Jones—Relation of Soil Temperature to Disease in Plants. 453 
the disease coincides with the optimum for the development of the 
fungus. The stimulating effect of the favorable temperature upon 
the parasite seems to be the determining factor with tomato wilt. 
In the case of the Rhizoctonia canker of the potato, a disease which 
is worse at the lower temperature, the condition has been inter¬ 
preted as resulting from the fact that the lower temperature so re¬ 
tards the development of the young potato sprouts as to give the 
parasite a longer period for its destructive underground action on 
the young shoot tips, whereas at the higher temperatures the rapid¬ 
ity of their growth enables them to escape relatively unharmed. 
The retarding effect of the low temperature upon the host seems to 
be the most important factor in determining the outcome with 
potato stem canker. 
Let us now recall the results with the tobacco root rot. Here, 
as with the potato canker, the disease is worse at the lower tem¬ 
perature and is checked at the higher. When we seek the explana¬ 
tion, however, the facts do not accord with those in either of the 
two previous cases. In the first place, the tobacco fungus in pure 
culture trials is found to behave almost exactly like the Fusarium, 
growing distinctly faster at 25°C. or above and lessening in growth 
rate as the temperature falls. The worst development of the 
disease occurs, therefore, at a temperature which seems less favor¬ 
able for the parasite, whereas the disease disappears at the soil 
temperature most favoring growth of the fungus. If now we turn 
to the temperature relations of the tobacco root (PI. XXXVI, fig. 
2), the case becomes the more puzzling since the most vigorous 
root development is found not at the higher temperature where the 
root growth is more rapid. Plainly here some explanation must 
be sought other than the direct influence of temperature upon the 
relative vigor or growth rate of either parasite or host. It is, of 
course, easy to make conjectures involving other possible variable 
factors which may be influenced by temperature, such as enzym 
or toxin development by the parasite or some products or activities 
of the host tissue which may contribute to its resistance. We 
have already referred to the interesting suggestion by Balls of the 
possibility of self-inhibition of the parasitic Rhizoctonia through 
auto-intoxication at certain temperatures. However one may in¬ 
cline toward one or another such possible theory, the truth of the 
matter is that we do not in any case know enough as yet about the 
intimate relations of parasite and host to justify the formulation 
