82 BULLETIN 934, U. S. DEPARTVIEXT OE AGRICULTURE. 
BIOLOGIC FACTORS. 
Mention has already been made of two strictly biologic factors 
which may influence the amount of damping-off. Taylor (138) and 
Eathbun (106) haye found Fusarium not only at considerable depths 
in the soil of pine seed beds, but yiable Fusarium spores without 
hyphee in the alimentary canals of earthworms and insect larvae in 
the soil, and they attribute to the migrations of these and to the 
tunnels which various animal forms make in the soil a possible im- 
portance in the distribution of damping-off Fusaria. A likely rela- 
tion between Corticium vagum epidemics in pine seed beds and the 
character of the weed flora has also been considered (66). 
The relation between the damping-off parasites and other micro- 
organisms in the soil is also a matter of some interest. The effect of 
the microfauna of the soil on the microflora in general has been 
considered b} 7 Russell and others in a number of papers. The effect 
of soil disinfection by heat in favoring the work of artificially intro- 
duced soil-inhabiting fungous parasites, apparently a rather frequent 
phenomenon and quite evident in the inoculation experiments with 
Pythium debaryanum reported hi the present bulletin, has been in 
other cases attributed to the removal of bacteria and other fungi 
which might compete with the parasites (36, 80). Heating soil is 
known to produce physical changes and also very considerable chemi- 
cal changes both in organic and inorganic substances. These must not 
be ignored in considering the effect of previous soil heating on para- 
site activity. "With a view to determining whether all the difference 
noted in the behavior of P. debaryanum in heated soil is due to the 
direct effects of the heating or in part to the elimination of com- 
peting microorganisms, an exjDeriment was conducted in 3-inch pots 
of autoclaved soil in which 111 of them were inoculated with agar 
cultures of the Pythium at one point in each pot shortly after seed 
sowing. The seeds sown in each pot approximated 136, considerably 
more than are used on equal areas of nursery seed bed. Of these, 
fifteen 5-pot units and one 3-pot unit had been inoculated broadcast 
with rice or nutrient agar cultures of various organisms supposed to 
be saprophytic on pines. These included Phoma betae, Phoma sp., 
Chaetomium sp. (from a maple root), Rhizopus nigricans, Tricho- 
thecium roseum, THchoderma Jeoningi, Aspergillus spp. (including 
one with black and one with bright-colored spore heads), Rosellinia 
sp. (from soil) , Penicillium sp., an undetermined bacterium, and three 
undetermined 'higher fungi. The whole 78 pots inoculated with P. 
debaryanum and saprophytes, the percentages being based on the 
total number of seeds in the case of emergence and survival and on 
the number of seedlings which appeared above ground for damping- 
off loss, as compared with those which had received the parasite only, 
gave results as follows : 
