DAMPING-OFF IN FOREST NURSERIES. 87 
(3) The most serious losses in conifers are ordinarily from the 
root-rot type of damping-off, occurring soon after the seedlings 
appear above ground and while the hypocotyls are still soft. Losses 
due to the killing of dormant or sprouting seed by parasites before 
the seedlings appear above the soil are also frequently serious, some- 
times necessarily more so than the later types, as in extreme cases 
more than half of the seed or young seedlings are destroyed in this 
way. Damping-off due to infections of parts above the soil surface 
is serious only under extremely moist atmospheric conditions. The 
late type of damping-off, in which the roots are rotted after the 
stem becomes too rigid to be easily decayed, is ordinarily less im- 
portant than the early types. Seedlings more than 2 months old are 
ordinarily able to recover from infections by the damping-off fungi. 
Even after the first month, seedlings with part of their root system 
killed often recover. 
(4) It is possible that damping-off has a certain value as a selec- 
tive agent by eliminating weak individuals in the seed-bed stage and 
allowing only the best trees to go into forest plantations. This 
value, however, is believed to be slight. Disinfectant treatments of 
seed beds, even when controlling early parasitic losses very well, 
allow a considerable percentage of disease during the last part of 
the damping-off period, often as much as occurs at the same stage of 
development in untreated beds. As it is only this late damping-off 
in which differences in individual resistance of the seedlings seem to 
be of importance in determining whether or not they succumb, it is 
believed that whatever selective value the disease may have will 
appear in a larger proportion of the damping-off in the treated than 
in the untreated beds. 
(5) Of the different conifers, reports are available as to the sus- 
ceptibility of 63 species. Species which are especially susceptible 
at some nurseries may prove more resistant than the average at 
others. Pinus resinosa, which is especially subject to loss at some 
nurseries, is believed to be so because its growth at these nurseries is 
slow and its period of susceptibility is therefore especially long. 
In its early stages it does not seem especially susceptible. Repre- 
sentatives of all the commonly grown genera of the Abietoidese have 
been reported by one observer or another as decidedly susceptible. 
The reports on junipers and other members of the Cupressoidese, on 
the other hand, have indicated a considerable amount of group re- 
sistance to damping-off. 
(6) The best control method appears to be the disinfectant treat- 
ment of the seed-bed soil before or immediately after seed is sown. 
Sulphuric acid has been found very useful for conifers, as they are 
apparently especially tolerant of acid treatment. No method has yet 
been worked out to a point at which all of its details are entirely 
