464 
Journal of Agricultural Research 
Vol. XXI, No. 7 
water relation was kept constant by frequent weighing and addition of 
water to restore that lost by evaporation from the soil or through tran¬ 
spiration by the plants. The weighings were usually made every two 
or three days; at the higher temperatures water was added more fre¬ 
quently as required. 
SOURCE OF FUNGUS 
A single strain 1 (201) of sterile stage of Corticium vagum was used in 
all the experiments in which artificial methods of soil inoculation were 
employed. The fungus was increased on bean pods and was introduced 
into the soil usually after about six weeks’ growth on this medium. In 
three experiments the soil was inoculated by sclerotia upon the surface 
of the tubers used for seed. 
SOIL, INOCULATION METHODS 
The methods of soil inoculation followed were essentially those de¬ 
scribed for the work on the pathogenicity of the fungus. They need 
not be discussed here. In general, where artificial methods were em¬ 
ployed the inoculum was placed below the potato sets so as to make 
sure that none of the material introduced would come in contact with 
the growing stems. The sets in all cases were planted at a depth of 5 
inches. After planting, the cans were allowed to remain under uniform 
condition for a period of six days, at the end of which time the tempera¬ 
tures were adjusted in the various tanks. 
In removing the plants at the conclusion of each experiment the soil 
in each can was first saturated with water for a period of 24 hours. The 
roots and stems were then washed free from dirt by a gentle stream of 
water from the hose. 
QUANTITATIVE INDEX 
Previous field and greenhouse experiments had shown clearly that 
owing to the peculiar nature of the attack of Corticium vagum ( 12 ) the 
percentage of the potato stems showing lesions gave no accurate idea of 
the damage resulting to the plants. The degree of the intensity of 
injury to the stems was used, therefore, as an index for determining the 
different temperature values (PI. 89-93). I n an attempt to express this 
relationship the diseased stems occurring at each of the various tempera¬ 
tures were divided into three categories: Slightly injured, severely in¬ 
jured, and cut off. All stems showing lesions ranging from the first 
indications of disease to those sufficiently serious to interfere with the 
physiological processes of the plant were listed as slightly injured. Stems 
1 This strain is the same employed in the previous experiments on pathogenicity, for the record of which 
the reader is referred to the earlier paper ( 12 ). The term “strain” is used here with no special morpholog¬ 
ical or physiological significance. It indicates, rather, the results of a single isolation from a definite 
locality. 
