14 



BULLETIN 1410, U. S. DEPARTMENT OF AGRICULTURE 



tahacum). Whether or not different species of tobacco will respond 

 differently has not been determined. 



INFLUENCE OF SOIL CONDITIONS 



The writers have naturally been led to make some observations 

 and to conduct certain experiments bearing on the relation of the 

 environment and other conditions in the soil to the occurrence and 

 severity of brown root rot. It has already been pointed out that 

 experimentally this disease is apparently very sensitive to desiccation 

 or aeration, acting jointly or separately. How far these factors in- 

 fluence the disease in the field has not been satisfactorily determined. 

 It is likely, however, that the prevalence of the disease may be ma- 

 terially influenced by the cultivation practices under certain weather 

 conditions, and this line of experimentation may prove profitable. 

 One is naturally led to consider the role played by this factor in ex- 

 plaining the striking influence of the rotation system on the disease. 



Fig. 12. — Influence of brown root-rot soil on the growth of varieties of tobacco, 

 showing eight varieties markedly different in resistance to Thielavia root rot. 

 These are all about equally affected in this diseased soil. Little, if any, varietal 

 difference in resistance to brown root rot appears to exist 



Though the writers have not conducted any experiments on the 

 relation of the physical condition of the soil to the disease, it seems 

 very evident that no close correlation exists. The disease frequently 

 occurs in spots in the field, but it does not seem to bear any con- 

 sistent relationship to topography. The condition occurs in coarse 

 sandy soils as well as in soils with a considerable percentage of silt 

 or clay. In the absence of a known causal agency, however, it is 

 not yet possible to say that the brown root rot found in the typical 

 tobacco soils of Kentucky and Wisconsin is identical with that 

 occurring in the Connecticut Valle} 7 . 



By means of screening and floating out the organic matter from 

 the soil, the writers were able to show that a large part of the inju- 

 rious agent of brown root rot exists in the organic matter; further 

 than this, no evidence was found as to its relation to the disease. It 

 may be said, however, that the disease appears to exist in soils rela- 

 tivelv very low in organic matter. 



