452 Wisconsin Academy of Sciences , Arts, and Letters . 
Text \Fig. 4. Soil temperatures for July, 1915, and July, 1916, at Mad¬ 
ison, Wisconsin, at depths of 2, 4, and 8 inches. The days of the month 
are indicated by the figures across the top. 
the loss from Thielavia root rot in Wisconsin alone ran into millions 
of dollars whereas in 1916 relatively unimportant losses occurred. 
The evidence seems conclusive as satisfactorily explaining the 
practical aspects of the problem. But as a matter of scientific in¬ 
terest we must persistently ask, “Why?” Why are the tobaco 
roots rotted at the lower temperature while they escape at the 
higher in soil equally infested? We naturally turn to our experi¬ 
ences with the other two diseases. In the case of the Fusarium 
wilt of the tomato, in which case the disease increased with rise of 
temperature to a certain point, we found a possible explanation in 
the fact that the optimum soil temperature for the development of 
