More About the Control of Nematodes

Albert L. Taylor.

The control of nematodes requires clean soil, clean planting stock, and sanitation. Plant parasitic nematodes are eliminated from the soil by crop rotation, chemicals, heat, bare fallow, and a few other methods.

The use of crop rotations to control nematodes is based on the fact that nematodes are obligate parasites and can neither live indefinitely nor reproduce unless they can feed on living plants. Furthermore, all have a degree of host specialization: Any given species can feed and reproduce only on certain plant species. Without those plants, the nematodes starve or succumb to parasites, predators, and diseases, even though other plants are grown nearby.

The main disadvantage of crop rotations for control is the time required and the loss of income if the rotation crops are less profitable than the main crop. When crops of low or moderate value are concerned, it is the only practical method of control.

Chemicals used to kill nematodes in the soil must be efficient for killing the nematodes and must leave no residue that can harm plants. They should be easy to apply and inexpensive. Many chemicals have been tested for the Purpose. Four are in general use:

Chloropicrin and mixtures that contain methyl bromide, chlorobromopropene, ethylene dibromide, or dichloropropene. One type of methyl bromide mixture is a gas at ordinary temperatures. The others are liquids. The liquids are applied by injection into the soil. The methyl bromide gas is released under a cover placed above the soil. In either case, the fumes permeate the upper layers of the soil, killing the nematodes by contact.

Each has advantages and disadvantages. Prices also vary, so the choice of one for any given plot of soil involves consideration of the organisms to be controlled, the local conditions, and the relation of the cost of the fumigant to the value of the crop.

Chloropicrin applied at 200 pounds an acre is an excellent nematocide and insecticide. Applications of two or three times that amount also control some fungi, bacteria, and weeds. Because chloropicrin does not penetrate undecayed plant material readily, it should be used only after the residues of a crop have had time to decay. Because its fumes in the air will damage plants, chloropicrin cannot be used in one part of a greenhouse while crops are growing in nearby sections a difficulty that may also be encountered outside.

After chloropicrin is applied, it must be confined to the soil. The usual method is to apply a "water seal" by sprinkling with enough water to wet the top inch or two of soil. An interval of 5 to 25 days must be allowed between application of the fumigant and planting. The exact time depends on the amount applied, the soil moisture, and the type of soil.

Extreme precautions must be used in handling chloropicrin. Small amounts of the fumes in the air will cause profuse watering of the eyes. Larger concentrations cause violent coughing, vomiting, or even death. Nevertheless, chloropicrin is not dangerous to use. In fact, the opposite is true to a certain degree, because the watering of the eyes gives preliminary warning of the presence of fumes before the more serious effects ensue. No one will voluntarily remain in even a low concentration of chloropicrin fumes. Gas masks of the proper type give full protection. Chloropicrin is not inflammable. The shipping containers are heavy cylinders for large quantities and sealed cans for 1-pound bottles.

The 98 percent methyl bromide fumigants, applied at rates of 1 or 2 pounds for each too square feet, give good control of nematodes, soil insects, and most weed seeds, fungi, and bacteria. The 10 to 15 percent methyl bromide mixtures are good nematocides and insecticides at the rate of 78 to 100 gallons to the acre. Methyl bromide penetrates undecayed roots readily. Small amounts of fumes in the air do not damage growing plants. Crops can be planted 2 to 4 days after application. It is particularly useful for greenhouse fumigation. If reasonable precautions are taken, methyl bromide is neither unpleasant nor dangerous to use. The 98 percent methyl bromide fumigants are obtainable in 1-pound cans. The 10-percent or 15-percent mixtures are shipped in drums.

Ethylene dibromide soil fumigants usually contain 41 percent to 83 percent of the chemical by weight, the diluent being naphtha. The rates usually recommended to control nematodes and soil insects are 10 to 20 gallons an acre of the 41-percent mixture and proportionately less of the 83-percent material. The latter is often diluted for convenience in application. Penetration of undecayed crop residues is good and small concentrations of fumes in the air are not toxic to plants. No water seal or cover is necessary. The soil can be planted 10 to 14 days after application of the fumigant. Ethylene dibromide fumigants are not dangerous or unpleasant to handle if used with care. The shipping containers are drums of various sizes.

Dichloropropene fumigants have about 50 percent of this chemical mixed with dichloropropane. Applications of 20 gallons an acre are used against nematodes and soil insects. The kill of nematodes in undecayed crop residues is good. Fumes in the air do not injure plants. At least 2 weeks must be allowed between application of the fumigant and planting of the crop. No cover or water seal is necessary. The shipping containers are steel drums.

