478 
Journal of Agricultural Research 
Vol. XXI, No. 7 
action of Corticium vagum on the potato stems. The recent work of 
Edson and Shapovolov (j) is interesting in this connection. 
From the foregoing studies and from previous work, much more exten¬ 
sive than outlined here, it becomes evident that the sterile stage of 
Corticium vagum is definitely capable, under natural conditions of potato 
culture, of producing all the primary effects on the potato plant which 
have been attributed to it, and that under normal field conditions other 
organisms, whatever they may be, are possibly of secondary or of more 
or less minor importance. Granting their pathogenic action under 
extreme conditions, we have no conclusive evidence as yet of their 
importance in general potato culture. It is evident, however, that this 
problem stands much in need of investigation. 
DISCUSSION 
The results obtained both with pure culture and with the natural 
method of soil inoculation disclosed the general fact that the organisms 
concerned are capable of producing lesions on potato stems over a wide 
range of temperature from 9 0 to 27 0 C. Corticium vagum becomes a 
serious factor in potato production, however, only at soil temperatures 
below 24 0 . While 18 0 proved the most favorable temperature for the 
pathogenic action of the fungus, it appeared evident that any wide varia¬ 
tion in the numerous factors involved might result in a temperature 
requirement for maximum pathogenicity at any point between 15 0 and 
21 0 . No definite critical temperature such as has been obtained for other 
soil organisms by Gilman (4), Tisdale (16), and Johnson and Hartman 
(5) was found to exist at the lower range of temperatures maintained in 
the experiments. In all cases within the range of temperature favorable 
for its pathogenic action Corticium vagum proved to be a very dangerous 
parasite. This range approximates very closely that found most favor¬ 
able for the best development of the potato plant. 
The divergence of the percentage and the intensity curves at the lower 
temperatures reveals a wide difference between the percentage of stems 
showing lesions at the various temperatures and the degree of injury to 
the plant subsequent to infection. This difference was found to be due 
both to a more severe type of tissue destruction subsequent to the initial 
infection at the lower temperature and to an attack on and frequent 
destruction of the growing point of the young shoots. This latter type 
of injury was seldom found at or above 21 0 C., while at temperatures 
lower than 21 0 it assumed decidedly serious importance. 
At 18° C. the fungus exhibits a maximum of activity both in the 
number of stems infected and in the degree of injury subsequent to 
infection. The exact relation of soil temperature to the divergence 
between these two processes at 21 0 and 24 0 as compared with the lower 
temperatures is not clear. It is possible that the rapid rate of growth 
