46 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 



liest report of pathogenicity (62), but have not been used as evidence 

 in the present bulletin, though the contaminating bacteria in one of 

 them, when tested independently, showed no evidence of parasitism. 

 In all the experiments mentioned in the foregoing as giving positive 

 results with Pythium the cultures used were apparently pure. 



Cultures from single chlamydospores should be reasonably easy 

 to secure, part of the chlamydospores in water cultures being separa- 

 ble from the mycelium by vigorous shaking, and further inoculation 

 tests with cultures so obtained are probably desirable. The experi- 

 ments so far conducted are believed to be sufficiently conclusive, how- 

 ever, for all practical purposes. For isolation of absolutely pure 

 lines of this or any other coenocytic fungus, it is evident, as pointed 

 out by Dr. W. H. Weston (146), that isolations should be made from 

 the uninucleate swarm spores. For the determination of the bare 

 fact of pathogenicity such a refinement would be superfluous. 



CROSS-INOCULATIONS. 



The physiological identity of the Pythium attacking coniferous 

 seedlings with the one which attacks dicotyledons is indicated by 

 the results of several inoculation experiments. The last two experi- 

 ments, one with jack pine and one with red pine for the host, are 

 the most comprehensive and give results sufficiently decisive so that 

 quotation of the corroborative evidence from earlier experiments 

 is considered unnecessary. The results appear in Table V. Each 

 unit consisted of five 3-inch pots except in the controls, in which 

 23 pots were used in the jack-pine experiment and 18 in that with 

 red pine. In the second experiment, separate records were kept of 

 the survival in each pot, and the probable error calculated from the 

 controls was less than two seedlings per pot for a single pot, less 

 than 0.9 for a mean of 5 pots, and less than 0.5 for the mean of the 

 18 control pots. While the number of controls was, of course, in- 

 sufficient to furnish an exact basis for such a calculation, the small 

 value found tends to confirm the impression gained from inspection 

 of the table that considerable confidence can be placed in the results. 



The difference appearing in Table V between jack pine and red 

 pine in point of susceptibility to germination loss from Pythium 

 agrees with field observations in Nebraska, the red pine at the Bessey 

 Nursery, though on the whole more susceptible than jack pine to 

 damping-off losses, having given indication of more resistance to the 

 disease for the first week or two. Inoculations in other experiments 

 on western yellow pine indicate that the strains which attack it are 

 identical with those attacking jack pine and red pine. 



The conclusion reached from the cross-inoculation results is that 

 the Pythium causing damping-off of the three species of pine men- 



