Jones—Relation of Soil Temperature to Disease in Plants. 459 
PLATE XXXV 
Plants which illustrate the relation of soil temperature to the Rhizoc- 
tonia injury of potato stems. The primary shoots developing in very 
early spring, when the soil was cool (about 15°C.), were cut off by the 
fungus. A little later the soil temperature rose to about 22°C., and the 
secondary shoots then developing escaped injury. (Photographs by B. L. 
Richards.) 
PLATE XXXVI 
Normal and diseased tobacco plants showing the effects of different soil 
temperatures. (Photographs by James Johnson.) 
Fig. 1. These two plants were stunted alike during an entire season 
by disease, and were then removed from the field and placed in the 
temperature control tanks. The plant placed at a soil temperature of 
30 Oi C. started to make a rapid recovery, whereas the one at 20°C. re¬ 
mained diseased. This is in accord with the field experience that a low 
soil temperature, which is favorable for the Thielavia root-rot disease, 
if followed by a high soil temperature, enables the tobacco plant to 
recover by sending out new roots which are not affected. 
Pig. 2. Root growth of tobacco plants in soil infested with Thielavia 
basicola as compared with growth in uninfested soil, at different soil tem¬ 
peratures. Root to right, in each pair, grown in infested soil. Root to left, 
grown in same soil sterilized. Note the reduction in amount of disease as 
the soil temperature at which the roots were grown is increased, also the 
reduction in size of the normal root system. 
PLATE XXXvn 
Tobacco plants showing the effects of variation of temperature on the 
development of the root-rot disease. (Photographs by James Johnson.) 
Pig. 1. a. Tobacco plant grown at 23°iC. in Thielavia-infested soil. 
d. Plant grown at 23°C. in same soil sterilized. 
Fig. 2. a. Tobacco plant grown at 26°iC. in Thielavia-infested soil. 
h. Plant grown at 26°C. in same soil sterilized. 
The Thielavia root-rot disease starts on a rapid decline at a soil tem¬ 
perature of about 26°C., whereas at 23°C. it is near its optimum for 
development. 
