DAMPING-OFF IN . FOREST NURSERIES. 
45 
pine in experiments 66, 67, and 68 combined. The figures are rel- 
ative, the mean survival of 47 different strains used in all three ex- 
periments being taken as 10. A survival figure above 10 therefore 
means that the strain was less destructive than the average Pythium, 
and a figure below 10 indicates more than average virulence. As 
strain 218 was not used in these three experiments, strain 345 can not 
be compared. 
Table IV. — Comparative virulence of original cultures and reisolated strains of 
Pythium deoaryanum in experiments 66, 67, and 68. 
Pythium strain. 
Description. 
Rela- 
tive 
sur- 
vival. 
Pythium strain. 
Description. 
Rela- 
tive 
sur- 
vival. 
No. 258 
16 
12 
12 
4 
4 
6 
No 409 
Reisolated from 338 
9 
No. 414 
Reisolated from 258 
do 
No. 347 
4 
No. 415 
No. 450 
Reisolated from 347 
7 
No. 295 
No. 348 
10 
4 
No. 338 
Reisolated from 295 
Reisolated from 338 
No. 419 
Reisolated from 348 
No. 408 
These figures are not absolutely consistent, but are to be viewed 
as contributing to the evidence furnished by the absence of damping- 
off in the control of experiments 58 and 62 that the cultures reiso- 
lated in those experiments were actually identical with the original 
strains. A further proof of this identity is in the fruiting tendencies 
of the strains. Both Nos. 414 and 415, the strains reisolated from 
original strain 258, exhibited the peculiarly sparse spore production 
which has been characteristic of strain 258 for the entire period dur- 
ing which it has been in culture. The other reisolated strains, taken 
from pots inoculated with normally fruiting strains, all showed 
normal spore production. 
PURITY OF CULTURES. 
A slight deficiency in the evidence as to the parasitism of Pythium 
debaryanum both in the writer's work and apparently in all previous 
investigations except those of Peters (100) and possibly Knechtel 5 
is the lack of single-spore cultures. The large number of strains 
which have remained apparently pure through numerous subcultures 
and have retained their individual characteristics as to virulence and 
fruiting tendencies (one strain having been carried on artificial 
media continuously for eight years without material change) give 
very strong justification for believing that the cultures used were 
pure. In three early inoculation tests the cultures used were after- 
wards found to have been contaminated by bacteria carried by mites ; 
the positive results obtained in these three were the basis of the ear- 
5 Knechtel's work in Rumanian has been available to the writer only in the German 
abstract, which makes an ambiguous statement on this point. 
