Consistently high yields followed oats or other small grain plus weeds, harvested Spanish peanuts, and non-leguminous weed fallow. Highest dollar returns followed those crops. Optimum yields as well as good leaf quality also followed ragweed and horseweed. Still higher yields occurred after leguminous crops such as crotalaria, beggarweed, and runner peanuts, but the tobacco was of poorer quality and dollar returns usually were reduced. Any increase in yield at the expense of quality is always undesirable.

The 1927-46 average yield of tobacco after weeds and harvested peanuts at Tifton exceeded 1,450 pounds an acre. Similar high yields, as well as top quality, followed oats during a 15-year period. As long as they effectively controlled root knot, the crops remained in this favorable position, whether they were grown in a two-crop system or included in sequence with still other crops.

All yields were approximately 200 pounds an acre less at McCullers than at Tifton. But at both locations total production was approximately 500 pounds less an acre after continuous tobacco than after good rotation crops. Yields after root knot susceptible cow-peas averaged 100 pounds an acre more than continuous tobacco. Thus almost any rotation proved better than none.

While low yields were associated consistently with very severe root knot, highest production did not necessarily accompany the most effective rotation. Although control of root knot was commercially perfect after bare fallow, yields here were fully 200 pounds an acre less than in good crop rotations. These lower yields were associated with lack of organic matter and poor physical condition of the soil. In the absence of sufficient organic matter, cover crops of rye or oats have increased yields 100 or more pounds an acre without affecting leaf grades. On the other hand, lower yields comparable to those after bare fallow followed heavy sods of Bahia and Dallis grass where the sod had not completely decomposed.

IN SUMMARY, nematode root diseases are serious problems throughout the flue-cured area, Virginia to Florida. They increase in seriousness with the more southern latitudes, being most destructive in Florida and least in Virginia. The basic system of crop rotation is adjusted to control nematode diseases, to maintain the soil in proper condition for production of quality flue-cured tobacco, and to aid in the control of other diseases, such as black shank, Granville wilt, and fusarium wilt.

For nematode control the small grains, especially oats, are helpful. Native weeds are good. Harvested Spanish peanuts are effective in most areas, but not if peanut-infesting nematodes are abundant.

Over long periods, a mixed rotation is better than any set system followed year after year, and almost any rotation involving nonlegumes is superior to continuous tobacco. There is also the possibility that as new varieties of rotation crops are released, their resistance to nematode diseases may differ from the old, and their value in the rotation will vary accordingly.

Maintaining the soil in proper condition to produce good tobacco involves careful watch over organic nitrogen residues in the soil. Crotalaria and runner peanuts, excellent nematode-controlling crops, must be used sparingly and only on the very poor soil types. Harvested Spanish peanuts gave the best results of any legume, but when used excessively they sometimes caused depressed stalk and leaf size and lowered quality of the top leaves. From the viewpoint of quality, small grains and native weeds have been best. It is noted, however, that such a native legume as beggarweed may become troublesome in a weed rotation because of excessive residues of organic nitrogen.

Rotation serves a double duty in relation to the control of black shank, Granville wilt, and fusarium wilt. It reduces the build-up in the soil of the organisms causing these fungus and bacterial diseases, and it reduces the nematode population, thus preventing root-infesting nematodes from opening the way to invasion by parasitic fungi and bacteria.

This over-all protective effect from rotation is not realized if the varieties of tobacco grown are highly susceptible to the other diseases. If nematode diseases are the major problem, rotation alone is adequate, but in the presence of black shank and wilt, it is necessary that rotation be combined with the use of resistant varieties. Rotation alone gave uncertain control of black shank in the 1930's, when only black shank susceptible varieties were grown. Even 5- to 6-year rotations were not completely successful. By contrast, in 1951 tobacco following 1 or 2 years of a rotation crop withstood black shank almost perfectly when the moderately resistant varieties Oxford I and Dixie Bright 101 were planted. The same situation now exists with respect to Granville wilt and the resistant varieties now available. Short rotations plus wilt resistance are highly effective in controlling the disease.

The fusarium wilt situation appears to be about the same, with rotation and resistance supplementing each other. An added factor here is that sweetpotatoes are subject to the same wilt and are unsafe to include on land to be used later for tobacco.

Lastly, it is a good policy not to grow rotation crops closely related to tobacco, and those include tomatoes, pepper, eggplant, and Irish potatoes.

J. G. GAINES, a graduate of Clemson College and Rutgers University, has been a pathologist in the Department of Agriculture since 1929 to investigate root knot and other diseases of tobacco.

F. A. TODD, a graduate of the University of North Carolina, is a pathologist in the Department of Agriculture. He has studied the control of tobacco diseases in Southeastern States for a long time.

Root knot on tobacco.