July i, 1921 Pathogenicity of Corticium vagum on the Potato 465 
showing lesions which either from depth of penetration or from extent 
of surface destroyed appeared definitely to interfere with the activity of 
the plant were classed as severely injured. In the third group were 
placed all stems showing definite growing-point destruction and also 
those which had actually been cut off by the operation of the fungus 
subsequent to the appearance of the stem through the soil. In order to 
obtain a quantitative expression of the intensity of injury at each tem¬ 
perature, the percentage of plants occurring in each class was determined 
and units 1,2, and 3 were allowed for each percentage in the three classes 
slightly injured, severely injured, and cut off. 
EXPERIMENTAL STUDIES 
In all the experimental work with the potato, the application of the 
results to the interpretation of the operation of the fungus under natural 
field conditions was kept constantly in mind. In order to eliminate all 
possible interfering organisms and to establish a firm basis for judgment, 
experiments were first conducted with pure cultures of the fungus in 
sterile soil. The results obtained under these extremely artificial con¬ 
ditions were finally supplemented by those obtained with the tempera¬ 
ture equally well controlled, but with other relationships of the host and 
the fungus approaching as nearly as possible those found under natural 
conditions. As a final control the temperature work was carried into 
the field, where plantings and inoculations were made at definite inter¬ 
vals and the temperatures recorded throughout the growing season. 
In addition, valuable temperature data were obtained during 1918 and 
1919 from the disease-control experiments conducted on a rather exten¬ 
sive scale in important potato-growing districts of northern Wisconsin. 
These latter field studies as stated will furnish the basis for a later report. 
Experiment i. —In the first experiment four tanks with eight cans 
each were used. The water in the individual tanks was held at approx¬ 
imately 16 0 , 19 0 , 23 0 , and 29 0 C. 
The soil used was sterilized with steam for 7 hours at two different 
periods 24 hours apart and at a temperature of approximately 97 0 C. 
Two half tubers of the Irish Cobbler variety, treated previous to cutting, 
were then planted with the inoculum in each of six of the eight cans in 
each tank. Two cans in each tank were left for the growth of control 
plants in uninoculated soil. 
Thirty-eight days after planting the plants were removed and examined. 
The total intensity of stem injury together with the percentage of stems 
injured is shown in Table I and graphically in figure 1. 
The most serious types of lesions occurred unmistakably at the two 
lower temperatures. Growing-point injury was found only at the lowest 
temperature of 16 0 C. The injury on the stems at both 24 0 and 29 0 was 
very slight. 
