DAMPING-OFF IN FOREST NURSERIES. 85 



With Pythium : 



Without saprophytes, 27.5 per cent dainped-off ; coefficient of variability, 



67±7.8 per cent. 

 With saprophytes, 16.9 per cent daniped-off ; coefficient of variability, 



39±2.4 per cent. 



This tendency has been frequently observed in experiments in 

 which inoculum is applied to the soil at the time of sowing. Even 

 in experiments in which a relatively small proportion of the seed- 

 lings are killed, some of the pots are nearly or entirely cleaned out. 

 It is taken as an indication that failure of inoculation to give results 

 is often due to the inability of the fungus to maintain itself in a 

 vigorous condition till the germinating seed is far enough along to 

 allow easy infection. It may also be in part due to lack of uni- 

 formity of the soil in different pots affecting virulence of parasites 

 or resistance of hosts. 



In addition to this experiment on autoclaved soil, a somewhat 

 similar experiment was conducted in a nursery in the Kansas sand 

 hills on soil which had been treated with sulphuric acid, followed 

 by lime raked into the soil. Saprophytes, for the most part the 

 same strains that had been used in the experiment in the greenhouse, 

 were added to 24 plats, each of one-half square foot, of Pinus hank- 

 siana and 24 of P. ponderosa, with 16 interspersed plats of each 

 species serving as controls. The saprophytes were growing on rice, 

 part of which was added to the plat with the inoculum in addition 

 to the fungous mycelium. Damping-off was rather heavy in this soil 

 from accidental infection or from parasites which survived the 

 initial acid treatment, no parasites having been artificially intro- 

 duced. The loss was probably due to Cortioium vagum or Fusarium 

 spp. rather than to Pythium debaryanum in this case. In both pines, 

 emergence was slightly better in the control plats than in those to 

 which the saprophytes had been added, the difference for Pinus bank- 

 siana being less than half its probable error and for P. ponderosa 

 slightly more than its probable error. Damping-off for the first few 

 days after emergence was somewhat less in the controls in one species, 

 but higher in the saprophyte-inoculated pots in the other. The 

 saprophytes therefore gave no evidence of effective competition with 

 the parasites on this acid-lime treated sand. 



While the competition for water which seems to be the form of 

 competition most common among green vascular plants is not likely 

 to be of significance between fungi such as those which cause damping- 

 off, a very little observation of the growth of mixed cultures of the 

 parasites and other organisms in Petri dishes is sufficient to make 

 one realize that the latter may very considerably decrease the activity 

 of certain of the parasites. In nutrient agar most of the fungi and 

 bacteria introduced from the soil in attempting parasite isolations, 



