50 
BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 
the horizontal line showing the location of the mean for experiment 
68 are much larger below the means than above it in the left-hand 
portion of the graph, while the reverse is true in the right-hand 
portion. To this extent the relative activity of the strains in this 
experiment agrees with the performance of the same strains in the 
two jack-pine experiments, as shown by the solid line. It can not 
be decided from an inspection of the graph whether there is a real 
agreement, in view of the large accidental variation present. How- 
ever, the correlation coefficient, 0.446±0.079, five and one-half times 
its probable error, indicates a considerable correlation, not as good 
as was found for the Corticium strains, but sufficient to establish a 
strong presumption that observed differences in activity of the dif- 
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Fig. 16. — Diagram showing the comparative virulence of 47 strains of Pythium debariianum 
in successive inoculation experiments on species of Pinus. The results in experiments 
Nos. 66 and- 67 (on Pinus lamksiana) are shown by the solid line, the strains being 
arranged from left to right in the order of descending virulence indicated by the number 
of seedlings surviving in those experiments. The results from the use of the same 
strains in experiment No. 68 (on Pinus resinosa) are shown by the broken line. Such 
correlation as there is between the two curves (coefficient 0.45 + 0.08) goes to indicate 
a real difference in virulence between the different strains. The strains indicated by 
the underscored numbers are original strains, and those not underscored are reisolations 
from the original strains in earlier inoculation experiments on pine seedlings. 
ferent strains in these inoculation experiments were in part actually 
due to differences in the capacity of the strains. 
It has been suggested in the foregoing that the difficulty in demon- 
strating constancy in the difference in virulence between the various 
strains of Pythium debaryanum is due in part to the lack of such 
extreme differences as were observed between the various Corticium 
strains. Figure 13 shows the distribution of the different original 
Pythium strains according to the virulence indicated in the three 
inoculation experiments of figure 16 (application to autoclaved soil 
at the time of sowing). Each value plotted is based on the average 
results in 15 pots. Of the strains used, 21 were from species of pine, 
1 from spruce, 2 from potato tubers, 2 from fenugreek, 3 from sugar 
beet, and 6 from soil direct. Despite the considerable number of 
