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



BIOLOGIC FACTORS. 



Mention has already been made of two strictly biologic factors 

 which may influence the amount of clamping-off. Taylor (138) and 

 Rathbun (106) have found Fusarium not only at considerable depths 

 in the soil of pine seed beds, but viable Fusarium spores without 

 hyphae 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 by 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 in 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 experiment 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, PJioma sp., 

 Chaetommm sp. (from a maple root), Rhizopus nigriccuns, Tricho- 

 thecium roseum, Trichoderma Jconingri, 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 : 



