Of intermediate value in Georgia (but effective in 2-year rotations in North Carolina) were cotton, velvet-beans, and weed fallow where crabgrass grew among the weeds. In one 20-year test, excess root knot occurred in tobacco six times after two successive years of weeds, seven times after velvet-beans, and eight times after cotton. Other crops of only intermediate effectiveness at Tifton were Korean lespedeza (Lespedeza stipulacea), common lespedeza (Lespedeza striata), root knot resistant soybean, pearlmillet (Pennisetum glaucum), and grain sorghum (Sorghum vulgare).

Corn was still less effective against root knot. It was followed by severe infestations in 16 tobacco crops out of 20. The test was on Tifton sandy loam soil. On Norfolk lowland sandy loam, where root knot was less destructive, corn rotations were more reliable. At no time did a maximum of 100 percent severe disease develop after corn, velvetbeans, and weeds. cowpeas resistant to root knot were not much better than corn; the succeeding tobacco showed severe root knot 12 out of 20 years. Sudangrass (Sorghum sudanense) behaved about the same as corn.

Least effective of the rotation crops were cowpeas that were susceptible to root knot and sweetpotatoes. The only difference between the two varieties of cowpeas was that the more susceptible variety permitted maximum disease to develop within 3 years instead of 6. After that, the same amount of root knot appeared after both varieties. Severe disease was the rule after perennial lespedeza (Les-pedeza cuneata), carpetgrass (Axonopus compressus), Bermuda-grass (Cynodon dactylon), chufa flatsedge (Cyperus esculentus), susceptible soybeans, tomatoes, pepper, squash, cucumber, okra, snap beans, and similar truck crops.

At McCullers, corn was slightly less effective than weeds, oats-weeds, peanuts, and cotton, but all were adequate to insure against severe losses throughout the 15-year test period. Redtop also was of value there. Thus it was easier to control root knot by rotation in North Carolina than in Georgia.

Small grains are the only reliable winter cover crops in tobacco rotations. Oats or rye, turned under in time for planting tobacco, have given slight but consistent reduction in nematode disease not enough, however, to prevent excessive damage if tobacco was grown every year in the same field. The practice is of slight value only as a supplement to a good rotation. Winter legumes, such as vetch, Austrian winter peas, and lupine, are highly susceptible to root knot and tend to increase damage from disease.

IN ORDER TO DETERMINE weed influences on root knot, a number of common weeds were cultivated in 3-year rotations with tobacco. It appeared that related species of weeds brought about different root knot responses. For example, sickle senna, or coffeeweed (Cassia tora), reduced root knot to a minimum, while another species, coffee senna (Cassia occidentalis), permitted severe disease in succeeding tobacco. The common Canada fleabane, or horseweed (Erigeron canadensis), was more effective than a similar fleabane (E. pusillus).

The weeds most effective in preventing root knot were no better than peanuts and crotalaria when used in 3-year rotations. Among the most resistant were coffeeweed, horseweed, ragweed, beggarweed (Desmodium tortuosum), and goldenrod (Solidago microcephala). Mexican clover (Richardia scabra) permitted slightly more root knot than peanuts or crotalaria but was more effective than dogfennel (Eupatorium capillifolium). Weeds that had intermediate value but permitted occasional excess disease were crabgrass, camphor plant (Heterotheca subaxillaris), bitterweed (Helenium tenuifolium), and fleabane (Erigeron pusillus). Of still less rotation value were dogfennel and bull paspalum (Paspalum boscianum). Weeds that permitted severe disease consistently in a 7-year experiment were spiny amaranth (Amaranthus spinosus), slim amaranth (A. hybridus), cocklebur (Xanthium chinense), lambsquarters (Chenopodium album), drug wormseed (C. ambrosiodes var. anthelminticum), and coffee senna (Cassia occidentalis).

The best natural weed fallow in North Carolina consisted of ragweed and horseweed, but crabgrass frequently predominated between annual row crops. Cultivation after a row crop or after small grain encouraged crabgrass. In Georgia, Mexican clover, crabgrass, bull paspalum, and camphor plant predominated after oats the first year, with occasional beggar-weed, horseweed, and ragweed. The second year of weeds showed less grass and Mexican clover, with more horse-weed, camphor plant, beggarweed, dogfennel, and goldenweed, Aplopappus divaricatus. The clean-cultivated cotton and peanuts discouraged all weed growth. Corn encouraged beggarweed, crotalaria, cocklebur, coffeeweed, crabgrass, and bull paspalum. Normally root knot susceptible weeds soon disappeared in a long weed rotation, but in short ones that did not occur. If soils are very fertile, a long period may be necessary to eliminate the susceptible weeds. Root knot has been observed in old abandoned barnyards 15 years later on lambsquarters and Jerusalem-oak (Chenopodium botrys). Such places, however, are not suitable for flue-cured tobacco.

CONCLUSIONS regarding rotation effects cannot be drawn from short-time experiments those continuing only 4 or 5 years.

