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



as well as to a less extent the paramecia, 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 entirely 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. Roy 

 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 

 Corticium 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 hetae 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. 



