SOIL TEMPERATURES IN PHYTOPATHOLOGY 233 



20°C. but not at 33°. This sugge.sts the possible importance 

 of temperature as a factor in explaining the variation in experi- 

 ence ^^dth Rhizoctonia on the potato. Certainly there is a wide 

 variety of potato-disease problems where we must know the re- 

 lations of soil temperature to both host and parasite before we 

 can fully understand them. And this holds true for parasites of 

 the most diverse tj^e. As illustrating this, one may consider 

 the regional limitations in the occurrence of the two bacterial 

 diseases of the potato, the brown rot {Bacterium solanacearum) 

 and black leg (Bacillus phytophthorus) . Both of these are dis- 

 tributed with the seed tuber hence have doubtless been intro- 

 duced equally widely and very generally. The e\ddence seems 

 conclusive, however, that in America the browTi rot is practi- 

 cally confined to the south-eastern states, whereas the black leg 

 is northern in its persistent occurrence. It is also of like import 

 to note the increasing evidence that the myxomycete causing 

 powdery scab (Spongospora) may be restricted to the northern 

 limits of the potato belt by its limited temperature endurance. 

 Again, I am at a loss to account for the curious fluctuations in 

 the occurrence and degree of development of common scab 

 (Oospora) which I have previously recorded (20) unless we 

 admit soil temperature as an influential factor; and certain 

 observations, both as to its relatively lesser development in 

 Europe and as to its seasonal and regional variations in America, 

 incline me to the theory that high soil temperatures at certain 

 critical periods are essential to its serious parasitism. Perhaps 

 the most puzzling problems in American potato pathology re- 

 late to the Colorado potato failures. Different observers have 

 in turn attributed the trouble to Rhizoctonia, to Fusarium, and 

 to degenerate seed stocks. WTiatever may be the causal inter- 

 pretation, all now seem agreed that high soil temperatures are 

 an important condition influencing the development of these 

 pathological results. 



Examples could be further multipUed, but these mil at least 

 suffice to show the reason for the hvely interest of phytopatholo- 

 gists in soil temperature problems. This is aheady leading such 

 investigators in at least two of our universities, Wisconsin and 