At McCullers, root knot remained severe during the first rotation cycle. Then there was a steady decline in disease until a minimum occurred in 1946, 10 years later. During the following 5 years, there was a slight but unimportant increase. Those trends occurred in all 2-year rotations with oats, weeds, peanuts, cotton, and corn, but root knot remained severe throughout the 1937-51 period in continuous tobacco plots.

Striking differences occurred at Tifton between some rotation crops during the 1925-51 period. All the tested crops in 3-year rotations there permitted an increase in root knot up to or beyond the danger level. In contrast, bare fallow consistently prevented an increase in disease, and at no time was there any indication that this starvation method would fail.

The time required for nematode populations to build up to a crop-destructive level varied with the different rotations. In these 3-year rotations, maximum root knot occurred after 2 or 3 years of sweetpotatoes, susceptible cowpeas, or tobacco; after 4 years of corn-corn-tobacco; 6 years of nematode-resistant cowpeas; and 9 years of velvetbeans or native weeds; but not until 27 years after peanuts-peanuts-tobacco.

The results with the cotton rotation were quite unexpected. Like sweet-potatoes, cotton permitted a rapid increase in disease the first 2 years. Maximum root knot occurred the third and fourth years, after which there was a steady decline in nematode activity until this rotation reached the bare fallow level of minimum root knot activity 14 years later. An average of 100 percent of the tobacco plants after cotton were severely affected by root knot in 1930-31. This was in contrast to only 2 percent during the 1942-46 period.

The long-time rotation experiments have shown many successive periods of ascending and descending disease developments, plus some trends that extend over long periods. At both McCullers and Tifton there was a low disease activity in 1946, followed by a higher peak of activity in 1951. Many factors exist in the crop-soil complex. Some of them we do not understand fully. Rotation effects need to be evaluated on a basis of averages and not on the basis of results of 3 or 4 years. Any rotation is considered dangerous, however, if it permits several successive years of severe root knot.

A good rotation crop may lose its value temporarily if grown too often. In the 1925-51 peanuts-peanuts-tobacco rotation, root knot damage was negligible for the first 15 years, but remained excessive after 1946. In additional rotations, begun in 1944 where peanuts had not been grown before, infestations were reduced by peanuts between 1946 and 1950 but were increased to the destructive level in 1951. When one rotation crop permitted excess disease damage, the substitution of another one reduced disease.

EXTENDED ROTATION experiments have shown that certain crops, notably oats and peanuts, have outstanding value and wherever possible the rotation should be varied. Behavior of the tobacco will indicate when the rotation crop should be changed. A number of crops, which are not highly effective in nematode control when used alone, can be used in combinations to good advantage.

For example, in the effective peanuts-oats-tobacco rotation for controlling root knot, corn can be substituted in place of peanuts. The substitution would make a better rotation in an area where Granville wilt is present, because peanuts are susceptible to it and corn is not. The crop sequence that places the most resistant crop immediately in advance of tobacco generally is to be preferred. Thus cotton-peanuts-tobacco is more effective than peanuts-cotton-tobacco.

Where it is practical to utilize a number of crops on the farm, it is desirable to change their sequence in succeeding 3-year cycles. For example, corn-oats-tobacco might be followed by cotton-peanuts-tobacco, and that in turn by peanuts-oats-tobacco. Millet, corn, and grain sorghum may be substituted occasionally with safety. Another plan might be to grow corn-oats-tobacco in succeeding cycles until that rotation begins to show excess root knot, and then change to another three-crop system. It is important to remember that any systematic two-crop or three-crop system, continued for long periods without change, may become less effective. For example, after 15 years in one location, peanuts-oats-tobacco rotations permitted severe root knot development. The expert grower will watch his tobacco crop closely each year and adjust his rotation to check disease build-up before it becomes destructive. He will also consider rotation in its relation to soil fertility.

WHEN TOBACCO FIELDS are abandoned and permitted to revert to weeds and finally to brooms-edge and pines, the soil again becomes suitable for flue-cured tobacco. No better plants are known to condition the soil for tobacco than the original native vegetation. Early growers felt that cropping practices that kept the soil close to its virgin state were best for producing good leaf quality. That was sound reasoning. Of the crops compared in rotation tests, the best leaf quality followed small grain, cotton, corn, and some non-leguminous weeds. Quality was almost as good after harvested Spanish peanuts.

While crotalaria is an excellent rotation crop for nematode control, its Use in a tobacco rotation is limited to Poor and sandy or low and wet lands that have insufficient organic matter. With soils of average fertility, one crop of crotalaria may cause poor quality in the following tobacco crop.

In a time when emphasis is placed on increasing soil fertility to insure bountiful yields of food and fiber, it may seem a paradox that the best flue-cured tobacco can be grown only on relatively infertile soils. One important reason is that flue-cured leaf of the desired composition, texture, and aroma can come only from relatively healthy plants that receive limited amounts of nitrogen. To promote best yields, sufficient readily available nitrogen is essential during the period of most active growth. After that the supply must diminish.