Jones—Relation of Soil Temperature to Disease in Plants. 435 
man later determined that about 17° C. (62° F.) constitutes a 
critical point below which even in the “sickest” soil the disease 
does not develop, whereas above this point its seriousness in¬ 
creases with rising soil temperature. Although his pioneer trials 
were made by relatively crude methods, later repetitions with 
more exact apparatus have verified his conclusions. Soon after 
this, while Dr. W. H. Tisdale as a graduate student worked upon 
the flax-wilt problem in the same greenhouses, he observed that 
the plants nearest the heating pipes succumbed first, and, fol¬ 
lowing this lead experimentally, he determined that with this 
disease also there is a critical point, about 16°-17° C., below 
which its development is inhibited. 
The practical significance of such conditions became evident 
when field records were secured which showed that in cool years 
like 1915 the soil temperature may remain through midsummer 
perhaps 5° C. lower than in hot summers like 1916. The difference 
year by year in the advent of higher soil temperatures in the 
spring and early summer is even more marked. Such results 
naturally stimulated the attempt at improved technique and further 
investigation into the relation of soil temperature to still other 
cases of soil parasitism. It has been recognized from the outset 
that an intelligent interpretation of these problems requires con¬ 
stant recognition of the fact that each such disease results from the 
vital interrelations of two organisms, the parasite and the host, 
both of which must be affected by variations in soil temperature. 
One must also constantly be mindful of the fact that variations 
in soil temperature tend to induce concomitant variations in soil 
moisture, aeration, and other environing factors. It is necessary, 
therefore, to aim either at avoidance of these other possible variables 
or at their adequate evaluation if one is not to be misled. To these 
ends considerable time was early given to perfecting experimental 
methods. 
From the outset these studies have been developed in helpful 
and mutually stimulating cooperation in which various staff mem¬ 
bers and graduate students have taken part. In several cases the 
results have reached print, in more, the work is still in progress. 
The writer therefore acknowledges the generous approval of several 
colleagues and students for the use of such material basic to this 
article. The chief problems undertaken thus far are listed below, 
together with the pertinent publications, so far as these are in 
print, which give the details essential to a fuller understanding. 
