DAMPIXG-OFF IX FOREST NURSERIES. 29 
been treated -with sulphuric acid followed later by lime. The other 
experiments included in the graphs were on autoclaved sandy loams 
in pots in the greenhouse. In these graphs are included all of the 
results in which the same groups of strains were used repeatedly in 
different experiments. In figure 1, the values plotted for experi- 
ments 36, 49, and 51 are for the number of seedlings which appeared 
above ground, the heavy inoculations and favorable conditions for 
damping-off in these experiments being such that even weak strains 
caused heavy losses and the survivals therefore do not give differ- 
ential results. Comparison of the survival data in the other experi- 
ments in figure 1 with the emergence data for the same strains in 
that figure and in figure 10 indicates that the strains best able to 
reduce survival are also the ones best able to reduce emergence. 
"While the data presented in the graphs are not entirely consistent, 
it is very evident from them that strains 147. 213, and in a lesser 
degree 206 were regularly more virulent than most of the strains in 
tests conducted several years apart on different species of Pinus. 
It is also evident that certain strains of 186 and 189 which appear 
in figure 2 are quite regularly of low or doubtful virulence. Strains 50, 
183, 192, 211, 212, 230. and 233. whose virulence is apparently inter- 
mediate, show a greater variability. In experiments 36. 45, 47, 49, 
and 51. in which conditions especially favor parasitism, they may 
cause practically as serious loss as the regularly virulent strains, the 
best differential results being shown in experiments in which the 
disease is less favored. The apparent variation in the relative viru- 
lence of such strains in different experiments may, of course, mean that 
their virulence is differently affected by different conditions. It 
seems rather more probable that the variation in relative activity is 
to be classed as accidental variation, necessarily great with small 
units which are subject to numerous uncontrollable variables. It 
seems entirely possible, however, that part of the observed differences 
in relative activity may be due to differences, not in virulence, but in 
the ability of the different strains to maintain themselves saprophyt- 
ically in different soils during the period between inoculation and 
the' commencement of germination. For example, strains 230 and 
233 came from a nursery in southwestern Kansas in which the soil- 
acidity exponent, as determined by Dr. L. J. Gillespie, of the United 
States Bureau of Plant Industry, is 8.4. It seems entirely possible 
that these strains, rather strongly parasitic in some of the experi- 
ments, including an experiment on the soil from which they were 
taken, might prove less able than strains from some other habitats 
to maintain themselves on some of the eastern soils used in the green- 
house tests. The source of strains 230 and 233 was furthermore a 
locality where high soil temperatures are to be expected. The fact 
