May 15, J919 Influence of Soil Environment on Rooirot of Tobacco 61 



T. hasicola practically nothing of a definite nature exists. Rather 

 obscure statements that high temperatures favor the disease have been 

 published by Gilbert {12), while, on the other hand, Clinton {8) states 

 that possibly unusually cold, wet spring weather has something to do 

 with determining whether or not the fungus does much damage. Gallo- 

 way (11, p. 174-17S) found that in the greenhouse the disease was appar- 

 ently more severe on violets on the approach of fall than in^ summer, 

 indicating a temperature relation. In Italy where very considerable 

 observation has been made on the disease, it is agreed that weather 

 conditions have much to do with its occurrence and severity. That 

 such was the case in Wisconsin was evident during the first season of 

 observation. The recovery of badly infected plants in large areas 

 during the course of only two or three weeks led to the desire to study 

 in more detail the environmental conditions bringing this about. It 

 was at first suspected that the moisture relations were the all-important 

 factor; but in connection with its study, temperature records of the 

 soil under field conditions were taken beginning in the spring of 191 5, 

 and continued for the seasons of 1916, 1917, and 191 8. 



In the fall of 191 6, following some interesting results by Tisdale (25) 

 on the influence of soil temperature on flaxwilt (caused by Fusarium 

 lini), the writers, under the advice and support of Dr. L. R. Jones, 

 undertook to have a large tank (PI. 2, I) constructed in which soil could 

 be held fairly constant at several different temperatures. This tank 

 has already been described and illustrated in some detail by Jones {16). 

 Further detailed description of the mechanical part of the apparatus 

 seems unnecessary, especially in view of the fact that improvements are 

 being gradually made on these tanks which will no doubt necessitate 

 further description of similar apparatus developed in the Department of 

 Plant Pathology of the University of Wisconsin. It should be said, 

 however, that by means of proper insulation of the compartments it 

 has been possible to maintain a fairly constant temperature of water 

 at any selected temperatures between approximately 5° and 40° C. 

 This has been done by the inflow of cold water from the taps in the winter 

 time and by heating the water to the higher temperatures with electric 

 bulbs or with steam. The expense of automatic temperature regulation 

 in a large number of chambers has discouraged the use of such apparatus 

 up to the present time, but personal attention and regulation two and 

 three times every 24 hours, in combination with good insulation, has been 

 found to give results sufficiently accurate for most needs. It was found 

 that although considerable ranges of temperature occurred at the 

 extremes (below 15° and above 30°) the temperatures between 15° and 

 30° could be held quite constantly within i degree. 



The soils used were placed in i -gallon battery jars and set on boards 

 suspended in the water in the tanks. Four jars could be placed in each 

 compartment with displacement of only a relatively small amount of 



