Mar. 3,1923 
Further Studies of Corticium vagum on Potato 
769 
damage the potato crop. However, from the standpoint of practical 
methods of control, except for the possible advantage gained by late 
planting, we may expect but little from temperature relation of the fungus. 
This is the more evident considering the fact, as pointed out in the earlier 
publications, that the fungus operates most vigorously as a parasite 
under such temperature as best favors maximum potato production. 
The generally accepted idea borne out by these field experiments that 
the early crop of potatoes is more severely damaged by Corticium vagum 
appears to be largely a question of the cooler soil temperatures main¬ 
tained during the early growth of the plants. What part may be played 
by the higher soil moisture content generally occurring during these 
earlier dates remains undetermined. Whether such increased soil 
moisture, if important, contributes directly to the pathogenic activity of 
the fungus or only indirectly by its influence on the temperature of the 
soil is not clear. From the greenhouse experiments in which the moisture 
factor was carefully controlled, it would appear evident that such varia¬ 
tions as frequently occur in the temperature values between early and 
late planting might adequately account for such differences as are noted 
in the pathogenicity of the fungus in question. Again, the greater sus¬ 
ceptibility to the attack of C. vagum frequently attributed to our stand¬ 
ard early varieties enters in as a possible factor in the greater damage 
to the early crops. Here again it is not improbable that the suggested 
susceptibility is induced by the particular environmental conditions to 
which these early varieties are exposed. The possible increased virulence 
of the fungus produced by a cooler soil evidently functions here as an 
important factor. 
Such determination of the various temperature relations of Corticium 
vagum made during these studies may add much by way of explanation 
of many of the conflicting opinions held regarding the pathogenicity of 
the fungus as a potato parasite. Seasonal variation in the severity of the 
stem canker is easily explained in view of the facts presented. Special 
significance is also given to the observation that the stem canker caused 
by C. vagum reaches its maximum severity in the northern potato sections 
of Canada and the States of Maine, Wisconsin, and Michigan, as also in 
the higher altitudes of the Western States. Clearly, however, the fungus 
may become especially virulent on the very early crops even in the warmer 
areas. 
SUMMARY 
(1) Special significance is given to the soil temperature relations of 
Corticium vagum established in the earlier greenhouse experiments by the 
field studies conducted during 1918 and 1919. These studies further 
emphasize the importance of the soil temperature as a controlling factor 
in tiie pathogenicity of the sterile stage of C. vagum on the potato. 
(2) Corticium vagum caused greater growing-point destruction and a 
more severe type of cortical injury on the early potato crop planted 
April 26 and May 7 than on the later plantings of May 17 and 30. This 
difference was due to a sudden rise in the soil temperature at such dates 
as to permit early growth of the later crops to take place in a very warm 
soil. 
(3) Growing tips of the young primary shoots grown from potato 
tubers planted at the early dates of April 26 and May 7 were destroyed in 
cold soil by Corticium vagum , while secondary primordia developed from 
such injured shoots later grew through warm soil uninjured. 
