8o Journal of Agricultural Research voi. xvii, no. 2 



Theoretically we are concerned with the reasons for the injurious 

 action of nutrient salts on the growth of tobacco in infested soil, or 

 more directly, the increased severity of the disease in the presence of 

 increased supply of nutrient salts. A root system reduced by T. hasicola 

 evidently is not able to increase its functions in the presence of increased 

 fertility, in substitution for the lost roots; but it still seems as though 

 this should be possible, provided other factors do not interfere. 



There are at least three plausible explanations for the observed be- 

 havior: (i) the increased concentration of the soil solution may favor 

 fungus growth; (2) increased food suppl}^ may favor increased suscepti- 

 bility to disease ; (3) the reduced root system in the presence of increased 

 concentration of soil solution may not have been able to furnish a suffi- 

 cient supply of water to the plants. The first explanation seems most 

 plausible and yet seemingly can not wiiolly account for the results 

 obtained. The second, that of increased susceptibility, seems least 

 plausible, since no one has yet satisfactorily shown that actual suscepti- 

 bility to disease is increased by increased fertilization. There is some 

 reason to suppose that the third hypothesis is a factor. In the unin- 

 fested soil a good illustration of the injurious action of high concentration 

 of soil solution on plant growth is found. This is explained as an osmotic 

 relation, the entrance of water to the plant being reduced, owing to the 

 high concentration of the soil solution. It seems probable, therefore, 

 that in the presence of a greatly reduced root system this condition 

 would be exaggerated with a resultant reduced growth. The water- 

 relation theory is strengthened by the observations on the relative time 

 and extent of wilting of plants on days favoring high transpiration. 

 Wilting of tobacco plants during periods of high transpiration on infested 

 soils and rapid recovery is quite common and indicates a delicate water 

 relation existing between the plant and the soil. 



INFLUENCE OF COMPACTNESS OF SOIL 



Field observations have seemed to indicate that in many instances where 

 the soil in infested fields is hard or compact, owing to poor preparation 

 of the soil or to other causes, the damage from T. basicola is more marked 

 than in loose soils. In fact, many farmers have been found who have 

 attributed poor yields to compact soils alone, when, as a matter of fact, 

 rootrot was undoubtedly the primary cause. It is, however, very difficult 

 to say just how much injury is due directly to the hard compact or baked 

 condition of the soil and how much is due to its influence on the progress 

 of the rootrot when present. 



A simple experiment to determine this point was carried out. The 

 naturally infested soil from the field was carefully screened and mixed 

 in a relatively moist condition. A 6-inch clay pot was filled mth the soil 

 in as loose a condition as possible. This held 2,000 gm. Another pot 

 was then filled with the same soil, with as heavy tamping as possible. 



