May 15, 1919 Influence of Soil Environment on Rootroi 0} Tobacco 63 



being brought up to about three-fourths saturation, the jars- were set in 

 the tanks at the different temperatures and allowed to remain there for 

 three to five days to permit the necessary changes of temperature. One 

 young seedling of tobacco, usually the susceptible White Burley variety, 

 was then transplanted into each jar. Subsequent attention then con- 

 sisted only in taking the temperature records twice a day, in maintaining 

 the proper temperature, and in watering the plants as required. In the 

 first experiments, when the temperature range was determined, 12 

 difi'erent soil temperatures were run at one time ; but in the latter experi- 

 ments, when a closer approximation of the critical temperature was 

 necessary, only 6 or 7 temperatures were used. 



Experiment I. — Twelve temperatures were used, ranging from 7° to 

 40° C, and the white burley variety was transplanted into the jars. 

 The plants in the uninfested or sterilized soil series grew best at tempera- 

 tures of 29° and 31°. Practically no growth occurred below 13°, and 

 again there was poor growth at 40°. iProm a physiological standpoint it 

 was interesting to note that a marked effect upon the shape of the plants 

 occurred especially at the higher temperatures. While the plants grew 

 low and stocky with broad but rather pointed leaves at the optimum 

 temperature for growth, the plants became tall and spindly, with short 

 and rounded leaves at a temperature of about 36° to 40°. ' 



In the infested soils at temperatures above 26° the plant growth ap- 

 peared to be almost as good as that in uninfested soil (Pi. 2, I). At the 

 temperatures 23°, 21°, 19°, and 17°, however, a very decided reduction in 

 growth occurred as compared with the uninfested soils at the same tem- 

 perature. 



Upon removal of the roots from the infested soil series it was found that 

 those at temperatures between 23° and 17° were heavily attacked by T. 

 basicola and that slight infection occurred at 7°, while at 26° relatively 

 few lesions occurred. The lesions were still less common at 29°, while at 

 31 ° only one lesion could be found. At the higher temperatures, 35° and 

 approximately 40°, no signs of Thielavia infection were found. 



Experiment II. — In this experiment the temperature range was 9° to 

 40° C. The toxic action of the heated soils at temperatures of 17° to 25° 

 became quite marked early in the experiment, and no doubt affected the 

 results. The total leaf area of each plant was determined at four different 

 times during the course of the experiment. The results lack uniformity, 

 however, owing to the toxic action of the heated soils. The infested soils 

 gave the best growth at 35°, with an average of 251 square inches, as com- 

 pared with 289 square inches for the sterilized soil at the same tempera- 

 ture. The poorest growth was at 19°, where a leaf area of only 19.2 square 

 inches was obtained in the infested soil, as compared with 206.4 square 

 inches in the sterilized soil. While the disease was quite marked at 24.5°, 

 71 square inches on infested soil as against 205 square inches on sterilized 

 soil, decided indication of reduced severity again appeared at 26^, 169 



