Using Chemicals To Combat Root Diseases

Jesse R. Christie.

Many of the myriad of organisms that inhabit the soil subsist on living plants. They may injure the roots and other underground parts of the plants and interfere seriously with growth. The damage they cause is likely to become more serious the longer the land is in cultivation, especially if the same crops are grown repeatedly in the same place.

Their control is a problem that we have not yet solved entirely. Suitable crop rotations and the elimination of weeds that may serve as the hosts for the disease organisms, the planting of resistant varieties, and other cultural practices help reduce losses of crops, but often they are not enough.

Control by the use of chemicals has met with considerable success, especially with vegetables. Volatile liquids that function as soil fumigants have come into use extensively since 1945. Thousands of acres are fumigated annually, and the acreage is increasing. Apparently growers have found the procedure profitable. Soil fumigation is costly, but the increased yield afterwards may have a cash value very much more than the investment. Sometimes fumigation makes the difference between a profitable crop and none at all.

To the bacteria, fungi, nematodes, and insects, to which most of the noxious organisms belong, it will be practical for us to add weed seed. Soil fumigation has been most successful in controlling nematodes; some of the fumigants kill other pests; some Other methods are as effective as fumigation and less expensive. When one has to fumigate for nematodes, however, the control at the same time of insects and other pests is important.

Soil fumigants are sold under various trade names, but only four distinctly different kinds are in general use. They are composed of (or have as their active ingredient) methyl bromide (bromomethane), Chloropicrin (trichloronitromethane), dichloropropene (1,3-dichloropropene), and ethylene dibromide, which is 1,2-dibromoethane.

For seedbeds, soil fumigation or any other chemical treatment should be effective against a variety of organisms.

Damping-off is notoriously serious in seedbeds, and failure to control it or reduce losses from it is a serious defect of a treatment.

A soil treatment that will kill weed seed and thereby eliminate the cost of weeding may effect savings that more than pay the cost of even an expensive chemical.

Root knot is an ever-present problem in the South. Its control in seedbeds is especially important. Other nematode problems may be equally serious in some regions for example, control of the stubby root nematode (species of Trichodorus) in the seedbeds Of certain celery-growing regions in Florida. Of the fumigants in general use, methyl bromide and Chloropicrin come nearest to fulfilling those requirements.

Methyl bromide, a highly volatile compound, has a boiling point of about 40 F. In order to inject it into the soil, in the manner that most fumigants are applied, it must be mixed with a diluent having a higher boiling point. Such mixtures have beer, in use for some years, but with them it is hard to keep the gas in the soil long enough to obtain the desired results.

A newer method makes it possible to utilize, to a much greater degree,the killing power of the chemical. Undiluted methyl bromide is evaporated in shallow trays placed on the surface of the soil under a gas-tight cover. The cover is held up a few inches by supports, so there is a shallow air space between it and the surface of the soil. The edges of the cover are buried. The rate for most purposes is 1 pound to 100 square feet, but for controlling the more resistant fungi, rates up to 4 pounds to 100 square feet are recommended. The cover should remain in place for 48 hours. Usually seed can be planted 2 to 3 days after the cover is removed. Penetration of the gas into the soil is surprisingly good. Under favorable conditions, the sterilizing effect extends to a depth of 12 inches or more. Most or all of the insects are killed. Miscellaneous soil nematodes are nearly eradicated. Control of root knot is usually good, although not always complete. All living plants are killed, including the most tenacious of the grasses and all but a few of the more resistant weed seed. On the basis of some tests, the control of fungi seems satisfactory. The main disadvantage of the method is cost of the cover and equipment needed to treat large seedbeds and the amount of time and labor involved in moving equipment from place to place.

Chloropicrin has been on the market longer than any other soil fumigant in general use. It is expensive and disagreeable to handle, but many vegetable growers prefer it for fumigating seedbeds. Properly applied, it will kill insects, nematodes, most weed seed, and all except a few of the more resistant fungi. Chloropicrin is injected into the soil in the same manner as most other fumigants. Injection points or continuous streams if power equipment is used should be spaced to inches apart. The recommended rate of application is 2 cubic centimeters to 3 cubic centimeters (about one-half teaspoonful) at each point.

