Soil Fumigation To Control Root Ills

J. G. Gaines, T. W. Graham.

The Coastal Plain from Virginia to Florida is a center of tobacco production. Its light, sandy soils and mild, humid climate are ideal for growing cigarette tobacco of high quality. But the soils and climate likewise are favorable for the development of soil-borne diseases, especially the ones caused by nematodes.

Many farmers do not have enough good tobacco soil to permit the long rotations required for control of root diseases. They need a safe and efficient soil treatment that will enable them to use shorter rotations.

Early experiments with chloropicrin, urea, and formaldehyde demonstrated that some diseases can be controlled by adding chemicals to the soil. But they were either too expensive and cumbersome to apply, or they adversely affected growth and quality of leaf. Cheaper and more practical chemicals later became available. They are not safe enough to be recommended without qualification, but some of them are being used increasingly on tobacco farms in the Southeast.

Gaseous chemicals, whose vapors penetrate to all parts of the soil, have been more satisfactory than those that require mechanical mixing. Such gas-forming chemicals are called soil fumigants. The present materials are liquids that volatilize when they are exposed to the atmosphere. One in common use is ethylene dibromide. Dowfume W-40 is one of several commercial mixtures of it. It is 4o percent by weight and 20 percent by volume of ethylene dibromide. Another one is dichloropropene-dichloropropane, which is supplied as D-D mixture.

To fumigate tobacco land, one of them is poured or injected into the soil under suitable conditions and with special equipment. The job must be done carefully, otherwise it will do no good. If it is done at the wrong time or if excessive amounts are used, the tobacco may be seriously damaged.

Nematode diseases are the principal ones controlled by soil fumigants. Treatment does not eradicate nematodes but reduces them sufficiently to make it possible to grow a crop without measurable loss from disease. The major nematode diseases are root knot and root rot.

Besides controlling nematode diseases, the fumigants have given some control of black shank, fusarium wilt, and stem rot. It seems that roots injured by nematodes are more readily invaded by other disease organisms. Consequently fumigation treatments most effective against nematodes may greatly reduce the amount of other soil-borne diseases.

Upper: Black shank attacks the roots of tobacco plants. In the bare patches plants wilted and died early. The fungus causing the disease survives for years in the contaminated soil. Lower: Tobacco plants affected by mosaic and wildfire. Wildfire caused spotting and dead areas on lower leaves; mosaic caused mottling of top leaves.

Extensive experiments with soil fumigants have been conducted at Florence, S. C., and Tifton, Ga. Careful data have been taken on the control of root diseases, yields, quality (as indicated by grade and price of the cured leaf), and rates of application.

Experiments in Georgia showed that fumigation with 40 percent ethylene dibromide reduced root knot from a disease index of 91 in untreated Soil to 29 in treated plots. (In the index numbers, too means all roots are severely diseased and o means all roots are healthy.) Under the same conditions D-D mixture reduced infestation to index 35. A great variation occurred because of soil condition and method and rate of application. Thus in treated plots the amount of disease varied from 0 to 70. The most effective control of root knot and root rot was obtained in Norfolk sandy loam soil,where tobacco had been grown for 22 consecutive seasons. At Florence, where root-knot infestations were less severe, similar results were obtained. Ethylene dibromide reduced root knot from disease index of 56 in untreated Soil to 20 in treated plots. Following D-D mixture it was 27. Thus, both in Georgia and South Carolina, the 40-percent ethylene dibromide mixture used was slightly the more effective. The difference was fairly consistent, but usually it was not enough to influence yields very much.

If there were no nematodes, fumigation did not increase yields. On the contrary, excess amounts of either fumigant have sometimes lowered yields because of injuries to roots. In diseased soil, however, marked increases in yield have been obtained consistently. In fact, fumigation gave just as effective nematode control and as large yield increases as were obtained with the very best crop-rotation practices. On nematode-infested Georgia land, the 6-year average yield from untreated soil was 920 pounds an acre. Ethylene dibromide fumigation increased the average to 1,590 pounds. D-D gave 1,468 pounds.

Increased yields in South Carolina were less spectacular but were significant, particularly following the more effective treatments. The average yield with untreated soil was 936 pounds an acre. That was increased to 1,255 pounds by ethylene dibromide and to 1,276 pounds by D-D. Increases in different experiments varied from 100 to 500 pounds an acre.

Wide fluctuations in results obtained from fumigation indicate that both soil and weather conditions, as well as rates and methods, have an important bearing on control and crop response. The land needs to be well pulverized and free of clods, roots, and litter that might clog the applicator. The soil should contain ample moisture but not be too wet to plow. Quiet, damp, or cloudy days are best for fumigation. A light shower immediately after applying the material is ideal, but not essential if the soil is in proper condition. It is important that the weather be mild and not windy and that the temperatures should remain above a minimum of 40 F. and below a maximum of 75 for the first 2 days after the application.

Row treatment involves applying a stream of the liquid directly under each tobacco row. Equipment may consist of one-row and two-row rigs for attaching to tractors.

For small tobacco fields, a simple one-horse rig attached to a single-foot plow stock has been cheaper and more practical. It consists of a small horizontal (preferably flat) tank, which rests between the plow handles or on the beam. An outlet in the bottom, provided with a 1/4-inch nipple and valve, permits the liquid to flow out at controlled rates. A 3/8-inch flexible or plastic delivery tube extends from the valve to the heel bolt or to the rear of the plow shank. The end of the tube should be a few inches higher than the plow point and almost against the rear of the shank. The shank is equipped with a small plow not over 2 1/2 inches wide. The tank is fastened to the beam or handles in such a way that the outlet is directly over the shank, so that the delivery tube will extend straight downward. This is necessary for best results with gravity flow rigs where there is no pressure to regulate the flow. A breather tube may be installed in the tank if desired, provided the filler cap can be fitted in an airtight position. This will keep the flow more constant if properly used. If no breather tube is used, a small opening will be necessary in the cap to permit air to replace the liquid which flows out. The flow from gravity tanks is not perfectly uniform, but the slight variation has caused no difficulty.