Jones-—Relation of Soil Temperature to Disease in Plants. 447 
conditions. Time did not permit Dr. Richards to make a complete 
determination of these conditions, including the possible rela¬ 
tions of soil moisture, aeration, and other variable factors aside 
from temperature. He did, however, secure fully convincing 
evidence that soil temperature has such a highly important influ¬ 
ence that under most conditions it would seem to be the domi¬ 
nant thing in determining the pathogenic results. The details 
must be left for Dr. Richards’ full account, but the convincing 
nature of this evidence may be judged from two examples. 
Potatoes were grown experimentally in a series of like pots 
of soil which were held at soil temperatures graduated from 9°C. 
to 30°C., the air temperature being for all alike about 18°-22°C. 
It was found that if fungus-free seed potatoes were used, the 
plants grew at all these temperatures except the very highest, 
but the best normal development was at about 18°C. At lower 
soil temperatures their growth was distinctly retarded, and at 
higher, although the aerial stem elongation was more rapid, the 
leaves were smaller and abnormal and tended to roll. In case 
the seed tubers bore sclerotia the fungus developed to some de¬ 
gree at all temperatures, but the serious stem cankering oc¬ 
curred at 15°-21°C. with a distinct and rapid lessening of the 
diseased condition at higher temperatures. 
Evidence of the same import was also secured by Dr. Rich¬ 
ards in a second way. This was by so varying the date of 
planting as to expose the potato plants to different soil tempera¬ 
tures during the early stages of development. The first plant¬ 
ing was made April 26 and later ones were made in May and 
June, the soil being uniformly inoculated in all trials. Fortu¬ 
nately for the purposes of this trial the surface soil temperature 
remained cool (most of the time below 15 °C. at a depth of 4 
inches) until about May 25, when the weather turned suddenly 
warmer and the soil temperature at once rose several degrees, 
mostly to 22° and above in the upper four inches, and remained 
uniformly warmer thereafter. The outcome as to type and de¬ 
gree of injury to the young sprouts was marked and convinc¬ 
ing. The tips of the primary shoots of the early planted series 
growing in cold soil were badly attacked by the Rhizoctonia and 
in many cases they were killed below ground. On the other 
hand, with the late planted tubers the sprouts developing at 
higher soil temperatures were almost free from this destruction 
of growing point. Even more striking, however, was the fact 
