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 



10 





- S 



&TOTION I 



6 e 



IO II 12 13 14- /6 16 17 



FIG. 12. Diagram 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 P H 7, indicating approximate neutrality 

 while Pn6 indicates ten, and PHO one hundred times as great a hydrogen-ion concentra- 

 tion as Pn7 ; 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 constituted a limiting factor. In other 

 words, conditions favored damping-off even in these two experiments 

 too much to permit completely differential results for the more viru- 

 lent strains. Despite this artificial limit preventing the full vari- 

 ability becoming evident, the coefficient of variability of the survivals 



