SOIL, DISINFECTION AVITH HOT WATER. 11 



tomato plants. Again, the check plants in infested soil showed 

 severe nematode and fungous infection and were much dwarfed and 

 yellowed. (See PI. IV, fig. 2.) Some of the plants grown in untreated 

 greenhouse soil, supposedly uninfested, showed mild nematode 

 infestation, while more than half were attacked by Rhizoctonia. 



Because of the partial infestation of the greenhouse soil, it was 

 not possible to determine how much of the increased plant growth 

 in the soil treated with boiling water was due to the killing of the 

 pathogenes and how much resulted from the stimulating effect of 

 the heating. 



A scrutiny of the results of the S-inch pot treatments given in 

 Table IV shows a verification of those secured in Experiment Series I ; 

 viz, a marked reduction of the diseases in the 2,000 c. c. treatments 

 and their elimination in the pots which received 3,000 c. c. of boiling 

 w^ater (PL V, fig. 1), and a considerable increase in the number, size, 

 and vigor of plants in treated pots above that shown in the check 

 pots (PI. V, fig. 2). As might be expected, the added 4,000 c. c. 

 treatment gave similar results. 



Again, on account of the nematodes and Rhizoctonia in the fresh 

 greenhouse soil, no conclusive results were obtained regarding the 

 degree of growth stimulation which might be caused by the heating 

 alone. It may be noted that in general the percentage of germina- 

 tion and the plant growth were larger in the fresh greenhouse soil 

 than in the old infested soil (PI. V, fig. 2). 



The results from the repeated bench experiment, as given in Table 

 V, show only data from tomatoes, smce the lettuce was destroyed 

 by mice shortly after the experiment was started. Here, again, sub- 

 stantial confirmation of the results of Experiment Series I was 

 secured. As in Experiment Series I, the treatment approximately 

 equivalent to 1,000 c. c. per 8-inch pot reduced the amount of disease. 

 In the 2,000 c. c. treatment of Experiment Series II only 11 per cent 

 of the plants showed mild root-knot mfection, as compared with 96 

 per cent of more severe root-knot in the check, while the 3,000 c. c. 

 application practically eliminated the trouble. 



A study of Table VI, which gives the soil temperature records, 

 shows that in 8-inch pots treated with 3,000 and 4,000 c. c. of boiling 

 water and in the bench section treated at the rate of 7 gallons per 

 cubic foot of soil, all of which treatments kiUed the pathogenes, a 

 relatively short time was required to raise the soil temperature to 

 the maximum in each treatment and that the soil retained the 

 heat for a strikingly long period. In all cases the temperature was 

 maintained above 60° C. for more than 30 minutes and above 55° 

 C. for 48 to 65 minutes, both temperatures being well above the 

 thermal death point of the root-knot nematode, as determined by 

 the first writer, and presumably above that of the fungi in question. 



