J. G. Gaines, F. A. Todd.
Crop rotation is of unusual importance in the production of flue-cured tobacco.
Rotation crops, such as sweetpotatoes and tomatoes, encourage the early establishment of destructive soil-borne diseases. The use of small grains often prevents disease losses. Other plants, such as crotalaria, may be effective in preventing disease, but have undesirable crop effects that result in poor quality of leaf.
Cultivated plants are not alone in exerting those influences. Weeds growing between seasons may be beneficial or harmful. Nor do the responses to crops and diseases remain constant. They have less influence in some seasons and more in others. So, too, with some soil-borne diseases, which may remain localized and be more destructive in some seasons and areas than in others.
Consequently, we have to consider a complex of crops, weeds, diseases, season, and location when we evaluate rotations in tobacco culture.
It used to be the custom in the Southeast to locate tobacco fields in areas newly cleared from pine forests and to grow tobacco continuously there until the soil failed to produce good crops. Then more new ground was cleared, and the process was repeated as long as any virgin land remained. Finally, growers were forced to use rotations, primarily because of root knot.
The fact that many growers in Georgia have reduced the amount of injury from root knot enough for successful tobacco culture for 30 years points to the value of crop rotation as a practical disease-control measure.
GRANVILLE WILT, or bacterial wilt, in North Carolina has been reduced by growing corn for 2 to 4 years. Redtop (Agrostis alba), hairy crabgrass (Digitaria sanguinalis), lespedeza, soybeans, and crotalaria also could be included in the rotation. Horseweed (Erigeron canadensis), ragweed (Ambrosia artemisiifolia), jimsonweed (Datum stramonium), and horsenettle (Solanum carolinense) were susceptible to infection, but the occasional growth of those weeds in a corn rotation did not nullify the beneficial effects of corn. Four years of bare fallow failed to eliminate wilt. Crop rotation was not uniformly successful when the soils were heavily infested. Limited short rotations and the use of a tobacco variety slightly resistant to wilt have proved adequate. The growing of tomato, pepper, peanuts, and potatoes and the continuous culture of tobacco encouraged wilt.
Fusarium wilt has been controlled in Georgia by 3-year crop rotations plus the use of the slightly resistant standard tobacco varieties. Rotations that failed to control root knot also failed to prevent wilt. Two successive years of oats plus weeds effectively controlled this root knot-wilt complex. Sweetpotatoes encouraged the establishment of fusarium wilt in tobacco soils. The same fungus disease attacks both crops. Wilt was limited in small areas before 1950. The rate of spread and establishment may be governed to a large extent by whether or not sweetpotatoes are grown in tobacco soils and by the degree of susceptibility of new tobacco varieties. There is also the possibility that more virulent strains of the causal fungus may appear in some areas.
Stem rot is influenced more by season than by rotation. Over a 20-year period, in a 3-year rotation test,however, less stem rot followed cotton and tobacco than cowpeas, corn, weeds, and peanuts. The greatest amount of disease, averaging 4 percent, followed velvetbeans. As much stem rot followed bare fallow rotations as corn and weeds. More stem rot occurred after leguminous cover crops if the residues remained than if the tops were removed. Increased stem rot was associated with severe root knot in those rotations.
Black shank also responds to crop rotation, but rotations alone are not always successful. Longer intervals are required between tobacco crops. Two years of clover, lespedeza, small grain, or grass crop followed by 2 years of corn (or cotton or harvested Spanish peanuts) have been reasonably successful. Limited rotations plus a moderately resistant tobacco variety have been adequate. Root knot sometimes influences the amount of black shank damage. At Attapulgus, Ga., the resistant Rg cigar-wrapper variety develops severe black shank if root knot is present. This variety remained highly resistant in the absence of root knot. Highly susceptible varieties of tobacco, however, succumbed to black shank in nematode-free soil. Thus control of root knot alone is not enough to control black shank, unless it is combined with a variety that is moderately resistant to black shank. Growth of tobacco, pepper, tomato, and related plants encourages the development of black shank, even if resistant varieties of tobacco are used.
Nematode root rot is a little-known disease complex that also has been associated with crop rotation. Three-year rotations with small grain plus weed fallow or bare fallow were helpful in South Carolina. Corn, cotton, and crabgrass, as well as continuous tobacco culture, permitted the disease to become destructive. The oats-plus-weeds rotation helped materially in controlling the nematode root rot-root knot combination.
