Black shank is spread by moving water, soil, and plants. The fungus is related to the water molds, and the disease spores can contaminate ponds or streams into which infested fields drain. In areas where black shank is present, it is unsafe to use stream or pond water in the plant bed or when setting out plants. Well water and city water offer less hazard.

The infection is readily carried in soil, and so it has often been spread by road workers, as indicated by infection coming in from the roadside. The disease can be carried from field to field in the soil on cultivators, tractors, and trucks. Plants also carry black shank. The best protection is for the grower without black shank to grow his own plants. Never should he get his plants from an area where black shank occurs. It is impossible to tell whether a bed is free of infection, because during the cool weather of the plant-bed season the disease is quite inactive.

For the grower who finds black shank in a field the first time, the best procedure is: Stop cultivating the field as soon as possible, because cultivation spreads the disease by moving the infection, injuring roots, and facilitating entry of the fungus. Keep out of the diseased field as much as possible to reduce spread to a minimum. Sow the affected field to grass or another non-cultivated crop as soon as possible, and do not bring it back into tobacco for 5 years or more. Locate future fields so as to avoid drainage from the diseased field.

The problem of varieties is acute in the flue-cured area wherever black shank occurs Virginia, North Carolina, and South Carolina. Epidemics of black shank are due largely to favorable weather and a build-up of infection. Variety may be an important factor. At the time of the original outbreak in Florida, it was noted that the disease had spread gradually until 1924, when the highly susceptible variety Round Tip was planted extensively for the first time. In 1924 the disease spread alarmingly. In North Carolina epidemic development has followed extensive planting of the 400 series of varieties-400, 402, Yellow Special, and Golden Harvest. Those varieties are tolerant to many diseases but are highly susceptible to black shank. Hicks and Gold Dollar are less susceptible. Growers who live in areas where black shank now occurs but do not have the disease on their farms might well plant one of the less susceptible of the common varieties.

Some of the resistant flue-cured varieties, developed in 20 years of effort, are Oxford I, Vesta 47, and Dixie Bright 101. Plant breeders have learned that flue-cured varieties highly resistant to black shank have been so low with respect to returns per acre that it has rarely been practical for the grower to plant them. The results with the moderately resistant varieties I listed have been better; Dixie Bright 101 has yielded on a level with such a successful susceptible variety as 402. Dixie Bright 101 is not equally well adapted to all areas, however.

In any event, growers of flue-cured tobacco must depend largely on varieties that are only moderately resistant to black shank; plant breeders may be able to develop varieties that probably will be better in yield and quality but not in resistance. There is no indication that it will be possible to produce flue-cured varieties resistant to black shank that can be planted continuously on the same land. A combination of resistance and rotation is the best answer at present to the problem for the grower of flue-cured tobacco; that probably will apply in other areas as resistant varieties become available there. The soil fumigants that are used against nematodes may have some value.

BACTERIAL (GRANVILLE) WILT in the field looks very much like blackshank. Plants wilt and die and the roots decay. In stalks of diseased plants that are sliced lengthwise 12 to 18 inches above the soil line, dark-brown, threadlike streaks show up in the woody tissue. They are a certain symptom of bacterial wilt.

The disease appeared first in North Carolina about 1900. Gradually it spread until thousands of farms were involved. Growers came to expect a loss of 20 to 25 percent of their crops each year.

Rotation became the standard recommendation for control, but failures were numerous.

The organism attacks many plants besides tobacco some common weeds, peanuts, potatoes, tomatoes, eggplant, and peppers.

An intensive search was made to find tobacco resistant to bacterial wilt. Finally it was discovered in a collection made in Colombia. It was crossed with the flue-cured type, and the first commercial wilt-resistant variety Oxford 26 was produced. It has since been used to produce Dixie Bright 27, 101, 102, and Golden Wilt.

The wilt-resistant varieties are not immune to the disease, although they stand up well under ordinary conditions. The parasite invades the roots and base of the stalk. However, if tobacco is grown continuously on the same land, infection may build up to the point that the tobacco is damaged. The remedy is to grow the resistant variety in a short rotation with other crops. Long rotations cannot be depended on to control the disease if the variety grown is wilt-susceptible.

BLACK ROOT ROT causes dark lesions that may be scattered over the roots. Above-ground symptoms are growth retardation plus wilting on bright days. Root rot is common in Tennessee, western North Carolina, and Virginia. It was the major disease of tobacco in 1910-20, when highly susceptible varieties especially of burley and Havana were grown. As long as new land was constantly cleared for growing tobacco, root rot did not accumulate, but as culture was discontinued On new land, the black root rot infection built up rapidly in the old fields. A contributing factor was the widespread use of lime, which encourages root rot. Liming tobacco land is advisable only if the land is very acid, below pH 5.0 to 5.5. After 1925 varieties resistant to root rot became available, among them 142, 21 1, 307, K1, and K2 (Havana) and Kentucky 16, 41A, and Burley 1 and 2 (burley).

