SOIL MICROORGANISMS AND PLANT PATHOGENS 303 



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 (408) to increase the antagonistic action 

 of the soil microflora against the parasitic fungi causing cereal root rot. 

 Various actinomycetes were shown to be antagonistic (908) to species 

 of Pythium, a root parasite of sugar cane. The phenomenon of antago- 

 nism was independent of the ^H 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 is illus- 

 trated in Figures 31 and 32. Clavacin (patulin) was found capable of 

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

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

 of exerting a strong fungicidal action upon Ceratostomella ulmi, the 

 causative agent of the Dutch elm disease j the last effect could be over- 

 come partly by certain nutrients in the medium, especially peptone 



(949)- 



Numerous soil microorganisms are moderately or strongly antago- 

 nistic to such pathogens as Hyfochnus centrijugus, Hy-pochnus sasakii, 

 and Sclerotium 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 (1003) to be inhibited 

 in rotted tissues by the antagonistic effects of secondary organisms. In 

 many cases, the rotting of fruits was suppressed by mixtures of organ- 

 isms but not by the pathogens alone j the type of rot was also modified, 

 depending on the temperature and the specific nature of the antagonists 

 (827). Phytomonas tumefaciens, the causative agent of crown gall in 

 plants, can be controlled by means of crude penicillin (92). 



The stimulating effect of mycorrhizal fungi on the host plant has 

 been attributed (297) to 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. 

 On the other hand, the toxicity of certain soils to mycorrhiza has been 

 explained (83) as due to the accumulation of antibiotic substances by 

 the fungus population of the soil. This concept is purely hypothetical 

 and is not based upon experimental evidence. 



Certain fungi are also known (340) to be antagonistic to ants and 



