May 10,1924 Effect of Soil Temperature and Soil Moisture on Clubroot 553 
ranging between 15° and 20° C. The variety Copenhagen Market was used in 
the first series and Wisconsin Hollander in all subsequent series. Both varieties 
appeared equally susceptible to clubroot. Plants were selected as nearly equal 
in size and general vigor as possible for all cans of each series. The soil was 
carefully removed from the roots and all long roots were broken off to a length 
of 2 inches before transplanting into the diseased soil. Four plants were set 
in each can and the soil pressed firmly about the roots. The experiments were 
all conducted in the same greenhouse, and each series was arranged so that the 
light, air, temperature, and humidity would be as nearly equal as possible for 
all plants. 
The cans used for the soil temperature series were 6 inches in diameter and 
10 inches deep. In all the temperature series the soil in half the number of 
cans was adjusted to a high moisture content, and the remainder was kept at a 
low moisture content. The water in the wet soil was added after transplanting 
to avoid danger of “ puddling’ 7 the soil around the roots at the time of transplant¬ 
ing. In the experiment in which the effect of soil moisture alone was studied, 
cans 5 inches by 5 inches were used. The procedure of preparing the soil, filling, 
and planting was essentially the same as that used for the temperature studies 
except that a circular glass plate 2 inches in diameter was used in place of the 
inverted pot. Water was added to these cans twice daily to avoid the neces¬ 
sity of adding very large amounts at one time. 
The plants were grown in the cans of infested soil for periods of three to eight 
weeks, depending on the age when planted and on the rate of growth. In general, 
they were grown until the plants in some of the cans were severely injured by 
the disease and had become badly wilted. In removing the plants care was 
taken to preserve intact as many of the roots as possible. However, since the 
amount of naturally infested soil available was limited, it was necessary to 
sacrifice part of the smaller roots to avoid losing too much soil. Since in all 
cases infection was most severe on or near the stem or main roots, the loss of 
the smaller roots did not materially diminish the value of the results. 
SOIL TEMPERATURE IN RELATION TO CLUBROOT INFECTION 
For the soil temperature experiments the cans were placed in 11 Wisconsin soil 
temperature tanks” which have been described fully elsewhere (11). By this 
method the aerial parts of all plants in the same experimental series were exposed 
to like atmospheric conditions while the soil temperatures for the various plants 
were varied according to the needs of the experiment. The soil moisture was 
also varied with the experimental culture cans. 
Series I: In the first series four tanks were maintained at 10°, 18°, 24°, 
and 30° C., respectively. Plants of commercial Copenhagen Market variety 
were used. Since the soil contained the organism of the yellows disease, Fusarium 
conglutinans , which is stimulated by high temperatures, most of the plants at 
the higher temperatures were badly injured by the latter disease. After three 
weeks they were removed and discarded since the presence of the two diseases 
rendered confusing results. However, since severe clubbing occurred on all 
plants grown at 18° C., while none were affected at 10°, the next series was 
maintained at temperatures of 9°, 12°, 15°, and 18° C. in order to determine 
the lowest temperature at which the disease would occur. 
Series II: In this series two cans each of A and B soils were used for each 
temperature. The soil in one can was kept at a water content of approximately 
35 per cent, and the soil in the other at 64 per cent of its water-holding capacity. 
After a period of one month the plants were removed and the roots examined. 
It was found that at the three higher temperatures uniform clubbing developed 
