768 
Journal of Agricultural Research 
Vol. XXIII, No. 9 
Fortunately, again, for the purpose of the experiments the temperature 
for the two years differed widely. June, 1918, was cool, averaging o. 11° C. 
below normal for the locality, while June, 1919, was the warmest that had 
been experienced in Hancock and surrounding districts for a long period 
of years, with an average of 2.6° above normal. This high average, it is 
evident from the figures submitted in Table IV, was due to uniformly 
high temperatures rather than to any period of excessively high tempera¬ 
tures. Mr. H. B. Hersey, Director of the Wisconsin Section of the U. S. 
Weather Bureau, states that— 
June was unusually warm. The average temperature for the State, 69.4° F., has 
been exceeded only once in June since records began in 1891. 
The actual daily mean air temperature for Hancock for the month of 
June was 19.i° C. in 1918 and 22 0 for the same month in 1919. The 
average mean daily soil temperature of 23 0 was determined for Plainfield 
during June in 1919 with a calculated soil temperature of 20.i° for 1918 
(Table IV), or a difference of 2.9 0 . For the second, third, and fourth 
weeks after planting the average daily mean temperature calculated for 
1918 was 19.4 0 with an actual daily mean of 22.8° for 1919, a difference 
of 2.4 0 . 
These temperature relations are especially significant when considered 
in view of the critical temperature for growing-point injury determined 
in the more carefully controlled greenhouse experiments. As previously 
indicated, no damage was found to occur to the growing points of the 
young potato shoots above 21° C., while below this to as low as 12 0 serious 
destruction of the young bud resulted. In 1918 the average mean soil 
temperature during the critical period of growth the shoots through the 
soil remained below this critical temperature, while in 1919 the average 
daily mean was well above 21 0 . Growing-point destruction was almost 
wholly responsible for the reduced stand and the decreased number of 
stems per hill in the control rows in the 1918 crop. It is also significant 
that the average daily mean of 20. i° maintained during June of 1918 
approaches closely the soil temperature (18 0 ) found optimum for the 
destruction of cortical tissue of the young stems, while during June, 1919, 
which included the entire time which the young shoots were growing 
through the soil, the average daily mean soil temperature approximated 
closely that temperature (24 0 ) found in the greenhouse experiments to 
be optimum for the early growth of the young potato shoots and at which 
temperature the fungus, due to some inhibitive factor, ceases to be seri¬ 
ously parasitic on potato stems. In view of the facts, it appears evident 
that the greater damage to the 1918 crop, amounting to three times that 
of 1919, was occasioned primarily by low soil temperature during that 
season. The high soil temperature, on the other hand, appears equally 
responsible for the inhibited action of the pathogen during 1919. 
DISCUSSION 
Results obtained from both the field and greenhouse experiments 
emphasize clearly the controlling influence of soil temperature upon the 
pathogenic power of Corticium vagum. While the fungus, as was shown 
in the earlier studies, may produce lesions on the underground parts of 
the potato over a relatively wide range of soil temperature, it has become 
evident that variations in the average mean soil temperature of 2 0 or 3 0 
above or below 21° C. and maintained for the first few weeks after plant¬ 
ing may determine accurately the degree to which this fungus may 
