DAMPING-OEF IN FOKEST NURSERIES. 79 
within reasonable limits, are expected to increase the speed with 
which the disease works, but these should also hasten the develop- 
ment of the host to a point at which infections are unable to cause 
death. It is the total amount of damage in the beds rather than the 
damage per unit of time which is of practical importance. For a 
number of reasons, then, the method followed in obtaining the data 
for these graphs can not give information of maximum value. - While 
data of the sort mentioned are of undoubted interest and would be 
of still more value if the records had been commenced when the first 
seedlings appeared instead of. a few days later, the relation of any 
specific factor to the total extent of the disease can be better deter- 
mined by comparing plats in series in which the factors are as far 
as possible controlled and varied one at a time. To vary soil moisture 
and soil temperature independently will prove somewhat difficult, 
but it can be done with the proper facilities. Some work with en- 
vironmental factors should be done under conditions of artificial in- 
oculation in the greenhouse, in which the different damping-off 
parasites can be experimented with separately, as it is obvious that 
the factors which favor the activity of one may not be favorable for 
another. • 
CHEMICAL FACTORS. 
Chemical factors are presumably also important, as the soil is 
in most cases the culture medium for both the parasite and the host. 
The much greater activity of Pythium debaryanum in autoclaved 
soil than in untreated soil may be due to the larger quantity of 
soluble organic matter commonly present in autoclaved soil. Pythium 
debaryanum has been found more sensitive to unfavorable substrata 
in artificial culture than Corticium vagum and is apparently more 
dependent on soil organic matter in the nurseries than is G. vagum. 
For example, in the normal humus-containing surface sand in the 
beds at Cass Lake, Minn., both Pythium and Corticium occurred 
frequently in the damped-off seedlings, while in beds a few feet 
distant, from which enough of the surface soil had been removed to 
leave no humus, nearly all the damping-off foci contained abundant 
Corticium, and no Pythium could be found. With both fungi and, 
in addition, with two species of Fusarium (68) heavy inoculation has 
been more successful in experiments at the time of sowing than 
light inoculation. This has been thought possibly due in part to the 
larger amount of nutrient substratum added in the heavy inocula- 
tions, allowing better saprophytic development of the fungus in the 
soil. In each of the two experiments with Pythium reported in 
Table XI, a 5-pot unit was treated with corn-meal infusion and 
another with prune infusion at the time of inoculation. In both 
experiments germination was lower, damping-off after germination 
higher, and the survival less than half as great in the pots with 
