As all of the fumigants are somewhat toxic to plants, they must be applied far enough in advance of planting to allow them to have their effect and then diffuse out of the soil. This aeration time depends on the type and amount of fumigant used, the type of soil, and the temperature and moisture conditions. Fumigants disappear more quickly from warm soils than from cold soils and more quickly from dry or moist soils than from wet soils. Plowing or otherwise working the soil will hasten aeration, but should not be done until at least a week after application. It should be emphasized that the number of days between application of fumigant and planting as given in the preceding discussion are minimum times. It is often convenient to fumigate the soil several months in advance of planting. Soil can be fumigated in the fall for planting in the spring without serious loss of efficiency.

The effect of successful soil fumigation is the elimination of enough of the nematodes and other soil pests so that the crop is not seriously damaged. Since the damage from nematodes is usually the formation of galls or the partial destruction of the root system, there will be a marked increase in the number and extent of the roots, improved growth and vigor of the plant, and a tendency toward more uniform growth. If other conditions are favorable, yields will increase. Yield increases of several hundred percent are not uncommon when heavily infested soil is fumigated, but usually yield increases are from 20 percent to 50 percent. With root or tuber crops, the percentage of culls drops. Such beneficial effects are most conspicuous on the first crop following fumigation, but are often seen on subsequent crops.

Optimum applications of soil fumigants are not assumed to eradicate the nematodes but to provide the degree of control that will produce the most cash return in proportion to the cost of the fumigation. Usually it is better to use a moderate amount of fumigant for each crop than to attempt a higher degree of control with a larger amount in the hope that several crops can be raised before it is necessary to repeat the fumigation.

Ethylene dibromide and dichloropropene fumigants are the least expensive. The average cost of moderate applications is about 35 or 40 dollars an acre. The two fumigants are widely used on crops of moderate to high value when the increase of salable produce will be more than twice or three times the cost of the fumigation. The cost of applying 500 pounds of chloropicrin to the acre is 400 dollars. Liquid methyl bromide fumigants applied at the usual rates cost about 175 dollars an acre, and 98 percent methyl bromide fumigants cost about 80 cents per 100 square feet when 1 pound per 100 square feet is applied. The use of these fumigants is confined to crops of very high value, to greenhouses, seedbeds, and nurseries. Weed control by fumigation, since it eliminates expensive hand weeding, is often an important consideration.

The principal question in the practical use of soil fumigants is that of the possible profit to be obtained from their use whether the increase in salable crops as a result of the fumigation will pay for the cost of the fumigant, the expense of application, and a reasonable profit. The best guide is experience with similar crops and conditions. Lacking that, trial plots can be used to compare yields from fumigated or unfumigated areas and to compare different fumigants or different rates of application of one fumigant. Such trials are advised if plant parasitic nematodes are known to be present in significant numbers, if yields of crops have declined over a period of years, and if the growth of crops is not uniform or root systems are poor.

All plant parasitic nematodes are killed almost instantly when heated to about 140 F. Several methods of heating soil for nematode control have been devised. The most common is steam released from pipes buried in the soil or under a steam pan. Steam pans, usually of metal, are about 8 inches deep and 6 to 8 feet wide by 8 to 10 feet long. They are closed above and open below. The edges of the pan are buried 3 or 4 inches deep. Steam is released under the pan until the top 6 or 8 inches of the soil is heated to the required degree.

Other methods of applying heat are used with small lots of potting soil, which are heated by steam in a closed chamber, exposed to dry heat in shallow layers, drenched with hot water, or heated by electricity.

Many species of plant parasitic nematodes can be killed by drying. Small lots of soil can be air-dried by spreading out in thin layers. In favorable climates, the method can be applied on a large scale, the usual procedure being repeated plowing of the soil during the dry season of the year. In any climate it is good practice to remove the roots of a nematode-infected crop from the soil as soon after harvest as possible and to allow them to dry before plowing under.

In low, flat fields, flooding is sometimes used. We have little information on the effect of the method, but farmers who use it generally agree that flooding for several weeks is necessary.

Another possible method is bare fallow. Keeping the soil free of all vegetation deprives the nematodes of the opportunity to feed and reproduce. But because of the labor required for Weed control and possible deleterious effects on the soil, this is seldom Practical even for small plots.

One of the main sources of nematode infestation is planting stock, particularly plants used for transplanting, bulbs, tubers, corms, and roots. Some species of nematodes, such as the wheat nematode, Anguina tritici, and related forms, may be located in galled kernels mixed with seed, cysts of the genus Helerodera may be mixed with seed, or nematodes such as the rice nematode (Aphelenchoides oryzae) may be found between the seed and its enclosing glumes. Soil clinging to roots of transplants may be infested.

Other important sources of nematode infestation are soil brought into a field on vehicles or farm implements, or washed in by running water. Compost made from infected plants may also be infested. If a field is fumigated, special care should be taken to see that the seedbed is also fumigated. In fact, seedbed fumigation or sterilization by other means is excellent practice under any conditions.

It is best to discard infected planting stock, although it is sometimes possible and worth while to attempt to kill the nematodes in it and so save exceptionally valuable material. The hot-water treatment is extensively used for killing nematodes in narcissus and other bulbs, which, especially when dormant, can stand enough heat to kill the contained or adhering nematodes without serious harm to the plants themselves. Narcissus bulbs are presoaked in water with a wetting agent added, placed in water heated to 110 F. for 4 hours, and immediately dried or planted. Similar treatments have been worked out for other bulbs and planting stock..

Attempts to kill nematodes in planting stock by means of chemicals have been made, but always so far with serious injury to the plants. When the nematode is one of the ectoparasitic species that is, one that does not enter the plant it can be removed from transplants by simply washing off the adhering soil with cold water.

Only a few satisfactory and practical methods of controlling nematodes on growing plants are known. Such methods would find widespread use in orchards, in growing perennial shrubs, and even for annual crops. Where orchards or perennial ornamentals are to be planted, the only precautions that can be taken are to make sure that the soil is not infested before planting and that the transplants are free of nematodes.

The type of cover crop used in peach orchards can have a considerable effect on the degree of attack by root knot nematodes and consequently on the growth and yield of the trees. In experimental plots in Georgia, trees on plots where root knot resistant cover crops were planted produced about six times as many peaches in four seasons as trees on control plots where cover crops highly susceptible to root knot were planted. Where no cover crops at all were used and the plots were kept free of weeds, about five times as many peaches were produced as on the control plots.

Some species of the nematode Aphelenchoides, which parasitize the above-ground parts of such plants as strawberries and chrysanthemums, can be controlled by repeated spraying of the plants with parathion.

Undoubtedly the simplest method of preventing nematode damage is the use of plant varieties or rootstocks which are not susceptible to attack. Examples are the Shalil, Yunnan; and S-37 peach rootstocks, which are highly resistant to attack by some of the most common species of root knot nematodes in this country, though not to all of the root knot nematode species. Some advances have been made in the development of varieties of other crops resistant to root knot and other nematodes, but progress is necessarily slow and it will be many years before satisfactory nematode-resistant varieties of all crops will be available.

ALBERT L. TAYLOR joined the division Of Hematology investigations of the Bureau of Plant Industry, Soils, and Agricultural Engineering in 1935. He did experimental work on soil fumigation in Tifton, Ga., until 1946 when he joined the Shell Chemical Company to do research and development work on the soil fumigant D-D. He returned to the division of Hematology in 1949 and is now stationed in Beltsville, Md.