44 BULLETIN 034, U. S. DEPARTMENT OF 'AGRICULTURE. 



it proved difficult to keep the control pots entirely free from damp- 

 ing-off. Cultures from seedlings Avhich damped-off spontaneously 

 in control pots indicated that Pythium as well as Fusarium may be 

 introduced by accident, even when insects, birds, and rodents are ex- 

 cluded. This agrees with the evidence of Hofmann (77) that 

 Pythium debaryanum is sometimes disseminated by wind,, despite 

 its apparent lack of adaptation to wind distribution. It is also in- 

 dicated, however, that unheated tap water increases damping-off 

 when used on control pots and probably carries this semiaquatic 

 fungus. Notwithstanding infections in the controls of a number of 

 the experiments, it is believed that the large number of pots whose 

 results have been considered in drawing conclusions, the fact that the 

 Pythium pots lost more heavily than the controls in every one of the 

 16 experiments, and the magnitude of the differences between both the 

 emergence and subsequent damping-off figures for the inoculated pots 

 and the controls in most of the experiments establish the parasitism 

 of the fungus in inoculation on autoclaved soil without it being neces- 

 sary to present all the evidence in detail. The pot series which in- 

 volved reisolation and reinoculation (Table III), together with the 

 results given for other purposes in Tables V and VI, seem sufficient 

 by themselves to establish a parasitic relationship. 



REISOLATION AND REINOCULATION. 



In a number of the experiments dead seedlings in the inoculated 

 pots were examined and typical Pythium hyphse and spores were 

 found. In three of the experiments in which the controls remained 

 entirely free from disease up to the time the experiment was closed, 

 reisolations and reinoculations were made in accordance with the 

 usual rules of proof. The results are given in Table III. 



From Table III it will be seen that five strains reisolated from 

 Pinus banJcsiana and one strain reisolated from P. ponderosa gave 

 positive results in pots of P. Ixmksiana. and P. resinosa. In addition 

 to the reinoculations shown in the table, the strain reisolated from 

 Pinus ponderosa (No. 338) was again reisolated in duplicate from 

 P. banksiana in experiment 62, and both these secondary reisolations 

 gave cultures which were parasitic on P. banksiana and P. resinosa 

 in subsequent inoculations. 



That the organisms reisolated were actually the same as those used 

 in the initial inoculation is indicated .not only by the absence of dis- 

 ease in the control pots of experiments 58 and 62, but by the distinc- 

 tive characters of some of the strains. In general, cultures reisolated 

 from strongly parasitic initial strains were themselves strongly 

 parasitic and vice versa. This is shown by comparing the figures for 

 the initial and reisolated strains, as shown in Table IV. Each figure 

 represents the average results in 10 pots of jack pine and 5 of red 



