550 
Journal of Agricultural Research voi. xxvm, No. 6 
piration at a time when the roots could not supply sufficient moisture because 
of the distortions due to the presence of the organism in their tissues. In inter¬ 
preting this evidence it should be kept in mind that the parasite probably made 
its entry early in the host’s development when the soil was relatively cool and 
moist, and that the disease had progressed to an advanced stage before the dry, 
hot period of midsummer. Halstead (7, 8 ) after noting the greater destructive¬ 
ness of clubroot on turnips during a hot season comments as follows: 
The extreme heat of midsummer, although detrimental to the normal development of turnips, seemed 
to have an opposite effect upon the parasite preying upon their roots. When harvested, the turnips 
were found to be distorted to a much greater degree than any grown upon the same land since the ex¬ 
periment began. 
These observations, although directing attention to temperature, were based 
upon field plots where the comparisons were between different seasons in which 
other factors may have varied greatly. 
Chupp ( 5 ) studied the earlier stages of the disease under controlled conditions 
and from his observations concluded that temperature might play an important 
part in spore germination and infection. In discussing his results on spore ger¬ 
mination he states that— 
Temperature conditions also influence germination of the spores. It was practically impossible to ob¬ 
tain infection in the greenhouse during the coldest winter months when the temperature was from 10° 
to 18° C. The‘spores also fail to germinate at ordinary room temperatures (from 16° to 21° C.) The 
optimum temperature for germination proved to be from 27° to 30° C. This, however, is not the case 
when spores are placed in test tubes on agar with young cabbage seedlings, for under such conditions 
infection takes place at a. temperature of from 16° to 21° C. The presence of the host seems in some 
manner to exert an influence which to a certain extent takes the place of that offered by a greater 
amount of heat. 
Chupp’s results indicate that studies upon the germination of spores apart 
from the plant will not necessarily apply to conditions as they exist in the soil. 
The introduction of cabbage seedlings indicates that his first results were influ¬ 
enced more by some unfavorable condition of his germinating medium than by 
temperature. This seems to be the only case in which temperature has been 
definitely designated as a limiting factor in the development of clubroot. 
The influence of soil moisture upon clubroot development has been given 
more attention by previous workers than has temperature. However, in these 
cases no one has clearly differentiated the possible influence of moisture from 
the other variable factors such as soil reaction, aeration, or humus content. 
Chupp ( 5 ), Reed ( 18 ), and Ravn ( 14 ) cite cases where the disease was more 
severe in the low areas of the field, and this accords with observations in southern 
Wisconsin. Drainage of such areas was recommended by Ravn ( 13 ) who cited 
cases where the disease had been checked by proper drainage alone. Anderson 
( 1 ) reports a similar case of control by drainage, although in some instances he 
found the land became more subject to the disease after being drained. Hal- 
sted (7, 8 ) observed that watering increased the virulence of the disease during 
a dry season. Whitehead ( 20 ), Reed ( 18 ), Cunningham ( 6 ) and Ravn ( 15 ) 
noted that the disease was more severe after periods of wet weather. Hum¬ 
phrey ( 10 ) working in Massachusetts found the disease worse on the heavy 
and moist soils, while Reed ( 18 ) in Virginia notes that— 
In soils which contain a large amount of organic matter (humus) and are warm and moist, the organ¬ 
isms find conditions very suitable for their growth. 
Ravn, who, with his associates, conducted field experiments in Denmark for 
many years, gave more critical attention to the possible influence of soil moisture 
on the disease than any previous worker had. However, his conclusions were all 
