DAMPING-OFF IN FOREST NURSERIES. 69 
most easily controlled by soil disinfection (see the bottom line in 
the last four columns of Table IX). Its poor adaptation for aerial 
dissemination would lead one to expect to find it seldom in beds 
treated with efficient disinfectants. The entire absence of Corticium 
in heated soil therefore seems somewhat significant. The rather 
high Corticium yield in the formaldehyde plats is of some interest 
in view of the reported inefficiency of formaldehyde in destroying 
Qorticium vagum on potato tubers (48, 50). As will be noted from 
the data given, more than one suspected parasite was often found in 
what appeared to be a single focus. This was probably in some 
cases due to independent foci being nearly concentric; it also in 
some cases undoubtedly means that one of the organisms found was 
only secondary. In the beet-seedling cultures by Busse and his 
associates, individual seedlings }uelded two or more potential para- 
sites in 100 of their nearly 1,300 examinations. It not infrequently 
happened in the work on pine seedlings that no fungus recognized 
as a likely parasite could be isolated. This was especially true 
in plate cultures when Ehizopus or Trichoderma happened to be 
abundant, as both are very fast growing and often suppress para- 
sites. This is an additional reason for the' development of some 
method as a dilution plate of lesion fragments for diagnosing damp- 
ing-off. 
Even an accurate and complete census of the organisms present in 
the different foci could not be directly interpreted in terms of rela- 
tive importance. None of the parasites so far used in inoculation 
have been vigorously parasitic under all conditions. Of both Corti- 
cium vagum and PytMum debaryanum some strains, microscopically 
indistinguishable from the others, are very weak as parasites. Only 
part of the Fusarium species are parasitic on pine, and data showing 
which . are and which are not parasitic are known for only a very 
few. There is therefore no fungus which can be said positively to 
be the cause of any particular damping-off " patch " simply because 
it was found in some of the dead seedlings in the patch. In an occa- 
sional exceptional case, such as the large Corticium patch in figures 
7 and 8, there is such a vigorous growth of the fungus that its pre- 
dominance is undoubted, but such cases are rather rare. A census 
throws light on the importance of the different fungi, but can be 
interpreted only in the light of inoculation results. 
For Fythium and Corticium the inoculation data do not permit 
• any simple comparison between the two, for the reason that neither 
is uniform. Each' has strains of high virulence and strains having 
practically no effect on pines. In the inoculations in autoclaved soil at 
sowing time the strongest' strains of Corticium vagum have on the 
whole caused more damage than any of the Pythium strains, but, on 
the other hand, there has seemed to be a higher proportion of very 
