May 15, 1919 Influence of Soil Environment on Rootroi of Tobacco 65 



is in line with the frequently observed fact that sterilized soil reinfested 

 is a very favorable medium for the progress of disease. At 31° to 

 32° C. a trace of infection was found in the sterilized soil, but no infection 

 occurred in the naturally infested soil. Nematode injury was most serious 

 at the higher temperatures. 



Experiment VI. — Six different temperatures ranging between 17° 

 and 32° C. were used. The best growth of the controls in uninfested 

 soil occurred at 31° to 32°, and the poorest at 17° to 18°. The best 

 growth in the infested series was also obtained from 31° to 32°, which 

 was practically equal to that of the controls. Almost equally good 

 growth occurred at 28° to 29°, but at lower temperatures the results 

 were again unfortunately interfered with by the toxic action of sterilized 

 soil, which, though it had been treated for the purpose of reducing the 

 toxicity, had not apparently sufficiently reduced the toxicity. Exami- 

 nation of the roots, however, which were carefully washed out, yielded 

 results in line with the previous experiments. 



Experiment VII. — In this experiment the soil in the uninfested series 

 was sterilized with a i to 50 formalin drench three weeks prior to its use, 

 in order to avoid further interference by the toxic action of the steam- 

 sterilized soils. Six temperatures ranging from 17° to 32° C. were again 

 used. This experiment was begun on January 18, 1918, using the "White 

 Burley variety in the same soil as previously used, and concluded on 

 February 26. Marked differences in growth on the uninfested and 

 infested soils w^ere already noticeable on Februar}/ 5; the plants in the 

 sterilized soil 20° to 21° and 23° to 24° were twice as large as those in the 

 infested soil; whereas the plants in the invested and uninfested soil at 

 31° to 32° were practically equal in size. The final results are shown in 

 Table VI A, in terms of air-dry weight of the stalks, leaves, and roots in the 

 infested and uninfested series, together w4th the amount of infection on 

 the roots. It may be seen that the best temperature for growth in this 

 case was apparently 28° to 29° for the above-ground portions of the plant, 

 but that the best root development took place in the cooler soil at 23° to 

 24°. In 'the infested soil a gradual increase in growth from the lowest 

 •to the highest temperature is evident. (PI. 2, II-III.) Practically the 

 same is true for root development. (PI. 2, IV.) 



Experiment VIII. — In this experiment an ordinary greenhouse soil 

 mixture, free from T. hasicola was used. No sterilization, therefore, 

 was used, and infestation with T. hasicola was produced by thoroughly 

 incorporating a heavy spore suspension of endoconidia of the fungus 

 from young cultures on agar. The experiment was now run as before, 

 except that the Connecticut Havana variety, which is relatively much 

 more resistant to T. hasicola than the White Burley, was used. The 

 young seedlings were transplanted on March 6, 191 8. On March 17 

 the plants in the inoculated pots already showed signs of heavy infec- 

 tion at the lower temperatures. On days of high transpiration the 

 108122°— 19 4 



