May 10,1924 Effect of Soil Temperature and Soil Moisture on Clubroot 559 
In the temperature series using the Wisconsin tank method with soil tempera¬ 
tures, ranging by gradation from 6° to 35° C. clubroot developed throughout the 
entire range except at the extremes, that is, from 9° to 30° C. Within this range, 
9° to 30° C., the effect of temperature upon the clubbing seemed directly corre¬ 
lated with its influence on the growth of the host tissues. The most severe injury 
to the plant occurs at a temperature somewhat above that for largest club pro¬ 
duction, wilting occurring sooner and dwarfing of the plant being more notice¬ 
able at 25 than at 20° C. The conclusion reached therefore is that temperature 
is probably not a direct limiting factor in clubroot development under field 
conditions. 
On the other hand soil moisture was found to be an important factor. The 
trials included a soil-moisture rajige graduated from 30 per cent of the water¬ 
holding capacity of the soil to full saturation. The disease developed on plants 
grown in soil maintained at a moisture content of 60 per cent of the water- 
holding capacity and all higher moistures but did not develop in soil kept at 45 
per cent or less of capacity. Injury to the plants increased with an increase in 
soil moisture above 60 per cent. Plants, however, are able to develop well 
(although not at their best capacity) with a soil moisture of only 45 per cent of 
capacity. 
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. This may be due to an absence of enough water to provide a film around 
the spore, a condition which appears (5) to be necessary for spore germination. 
Prolonged periods of drought may even destroy the organism in the soil. 
An important factor under conditions of excessive moisture is that of secondary 
decay. 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 destroyed by secondary decay organisms 
and the plant dies. 
Under natural field conditions a long period during which the soil is so dry that 
no clubbing, or only small clubs, would develop, may give the plants an opportu¬ 
nity to produce a good crop regardless of short wet periods. Such periods of low 
soil moisture occur during many seasons in most of our cabbage-growing sections 
and this no doubt explains the difference in severity of the disease in the same 
field in different seasons. 
On comparing previous literature on clubroot with the evidence here presented 
as to the influence of soil temperature and soil moisture it seems probable that a 
consideration of these factors, and especially of soil moisture, may help to ex¬ 
plain the conflicting results secured concerning the use of lime and the relation 
of soil acidity to the clubroot disease. 
LITERATURE CITED 
(1) Anderson, A. 
1855. REPORT ON THE DISEASE OF FINGER AND TOE IN TURNIPS. In Trans. 
Highland and Agr. Soc. Scotland, ser. 3, v. 6, p. 118-140. 
(2) Briggs, L. J., and McLane, J. W. 
1907. THE MOISTURE EQUIVALENTS OF SOILS. U. S. Dept. Agr. Bur. 
Soil Bui. 45, 23 p., 1 fig., 1 pi. 
(3) -and Shantz, H. L. 
1912. THE WILTING COEFFICIENT FOR DIFFERENT PLANTS AND ITS IN¬ 
DIRECT determination. U. S. Dept. Agr. Bur. Plant Indus. 
Bui. 230, 83 p., 9 fig., 2 pi. 