No accepted variety of tobacco has pronounced resistance to root knot,and since the control of root knot is an important tool by which it is possible to keep other soil-borne diseases in check, rotation becomes more urgent than ever, especially in districts where black shank and fusarium wilt develop along with nematode diseases. Root knot nematodes occur over the entire flue-cured region from Virginia to Florida. Thousands of tobacco growers, particularly in the Carolinas, are faced with the grave problem of attempting to produce profitable crops on lands infested with two or more diseases. Good varieties of tobacco, which have adequate multiple resistance to these infestation complexes, are not available; existing varieties have to be supplemented by rotation. On the other hand, limited rotations are inadequate without the use of moderately resistant tobacco varieties.
To complicate matters, there are several recognized species of root knot and root rot nematodes. Some of them vary in ability to infest different crops. Closer attention than ever must be paid to them by grower and research worker alike. If previously effective crops fail to protect tobacco, one has to know that quickly so that other crops can be substituted in time. Also nematodes in time may become adapted to crops that are considered resistant.
Because root knot is more widespread than other recognized nematode diseases, extensive experiments designed for nematode control have been made at McCullers, N. C., and Tifton, Ga. A number of field crops were grown in continuous rotations with tobacco between 1925 and 1951 at Tifton and between 1937 and 1951 at McCullers. Experiments of shorter duration were conducted at both locations and at Florence, S. C. Annual records were kept of root knot in tobacco at the close of harvest. Yields, leaf grades, and occurrence of other important diseases were recorded. In general, wherever peanuts, soybeans, cowpeas, velvetbeans, and Similar cultivated legumes were grown,the tops were removed in an effort to avoid the danger of adding objectionable amounts of nitrogen to the soil. Uniform fertilizer and cultural practices accepted in the respective States were followed throughout.
Leaf quality and yield usually are not affected by the equivalent of less than 50 percent severe root knot at the close of harvest. Unlike black shank, fusarium wilt, stem rot, and Granville wilt, which may cause death of the plant from a single infection, severe nematode infestation after mid-season may not cause measurable reduction in leaf grade or yield. Sometimes a root disease of 80 percent severity causes no measurable crop loss. If a well-developed root system is attacked by root knot nematodes and the roots do not break down readily, little damage may be done. Only when the weakened roots break down from secondary decay, caused by common soil organisms, do serious losses generally occur. If seedling roots are infested, marked stunting of plants may be evident from the outset. The earlier the attack the likelier is the secondary breakdown before plant maturity is reached. Usually the breakdown is slow acting, but when weakened roots are attacked by black shank and fusarium wilt, early death is more certain.
Root knot and other root nematode diseases commonly cause reductions in yield of 200 to 400 pounds an acre. Occasionally maximum losses resulting from root knot exceed 1,000 pounds an acre. Leaf grades, as well as yields, are materially reduced when a big part of the root system becomes affected by secondary decay.
Leaves harvested from these wilted and stunted plants are immature and may cure with a green or dull cast. If they are left on the stalk until ripe in appearance, they will be trashy, dark, lifeless, and of very low value when cured. Such immature leaves from severely wilted plants may contain excessive nicotine and too little sugar.
Crop rotation controls nematodes by partial starvation through growth of resistant crops. Root knot nematodes require susceptible growing roots in which to develop and reproduce. The resistant crop plants are readily invaded by the nematode parasites, but the nematodes fail to mature and reproduce in them.
Two-year rotations, with tobacco grown alternately with a single resistant crop, are adequate in North Carolina and Virginia. Farther south, where the seasons are longer and conditions more favorable for year-round nematode and plant development, 3-year rotations are needed to give the same reduction. If an intermediate or susceptible crop is included in the rotation, longer intervals than 2 years are required between tobacco crops. Two years of bare fallow, used as an experimental check to compare with crops, were adequate to insure against root knot damage, but at least 3 or 4 years were needed to eliminate all infestations.
Crop rotations in which a resistant crop is grown may reduce root knot but will never eliminate it.
ROTATION CROPS are rated according to their ability to prevent root knot in tobacco that follows them. Crops most effective in reducing subsequent nematode occurrence are regarded as most resistant.
Peanuts and oats, or other small grains, were the most effective field crops tested. Weeds were permitted to grow in the summer and fall after the small grain. Spanish (bunch) and runner peanuts were of equal value against disease. Over a 20-year period of systematic 3-year rotations in Georgia, harvested Spanish peanuts and oats-weeds permitted infestations to reach or exceed the danger level, 70-percent infestation, only once. The average for peanuts was 30 percent and for weeds 56. While those differences were highly significant in favor of peanuts, there were no differences in yields.
The bare fallow rotation (2 years of fallow and 1 year of tobacco) showed only 23 percent of slight to moderate root infestation, the lowest average of any rotation.