30 Levi Otto Gratz 



since there was the same amount of soil in the two systems. While results 

 were obtained, this method of moisture control is too cumbersome to be 

 used on a large scale. 



The soil was sterilized by autoclaving for from two to three hours under 

 12 to 18 pounds pressure. The inoculum was mixed into the soil, the 

 system was set up, and the moistures and temperatures were adjusted. 

 Four seedlings were transplanted into each culture vessel. The seedlings 

 were transplanted rather than started in the vessels, because if the seed 

 had been sown directly the seedlings would have been attacked by the 

 fungus before they could push through the soil. Furthermore, several 

 weeks could be gained in this way. This method, however, presented 

 several disadvantages. First, the transplanting was likely to be a definite 

 shock to the plant, and second^, with the fungus directly in the soil it 

 was impossible to know whether the plant was directly in contact with 

 the inoculum or some distance away from it. Where the difference in 

 the rate as well as the total amount of damping-off was to be compared 

 at the various temperatures and moistures, this represented a large source 

 of error. Also, when the plants were transplanted into the infected soil, 

 a small amount of water had to be given them to insure growth, and this 

 favored the growth of the fungus about the roots. Four jars in each tank 

 were kept as controls. The moisture content was determined after the 

 conclusion of several experiments to insure the uniform operation of the 

 systems for each of the experiments. 



During the winter of 1921-22, six trials were made with a total of 

 eighteen temperatures (table 9). In general, the results were similar for 

 all six trials. Damping-off occurred at all of the temperatures tried, 

 but the rate decreased above 30° C. Low temperatures and low moistures 

 were unfavorable for the development of the disease. Determinations 

 made at the end of the trials showed that slightly different moistures 

 resulted at the different temperatures. This was probably due to a more 

 rapid rate of evaporation and transpiration at the higher temperatures. 

 .Moisture determinations were made at the conclusion of trials I, II, 

 and VI. In trial VI both irrigation systems became clogged, resulting 

 in a lower moisture content of the soil in both series. For trial IV 

 another set of plants was transplanted into the soil used in the previous 

 experiment without disturbing it any more than was necessary. In this 

 trial, tank 3, which was held at 32° C. in the previous experiments, 

 was lowered to 19.5° C. There was a definite decrease in the rate of 

 damping-off. Whether or not this was due to a decrease in the virulence 

 of the fungus because of the previous high temperatures was not ascer- 

 tained. For trials V and VI, the upper three inches of the soil was re- 

 moved from the jars, thoroughly mixed to insure equal distribution of the 

 inoculum, and replaced. 



