SOIL MICROORGANISMS AND PLANT PATHOGENS 249 



general soil microflora j this was determined by comparing infection in 

 an unsterilized soil with that obtained in a sterilized soil. An increase in 

 soil temperature was found (312, 398) to increase the antagonistic ac- 

 tion of the soil microflora against the parasitic fungi causing cereal root 

 rots. 



Various actinomycetes were shown to be antagonistic (884) to species 

 of Pythiuniy a root parasite of sugar cane. The phenomenon of antago- 

 nism was independent of the f¥l changes j it has been ascribed to the 

 formation of a toxic, partly thermostable, principle. A marked influ- 

 ence of the soil microflora on grass diseases caused by Pythium (691 ) is 

 illustrated in Figures 32 and 33 (906). Clavacin (patulin) was found 

 capable of inhibiting the growth of various species of Pythium (cause of 

 damping-off disease of seedlings) in dilutions of about 1 1400,000 (22), 

 and of exerting a strong fungicidal action upon Ceratostomella ulmiy 

 the causative agent of the Dutch elm disease j the last effect could 

 partly be overcome by certain nutrients in the medium, especially 

 peptone (928). 



Numerous soil microorganisms are moderately or strongly antago- 

 nistic to such pathogens as Hyfochnus centrifugus, Hyfochnus sasakii, 

 and Sderotium oryzae sativae (241 ) j culture filtrates from some of the 

 antagonistic fungi were also able to reduce the damage caused by the 

 pathogens. 



Phytofhthora cactorum was found (974) to be inhibited in the rotted 

 tissues by the antagonistic effects of secondary organisms. In many 

 cases, the rotting of fruits was suppressed by mixtures of organisms as 

 compared with the pathogens j the type of rot was also modified, de- 

 pending on the temperature and the specific nature of the antagonists 

 (790). 



The stimulating effect of mycorrhizal fungi on the host plant has 

 been explained (298) by the capacity of the fungi to inactivate, destroy, 

 or absorb certain plant-retarding principles found among the organic 

 constituents of peat and other humus materials, or produced by fungi. 



Certain fungi are also known (332) to be antagonistic to ants and 

 their fungal symbionts. These antagonists are distributed by the insects, 

 thus spreading agents that are destructive to themselves and to their 

 fungus gardens. 



