May 10,1924 Effect of Soil Temperature and Soil Moisture on Clubroot 555 
Series V. In order that a comparison of the temperature influence over the 
whole range might be obtained in a single series, the lot of soil A which was free 
from Fusarium conglutinans was mixed with one-fourth its volume of quartz 
sand to make enough for 16 of the 6-inch cans. These cans were planted with 
plants more than two months old which had been somewhat checked in growth 
by being grown in small pots. The moisture was kepfr at 75 per cent of the 
water-holding capacity, and mineral wool insulation was used on the surface of 
the soil. Duplicate cans were placed in tanks kept at 6°, 9°, 12°, 15°, 20°, 25°, 
30°, and 35° C. Wilting due to clubroot first occurred at 25°, and soon after¬ 
wards at 20° C. The growth of plants in these two temperatures was checked 
completely by the time the plants were removed. But plants at 15° and 30° 
seemingly were not checked in any way. Growth was very slight in the 35° 
tank. Failure to maintain the original low temperatures concluded the experi¬ 
ment before the disease had time to develop to the extremes which it might have 
attained in a more prolonged period. The plants were so large when transplanted 
that the relative growth of the tops did not indicate the actual effect of the dis¬ 
ease and therefore were omitted from the photograph. The roots gave the best 
indication of the effect of the various temperatures upon development of the 
disease. 'Plate 4 shows two representative root systems taken from each tem¬ 
perature. The greatest development of clubs occurred at 20° and 25° C. There 
was also some clubbing at 15° ajid 30°, but none at the highest temperature 35° 
nor at the lower end of the series, 6°, 9°, and 12°. The results of this experiment 
are thus in accord with previous results. 
The interpretation of the above results must take into account the effect of 
temperature on the host as well as on the organism. While temperature may 
limit infection by its direct effect upon the organism, it is obvious that with such 
a disease as clubroot its severity, as estimated by size of clubs, will depend to a 
great extent on the growth of the host immediately after infection. Tisdale (19) 
in his studies of the relation of soil temperature to the development of cabbage 
roots and tops has shown that the greatest root development occurs at about 
20° C., with decided reduction above 25°; the present results are in accord with 
his findings. With regard to infection it appears from these results that club¬ 
root infection will occur over nearly as wide a range as that in which growth of 
cabbage occurs. At the higher points the limits seem nearly to coincide. At the 
lower end of the range there seems to be a point (below 9° C.) at which infection 
does not occur and the host does grow very slowly. Between these two points 
(35° C. and 9° C.) there is a great difference in the severity of the disease. The 
largest clubs occurred at 20° C. which is close to the optimum temperature for 
root development. It may, therefore, be said that clubroot infection may occur 
at any temperature which is favorable for growth of cabbage and that the disease 
develops most rapidly and becomes most severe at about the optimum tempera¬ 
ture for root development of this particular host plant. 
In the individual series all plants were selected for uniformity of size and 
vigor; all were of the same age. However, there was a difference in age, size, 
and vigor between the different series. In general, it was observed that infection 
was less severe and development of clubs was slower in the plants that were 
older and less vigorous. Plants that had been temporarily checked before 
transplanting into the infested soil were slower in starting a vigorous growth in 
the new soil, and the disease was correspondingly delayed and less severe. This 
slow growth may explain some of the results of earlier attempts to produce 
the disease on old plants which led to the belief that infection occurred only on 
plants at the seedling stage. Kunkel (12) concluded that “old plants are almost 
as susceptible as young ones, provided they are growing.” Although no plants 
