558 
Journal of Agricultural Research voi. xxviii, No. 6 
He also included observations on seasonal rainfall which aid in interpreting 
his results. He states that in 1913 the seasonal rainfall was only two-thirds of 
that of 1912, although both were below normal. The rainfall in 1912 was more 
evenly distributed and there was more cloudy weather. This was supposed to 
have stimulated the formation of adventitious roots. From this information 
we may infer that in 1912, the year in which clubroot was most severe, the soil 
was kept more moist by the even distribution of rainfall and by cloudy weather. 
Roots, - therefore, probably became diseased in the flat soil which was moist 
enough to favor infection. The hilling process would provide drainage for the 
top layer of soil and probably reduced the moisture below the point where infec¬ 
tion could occur. The adventitious roots developed in this hilled soil escaped 
infection because of the low moisture content of the soil immediately surrounding 
them. Enough moisture was present, however, so that the plants developed 
heads. The dry season of 1913 gave different results, probably because of exces¬ 
sive drying out of the soil beyond the point where roots could get enough moisture 
for growth. “The 1912 yield on some hilled plots was increased more than 
tenfold.” 
Similar development of adventitious roots was observed by the writer during 
the summer of 1920 in the Racine, Wis., district. Plants were badly clubbed in 
parts of some fields during the early growing period, and on hot days early in 
August these showed decided wilting. There was so little rainfall after July 1, 
that the soil became dry even in the lower areas. Adventitious roots, sent out 
from the stems above the clubbed areas, grew down into the soil beside the 
older, badly clubbed roots. These new roots did not become infected and in 
time were able to supply sufficient moisture to produce marketable heads; which 
were, of course, much below the average weight of heads produced on uninfected 
plants. It has been frequently observed in cabbage fields in various localities 
that when clubroot occurs regularly that it is usually most severe in the lower 
areas or depressions in a field. The continual favorable moisture conditions in 
such areas makes it possible for the organism to develop whenever wild or culti¬ 
vated crucifers grow there and so the organism becomes abundant in such soil. 
On the other hand epidemics of the disease occur on higher and well drained soils 
which may be explained by the fact that the spores of this organism are known 
to be long lived and when the soil is once infested it only needs a season of abun¬ 
dant rainfall to make the disease severe even on this type of soils. 
SUMMARY AND CONCLUSIONS 
It has long been recognized that clubroot is influenced by various environ¬ 
mental conditions. In recent years the work on this disease has been largely 
with the influence of soil acidity and the effect of liming. This has been espe¬ 
cially true with studies in America, where liming has become the principal means 
of control. However, it has been frequently observed that the disease is not 
checked by liming the soil. These results indicate that there are factors other 
than soil acidity which largely influenced the severity of the disease, so experi¬ 
ments were conducted to try to determine what these factors might be. The 
work here reported shows the effect of soil temperature and of soil moisture as 
related to the occurrence of clubroot. 
The trials were made by planting cabbage seedlings in naturally infested soil 
in the greenhouse. The air conditions were alike for all plants but the soil con¬ 
ditions were varied experimentally. The earlier trials aimed to determine the 
possible influence of different soil temperatures upon disease development, the 
later ones to determine the possible influence of different soil moistures. 
