556 
Journal of Agricultural Research v 0 i. xxviii. No. 6 
that could properly be called old were used in this work, it was noted that check¬ 
ing growth in general delayed development of the clubs. From the nature of 
the development of the organism within the host this dependence on host develop¬ 
ment seenjis probable. 
SOIL MOISTURE IN RELATION TO CLUBROOT INFECTION 
In a preliminary test, using ordinary flower pots placed on a greenhouse bench 
and watered with a sprinkling can, there was a wide variation in amount of 
clubbing in the different pots, indicating the influence of soil moisture since other 
factors were equal for all pots. 
Series I. In the more carefully conducted temperature series, however, watering 
was done with greater care, using 6-inch cans maintained at high and low soil 
moistures, as described in Series II and IV above. All of the plants grown in soil 
with the low water content remained entirely free from disease, while all of those 
grown in the same soil with the higher water content became diseased within the 
favorable temperature range. Plate 1 shows a representative plant chosen from 
each can of the higher moisture series in soil A. Plate 3 represents the corres¬ 
ponding series in soil B. An examination of Plates 2 and 3 will show some 
difference in plant development due to difference in water content in the cans of 
soil where they were grown. Undoubtedly the plants grow more vigorously in 
soil with the higher moisture, provided no clubbing occurs to check growth. 
However, it is seen that in soil containing insufficient moisture for disease 
development the plants could-obtain enough water for considerable growth. 
All plants were of equal size when planted. Those grown at 6°, especially those 
in the low-moisture cans, made little growth and may be used for comparison 
to indicate the relative amount of growth in cans with different known moist¬ 
ure content. 
Series II. To test further the influence of soil moisture on clubroot a series was 
prepared with cans containing an amount of water equal to 30, 45, 60, 75, and 
105 per cent of the water-holding capacity of the soil in the can. Plants in the 
soil containing only 30 per cent of the water-holding capacity were not protected 
for a long enough period to enable them to become established; as a result they 
wilted and died. Those in the soil containing water to the extent of 45 per cent 
of its water-holding capacity had little difficulty in becoming established and grew 
well. The plants in the two cans with higher water content wilted badly during 
the day for some time before they were taken out. When they were removed, it 
was found that clubs had developed on all the plants except those in the 45-per 
cent can. The roots of the plants in the 105-per cent cans were almost completely 
decayed. Those in the 90-per cent can had become so soft that they could not be 
taken out of the soil without breaking, but the others were still firm. Plate 5 
shows one plant from each can and illustrates well the dwarfing of the plants at 
the higher moisture content and the relative clubbing in each. 
From the results of these investigations it seems that the disease develops only 
occasionally, if at all, in soils with a water supply less than half of the water¬ 
holding capacity. This limit is, no doubt, somewhat higher or lower in soils 
rich or poor in organic matter. With amounts of water above this minimum 
there is apparently a direct ratio between severity of the disease and increase in 
water content. At the higher moisture content it is probable that the severity 
of the disease is not limited to development of the clubs but is also due to a 
tendency of the clubbed roots to succumb more promptly consequent to their 
invasion by secondary organism. The optimum water content for development 
of cabbage plants is in general about 75 per cent of the water-holding capacity 
of the soil. This influence of excessive moisture in stimulating the clubbing 
