40 



The treatment of the soil by hot water, which raised the tempera- 

 ture of the surface from 176° F. to 186° F. to a depth of 4 in., 

 reduced the amount of Drop 76% and completely killed the 

 Rhizoctonia. 



Treating the surface with a steam rake raised the temperature of 

 the soil to 168° F. to a depth of 4 or 5 in. This treatment suc- 

 ceeded in reducing the Drop only 5% ; the Rhizoctonia being 

 reduced 57 %. 



The amount of heat necessary to kill the Drop is about 160" F.; 

 that for the Rhizoctonia appears to be somewhat less. 



This method of treatment possesses no value for such diseases as 

 the Botrytis, Mildews, Bacterial-Rot, etc., which can be controlled 

 by proper management of the crop. 



Other than the methods of heating the only alternative is chang- 

 ing the soil. 



The application of such substances as lime, sulphur, and charcoal 

 to the surface of the soil exerts no repressive influence on the Drop 

 or other lettuce diseases. 



Coatings of sawdust, coal ashes and sand applied to the surface 

 of the soil exert only a slight controlling influence upon the Drop. 

 The last substance, however, showed generally less Botrytis-Rot. 



Sub-irrigation, by reducing the surface moisture, has a tendency 

 to lessen the amount of rot. 



Experiments with various gases showed that while some are capa- 

 ble of killing rot fungi when freely exposed to their influence, they 

 are all powerless when the organisms are superficially embedded in 

 plant tissues or in the soil. 



Freezing the soil has no detrimental effect upon the Drop ; on the 

 other hand the development of the sclerotia is accelerated by 

 freezing. 



Desiccation exerts a remarkable accelerating influence upon the 

 development of the sclerotia ; in one experiment the amount of Drop 

 was increased 64%. 



The period in the development of the crop at which the greatest 

 loss occurs is at about the time of maturity. 



The optimum conditions for the development of the Drop fungus 

 are practically the same as those for lettuce. 



