27 



laid 8 in. deep and iS in. apart, but far better results could be 

 obtained in having the tile 9 or lo inches apart. The expense of 

 treating the house once for all is far less in the end than 

 continual treatment of the surface layer, and while the sterilization 

 method is more or less of a bother, the only alternative at the pres- 

 ent time is a complete change of the soil, which would be more 

 troublesome and expensive to most lettuce growers than steriliza- 

 tion. Where a bed is properly tiled and where a large steam boiler 

 is at hand, the heating of the soil can be more readily done than 

 mos! people imagine. We are of the opinion that when a house is 

 once free from Drop it can be kept in this condition for some time 

 providing that care be taken not to use tools which have been in 

 infected houses, and also when applying manure to see to it that it 

 does not contain the germs of Drop. In all probability, horse 

 manure taken from localities where the refuse from lettuce and 

 cucumber houses has not been thrown would be free from Drop 

 germs. 



Irrigation. 



In connection with the preceding experiments that have been 

 described there were carried on some investigations relating to the 

 effects of sub-irrigation upon the suppression of the Drop. The 

 results of these experiments were not sufficiently marked for drawing 

 any definite conclusions, though in almost every case the sub-irrigated 

 plots showed less of the disease. 



Rotation. 



The rotation of crops in soil containing the Drop does not appear 

 to reduce the amount of infectious material. On the other hand, 

 during rotation tlie mycelium of the fungus increases and becomes 

 more generally distributed. 



Results of Treating the Drop with Chemicals. 



In the treatment of pathogenic(disease producing)organisms located 

 in the soil by chemical substances we have a problem to deal with 

 quite different from treating foliage. Such a problem is much more 

 difficult than when we have to deal with foliage, inasmuch as it is 

 not impossible in most instances to cover the latter with an adhesive 

 mixture that will act to a greater or less extent as a prevention to 



