DAMPING-OFF IN FOREST NURSERIES. 73 
pots, as there were still present a number of soft-stemmed seedlings 
from seeds which were slow in germinating. These younger seed- 
lings were excluded in counting the dead, the rule being to include 
only plants which had developed a sufficiently rigid stem to remain 
upright after death. Comparison of the percentage of killed with 
the total percentage attacked for the two pines is rather interesting. 
As has already been pointed out, while Pinus resinosa suffers very 
heavy damping-off losses at a number of nurseries it seems to be 
less susceptible than some other species to parasitic injury during 
the sprouting period, before the seedlings appear above the soil sur- 
face. Observation of beds of this species during different seasons 
has indicated that it has not a greater susceptibility, but rather the 
fact that its susceptibility lasts longer, which causes it to suffer as 
seriously as it does at certain nurseries. It is indicated in Table X 
that the succulent root tips of Pinus ponderosa are just as easily 
attacked by damping-off parasites as those of P. resinosa — in fact, 
considerably more easily attacked, as indicated by the figures in col- 
umn 8. With the P. ponderosa seedlings, however, the older parts 
of the roots had become resistant at this age in nearly all cases, while 
of the affected P. resinosa seedlings more than one-third were still 
unable to limit the lesions, and death resulted. 
In general, this experiment indicates that Corticium vagum and 
Pythium debaryanum are able to cause the death of some pine seed- 
lings which have developed rigid stems and that both are also able, 
as has been found by other workers in the case of sugar beets, to 
cause " root sickness," the rot of the younger portions of, the root 
systems, in seedlings which have developed too much resistance to 
be killed. The evidence for the parasitism of the two Fusarium 
species on these older root systems is not so good; as in the experi- 
ments on younger seedlings, their ability to attack the pines is prob- 
ably less than that of the other two fungi. Further inoculation ex- 
periments are desirable both with these fungi and with others on the 
roots of seedlings too old to succumb to the more ordinary types of 
damping-off. 
RELATION OF ENVIRONMENTAL FACTORS TO DAMPING-OFF. 
- In the earlier section dealing with disease control, mention was 
made of the. general belief on the part of men who have had experi- 
ence with seedling diseases that damping-off is favored by thick seed- 
ing, by much organic matter, especially by poorly rotted manure in 
the soil, and by excessive moisture in the air and soil. It is also 
commonly stated that high temperature favors the disease ; on this 
point there is perhaps a less general agreement. Practically all the 
evidence on these points is observational. 
