DAMPING-OFF IN FOREST NURSERIES. 73 



pots, as there were still present a number of soft-stemmed seedlings 

 from seeds which were slow in germinating. These younger seed- 

 lings were excluded in counting the dead, the rule being to include 

 only plants which had developed a sufficiently rigid stem to remain 

 upright after death. Comparison of the percentage of killed with 

 the total percentage attacked for the two pines is rather interesting. 

 As has already been pointed out, while Pinus resinosa suffers very 

 heavy damping-off losses at a number of nurseries it seems to be 

 less susceptible than some other species to parasitic injury during 

 the sprouting period, before the seedlings appear above the soil sur- 

 face. Observation of beds of this species during different seasons 

 has indicated that it has not a greater susceptibility, but rather the 

 fact that its susceptibility lasts longer, which causes it to suffer as 

 seriously as it does at certain nurseries. It is indicated in Table X 

 that the succulent root tips of Pinus ponderosa are just as easily 

 attacked by damping-off parasites as those of P. resinosa — in fact, 

 considerably more easily attacked, as indicated by the figures in col- 

 umn 8. With the P. ponderosa seedlings, however, the older parts 

 of the roots had become resistant at this age in nearly all cases, while 

 of the affected P. resinosa seedlings more than one-third were still 

 unable to limit the lesions, and death resulted. 



In general, this experiment indicates that Corticium vagum and 

 Pythium debaryanum are able to cause the death of some pine seed- 

 lings which have developed rigid stems and that both are also able, 

 as has been found by other workers in the case of sugar beets, to 

 cause " root sickness," the rot of the younger portions of the root 

 systems, in seedlings which have developed too much resistance to 

 be killed. The evidence for the parasitism of the two Fusarium 

 species on these older root systems is not so good; as in the experi- 

 ments on younger seedlings, their ability to attack the pines is prob- 

 ably less than that of the other two fungi. Further inoculation ex- 

 periments are desirable, both with these fungi and with others on the 

 roots of seedlings too old to succumb to the more ordinary types of 

 damping-off. 



RELATION OF ENVIRONMENTAL FACTORS TO DAMPING-OFF. 



In the earlier section dealing with disease control, mention was 

 made of the general belief on the part of men who have had experi- 

 ence with seedling diseases that damping-off is favored by thick seed- 

 ing, by much organic matter, especially by poorly rotted manure in 

 the soil, and by excessive moisture in the air and soil. It is also 

 commonly stated that high temperature favors the disease ; on this 

 point there is perhaps a less general agreement. Practically all the 

 evidence on these points is observational. 



