554 
Journal of Agricultural Research 
Vol. XXVIII, No. 6 
on all plants in soil with the higher moisture content, but that only scattered 
infection occurred in soils with the lower moisture content. No case of clubbing 
was found in the soil kept at 9° C., even at the higher moisture. 
Series III: Since the plants grown in soil maintained £Ct 12° C. showed 
severe clubbing, only temperatures below 16° were included in later series with 
these soils. By doing this the soil was kept below the temperature range for 
infection by the Fusarium. Therefore, the third series was run in tanks main¬ 
tained at 6°, 9°, 12°, and 15° C. As soil moisture appeared to be exerting 
a marked influence on the disease, the range in water content was increased. 
Soil A was maintained at 75 per cent and 30 per cent while soil B was maintained 
at 90 per cent and 30 per cent of the water-holding capacity. Since the plants 
were transplanted when considerably smaller than those used in previous experi¬ 
ments, they were allowed to grow for a longer period, approximately eight 
weeks. Upon examination of the roots it was found that in both soils with 
high moisture content all the plants were badly clubbed at 12° and 15° C. No 
cases of infection were found in either soil at 6° C. At 9° the plants grown in 
soil A showed no evidence of infection but each of the plants in the corresponding 
can of soil B in the same tank showed slight clubbing (Plate 3). Plate 2 shows 
the effect of these conditions on plants grown in clean soil. In no case of low 
soil moisture (either A or B) was there any evidence of clubbing at any tem¬ 
perature. Plate 1 shows one representative plant from each can of soil A at 
the higher moisture content. 
Series IV: In order to determine the influence of higher temperatures, 
without confusion with yellows disease, four lots of soil were secured infested 
with Plasmodiophora brassicae but free from Fusarium conglutinans. One 
sample was obtained from the same garden as soil A and kept free from con¬ 
tamination with the Fusarium in the greenhouse. The other three were from 
Vermont, New York, and Minnesota, respectively, 3 from fields where clubroot 
was known to occur, but where yellows had not been observed. These soils 
were used for planting in tanks at 20°, 25°, 30°, and 35° C. The plants used in 
this series were larger than those in the previous temperature series and the 
period of growth in the infested soil was shorter. 
Soil A was used at a water content of 30 and 75 per cent as in the lower tem¬ 
perature series. No clubbing occurred at any temperature in the soil maintained 
at 30 per cent of the water-holding capacity. In the soil maintained at 75 per 
cent of the water-holding capacity severe clubbing occurred at 20° C. but there 
appeared to be a reduction in the amount and severity of the disease as the tem¬ 
perature rose. Since there was a tendency for the soil to become dry more 
quickly at the higher temperatures due to the fact that the cans used for this 
series were smaller, and the plants were larger, it is possible that these conditions 
contributed to the results. The soils from Vermont, Minnesota, and New 
York were used in small cans (5 inches in diameter and 5 inches in depth) and 
kept at a moisture content of about 75 per ceht of the water-holding capacity. 
The large plants, together with the limited amounts of water in these small 
volumes of soil and the fact that no insulation was used, made it so difficult to 
hold the moisture constant, especially at the higher temperature, that the results 
were not considered final. However, they were uniform in showing that infec¬ 
tion was most severe at 20° C. and decreased as the temperature rose. Clubbing 
in all parts of the root system occurred at all these higher temperatures except 
35°, in which case it occurred only on the main stem at the surface of the soil 
where its contact with the air may have resulted in a somewhat lower temperature. 
3 These were obtained through the courtesy of B. F. Lutman, Chas. Chupp, and E. C. Stakman, 
respectively. 
