Jones—Relation of Soil Temperature to Disease in Plants. 449 
these early stages and thus permanently to evade injurious at¬ 
tack, whereas in colder soils the shoot develops so slowly that 
the fungus gets in its deadly work before the tip can escape 
through the surface. 
The Tobacco Root Rot 
Attention may now be directed to the results with the tobacco 
root-rot disease. This is caused by the parasitic soil fungus 
Thielavia basicola, which is very widespread in its occurrence 
and attacks the roots of various plants besides tobacco. Trouble 
with this root rot is frequent in the older tobacco sections of 
this country, and in southern Wisconsin it is the most serious 
tobacco disease. A noteworthy thing about it is that even on 
old tobacco soils where the fungus is almost omnipresent the 
severity of its attacks varies extremely from season to season. 
The farmers might attribute these variations simply to weather 
but for the fact that on new soil they do not occur. On virgin 
fields a sound crop develops each season regardless of weather, 
the failures occurring under certain conditions on the old to¬ 
bacco soils. Since tobacco is reputed to be a hard crop on the 
soil, the growers have naturally attributed their failures to soil 
worn out through exhaustive cropping. They have been faced, 
however, with the puzzling experience that in certain seasons 
the crop starting well would later fail, or, conversely, that when 
making a poor promise during June and July, it might suddenly 
mend in August and mature a profitable yield in September. 
Clearly, the explanation of these anomalies required expert at¬ 
tention. 
Dr. James Johnson, who has been working upon Wisconsin to¬ 
bacco problems for over a decade, was stimulated by the early 
results with cabbage yellows to attempt critical analysis of the 
relation of the various environmental factors including soil tem¬ 
perature to the development of root rot. In this work he had 
the cooperation of R. E. Hartman, a graduate student assistant. 
The results of these studies, which they have recently published, 
show conclusively that while soil moisture and other factors play 
a part, soil temperature remains the deciding factor, in most 
cases clearly determining the severity of the root rot. Trials 
made in the Wisconsin soil temperature tanks have given uni¬ 
form and convincing evidence as follows: At the lower tem- 
29—S. A. L. 
