May IS. r9i9 Influence of Soil Environment on Rootrot of Tobacco 83 



occurrence of disease, different results no doubt would have occurred had 

 Gilbert used a more susceptible variety. 



Practical advice on the use of infected seedlings will, then, vary with 

 the variety used. Injected seedlings should never be used if it is possible 

 to avoid it, especially on soils which are not infested, since this will only 

 hasten the time when all the soil will become so thoroughly infested as 

 to make a change to newer soils necessary. On the other hand, it fre- 

 quently is necessary to risk infected plants, as others may be unobtain- 

 able. In such instances it is much less likely that serious injury will 

 result if the infection is on a resistant variety. Again, it should be re- 

 membered that infected seedlings of even a susceptible variety trans- 

 planted into a heavily infested soil may produce a normal crop under 

 favorable conditions, such as a very warm season and a relatively high 

 soil temperature persisting for a long time. 



SUMMARY 



(i) The rootrot of tobacco, caused by Thielavia hasicola, is marked by 

 the stunting of plants in various degrees due to a reduced root system. 

 The extent of the damage is determined in a large measure by the environ- 

 mental conditions surrounding the roots of the host. 



(2) A study of these environmental conditions is essential to the proper 

 understanding of the occurrence and distribution of the disease in general 

 and local areas, and to good judgment in recommendation for control 

 measures. 



(3) There seems to be no variation in the pathogenicity of the rootrot 

 fungus secured from different sources. The amount of disease is deter- 

 mined entirely by the susceptibility of the host, the amount of infection, 

 and the soil environmental factors surrounding the roots of the host. 



(4) The factors especially studied were the amount of infestation in the 

 soil, the soil moisture, soil temperature, soil reaction, physical structure, 

 and fertility. An analysis of these f|actors separately as related to rootrot 

 frequently is very difficult, if not impossible. Under normal conditions 

 the end result in injur}^ by rootrot is the sum total of the favorable and 

 unfavorable action of these factors on the disease. Some of these factors 

 are much more important than others. 



(5) Other factors aside, the extent of infection and injury from tobacco 

 rootrot is directly proportional to the amount of infestation of the soil. 



(6) Rootrot is seemingly capable of developing in relatively dry soils. 

 Increasing the moisture content of the soil up to three-fourths of its 

 water-holding capacity does not materially increase rootrot. Saturated 

 soils are, however, considerably more favorable for the disease than 

 unsaturated ones. 



(7) The temperature of the soil is undoubtedly the most important 

 factor determining the extent of the rootrot of tobacco, other factors 



