Orchard Soil Fumigation

T. J. Smith, WSU Extension Agent for Chelan, Douglas, and Okanogan counties

Pathogenic soil organisms in most mature orchards often reduce root growth of young fruit trees when the site is replanted. Poor root development leads to reduced vegetative growth and poor fruit yields throughout the life of the replanted orchard. This is especially common in north-central Washington. Replant disease is most common when apples or pears are planted after either apples or pears or when cherries are planted after cherries.

Certain soil fumigants have controlled the specific orchard replant disease when properly applied. Positive effects can be measured the first season and even 20 years after treatment. No soil treatment effectively controls replant disease problems after planting.

Nonchemical control options include rotation 5 years or more out of orchard (removing most of the tree roots the first year), or replacing soil in the planting hole with good soil from a nonorchard source. Soil replacement is not at all practical on a large scale. To be effective, the soil replacement should fill a planting hole 7 feet across and 2.5 feet deep. Using less soil helps the first season or two but is not a long-term cure. Despite research efforts and a great body of theoretical control recommendations, no practice or product other than long rotations or fumigation has proved effective in orchard replant disease management.

Demonstration trials and thousands of acres of grower experience over the past decade have shown that soil fumigation does not always lead to excellent tree growth, but it usually does. Unfumigated orchards usually are difficult to manage when young and take several extra years to break even, if they ever do. If you are not absolutely certain that replanted trees grow very well in your orchard, you may skip a procedure that costs $400 to $600 an acre and suffer a reduction in gross returns of $40,000 an acre over the next 10 years (actual data).

Many soil fumigants, fungicides, fertilizers, and soil amendments have been tested for effect on orchard replant disease, only three have shown long-term growth and yield benefits in Washington orchard trials: methyl bromide, metam sodium, and fumigants containing chloropicrin. The other treatments may help early tree growth, but the effect of replant disease can be seen during the second or third leaf, resulting in slightly larger, but sick, trees.

Some fumigants must be custom applied, others may be applied by a certified private applicator. If you are unfamiliar with the product, pay special attention to use and safety information. Used improperly, fumigants can be quite hazardous to the applicator and to the crop and will not effectively control orchard replant disease. Some application options listed on fumigant product labels have not controlled replant disease, so closely follow the label methods that have proved successful in Washington orchards, as described below. Follow soil temperature and preparation guidelines on the product label. Growers usually try to fumigate in late fall after harvest or in early spring before planting; soil conditions may not be optimum at those times. Fumigants are more effective if soil condition is not at the bare minimum at application. In general, colder, wetter, and more finely textured soils retain fumigants longer. Soil usually is in best condition for fumigation in October and early November and in April or later in spring. Complete fall treatment well ahead of the time that soil temperatures drop below the minimum recommended on the label.

To reduce the potential of fumigant damage to the newly planted trees’ roots, dig planting holes or disturb the planting-area soil a few days before planting. Working the soil a few days before planting is especially important when planting by machine after a spring fumigant application. Fumigants tend to remain longer in unripped, compacted soils.

It is far better to plant later than usual in spring than to risk tree damage by planting while fumigant residues remain in the soil. Skipping fumigation because it sets back the planting date is a poor choice. A tree planted in May in fumigated soil usually outperforms a March-planted tree suffering from even a mild case of replant disease. Long-term productivity should be the main concern, not date of planting.

Fumigants are safe and effective when properly used, but special training is highly recommended for first-time users. Having used other pesticides or fumigants does not qualify as adequate user experience, because each fumigant has unique properties. Before using any fumigant, carefully read and follow safe handling and protective equipment information on the label. Special respirator canisters and vaporproof eye protection may be required.

Product Choices

Methyl Bromide This product is stored as a liquid under pressure, but it turns to a gas when released under the soil surface if the soil temperature is over 45°F. It moves through the soil as a gas, in the air spaces between soil particles. It is most effective when applied to relatively dry (50% of field capacity), warm (50 to 60°F) and well-plowed or well-ripped soil. It may remain in the soil 6 to 8 weeks under cool, wet conditions, so fall treatment is highly recommended. Spring treatment is possible but should be professionally monitored to determine that the product is at safe levels prior to planting. Keep application 20 feet or more from established plantings, especially if soil is warm, sandy, and dry.

For widely spaced trees, such as cherries or most pears planted at less than 120 trees per acre, spot-treating each individual future tree site may be economical. Inject 0.5 to 1 lb methyl bromide by special probe about 18 inches below the soil surface at each future tree’s planting site. Use a wooden stake or a shovelful of soil to plug the injection hole as soon as you remove the probe. Do not use your foot to press the hole closed. Injecting the product is complicated by cold temperatures and by compacted or wet soils.

