DAMPING-OFF IN FOREST NURSERIES. 
49 
same strain, No. 131 having been twice used in his inoculation ex- 
periments on sugar beets and strain 295 recovered from the second 
experiment. The apparent difference between this original strain 
and its supposed reisolation may possibly be due to the treatment 
given strain 131. Before it was used in any of the experiments 
shown but after it had been used by Dr. Edson, it was allowed to 
get very dry and was revived with great difficulty, growth being 
very slow. While it apparently recovered all of its normal growth 
qualities after one or two transfers, it is thought that this may 
possibly explain the 
apparently decreased 
virulence in the later 
experiments. 
The failure to se- 
cure as definite indi- 
cations of constant 
virulence differences 
as were obtained for 
several of the Cor- 
ticium strains is be- 
lieved to be in part 
due to a smaller 
actual difference be- 
tween the different 
Pythium strains ap- 
pearing in the graphs 
and in part to a 
larger accidental va- 
riation between re- 
sults in pots inocu- 
lated with the same strain. The growth of Pythium on agar media is 
much more affected by variations in the substratum than is the growth 
of Corticium, and it is rather natural to expect greater variations when 
the two fungi are added to autoclaved soil. In experiments 66, 67, and 
68 a number of strains not used in the earlier experiments were tested, 
in addition to the strains previously used. The survival results for 
all. the different strains, both original and reisolated, 47 in all, are 
shown graphically in figure 16. The results in experiments 66 and 
67, both on Pinus banksiana, are averaged and taken as the subject, 
while the results with the same strains in experiment 68 are made 
relative and shown by the broken line. The correlation between the 
performance of the same strains on the two species of pine is by 
no means as clear in the graph as it was in the case of the Cor- 
ticium strains (fig. 11). The areas bounded by the broken line and 
19651°— Bull. 934—21 4 
WST\ 
year 
EXPT. 
Sfco 
Njjj 
P//WS SSMKS/tf/S/A 
/9/6 
/9/6 
1917 
/9/7 
eeo 
66 
67 
66 
Rf am© Qq^ 
m 
t 
die 
2 
Fig. 15. — Diagram showing the results of inoculations with 
strains of Pythium debaryanum. This figure supplements 
figure 14, giving the results for original and reisolated 
strains independently. Each point plotted is based on 
the results in five pots. The object of this diagram is to 
give an idea of the degree of variability in the success of 
inoculations. An explanation of the symbols used will 
be found in the legend of figure 14. 
