benefit ratio. Knowledge of biologyand habitat of the nematode pests involved willhelp determine the appropriate chemical andoptimum time, rate and method of application.

Many environmental factors can positivelyor negatively affect the activity of fumigants.Soil type, soil temperature, soil moisture, organicmatter content, and certain soil cations can

affect the rate of chemical conversion, distancemoved, and the rate of movement.

Clay soils tend to restrict movement of alltypes of fumigants. Although clay soils slow theconversion of Telone II to the gas phase, theyactually increase the rate of metam-sodium con-version to MITC, the liquid biocidal state. Infine-textured clay soils, pore spaces are muchsmaller than those in sandy or sandy-loam soils.Such small pores are likely to be blocked byexcess moisture or compaction, making it moredifficult for vapors to diffuse. As a consequence,fumigation may be incomplete, especially if thenematicide is highly volatile or short-lived in thesoil. Sandy soils, however, contain large poresthat are less likely to be blocked by excess mois-ture or compaction; but an effective surface seal,necessary to prevent rapid loss of vapor, is moredifficult with such coarse-textured soils.

High organic matter tends to restrict move-ment and tie up fumigants, sometimes necessi-tating the use of higher rates. For effectivenematode control, peat and other organic soilsrequire two to three times the amount of nemati-cide needed for mineral soils. The high rates arenecessary to compensate for the adsorption ofthe nematicides on the organic matter of thesoil.

Temperature and weather conditions alsoinfluence the effectiveness of fumigants. As soiltemperature increases, the rate of conversion ofboth Telone II to the gas state and metam-sodium to MITC increases. High soil temperaturespeeds the gaseous diffusion, thus shortening theexposure of nematodes to the toxic fumes. Theresult is poor fumigation and more nematodedamage to plants. Low soil temperatureincreases the retention of the gas, thus prolong-ing exposure of nematodes to toxic fumes andresulting in more effective fumigation and lessnematode damage to plants.

Freezing temperatures during the winter afterfumigation will kill more nematodes that havesurvived the fumigation treatment, makingfumigation more effective. On the other hand,early snowfalls before the freeze will providesome insulating protection for the survivingnematodes, reduce winter-kill, and thus reducethe fumigant’s effectiveness. Warmer conditionsin the spring and summer will stimulate thenematode eggs to hatch early in the spring. Thismeans more generations during the growing

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season. The number of surviving nematodes willincrease faster, and the population may climbhigh enough to cause damage.

Soil PreparationProper soil preparation is vital to maximizeproduct performance.

1. The soil must be free of clods, since fumi-gants are unable to penetrate them and agood soil seal is not possible with toomany clods.

2. Previous crop residues must be thoroughlychopped up, incorporated into the top 4inches of the soil profile, and allowed todecompose before fumigant is applied.

3. The soil must be loosened to aid themovement of either liquid or gas throughthe desired treatment zone.

4. Soil moisture must be regulated to thelevel specified on the label for the fumi-gant being used.

5. The surface seal is important to increasethe concentration of the vapor in the sur-face inch or two of soil.

6. The soil should not be disturbed for atleast 3 weeks, or approximately 1 day forevery gallon of true fumigant applied peracre, after treatment.

Both soil temperature and moisture contentare important to successful fumigation. The soiltemperature below the surface should bebetween 40 and 80°F. In addition, these condi-tions help ensure that the target plant parasiticnematode is in a stage susceptible to the fumi-gant.

The depth at which volatile fumigants areapplied varies with dosage, soil temperature, soilmoisture, soil type, nematode species to be con-trolled, and desired depth of control. For mini-mum dosage rates under optimum soilconditions (light, loose soil with low organic con-tent and 60% field capacity moisture), applica-tion is generally made at a depth of 12 to 18inches. If the application is too deep, the vaporswill not reach the surface inch or two of soil; if itis too shallow, the vapors are lost through thesoil surface.

As dosage is increased, depth of applicationalso should be increased. For example, Telone II

at 12-15 gallons per acre should be injected at 8-12 inches, while 18 - 25 gallons per acre shouldbe injected at 12-15 inches. Increased depth andrate of application are associated with highnematode kill to deep soil levels and little loss ofvapors from the soil surface. If soil temperatureis above 80°F and soil moisture is considerablybelow field capacity, depth of application shouldbe increased and, to retain the vapors in the soil,the surface of the soil should be well compressedfollowing application.

Application RateThe rate of chemical application depends on thefollowing factors.

1. Previous crop history should be consid-ered, since higher rates should be usedfollowing a perennial crop (such asalfalfa).

2. Soil type is important because lighter soilsrequire lower rates than heavier soils.

3. Deep application requires the use ofhigher rates; broadcast application meth-ods use higher rates than row applica-tions.

Growers can decide the rate of application byconsulting with the sugar company agronomist.

Time and Methods of ApplicationThe optimum time for soil fumigation in thePacific Northwest is in the early fall. Springfumigation can be done, but is not as effective asfall application. Since most of the fumigantnematicides are phytotoxic (poisonous toplants), they must be applied to the soil beforeplanting. The time of application should bedecided by the grower in consultation with thesugar company agronomist.

