Apr. 7,1933 
Influence of Soil Temperature on Fusarium in Cabbage 7 7 
the percentage of disease. Plantings made after the latter date showed 
a rapid decrease in percentage of disease until about September 15, after 
which the disease did not develop. It may also be observed (fig. 7 and 8) 
that this rise and fall in the percentage of disease correlates in general 
with the soil-temperature curve. The disease did not appear in the 
early part of the season until the soil temperature had remained above 
17^ C. for several days, and it ceased to develop in the latter part of the 
summer as soon as the temperature dropped below this point. By com¬ 
paring the temperature curves in figures. 7 and 8, it may be observed 
that file soil did not warm up as early in 1917 as in 1919 and that the 
development of yellows was deferred in a corresponding manner. 
During the hot, dry period, which included the greater part of July 
and the first half of August, the disease developed as quickly as, and 
even more destructively lian, at the high temperatures in the greenhouse. 
This result was probably due to the combined influences of high soil 
temperature and low soil moisture, each of which was found to favor 
rapid development of the disease under greenhouse conditions. On the 
other hand, the disease was found to develop more slowly in the field 
at the lower temperatures than in the greenhouse at constant similarly 
low temperatures. This difference was probably due to the effect of 
the wide daily fluctuation of the soil temperature upon the fungus under 
field conditions—that is, the short period of higher temperature might 
have been insufficient for the fungus to recover from the effect of the 
longer duration of low temperature. Often when the mean tempera¬ 
ture was about 20° C. the range was probably below 20° all night and a 
large portion of the day, while it remained above the mean only a few 
hours. 
In several instances, during both 1917 and 1919, the plants were left 
in the beds for the greater part of the season and observations were 
made upon them from time to time. By this method it was learned 
that most of the Wisconsin Hollander plants which were healthy at the 
end of the first 30 days remained so fiu*oughout the experiment. The 
number of plants thus escaping the disease was greater when they were 
started in May than when started in July or in August when the soil 
was hot and dry. The Wisconsin Holknder plants which were started 
on May 15, 1917, showed no yellows at the end of the first 33 days, and 
only 15 per cent 62 days after planting. There was no increase in 
percentage of disease between July 18 and July 26. Conversely, the 
plants which were started on July 14, 1917, showed 96 per cent of yellows 
on August 4, 21 days after planting. Sixteen days later the disease had 
increased to 98 per cent. These data confirm the results obtained in 
the greenhouse, that plants which were permitted to establish them¬ 
selves before being exposed to the Fusarium at a temperature favorable 
for its attack were more resistant than plants started in “sick” soil 
at the optimum temperature for the fungus. In other words, the plants 
became more resistant with age. 
