i8 



otia confined entirely to the soil, the problem of treating it is there- 

 fore one of treatment of the soil. Were we dealing with a disease 

 caused by a fungus capable of producing thousands of spores in the 

 course of a few hours, which need only a slight current of air to 

 waft them about in every direction, then the question of treatment 

 would be quite different from that of the disease under consideration. 



From numerous cultures of the Drop fungus during the last three 

 years and from many observations in our greenhouse and others, we 

 have never observed that the fungus is propagated in any other 

 manner than by the growth of mycelium through the soil. It is 

 undoubtedlv disseminated from bed to bed and from one house to 

 another by means of tools, etc. and by transplanting plants derived 

 from infected propagating houses. At the beginning of our investi- 

 gations of the Drop in lettuce, our greenhouse which had been 

 recently remodeled and newly filled with fresh soil, was entirely free 

 from infection. Another house in this vicinity did r.ot contain any 

 of the Drop until some young lettuce plants were introduced into it 

 from Arlington. Since then, however, it has been common enough, 

 as no means have been taken to check it. In order to obtain a con- 

 siderable amount of the Drop in our lettuce house, we obtained 

 about one peck of badly infested soil from Arlington and in our first 

 experiment we inoculated three rows of plants, the infected soil 

 being put two or three inches deep around each plant. The results 

 of this inoculation can be seen in Diagram I, where the various plots 

 which received dififerent treatment are represented. Each plot con- 

 tained 55 plants set 8 inches apart in a ground bed made up of one 

 foot of typical Amherst loam, containing from 8 to io% organic 

 matter. The various plots were separated from each other by means 

 of boards, and except for surface treatment the soil was identical. 

 The conditions for all of the experiments which we shall presently 

 describe were nearly alike and practically the only condition offering 

 any variations was that of the temperature of the house and soil, 

 which always gave rise to ditTerences in the maturity of the plants. 

 The plot n. 3. (Diagram i), for example, would usually register from 

 four to six degrees higher than plot a, and uniform gradations of 

 temperature existed in the intermediate plots. 



The variety of lettuce used in all the experiments was a headed 

 form, and the seedlings were transplanted in the usual manner, 

 except that seed was sown in sterilized soil, and they were subse 



