May 10,1924 Effect of Soil Temperature and Soil Moisture on Clubroot 557 
is apparently the same throughout the range of temperatures at which the disease 
is capable of development. It is noteworthy that whereas in the case of tempera¬ 
ture the influence upon disease development was correlated with that upon host 
growth, by contrast in the case of moisture this does not obtain. In soils that are 
saturated with water growth of cabbages is practically stopped. The stimulating 
influence can be explained partly by the fact that excessive soil moisture favors 
the activities of the parasite and partly by the fact that it increases the damage 
due to secondary decay of the clubs. The disease may develop on the roots 
without severely checking the growth of the plant when the soil moisture is 
limited, but when the soil moisture is excessive the diseased roots are soon de¬ 
stroyed by secondary decay organisms and the plant dies. 
The host tissue in tops and roots of plants grown in soil with this low water 
content appears to develop normally, although perhaps not quite as vigorously as 
that in soil with more available moisture. Some plants grown in infested soil at 
the low moisture content, on which the disease did not appear at the conclusion 
of the experiment, were grown in nutrient solutions for a period of three weeks 
longer after having the soil carefully washed from their roots. On these plant# no 
galls developed whereas similar plants on which galls had started while growing in 
the wetter soil continued to develop the clubs after being placed in the nutrient 
solution. Therefore, it seems likely that the disease was avoided by preventing 
penetration of the organism rather than by checking the growth of the gall. 
After attempting to germinate spores on the surface of agar, Chupp (5) reports 
that “unless the spores were immersed in water there was no development .’* 
It is probable that failure of the disease to develop on plants grown in infested 
soils with low moisture content is due to insufficient moisture for spore germina¬ 
tion. It has been observed that extended desiccation will check the viability 
of spores and prolonged periods of drought may destroy the organism in the soil. 
RELATION OF TEMPERATURE TO FIELD CONDITIONS 
The indications are that under field conditions temperature is not a factor 
limiting the appearance of the disease, although it may exert an important 
influence on the severity of the disease if other factors are favorable. 
Although the water content of the soil below which clubroot will not develop 
is insufficient for the most vigorous development of cabbage, it nevertheless 
provides moisture adequate for moderate growth. Under field conditions it is 
probable that there is frequently not sufficient soil moisture in the upper layer of 
soil to favor infection, and healthy plants at this time probably depend more on 
the deeper roots for their supply of water. 
With these results in view it is interesting to note experiments conducted by 
Cunningham (6) in Vermont on the effect of “hilling up” cabbage plants as a 
method of controlling clubroot. He reports a method suggested by a Vermont 
grower, which consisted of hilling up the soil around the diseased plants to 
induce them to send out adventitious roots above the clubbed region. In 
explanation of the process, Cunningham notes that “The adventitious roots 
formed as a result of this practice on the stem above the diseased root are rel¬ 
atively disease free and function in the place of the affected roots which are 
practically useless.” Further, he notes that “any procedure which induces 
club-free roots to form above the diseased roots greatly lessens the damage 
caused by the club.” He conducted experiments during the course of two years 
in order to observe the effect of this procedure. “Decidedly beneficial results 
were secured in 1912, but in 1913 a decided injury was done.” He, therefore, 
concludes that “hilling is most likely to prove helpful when the disease is most 
severe.” 
