480 
Journal of Agricultural Research 
Vol. XXI, No. 7 
made possible. On the other hand, where total absence of temperature 
favorable for the action of the fungus occurs, as was found in the higher 
temperatures, no effect on the number of stems will result. 
SUMMARY 
(1) The power of the sterile or “ Rhizoctonia ” stage of Corticium vagum 
to produce lesions on potato stems varies greatly in different localities 
and under varying soil and climatic conditions. A study of the factors 
affecting the pathogenic action of the fungus is essential to a better 
Understanding of the disease and to the formulating of effective control 
measures. 
(2) Corticium vagum causes the greatest damage to the potato at a 
very early stage in the development of the host, probably before the 
young shoots appear above the soil. Two types of stem injury are 
especially recognized—the cankering of the cortex of the basal portions 
of the stem and the destruction of the primordia of the young shoots 
before they push through the soil. This latter type is considered the 
more serious. 
(3) Stem lesions produced by pure cultures of Corticium vagum were 
indistinguishable from those obtained in soil inoculated with the sclerotia 
of the fungus. Other fungi are recognized as possible complicating fac¬ 
tors under natural conditions. The fungus attacked the potato stems 
more vigorously in unsterilized than in steam-sterilized soil. 
(4) The percentage of stems injured does not give a true index to the 
degree of damage produced by the fungus. The intensity of injury is used 
to express the relative value of the different soil temperatures. 
(5) The various strains of Corticium vagum introduced into the soil as a 
pure culture or as sclerotia produced lesions on the basal portion of 
potato stems throughout the same range of soil temperature of from 9 0 
to 27 0 C. The greatest damage occurred between 15 0 to 21 °, while 18 0 
proved to be the most favorable temperature for tissue destruction as 
well as for growing-point injury. Serious destruction of tissue resulted 
at 9 0 . The severity of attack decreased rapidly above 21 0 until at 24 0 
C. vagum proved to be of minor parasitic importance. Few typical 
lesions occurred at or above 27 0 . 
(6) Growing-point destruction was confined to temperatures at or 
below 21 0 C. At 21 0 this type of injury was slight. On the other hand, 
it was found that it may reach its maximum expression at a temperature 
as low as 12 0 . 
(7) Growing-point injury is dependent upon the rate at which the 
young shoots grow through the soil. At temperatures above 21 0 C. 
the rapid growth of the potato, together with the decreased pathogenic 
power of the fungus, was found to permit the primordia of the young 
shoots to escape uninjured. 
