86 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 
as well as to a less extent the paramenia, nematodes, and amoebae 
which develop in such plates, exert a very considerable limiting in- 
fluence on the growth of most of the damping-off fungi. That they 
should also limit the growth of parasites in soil, whether by the 
production of toxic compounds, the exhaustion of food materials, or 
in other ways, seems entirety reasonable. The results in the writer's 
experiments on heated soil warrant the suggestion that further trials 
should be made of the introduction of vigorously growing bacteria 
or molds, preferably mixed cultures containing a number of different 
organisms, on seed beds which have been disinfected by some such 
method as steam or hot water, which leaves the soil in a favorable 
condition for the development of accidentally reintroduced parasites. 
If such treatment should be successful in improving the rather dis- 
appointing results with soil heating at some nurseries, it might 
easily become of practical value, as the cultivation of certain of the 
more easily grown saprophytes on a scale large enough to yield con- 
siderable quantities of bacterial or spore suspensions should be fairly 
easy and entirely practicable in an operation as intensive as that of 
raising coniferous seedlings. 
ACKNOWLEDGMENTS. 
The writer wishes to express his obligations to Dr. H. A. Edson, 
of the Bureau of Plant Industry, United States Department of 
Agriculture, and to Prof. W. T. Home, of the University of Cali- 
fornia, for suggestions during the progress of this work ; to Mr. Eoy 
G. Pierce and Mr. Glenn G. Hahn, of the Bureau of Plant Industry, 
for assistance in a number of the inoculation experiments; and to 
other members of the staff of the Office of Forest Pathology for 
assistance and suggestions at various times. 
SUMMARY. 
(1) Damping-off in nurseries is caused mainly by seedling para- 
sites which are not specialized as to host ; Pythium debaryanum and 
Corticiwn vagum are probably the most important of these. Damp- 
ing-off of various herbaceous hosts, including ferns, is often caused 
by specialized parasites which are limited to a particular host or 
group of hosts. Phoma betae is a rather extreme example of such 
specialization. For the conifers all the damping-off appears to be 
due to parasites of the generalized type. 
(2) Damping-off of trees, as of herbaceous plants (with the ex- 
ception of the cases caused by specialized seed-carried parasites), is 
ordinarily serious only in seed beds or cutting beds in which large 
numbers of plants are crowded together in a small space. In most of 
the forest nurseries it is a much more serious matter in conifers than 
in dicotyledonous seedlings. 