ROOT KNOT used to be thought the only nematode root disease of tobacco. It is characterized by swollen roots, which have numerous galls that later decay. It is caused by a group of nematode species, all of which attack tobacco. One of the species also attacks peanuts; so, in places where peanuts suffer from root knot, peanuts are not a good crop to rotate with tobacco.

Rotation and fumigation are effective ways to control root knot. Nematode root rot (Pratylenchus), the second important root nematode disease to be identified, is especially common in South Carolina and other Coastal Plain areas. The nematodes burrow through the smaller roots and cause a red-brown decay. They freely attack crabgrass, corn, cotton, and other crops, and are not so effectively controlled by rotation.

Two more recently identified nematode parasites that attack tobacco are the Tylenchorynchus and Helicotylenchus. They do not enter the roots, but they thrust in their sucking tubes and feed on the roots. Both nematodes sharply reduce the growth of tobacco plants, but the only general symptom is failure of the plants to make normal growth.

WILDFIRE AND BLACKFIRE leaf spots (bacterial leaf spot) are quite common in plant beds. Wildfire is especially harmful. The lesions it makes on the leaves are yellow and usually have a small white area of dead tissue in the Center. The blackfire lesions in the field are large, often angular, and dark. In the field, wildfire lesions show much less of the yellow border, so conspicuous in the beds. The lesions generally are more rounded and lighter colored than the blackfire lesions. Aside from the difference in appearance, the two diseases may be considered as a single bacterial leaf spot disease.

The leaf spots have long been known under such names as red rust and black rust. Between 1917 and 1927, this leaf spot trouble was epidemic in every major tobacco area. Damage was heavy. In some areas notably central Tennessee and Pennsylvania wildfire remained severe until 1938. Beginning in 1947 and continuing each year since wildfire has been steadily building up throughout the burley area of Kentucky and Tennessee. The disease was widespread and destructive in 1952 and spread into western North Carolina and Virginia.

As to control in plant beds by chemicals: Applications of copper, as a drench or spray, will protect plants in the beds. The recommended mixture is bordeaux 3-4-50 (3 pounds of copper sulfate and 4 pounds of hydrated lime to 50 gallons of water) or 4-6-50. The first is applied as a drench with a sprinkling can at the rate of 25 gallons to 100 square yards of bed. The second, stronger bordeaux mixture is applied with a sprayer. Either way is effective if properly done. A commercial fixed copper (Copper A or Tennessee Tribasic) may be substituted for the bordeaux-1.5 to 2 pounds of a 50-percent material to 50 gallons of water.

Effective treatment depends on an early beginning. The initial application must be made as soon as plants are above ground. Then it is important that the framing boards or logs, the cotton (unless it is new), the soil surfaces, and, of course, the plants be wet thoroughly. A second treatment should follow in 7 to 10 days, and a third as much later.

The three-treatment program has been adequate in Tennessee, Kentucky, and Maryland. In Pennsylvania and Wisconsin it has been found necessary to continue weekly treatments until setting time. Keeping plants free from wildfire-blackfire in the beds helps field control and should be a regular practice in places where the diseases occur.

The breeding of varieties immune to wildfire and blackfire is well advanced.

MOSAIC causes leaf chlorosis and mottling. Symptoms are most pronounced in the young leaves. Mosaic is found wherever tobacco is grown. It has been rare in Georgia but prevalent in Maryland. It also is common in Kentucky, Tennessee, and Virginia.

Its virus can be spread from plant to plant merely by rubbing first diseased and then healthy leaves. The infection spreads throughout the plant. Cured leaves of diseased plants may carry infectious virus for 25 years. Cured tobacco is an important source of infection because growers often handle the leaf of the previous crop in the spring when stripping and grading it; at that time plants for the new crop are in the bed. Plants often become contaminated when workmen chew natural leaf, get juice on their hands, and then handle plants. It is important to avoid infecting plants in the beds or at transplanting time.

Adequate resistance to mosaic is now available. Mosaic-resistant types of all sorts are being developed. A few varieties have been introduced.

IN SUMMARY: Some progress has been made in efforts to control tobacco diseases, but the situation is far from stabilized. It seems certain that farmers in many areas are going to suffer heavy disease losses for years to come from wildfire leaf spot, nematodes, and black shank. New diseases have appeared in recent years; others may appear.

E. E. CLAYTON is a pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering.