1898.] PUBLIC DOCUMENT — No. 33. 57 



every grower has the germs of the disease in his lettuce soil 

 to a greater or less extent, but the conditions giving rise to 

 their excessive development are not always present. Some 

 claim that manure is the principal source of infection ; yet, 

 on the other hand, while all use manure, all are not troubled 

 in the same degree. As a remedy for the drop, some have 

 resorted to the practice of sanding the surface of the soil or 

 putting on a layer of yellow loam. This is for the purpose 

 of giving a clean, uninfested surface to the soil surrounding 

 the plants. In regard to the effect of this treatment, it may 

 be stated that opinions difter considerably. Whether the 

 method of applying a superficial layer of sand or subsoil to 

 the surface will be of any assistance in keeping the drop in 

 check appears somewhat doubtful, from an experiment made 

 of burying some infested plants to a depth of three or four 

 inches in a pot of yellow loam subsoil. It was found that 

 the fungus made its way to the top in a ver}^ few days, as 

 was evident from the mould-like growth of the mycelium 

 upon the surface of the soil and the death by drop of plants 

 which had been set in the pot. Neither can we expect much 

 from the application of chemicals, as any such treatment 

 would interfere with the growth of the plant, and hence 

 become objectionable. Some experiments are now being 

 made with gases, with the idea of killing the organism 

 by fumigation ; but this method does not promise much 

 success. 



The application of live steam to the soil, and thus steril- 

 izing it, w^ould undoubtedly destroy the germs of the dis- 

 ease. To do this would necessitate laying two-inch tile at 

 a depth of eight inches or a foot below the surface of the 

 soil, and at a distance of one or two feet apart, and driving 

 in steam under pressure and allowing the same to permeate 

 the soil. This method can be employed on a small scale 

 with good results, but the larger area of a lettuce house 

 would render its practical application uncertain. Another 

 method of treatment by steam, which would be far cheaper, 

 would be to sterilize the surface of the soil to a depth of 

 three or four inches or more. This can be done by con- 

 structing a pit in the lettuce house and covering the bottom 



