6 BULLETIN 765, U. S. DEPARTMENT OF AGRICULTURE. 
several years and which is not “tobacco sick,” the Burley tobacco 
will produce as great or a greater weight, plant for plant or acre for 
acre, than the Connecticut Havana. If we pull up and examine the 
roots of plants of both varieties grown on healthy ground, they are 
found to be large and white. On sick soil, however, a Burley plant will 
usually have only relatively few stubby bfack roots, as compared with 
the Havana tobacco, although the latter variety also may show some 
signs of the disease. The difference in yields between two varie- 
ties can be made even more striking, since we have varieties two 
and three times as resistant as the Connecticut Havana. No 
variety more susceptible than the ordinary White Burley strains 
grown in the Burley section has been found. Such a test can be made 
easily on any soil, and in connection with this study it has been re- 
peated a great many times, with the expected results in nearly every 
instance. Where the expectations were not realized there has been 
good reason for suspecting disturbing factors other than the Thie- 
lavia root-rot. 
The behavior of resistant and susceptible varieties on sick soils is 
regarded as positive proof of the extent of the injury attributed to 
the root-rot. It can be satisfactorily explained in no other way. The 
accumulation of data upon the subject over a period of five years in 
Wisconsin, Connecticut, Kentucky, and in Canada, together with 
field surveys in other States, has left no doubt of the widespread 
occurrence and economic importance of the disease. 
With the marked differences in resistance to the root-rot mani- 
fested by different varieties of tobacco clearly in mind, it will be 
readily seen that these differences satisfactorily account for the 
marked contrast in methods of handling tobacco lands in some of the 
northern cigar-tobacco districts and in the Burley district. The ex- 
treme susceptibility of the Burley variety permits of no other system 
than a short cropping period for tobacco and a long “rest ” for the 
land. In the northern cigar-tobacco districts tobacco culture has in- 
volved a struggle between old sick soils and the resistance of the 
varieties grown. The influence of this struggle upon agricultural 
practice has been threefold. First, the growers have applied enor- 
mous quantities of fertilizers, hoping thereby to remedy the worn-out 
condition of the soils, which has resulted in more or less wastage of 
fertility, since sick soils can not be benefited appreciably by ferti- 
lizers except under special conditions (by the use of resistant types, 
through prevailing high soil temperatures, etc.). Inthesecond place, 
resistant strains apparently have been unknowingly selected and de- 
veloped on account of their adaptability to these soils. In the third 
place, the disease in many instances has made such progress that 
growers have been compelled to change to new soils frequently, and 
