. [Bulletins on forest pathology : from Bulletin , Washington, , 1913-1925]. Trees; Plant diseases. 32 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. remaining, giving the mean of the results in experiments 71 and 72, are shown graphically in figure 13, together with the results of some of Peltier's experiments in which other strains were used. Per- centages of seedlings damped-off after germination are not included in these and most of the other data on pines because the most viru- lent strains often entirely prevent germination, and no value for sub- sequent loss is obtainable. T
. [Bulletins on forest pathology : from Bulletin , Washington, , 1913-1925]. Trees; Plant diseases. 32 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. remaining, giving the mean of the results in experiments 71 and 72, are shown graphically in figure 13, together with the results of some of Peltier's experiments in which other strains were used. Per- centages of seedlings damped-off after germination are not included in these and most of the other data on pines because the most viru- lent strains often entirely prevent germination, and no value for sub- sequent loss is obtainable. The grouping of most of the writer's strains at the least virulent end of the register (that is, the one with the highest number of living seedlings) is of some interest. The distributions based on the two experiments considered separately. &T&TION I /2 13 /*â /e 16 17 Fig. 12.âDiagram showing the relation between damping-off of conifers (broken line) and soil acidity (solid line). The acidity of soil samples from the different nurseries, determined by Dr. L. J. Gillespie, is reported as PH7, indicating approximate neutrality while PH6 indicates ten, and PH5 one hundred times as great a hydrogen-ion concentra- tion as PH7 ; therefore the lower the hydrogen-ion exponent line, the greater the acidity. The seriousness of damping-off at each nursery is on an arbitrary scale in which nurseries with negligible loss are rated as 1, and the nursery which suffered most is rated as 10. These values are estimates, though for some of the nurseries extensive counts were available on which the estimates were based. agreed very well in this grouping. The minor group at the end of extreme virulence is not taken to indicate an actual grouping but, rather, an artificial one, due to the fact that both the strongest strains and some less strong were thrown into the same group by the lack of additional seedlings for the stronger strains to kill. This lack of additional seedlings constitute
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