DAMPING-OFF IN FOREST NURSERIES. 



45 



pine in experiments 66, 67, and 68 combined. The figures are rel- 

 ative, the mean survival of 47 different strains used in all three ex- 

 periments being taken as 10. A survival figure above 10 therefore 

 means that the strain was less destructive than the average Pythium, 

 and a figure below 10 indicates more than average virulence. As - 

 strain 218 was not used in these three experiments, strain 345 can not 

 be compared. 



Table IV. 



-Comparative virulence of original cultures and reisolated strains of 

 Pythium debaryanum in experiments 66, 67, and 68. 



Pythium strain. 



Description. 



Rela- 

 tive 

 sur- 

 vival. 



Pythium strain. 



Description. 



Rela- 

 tive 

 sur- 

 vival. 



No. 258 





16 

 12 

 12 

 4 

 4 

 6 



No 409 



Reisolated from 338 



9 



No. 414 



Reisolated from 258 



do.. 



No. 347 



4 



No. 415 



No. 450 



Reisolated from 347 



7 



No. 295 . . 





No. 348 



10 



No. 338 



Reisolated from 295 



Reisolated from 338 



No. 419 



Reisolated from 348 



4 



No. 408 











These figures are not absolutely consistent, but are to be viewed 

 as contributing to the evidence furnished by the absence of damping- 

 off in the control of experiments 58 and 62 that the cultures reiso- 

 lated in those experiments were actually identical with the original 

 strains. A further proof of this identity is in the fruiting tendencies 

 of the strains. Both Nos. 414 and 415, the strains reisolated from 

 original strain 258, exhibited the peculiarly sparse spore production 

 which has been characteristic of strain 258 for the entire period dur- 

 ing which it has been in culture. The other reisolated strains, taken 

 from pots inoculated with normally fruiting strains, all showed 

 normal spore production. 





PURITY OF CULTURES. 



A slight deficiency in the evidence as to the parasitism of Pythium 

 debaryanum both in the writer's work and apparently in all previous 

 investigations except those of Peters (100) and possibly Knechtel 5 

 is the lack of single-spore cultures. The large number of strains 

 which have remained apparently pure through numerous subcultures 

 and have retained their individual characteristics as to virulence and 

 fruiting tendencies (one strain having been carried on artificial 

 media continuously for eight years without material change) give 

 very strong justification for believing that the cultures used were 

 pure. In three early inoculation tests the cultures used were after- 

 wards found to have been contaminated by bacteria carried by mites ; 

 the positive results obtained in these three were the basis of the ear- 

 's Knechtel's work in Rumanian has been available to the writer only in the German 

 abstract, which makes an ambiguous statement on this point, 



