SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 



815 



and Fusarium lycopersici. 71 When soil is steamed, the fungi are readily- 

 destroyed; but once a parasitic organism like Pythium debaryanum is 

 introduced, it will readily develop in the treated soil and may even cause 

 a larger amount of infection. This parasitic activity can be decreased 

 by inoculating the treated soil with various saprophytic fungi. Treat- 

 ment of soil with a disinfecting agent followed by inoculation with sapro- 

 phytic fungi may prove to be most efficient in increasing the value of the 

 treatment. 72 



In genera] the methods of control consist, on the one hand, of crop 

 rotation, seed treatment and selection of resistant varieties, on the other 

 hand, of soil sterilization by heat or by chemicals, change in soil reaction, 

 or other chemical treatment. For the sterilization of greenhouse soil, 

 the following temperatures are required: 73 



INFECTING ORGANISM 



Nematodes 



Pythium 



Rhizoclonia 



Sclerolinia 



Septoria lycopersici spores 



Anthracnose beans, spores. . . 

 Anthracnose beans, mycelium 

 Corn root rot 73a 



TEMPERATURE OP CONTROL 



Further information on the steam disinfection of soil is given elsewhere. 74 



71 See Foex, E. Protection of seeds and young plants against diseases by soil 

 sterilization. Jour. Soc. Natl. Horticult., France, 4, Ser. 22: 242-254. 1921. 



72 Hartley, C. Damping off in forest nurseries. U. S. Dept. Agr. Prof. Paper 

 Bui. 934. 1921. 



73 Brown, H. D., Baldwin, I. L., and Conner, S. D. Greenhouse soil steriliza- 

 tion. Purdue Univ. Agr. Exp. Sta. Bui. 266. 1922. 



73a Valleau, W. D., Karraker, P. E., and Johnson, E. M. Corn root rot— a 

 soil-borne disease. Jour. Agr. Res., 33: 453-476. 1926. 



74 Hunt, N. R., O'Donnell, F. G., and Marshall, R. P. Steam and chemical 

 soil disinfection with special reference to potato wart. Jour. Agr. Res., 31: 

 301-363. 1925; Beinhart, E. G. Steam sterilization of seed-beds for tobacco and 

 other crops. U. S. Dept. Agr. Farm. Bui. 996. 1918; Byers, L. P., and Gilbert, 

 W. W. Soil disinfection with hot water to control the root-knot nematode and 

 parasitic soil fungi. II. S. Dept. Agr. Bui. 818. 1920. 