Chlorobromopropene can be used effectively against nematodes, insects, and soil fungi.

All soil fumigants are poisonous to man and animals. They must be handled with care lest the liquid come in contact with the skin or clothing and the fumes inhaled. If the fumigants are accidentally splashed on the skin, they should be washed off immediately with soap and water. If clothing or shoes become wet, they should be removed instantly and not worn again until clean. Stored fumigants should be kept tightly sealed. With the ones that are inflammable or have inflammable diluents, precautions should be taken against fire or explosion. Most of the soil fumigants are corrosive to metals, particularly in moist air, so applicators should always be cleaned thoroughly after use and partly empty containers should be tightly closed.

Besides fumigants, a few other materials have limited use in the control of nematodes, the greater part of the applications being to soil used for tobacco and other seedbeds. Urea controls nematodes when applied at rates of 8 ounces to 1 pound the square yard and is often mixed with calcium cyanamide for weed control. Sodium azide is also an effective nematocide when used at the rate of 4 ounces to the square yard. These materials are used in powder form, which is mixed with the upper layers of the soil.

Liquid soil fumigants are put 6 to 8 inches beneath the soil surface. The applications are made 10 to 12 inches apart horizontally. On a small scale that can be done with improvised equipment, but large-scale applications require special applicators. For areas of less than an acre, hand applicators that have a calibrated pump to deliver measured amounts of the fumigant through a hollow spike thrust into the soil are satisfactory. For larger areas, applicators drawn by tractors or mounted on tractors are used.

They are of two general types. One type delivers the fumigant in continuous streams behind shanks that run through the soil. The other delivers the stream of fumigant ahead of a plow which immediately turns the soil to cover it. If a shank applicator is used, the soil is prepared in advance by plowing, harrowing, and leveling. If a plow applicator is used, the harrowing and leveling follow immediately after application. Shank applicators can be made in any convenient size, but usually have six to eight shanks and can cover an acre or more an hour.

The essential points in soil fumigation are good preparation of the soil, application of the exact amount of fumigant desired with correct spacing and at the proper depth, and promptness in carrying through the operations necessary after application. Soil preparation includes cutting up of weeds, trash, and crop residues, which might interfere with the smooth operation of the applicator. After the application of the fumigant, the soil should be left smooth with all clods well broken up. This is usually accomplished by a drag behind the shanks of the applicator or by a harrow and drag after use of a plow applicator.

GASEOUS FUMIGANTS, such as 98 percent methyl bromide, are applied in a different manner. The soil is prepared as for planting. A gas-impervious cover, usually a specially treated paper, such as Sisalkraft, or plastic tarpaulins (Fumi Cover and others), is placed over the area to be fumigated. The cover is not in contact with the soil surface, but is supported a few inches above it. The edges of the cover are buried. The methyl bromide is then introduced by means of a plastic tube into an open container placed underneath this cover on the soil surface. Inexpensive applicators especially made for the purpose make it a simple operation. The cover is left in place for 24 to 48 hours. This method is limited to rather small plots.

It is used mostly to fumigate seedbeds, greenhouses, and nursery plots.

When the best possible control of nematodes or other soil pests is desired or when the crop is to be planted in rows less than 24 inches apart, the fumigant is applied to the whole area to be planted. This is called solid, area, over-all, or broadcast fumigation.

Row fumigation is used where crops are to be planted in rows more than 24 inches apart. One or two lines of fumigant are centered on the row. It requires some definite method of marking the rows so that they can be located for planting. The usual procedures are to form a raised bed when the fumigant is applied, to mark the rows by shallow furrows, or to locate them with reference to the tracks left by a tractor applicator.

Strip applications of fumigant may be used when orchards are to be planted. A strip of soil 6 to 8 feet wide is fumigated for each row of trees.

Site fumigation is used in orchards or when individual trees or shrubs are to be planted. An area 6 to 8 feet in diameter centered on the planting spot is fumigated by means of a hand applicator.

If crops are to be planted in widely spaced hills, spot fumigation can be used. The locations of the hills are marked and the fumigant placed with hand applicators.

The advantage of strip, row, site, or spot fumigation is the saving in the amount of fumigant required. At the same time the plants are protected from serious infection when they are small and most vulnerable. Often they are the most advantageous methods of using soil fumigants.