6 CIRCULAR 692, U. S.-DEPARTMENT OF AGRICULTURE 
In considering the length of rotation, it was demonstrated in 1939 that a 
period of 3 vears between tobacco crops was more effective than a period of 2 
vears. Tobacco grown after 3 years of corn or soybeans had 31.5 and 17.5 percent 
wilted plants, respectively, whereas tobacco grown after a 2-year period (1 year 
of soybeans and 1 year of corn) had 72.0 percent wilted plants. On comparison 
between different cropping periods, however, with data obtained during separate 
seasons, the results were not consistent. For example, tobacco grown in 1937, 
1938, 1939, and 1940, following 1, 2, 3, and 4 years of corn, averaged 6.5, 2.0, 
FiGcureE 1.—Control of bacterial wilt by crop rotation on small plots: A, Tobacco 
grown after tobacco every year since 1935; B, tobacco grown after 3 years of 
corn; C, tobacco grown after 3 vears of soybeans. Photographed August 7, 
1939. 
