May J 5, 1919 Influence of Soil Environment on Rooirot of Tobacco 57 



fering with the general conclusions to be drawn from the experiment, 

 and the writers have again shown that the highest acidity practically 

 eliminated damage from rootrot, but that heavy infection still occurred 

 in fairly acid soil. 



It was believed that the abnormal behavior of the two soils in series 

 5 and 7 might be due to reduced infestation. All the crocks, therefore, 

 were resterilized and the two of each series reinfested with 100 cc. of a 

 fairly heavy suspension of endoconidia of T. hasicola in water. This 

 was thoroughly incorporated throughout the 10 kgm. of soil of each 

 crock. Maryland Broadleaf tobacco, a variety almost as susceptible as 

 White Burley, was then transplanted into them soon after inoculation. 

 The results of 65 days of growth are shown in the air-dry weight under 

 experiment III in Table III. A heavy infestation apparently reduced 

 the efficacy of the acid soils to nothing, at least in the presence of a 

 susceptible variety. The disease appeared, in fact, more virulent in 

 the most acid soil. 



The soils were now again replanted to Connecticut Havana, the semi- 

 resistant type. The actual amount of infestation was also probably 

 somewhat reduced, as many of the spores originally introduced must 

 have spent themselves, although it is probable that the fungus was liv- 

 ing in the soil as a saprophyte. Results similar to those obtained in 

 experiment I were now secured, indicating that partial recovery from 

 infestation had occurred in series i and 2. 



The question arises as to just what effect soil reaction has upon the 

 occurrence of the disease. High acidity may increase the resistance of 

 the host plant; or it may act deleteriously upon the germination of the 

 spores or the growth of the parasite. If we assume that acidity in- 

 creases the acidity of the cell sap and, hence, the resistance to disease, 

 as suggested by Comes {10) for cereal diseases, we have a working hy- 

 pothesis which is, however, difficult to establish definitely. It has been 

 shown that T. hasicola (as do most fungi) grows best on an acid medium 

 (about I per cent). Water extracts of the soils from the various series 

 were made which represented approximately the concentration of the 

 soil solutions. Germination tests of endoconidia in these extracts showed 

 better germination in the acid end than in the alkaline end. Other soil 

 extracts tubed with agar showed better growth of T. hasicola at the 

 acid than at the alkaline end. Although the acidity from the higher 

 acid series was such as would not permit the growth of bacteria, yet 

 tests of this nature probably fall short of resembling the actual acidity 

 in the soil. The results in experiment III show, of course, that spore 

 germination and fungus growth are not completely inhibited by an 

 acidity requiring 9 to 10 tons of lime per acre. The writer is inclined to 

 believe, however, that the beneficial action of soil acidity in reducing 

 infection by T. hasicola is due to a gradual depressing effect upon the 

 fungus. 



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