INCREASING SOIL ACIDITY AS A MEANS OF 

 CONTROLLING BLACK ROOT-ROT OF TOBACCO 



By William L. Doran, Research Professor of Botany 



INTRODUCTION 



It has been established by several investigators (5, 24, 13)' and it is now 

 well known that a soil made less acid, as by the application of lime, becomes 

 favorable to increased infestation by Thielavia basicola (Berk, and Br.) Zopf 

 and to black root-rot of tobacco caused by this fungus. Soils with a pH 

 value of about 5.9 or higher, depending somewhat on soil temperature (l-l), 

 are therefore unsafe for tobacco, and so they remain until the hydrogen-ion 

 concentration of the soil is sufficiently increased or until the infestation of 

 the soil by the fungus, because of this or some other cause, subsides. 



Strains of tobacco resistant to black root-rot may, however, be grown in 

 such soil, and so far as resistant strains, now or later available, are .satis- 

 factory to growers and to manufacturers, this is the solution of the problem. 



It has sometimes been considered necessary, when such strains were lack- 

 ing, to use infested soil for some other crop than tobacco until the infestation 

 subsided; but, in the case of a soil otherwise good for tobacco, this is an un- 

 satisfactory solution, for, according to Johnson and Milton (20), it may re- 

 quire as many as ten years for Thielavia to die out in the soil even in the 

 absence of tobacco. 



Basic constituents are in the course of years removed from the soil by 

 leaching and in the crops, but the return of an over-limed soil to a sufficiently 

 acid condition to permit good growth of tobacco may be a \ery slow process, 

 as is illustrated subsequently in the present paper. 



The primary object of the work here described was to study means of 

 more quickly lowering the pH value of soil infested with Thielavm basicola 

 and thereby decreasing the loss caused by black root-rot of tobacco. It is 

 well known that the pH value of a soil may be lowered by the application of 

 sulfur or acids, and one of the principal objects was to determine to what 

 extent this method is effective and practical in protecting tobacco against 

 black root-rot. 



The possibility of using sulfur or an acid for this purpose has already 

 received some attention. In the experiments of Briggs (9), tlie application 

 of hydrochloric acid to a soil infested with Thielavia basicola resulted in a 

 marked reduction in the severity of black root-rot and a small increase in the 

 growth of tobacco. Chapman (12) found that ai)plications of sulfuric acid 

 or sulfur resulted in less black root-rot of tobacco and no interference with 

 growtli of the plants. Routt (27) found sulfuric acid beneficial to tobacco 

 (in some cases) when applied to soil infested with Thielavia. According to 

 .Vnderson and Swanback (7), about 500 pounds of sulfur per acre caused an 

 opthiuim increase in soil acidity, but they did not record the effect of this 

 treatment on black root-rot or on the yield or (juality of tobacco. 



^Reference is made by number to "Literature Cited," p. 138. 



