Jones—Relation of Soil Temperature to Disease in Plants. 445 
In this respect again the temperature range for development of 
the disease is well correlated with that for the growth of the 
parasite. In other words, the disease behaves about as one would 
anticipate, having knowledge of the temperature ranges of host 
and parasite, since, although they have their optimum at ap¬ 
proximately the same point, the tomato has a somewhat wider 
range of temperature tolerance than has its parasite. 
The Potato Stem Canker 
There is no plant of which the diseases have been more in¬ 
tensively studied in the last generation than the potato. This is 
partly because it is of such widespread economic interest and 
partly because potato disease problems are so serious. Yet upon 
some of the most common and obvious of these there have been 
for years differences of judgment as to the nature and cause. 
This has been especially the case with the cankering of the basal 
stem and the scurfing of tubers associated with the fungus Rhizoc- 
tonia, which in its perfect stage is known as Corticium vagum. 
This fungus develops conspicuous resting masses (sclerotia) on 
the surface of the tubers in late autumn, aptly described as the 
“dirt that won’t wash off” (Pi. XXXIY, fig. 1). 
These sclerotia are, however, purely superficial. Examination 
of the surface of the freshly dug tuber under a magnifying glass 
shows that they arise from the massed growth of the minute 
thread-like strands (hyphae) of the fungus, which ramify in 
abundance over the outside of the tuber, but here again always 
strictly on the surface and ordinarily causing no apparent harm 
to the host. All pathologists are agreed that with such tubers 
the Rhizoctonia exists practically as an epiphyte like the moss 
or lichens on the sound bark of a tree. If tubers bearing such 
sclerotia are planted, the pathological problem begins. The rest¬ 
ing fungous mass promptly originates an abundant new growth 
which creeps over the surface of the tender young potato sprouts 
and often finds an especially favorable nidus in the reentrant 
angle of the apex of this shoot, which is recurved, or hook-like, 
before it breaks through the surface of the soil. The complica¬ 
tions for the pathologist arise from the fact that, whereas in some 
cases the Rhizoctonia apparently behaves as a cankering parasite 
(Pl. XXXIV, fig. 2), killing off so many of the shoots before they 
emerge as to cause a serious loss of stand in the potato field 
