312 CONTROL OF SOIL-BORNE PLANT DISEASES 



The only important procedure which has thus far found a practical 

 application is the treatment of the soil in such a manner as to modify its 

 microbiological population, which in its turn can destroy or in other 

 ways control the activities of plant pathogens. Many such instances can 

 be cited for illustrative purposes. In the case of root rots of the straw- 

 berry, it was found that carbohydrate decomposition induces a favor- 

 able change in the soil microflora from pathogenic or potentially harm- 

 ful organisms to beneficial or rather innocuous types (424). Studies of 

 the eradication of cotton root rot led to recommendations of treatments 

 of soil with organic materials, especially during the period of increased 

 microbial activity (650). The survival of the fungus is considered 

 to be limited by microbial interrelationships rather than by food ex- 

 haustion. During the early stages of incubation, viable sclerotia are de- 

 stroyed more rapidly than dead sclerotia in soils treated with organic 

 matter. It was suggested, therefore, that the germination of the scle- 

 rotia is an important factor for their elimination from the soil. Field 

 application of organic materials, accompanied by early October plowing, 

 results in an increase in microbial activity. This brings about a reduc- 

 tion of the incidence of dead cotton in the succeeding crop and greater 

 difficulty of finding sclerotia. 



It was reported recently (65) that R. solani is able to cause 100 per 

 cent damping-off of radish seedlings planted at a distance of 4 cm. from 

 the inoculum 5 when the seeds were planted at a distance of 9 cm. the 

 damage was 40 per cent. The addition of i per cent ground wheat 

 straw or dried grass to the soil had a marked depressing eifect upon the 

 growth of the fungus. This was ascribed to the nitrogen starvation of 

 the mycelium, accompanied by the rapid utilization of the available soil 

 nitrogen by the cellulose-decomposing microorganisms multiplying at 

 the expense of the fresh organic material j carbon dioxide produced by 

 the cellulose-decomposers was believed to cause a marked fungistatic 

 action on the Rhizoctonia. 



Treatment of the soil with organic materials, which results in the de- 

 struction of certain plant pathogens by stimulating the development of 

 saprophytic microorganisms, may be designated as "partial disinfec- 

 tion." To what extent antibiotic agents produced by plants, such as 



