DAMPING-OFF IN FOREST NURSERIES. 



49 



same strain, No. 131 having been twice used in his inoculation ex- 

 periments on sugar beets and strain 295 recovered from the second 

 experiment. The apparent difference between this original strain 

 and its supposed reisolation may possibly be due to the treatment 

 given strain 131. Before it was used in any of the experiments 

 shown but after it had been used by Dr. Edson, it was allowed to 

 get very dry and was revived with great difficulty, growth being 

 very slow. While it apparently recovered all of its normal growth 

 qualities after one or two transfers, it is thought that this may 

 possibly explain the 

 apparently decreased 

 virulence in the later 

 experiments. 



The failure to se- 

 cure as definite indi- 

 cations of constant 

 virulence differences 

 as were obtained for 

 several of the Cor- 

 ticium strains is be- 

 lieved to be in part 

 due to a smaller 

 actual difference be- 

 tween the different 

 Pythium strains ap- 

 pearing in the graphs 

 and in part to a 

 larger accidental va- 

 riation between re- 

 sults in pots 



M>S7] 

 YEAR 

 EXPT. 





/°//V6/<? &/<?NK&/tfNrt 



fi>/A/i/S 



/a/6 



19/6 



1917 



/a/7 



62P 



66 



67 



63 



5 • 



■ 



"* 



■ 



* 



2 



Fig. 15. — Diagram showing the results of inoculations with 

 strains of Pythium debaryanum. This figure supplements 

 figure 14, giving the results for original and reisolated 

 strains independently. Each point plotted is based on 

 the results in five pots. The object of this diagram is to 

 give an idea of the degree of variability in the success of 

 inoculations. An explanation of the symbols used will 

 be found in the legend of figure 14. 



m pots inocu- 

 lated with the same strain. The growth of Pythium on agar media is 

 much more affected by variations in the substratum than is the growth 

 of Corticium, and it is rather natural to expect greater variations when 

 the two fungi are added to autoclaved soil. In experiments 66, 67, and 

 68 a number of strains not used in the earlier experiments were tested, 

 in addition to the strains previously used. The survival results for 

 all the different strains, both original and reisolated, 47 in all, are 

 shown graphically in figure 16. The results in experiments 66 and 

 67, both on Pinus banksiana, are averaged and taken as the subject, 

 while the results with the same strains in experiment 68 are made 

 relative and shown by the broken line. The correlation between the 

 performance of the same strains on the two species of pine is by 

 no means as clear in the graph as it was in the case of the Cor- 

 ticium strains (fig. 11). The areas bounded by the broken line and 



19651°— Bull. 934—31 4 