The method of application varies with:

1. Type of chemical and mode of action

2. Degree of nematode infestation

3. Type of crop preceding the sugar beet crop

4. Cost/benefit ratio

5. Soil type and soil conditions

6. Type of applicator available

7. The availability of water at time of appli-cation

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There are two major types of fumigant appli-cation methods commonly used for nematodecontrol in sugar beet fields. In type I, solid orbroadcast treatment, nematicides are applied tothe whole area of a field with the intention ofcontrolling the nematode population through-out the soil profile. Telone is injected at a shankspacing of 20-24” on broadcast applications.Fumigants should be used at the higher labelrate to obtain a uniform nematode control. Thedepth of the application is also very critical.Deeper injection allows the gas to penetratemore soil, and prolongs the exposure time, thusincreasing the effectiveness of the fumigationtreatment.

Type II is a less expensive method of treatingcrops grown in rows, and uses simple applica-tion devices for applying the chemical along theplanting row. However, row treatments do notequal overall treatment for obtaining maximumnematode control. The disadvantage of rowtreatment is that areas between rows leftuntreated create the source of reinfestation bysurviving nematodes.

With either method, the application equip-ment of the professional fumigation companyshould be in good working condition to achievea uniform application and avoid untreatedstrips. Figures 1 and 2 show machinery used toapply Telone II.

Sealing Soil Surface AfterFumigation and Waiting PeriodA soil surface seal is desirable following applica-tion of any volatile nematicide, regardless of soiltype, temperature, moisture, or preparation. Theseal improves nematode control in the surfaceinch of soil and, in addition, increases retentionof the fumigant in the soil.

To create an effective seal, the shank tracesleft by the application equipment must be dis-rupted and the soil surface compacted.Disruption of shank traces can be accomplishedusing equipment that uniformly mixes the soilto a depth of 3 to 4 inches. A tandem disc (orsimilar equipment) in combination with a ringroller or cultipacker can maximize soil sealing(figure 3).

Figure 1. Ripper type soil fumigator for Telone II application

Figure 2. Ripper shank for Telone II application

Figure 3. Telone II applicator, disc and soil compacter

The waiting period between application andplanting to avoid injury to the crop and increasefumigation efficiency depends on the rate atwhich the fumigant was applied, the type offumigant, soil temperature, and soil conditions.The fumigant leaves the soil quickly when theweather is hot and dry, and may remain longerwhen it is cold and wet. Soils high in organicmatter are especially difficult to aerate. Underoptimum conditions, 2 to 3 weeks are usuallyadequate.

ReinfestationReinfestation may occur following fumigation ifextreme care is not taken. For example, movingequipment from infested fields to treated fieldswithout cleaning, or leaving untreated stripsduring fumigation, then moving infested soilfrom these areas into treated areas, are commonsources of recontamination. Avoid using wasteor runoff water. Field ends should be treated;even though not planted, they can be a sourceof reinfestation while performing normal culti-vation practices. Weed control is important toreduce the number of nematode host plants in agiven field.

Finally, even if we do our part absolutely cor-rectly, the effects of Mother Nature, which areout of our control, on nematode survival andrate of reproduction may interact to make con-trol less than adequate.

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Summary of soil fumigationto control sugar beet cystnematodes:

• Fields should be fumigated if the soilcontains 5 or more viable cysts per500 cubic centimeters of soil, and theperiod between sugar beet crops is 3years or less.

• Consult with the sugar factory agron-omist to select the correct nematicides.This depends on the availability ofapplication equipment, time of yearapplied, and the cost/benefit ratio.

• Apply the recommended chemical atthe proper time and rate.

• Maintain proper soil conditions (mois-ture and temperature) before and dur-ing the application.

• Allow a sufficient waiting period afterfumigation to give enough exposuretime to kill nematodes adequately.

The authors—Saad Hafez is an extensionnematology professor and P. Sundararaj is aresearch associate. Both are with the University ofIdaho Parma Research and Extension Center.

ALWAYS read and follow the instructionsprinted on the pesticide label. The pesticide rec-ommendations in this UI publication do notsubstitute for instructions on the label. Due toconstantly changing pesticide laws and labels,some pesticides may have been cancelled orhad certain uses prohibited. Use pesticides withcare. Do not use a pesticide unless both the pestand the plant, animal, or other application siteare specifically listed on the label. Store pesti-cides in their original containers and keepthem out of the reach of children, pets, andlivestock. Trade names are used to simplify theinformation; no endorsement or discriminationis intended.

July 2009 © 2009 by University of Idaho

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with theU.S. Department of Agriculture, Charlotte V. Eberlein, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. TheUniversity of Idaho provides equal opportunity in education and employment on the basis of race, color, national origin, religion, sex, sexual orien-tation, age, disability, or status as a disabled veteran or Vietnam-era veteran, as required by state and federal laws