May IS. 1919 Influence of Soil Environment on Rootrot of Tobacco 47 



structure of the soil, including the relative amount, the state of the vege- 

 table matter, and the size of the mineral particles; (6) available chemical 

 fertility; (7) state of cultivation as regards compactness and aeration. 



To the soil physicist, chemist, and biologist it will appear that the 

 entire scope of soil science may be concerned in the production of disease 

 in the roots of plants, and such seems to be the case. On consulting the 

 branches of soil science it is at the same time both encouraging and dis- 

 couraging to find many of these factors influenced by a number of other 

 interrelated factors under normal conditions. For instance, soil tem- 

 perature as such can not be thought of without also considering the 

 influences of the air temperature, specific heat, moisture content, exposure, 

 and color of the soil upon such temperature. With soil moisture it 

 becomes essential to regard moisture-holding capacities, rainfall, drain- 

 age, cultivation, humidity, and temperature; or, in the case of soil fer- 

 tility, to consider along with the natural fertility, its cropping history, 

 applied fertility, and various other modifying factors. 



With these things in mind, however, it has become increasingly possible 

 to account for, if not to explain fully, seeming contradictions and lack of 

 accord with established principles of infection which have come under 

 the writer's attention during the past five years in the case of the rootrot 

 of tobacco. The occurrence and economic importance of the disease in 

 one State and not in another, on one farm and not on the neighboring 

 farm, or on the hilltop in one field and in the low spots of another, as 

 well as the total failure of a crop in a field one year followed by a com- 

 plete success the following year, or the change of crop prospects from 

 failure to 100 per cent yield within the period of two weeks, are all more 

 or less subject to scientific interpretation from this viewpoint. 



With respect to those factors, aside from environmental conditions 

 which may influence experimental results, it should be said that as far 

 as evidence from literature, or as far as the observation of the writer is 

 concerned, there is nothing to indicate that specialized races of T. basi- 

 cola occur, or that the fungus varies in any way in virulence owing to 

 differences in strain or age of cultures. It may be said with considerable 

 certainty, therefore, that we are dealing with a relatively constant organ- 

 ism as to pathogenicity. With respec+ to host differences it has been 

 shown (13, 14) that very decided differences in susceptibility in host 

 plants, and in varieties and strains of tobacco occur. By using pure 

 strains of seed experimental error from this source may be eliminated. 

 It should be remembered, however, as will be shown in the data here 

 presented, that because of these differences in susceptibility the critical 

 points in disease occurrence and severity may be shifted in one direction 

 or another to some extent, a fact which makes it important that the sus- 

 ceptibility of the variety used for experimental work be taken into 

 account in any interpretation of results. 



