DAMPTNG-OFF IN FOREST NURSERIES. 57 



experiments the losses were heavier in the Rheosporangium pots. 

 Especially in experiments Gl and 62A the evidence indicates very 

 strongly that both germination loss and subsequent damping-off of 

 the seedlings which come up can be caused by inoculation with Rheo- 

 sporangium on jack pine under favorable inoculation conditions. It 

 is, however, obvious that in all of the experiments the parallel in- 

 oculations with Pythium debaryanum gave much more positive re- 

 sults. The Pythium was active under conditions in which the Rheo- 

 sporangium gave no evidence whatever of parasitic capacity. It 

 furthermore appears that the two strains of Rheosporangium, though 

 probably identical originally, differed in virulence at the time of 

 their comparison in these experiments. The greater virulence of 

 strain 351 was quite distinct in most of the comparative tests on beets 

 as well as on pines. The possibility that the original culture was 

 really a composite of two or more strains, of which different ones 

 survived in the subcultures kept at Washington and Madison, re- 

 spectively, seems worth considering. Such an accident might also 

 have been responsible for the divergence of Pythium strains 131 and 

 295 referred to in another section. 



Further evidence of the parasitism of Rheosporangium was ob- 

 tained in inoculations with cultures reisolated from seedlings killed 

 by the original strains in experiment 62. Typical Rheosporangium, 

 identified by presporangium formation, was easily recovered from 

 the damped-off seedlings in pots of pines only, those of beets only, 

 and the pots in which both hosts were sown. The recovery of a 

 virulent Pythium strain from a single one of the pots inoculated with 

 the weaker Rheosporangium shows that despite the absence of dis- 

 ease in the controls a slight amount of contamination did occur. 

 However, the comparative ease with which the Rheosporangium was 

 isolated from seedlings in other pots inoculated with it and the fact 

 that it has never been obtained in the numerous cultures made from 

 controls and from pots inoculated with other organisms leave little 

 room for doubt that the strains isolated were really recoveries of the 

 Rheosporangium used in the original inoculations. The results of 

 reinoculation with these strains are shown in Table VII. 



From Table VII and by comparison with Table VI it appears 



(1) That in one experiment each on jack pine and red pine the reisolated 

 Rheosporangium strains gave positive results. In a second experiment on jack 

 pi no (No. 67) the difference between the Rheosporangium pots and the con- 

 trols was not significant. 



(2) That, as in Table VI, the Pythium strains used proved on the whole 

 decidedly more parasitic than the Rheosporangium strains. In experiment 66 

 this is not shown by the percentage of seedlings damped-off, but is sufficiently 

 evident when the germination loss as well as the subsequent damping-off per- 

 contage is considered, the survival being here, as in most other cases in which 



