Mar., 1923] CLAYTON — SOIL MOISTURE AND FUSARIUM WILT 
143 
Thus the disease could be brought on by making the plants from the 
saturated soil (35 percent moisture) dry, or the plants from the dry soil 
(13-20 percent) moist. The reduction in amount of disease with dry soil 
and saturated soil appears in each case to be due to, or at least correlated 
with, detectable differences in the host plant. The resemblance between 
the two ceases, however, at this point. 
When grown at a temperature favorable for the disease, and with dry 
soil, plants are infected and the fungus progresses slowly up the stem. 
If the temperature conditions are optimum for a sufficiently long period, 
these plants are killed by the wilt. In experiments IV and V the shortest 
incubation period for plants growing with 23 to 33 percent of soil moisture 
was, in each case, 15 days; the shortest incubation period for plants grown 
in the driest soil was 35 days. In numerous instances, apparently healthy 
plants growing with 13 to 14 percent soil moisture have been cut and found 
to have vascular discoloration extending far up the stem. The fungus was 
readily isolated from such plants, and in addition it was isolated in a num¬ 
ber of instances from the tissues of plants which showed no browning of the 
bundles. These investigations have indicated that a very large majority 
of the low-moisture plants which do not show external symptoms of the wilt 
are nevertheless infected. 
The presence of such infection can be readily shown in another manner. 
The temperature conditions were such that, with a soil moisture of 25 to 30 
percent, the disease made its appearance in about 15 days. At the con¬ 
clusion of experiment V, as already noted, four dry-soil plants which showed 
no disease were made moist; all four showed the disease in 10 days. Since 
it had been definitely proven that under the given conditions 15 days was 
the least possible time in which the fungus could infect, grow up the stem, 
and produce visible wilt, the only explanation for this result in 10 days is 
that infection had already occurred during the long period of weeks when 
the plants were growing in very dry soil, but at a temperature optimum for 
the disease. 
Since the dry-soil plants are readily infected, and the fungus, once 
within the host, produces the disease very slowly, it seems reasonable to 
suppose that the host as a whole is resistant and that this fact is responsible 
for the great reduction in the amount of disease. 
In the plants grown in saturated soil, infection did not occur, and the 
incubation period was not reduced by exposing the plants to a soil tem¬ 
perature between 25 0 and 30° C., for a long period with the soil saturated. 
There was no evidence that the disease would ever develop, or infection 
occur, so long as the saturated condition was maintained. For these reasons 
the dry-soil plants are characterized as resistant, while the saturated-soil 
plants are said to be immune. 
