May 15, 1919 Influence of Soil Environment on Roolrot of Tobacco 67 



other hand, at temperatures of 26° and above, the amount of infection 

 and the extent of the injury done are gradually reduced until at about 

 30° no appreciable injury results, and at at 31° to 32° it is permissible 

 to say that practically no infection whatever occurs. 



The results having shown that the rootrot can be practically con- 

 trolled by high soil temperatures, which at the same time are favorable 

 for the growth of tobacco, the question naturally arises as to how far 

 a plant may recover from serious root infection, provided a change of 

 soil temperature from one favorable to disease to one unfavorable to 

 disease is brought about. Eight White Burley plants which had been 

 planted to the infested tobacco field in June but which had made no 

 appreciable growth during the entire season in the field were taken up 

 late in September with their adhering soil and transplanted into the 

 jars with the infested soil. Four of these were then set in the tem- 

 perature tanks at a low temperature (20° to 21°) and four at a high 

 temperature (30° to 31°). After remaining at these temperatures for 

 a month the roots were dug out as carefully as possible, and the results 

 are illustrated in Plate 3. The plants had almost no roots when placed 

 in the tank, and one must marvel at the wonderful persistence of tobacco 

 plants in maintaining themselves with an almost complete lack of root 

 system. At the higher temperature, however, new roots were forced 

 ©ut through the blackened bases of the stalks and remained uniformly 

 clean, white, and free from disease. This experiment was repeated 

 with even more striking results by mo-^ng jars with badly diseased 

 plants from the low temperatures to the high temperatures in the tanks. 

 In the space of three or four days the plants seemed to have taken 

 on renewed vigor and growth. These experiments seem to prove 

 beyond doubt that similar conditions may happen in the field under 

 practical conditions, and that the phenomena of recovery of a badly 

 diseased crop, so frequently noted in the field within a short period of 

 time, is no doubt due in large measure to natural changes in tempera- 

 ture relations of the soil. 



SOIL TEMPERATURES IN THE FIELD 



It now remains to ascertain how far the soil temperatures occurring 

 under normal conditions in the field may influence the actual amount 

 of infection and damage from rootrot. It is necessary, therefore, to 

 determine the actual soil temperatures occurring during the growing 

 season in order that a knowledge may be obtained of the change occur- 

 ring at different times in the same season and during different seasons 

 taken as a whole. Unfortunately not a great many reliable data upon 

 soil temperatures for summer months in various sections of the country 

 exist. Such as do exist, however, may have a bearing upon future 

 studies of the influence of soil temperatures upon the occurrence of 

 disease. It is to be expected that soil temperatures have a fairly con- 



