248 CONTROL OF SOIL-BORNE PLANT DISEASES 



ANTAGONISM OF SOI L- I N H AB ITI NG MICRO- 

 ORGANISMS TO PLANT PATHOGENS 



The antagonistic interrelationships among the members of the micro- 

 biological population of the soil have received particular attention from 

 the point of view of modifying the virulence of those plant pathogens, 

 especially the fungi, that find temporary or permanent habitat in the 

 soil (23,310,554,696). 



In the infection of wheat seedlings by O. graminis, a number of 

 fungi and bacteria are able to exert a marked antagonistic action against 

 the pathogen (784). Not only the living cultures of the antagonists, 

 but, in many cases, the culture filtrates are also effective (511, 992). 

 The growth of H. sativum and F. graminearum upon sterilized soil 

 was completely suppressed (397) by the addition of small amounts of 

 unsterilized soil or by the simultaneous inoculation with harmless fungi 

 and bacteria, with the result that no infection occurred when wheat seeds 

 were inoculated with this soil. Although H. sativum is able to sporulate 

 readily in sterilized soil, this does not take place in nonsterilized soil, 

 sporulation being inhibited by the soil microorganisms. Virulence of 

 H. sativum on wheat seedlings was reduced by 11 to 57 per cent by cer- 

 tain cultures of Penicillium; Trichoderma reduced virulence by 50 to 

 58 per cent, Absidia glauca by 39 per cent, and A. nidulans by 30 per 

 cent. Many fungi, however, had no effect on the virulence of the patho- 

 gen, and some even increased it (7B5). The fact that root-rot diseases 

 are less severe on wheat grown on summer-fallowed land than on land 

 cropped to wheat for several years was believed to be due to the soil 

 saprophytic microorganisms, which in bare fallow have an advantage 

 over the pathogenic organisms in competition for food (Figures 30 

 and 31). 



The infection of wheat seedlings by O. graminis in sterile soil was 

 found to fall off rapidly with the reestablishment of the original soil 

 microflora (86). It was emphasized, however, that the effect of various 

 organisms upon the pathogen grown in artificial culture media is no 

 proof that the same organisms will be able to suppress the virulence of 

 the pathogen on wheat in soil. An inverse correlation was shown (617) 

 to exist between the degree of infection and the protective effect of the 



