GENERAL METHODS OF CONTROL 307 



biological balance of the soil population. Similar results have been ob- 

 tained (992) in efforts to control R. solani, or the damping-off of citrus 

 seedlings (Figure 33), by the use of T. lignorum, and in the action of 

 B. simplex upon Rhizoctonia in the soil ( 1 54). 



A number of antagonistic bacteria were found (502) to be able to pre- 

 vent scab formation by S. scabies on potatoes. Daines (167) found that 

 T. Ugnorum produces a diffusible substance which is toxic to S. scabies 

 in an artificial liquid medium. However, the toxic principle added to 

 potato soils is rapidly destroyed there by aeration j it can be removed 

 from solution by charcoal and by soil, where it is destroyed. It was sug- 

 gested, therefore, that it is highly doubtful whether antagonists will 

 be found to be of much assistance in combating potato scab in soil. The 

 physical and biological environments encountered in many cultivated 

 soils offer an important barrier against the establishment of the antago- 

 nist. When the latter was added to a 5-day-old culture of S. scabies, it 

 was greatly inhibited by the scab organism. Soil bacteria are also able to 

 produce substances toxic to both Trichoderma and Streftomyces alike. 

 In such a complex physical, chemical, and biological environment as the 

 natural soil, these antagonistic relationships may thus be modified or 

 even entirely destroyed. 



The application to the soil of organic materials which favor the de- 

 velopment of antagonists has given much more favorable results than 

 the use of pure cultures. Fellows (251) obtained field control of the 

 take-all disease of wheat in Kansas by the application of chicken and 

 horse manure, alfalfa stems and leaves, boiled oats and barley, as well 

 as potato flour. Garrett believed (316, 317) that the factor chiefly con- 

 trolling the spread of pathogenic fungus along the roots of the wheat 

 plant was the accumulation of carbon dioxide, with a corresponding 

 lowering of oxygen tension in the microclimate of the root zone. A high 

 rate of soil respiration was, therefore, said to check the growth of O. 

 graminis. This can best be maintained, of course, by periodic additions 

 of organic manures. Materials low in nitrogen were found to be more 

 effective than those high in nitrogen. Garrett, therefore, postulated the 

 hypothesis that the soil microflora used the mycelium of the pathogen as 

 a source of nitrogen, in the process of decomposition of the nitrogen- 

 poor materials. The addition of nitrogenous substances, in either an or- 