A water seal must be applied immediately. For best results, the area should be covered with burlap sacks, canvas, newspaper, or something like them, which, in turn, should be sprinkled with water. The cover, when it is used, may be removed after 4 or 5 days. The soil is allowed to aerate. Seed must not be planted until every trace of the gas has disappeared, which usually takes 8 to 12 days; in wet, cool weather it may take longer.

Chloropicrin forms a gas that is extremely toxic to plants, in both the soil and the air. Under certain weather conditions, a blanket of gas may collect over a fumigated bed near the ground, then drift slowly over a nearby area, and cause severe injury to the plants growing there, especially at night when foliage is wet with dew. That does not always happen, but it is a risk that should be remembered when one fumigates seedbeds near growing crops.

Chlorobromopropene, commonly called CBP, is a promising new chemical for the treatment of seedbeds. When emulsified with water and applied as a drench it has given good control of nematodes, fungi, and weeds in tests conducted on the sandy soils of central Florida. Equipment that has a small gasoline-driven pump, which mixes the chemical with water and delivers the mixture through a plastic hose to a sprinkling nozzle, makes application quite easy.

Mixtures containing dichloropropene or those containing ethylene dibromide are also used for fumigating seedbeds. They give good control of nematodes and soil insects but neither has much effect on weed seed or fungi.

The practice in some regions is to prepare beds that are 6 inches or more higher than the walkways between. When methyl bromide, chloropicrin, or CBP are used, the beds are first prepared and then treated. The walkways are not treated. When ethylene dibromide or dichloropropene are used, a common practice is to fumigate the entire area, and prepare the beds afterward. The rates usually recommended are higher than those for ordinary field applications: 35 to 45 gallons an acre of a dichloropropene fumigant or 30 to 40 gallons of 40 percent ethylene dibromide.

Ethylene dibromide and mixtures containing dichloropropene are generally used for fumigating fields. Both are excellent nematocides and good insecticides, but at ordinary rates neither has much value as a fungicide or herbicide. Ethylene dibromide is more effective against wireworms. Either fumigant substantially reduces the population of the mole crickets in Florida by killing the insects or else driving them to the surface, where they are eaten by crows and blackbirds.

All dichloropropene fumigants have about the same strength the same percentage of active ingredients. Ethylene dibromide fumigants are mixtures of ethylene dibromide and a diluent, usually naphtha. The diluent adds nothing to the efficacy. It is used merely to increase volume and thereby facilitate accurate and uniform application. The different trade-marked fumigants of this kind differ in the amount of ethylene dibromide they contain. The two most commonly used for field-scale applications contain either 41 percent or 83 percent of ethylene dibromide by weight. The 83 percent fumigants usually are diluted by the dealer or by the farmer before applying. If proper equipment is available, they may be applied without diluting.

The entire area of a field may be fumigated. The procedure is called solid, or broadcast, application. Or the fumigant may be applied only in the rows or hills where the plants are to grow. For solid application, standard practice is to space injections 12 inches apart. The recommended dosage for a dichloropropene fumigant is 20 to 25 gallons an acre. For a 41-percent ethylene dibromide fumigant it is 15 to 20 gallons an acre. For row fumigation a single stream is applied along each row. The usual rate for either of the materials is about 2 cubic centimeters (about one-half teaspoonful) per linear foot of row or 1 gallon to 1,900 feet of row. If rows are 3 feet apart, an acre will require about 8 gallons. Row fumigation has given satisfactory control of root knot on tomatoes and some other crops.

A few tomato growers have adopted the practice of applying two streams, 12 inches apart, along each planting row, but no one seems to have demonstrated that applying two streams is very much more effective than applying about the same amount of fumigant in one stream. Good control of root knot on watermelons has been obtained by making a single injection in each hill.

Row or hill fumigation will prove effective, no doubt, for controlling many but not all kinds of nematodes. In experiments conducted in Florida for controlling the stubby root nematode on sweet corn, row fumigation failed completely. Those nematodes move into the fumigated area from the surrounding unfumigated soil so quickly that the plants are given only very brief protection.

Soil fumigation has been in use long enough and has been tested extensively enough to demonstrate that it is an effective and useful method, but requirements for success are somewhat exacting and the factors influencing efficacy are numerous and not fully understood. Failures may occur for many reasons, among them faulty diagnosis (attempting to correct troubles not caused by organisms that fumigation will control), faulty application, and insufficient aeration of the soil between application and planting.