For closer plantings, methyl bromide most often is applied com-mercially at 400 to 600 lb/A. Higher rates are necessary when soil conditions are less than optimum. Contact the custom applicator well ahead of treatment, and carefully follow directions on soil preparation. The applicator usually will request a cleared, ripped, and smooth orchard surface before application. The better the soil is worked, the more complete the treatment and the faster the product leaves the soil. Working the soil also gives a once-in-20-years opportunity to properly lime and fertilize and to break up compacted layers. If the site is so rocky it’s impossible to rip properly, see the metam sodium section below. That product may be a more practical choice.

Chloropicrin Mixtures Chloropicrin (tear gas) was the first soil fumigant found to effectively control replant disease. Its use is relatively new in orchards only because properly designed custom application equipment was not available in most areas until the past few seasons. This product moves not much more than 9 to 12 inches from the point of injection, so it must be custom applied with special equipment. A large volume of the trees’ future root zone must be treated to assure long-term benefits. Thus, the equipment must apply the product so that the future tree row is treated in a band at least 7 or 8 feet wide and 2 to 3 feet deep.

The chloropicrin usually is mixed at high rates either with methyl bromide (at lower rates than used for straight methyl bromide fumigation) or with 1,3-dichloropropine. The 1,3-D + chloropicrin mixture often is sold under the trade names of Telone C-17 or Telone C-35. These mixtures have performed very well in north-central Washington trials. Telone II, a 1,3-D product without chloropicrin, is not recommended for orchard replant disease control. The lower rate of methyl bromide or the 1,3-D in the chloropicrin mixture aid in nematode and soil insect control. These pests would not be controlled adequately with only chloropicrin.

Follow the critically important guidelines on soil condition, soil preparation, and application timing described in the methyl bromide section, above.

Metam Sodium Originally sold as Vapam, this product is now also available under several other trade names, including Sectagon 42 and Metam CLR. Metam sodium is a water-soluble liquid that moves via irrigation water into the zone of the soil that you wish to treat. After the fumigant-water mixture stops moving down in the soil, the metam sodium converts into a more toxic fumigant gas (methyl-isothiocyanate). This gas moves only a few inches from the zone treated with the water mixture, so it is critical that the metam-water mix penetrate the future tree root zone 2.5 to 3 feet—but no more. Broadcasting the product with sprinkler irrigation or treating a band at least 8 feet wide along the future tree row has resulted in long-term tree growth and yield improvement. The most effective rate is 75 gal product per treated surface acre. Lower rates have resulted in reduced tree growth and lower fruit production. In trials, higher rates have not increased growth and production.

Before application, soil should be 45 to 75°F and relatively moist (over 85% of field capacity). Irrigate the field if the soil is even moderately dry. Use approximately 0.5 to 1 inch of sprinkler irrigation water to drive the fumigant to the desired depth. Without immediate incorporation with water, the product will evaporate rapidly, creating a drift and applicator hazard. However, using too much water during application will overdilute the product in the soil and greatly reduce the fumigant effect. The goal during application should be to drive the product in no less than 2 feet but no more than 3. Sandy, wet soils require the lesser rate of water; more finely textured, plowed or ripped, and drier soils require the higher amount. Measure the irrigation system application rate to determine the hours of irrigation that will apply the proper amount of water. Most systems should run 2 to 5 hours during application.

It is not always practical to work orchard soil before treatment. If the soil is prepared for planting after treatment, do not mix untreated soil into the fumigated area.

Note Metam sodium product labels describe a number of application methods. The only practical and effective methods for orchard replant disease involve driving the product into the soil with sprinkler irrigation water. Shanking or rototilling the product into the soil or filling planting holes with large volumes of water mixed with a per-site rate of the fumigant has not been effective on sites with moderate to severe replanting disorder. (Some newly designed shank applicators being tried in the Columbia Basin may be effective but have not been fully tested relative to treatment of orchard replant disorder). If you do not have sprinkler irrigation, I do not recommend this product. Filling shallow, level basins 7 feet wide constructed at each planting site with 12 oz of metam in 35 to 45 gal water is effective, if properly done. This method is far too labor intensive to be economical on a large scale but may be useful for limited tests. However, it is so labor intensive, it is rarely done properly! Follow directions on a 24(c) special local needs label for banding the fumigant during sprinkler application.

Fall treatment allows planting in the treated site in late winter or early spring. If fall weather or lack of irrigation water after harvest delays treatment until spring, wait 21 to 30 days between treatment and planting. The site and soil can be prepared for planting starting 10 to 14 days after treatment. Digging planting holes or disturbing the soil a few days before planting speeds release of fumigant residues that may remain. Nontoxic but unpleasant sulfurous odors may be in the soil several weeks after treatment.