2 CIRCULAR 692, U. S. DEPARTMENT. OF AGRICULTURE 
Control experiments, however, have inciuded studies of crop rota- 
tion, the method now in use, and these studies have provided informa- 
tion of immediate value to the tobacco producers who must continue 
to depend on crop rotation for wilt control during the next few years. 
The crop-rotation findings are thus emphasized in this circular, the 
contents of which may be further summarized as follows: 
Bacterial wilt was not eliminated from the soil by 1 to 4 years of 
bare fallow, but considerable variation in the degree of control was 
obtained by rotations with different immune crops. Thorough 
elimination of weeds susceptible to wilt did not increase the effective- 
ness of rotation. The results showed that control by crop rotation 
does not depend entirely on the wilt resistance of the plants grown. 
Soybeans, previously considered susceptible to wilt, can be used 
safely in rotation on wilt-infested land, a discovery particularly 
important when soybean culture is being rapidly expanded. Under 
exceptionally severe wilt conditions in 1939, rotations of corn, soy- 
beans, and redtop proved more effective than rotations of sweet- 
potato, crabgrass, or native weeds. 
In the 1939 season a rotation period of 3 years between tobacco 
crops was more effective than 1 to 2 years. Rotation periods of 1 or 
2 years, however, gave satisfactory control in seasons when the disease 
was less severe. 
The use of wilt-resistant tobacco strains in combination with crop 
rotation showed that increased wilt control is obtained by growing 
shghtly resistant strains on land rotated to corn for 1 year, but little 
increase in control was obtained where strains that were moderately 
or highly resistant were grown on rotated land. 
Soil treatment with urea at a rate supplying 420 pounds of nitrogen 
per acre gave effective wilt control, and tobacco grown on treated 
soil in rotation with 1 year of corn produced a normal crop of high- 
quality leaf. Treatment with chloropicrin also gave effective wilt 
control, but the cost is too high and the technique of application too 
inconvenient for large-scale use in the field. Soil treatment with a 
large number of complex organic chemicals and several modifications 
of cultural practice did not lessen wilt severity. 
Crop rotation, though heretofore only moderately effective, must 
continue to be used for the control of bacterial wilt. Improved 
rotation practices are now developed, but even so, this method will 
not be consistently effective. To obtain the best results, tobacco 
should be grown after at least 3 years of corn, soybeans, or redtop. 
GEOGRAPHIC DISTRIBU TION OF BACTERIAL Wie 
Bacterial, or Granville, wilt (Bacterium solanacearum E. F. Smith)? 
when first described by Stevens and Sackett in 1903 (7),? was found 
only in restricted areas of southern Granville County, N. C. At the 
present time the disease is uniformly distributed and highly destruc- 
tive in southern Granville County and in adjoining areas of Wake and 
Durham Counties. It has been reported also from every county in 
the State growing flue-cured tobacco, but the most rapid spread during 
recent years was in several areas of the Coastal Plain. Bacterial wilt 
has been reported also from Maryland, Virginia, South Carolina, 
2 Phytomonas solanacearum (E. F. Smith) Burgey et al. 
3 Italic numbers in parentheses refer to Literature Cited, p. 15. 
