July i, 1921 
Pathogenicity of Corticium vagum on the Potato 
463 
sufficiently to counterbalance the amount of variation above or below 
that which was desired. 1 
At the highest temperature the water was maintained at an extreme 
range of variation of 3 0 , or from 28.5° to 31.5^'C. At 27 0 this range of 
variation was less than 1.5 0 , while at 24 0 and at those temperatures 
approaching that of the greenhouse the total temperature variation 
rarely exceeded i°. With a mat of wet sphagnum on the cover of the 
tanks and with high collars around each can, the variations at the lower 
temperatures of 9 0 and 12 0 were held within a range of less than 2 0 . 
Under the system of regulation, the extremes of the variations were of 
relatively short duration. 
Owing to the direct exposure to the surrounding atmosphere the surface 
of the soil in the cans held at the extreme high and low temperatures 
varied somewhat below and above that of the water in the respective 
tanks. These differences could not be detected at a soil depth below 2 
inches. Since the potato sets were planted at a depth of 5 inches and 
since no variation in the vertical distribution of stem lesions could be 
detected, the average mean temperature of the water in each tank was 
taken as the temperature of the soil 2 at which the plants were grown. 
The air temperatures in thee greenhouse at which the plants were grown 
were maintained in general between 17 0 and 22° C. 
SOIL MOISTURE 
Determinations made at the beginning of each experiment showed the 
moisture content to vary in all experiments between 20.4 per cent and 
24.8 per cent of the dry weight of the soils used. This range of soil mois¬ 
ture had been previously determined to permit of uniform infection of 
potato stems, and no further precaution was taken to regulate more 
definitely the relation of the moisture content in the soils in the various 
experiments. The maintenance of a constant water relation throughout 
any one series, however, became a much more complicated process. 
The large number of cans used in the experiment and the relatively 
short period of growth of the plants rendered the use of the Livingston 
auto-irrigators (7) impracticable. The direct weight method was finally 
adopted. In this method precautions were taken at the beginning of 
each experiment to obtain a uniform water content of the soil used in 
any experiment. In the process of planting, all cans were then filled to 
exactly the same weight, and during the progress of the experiment the 
1 At the higher temperatures, the variation in the temperature of the water in all cases fell below those 
desired. The reverse was true for those below the temperature of the greenhouse. The adjustments 
found necessary were as follows: 
Desired temperature—30 0 , 27 0 , 24 0 , 21 0 ,18 0 ,15°, 12 0 , 9 0 C. 
Adjusted temperature—31.5 0 , 28 0 ,24.5 0 ,21 0 ,18 0 ,15 0 , n.5 0 . 8° C. 
2 The figures recorded in all tables must be considered as the exact calculated average of the soil tempera¬ 
tures at which the crops were grown and should not be assumed to indicate a degree of accuracy in tem¬ 
perature control as might be implied by the retention of the fractions given in the tables. 
