CHAPTER I 3 



MICROBIOLOGICAL CONTROL OF SOIL-BORNE 

 PLANT DISEASES 



The possibility of controlling microorganisms, especially fungi, in the 

 soil by favoring the development of antagonistic microorganisms is sig- 

 nificant for several reasons: fungi are causative agents of some of the 

 most important diseases of plants and are added constantly and often 

 quite extensively to the soil, in plant residues and in diseased plant 

 products J fungi capable of causing certain diseases of animals and of 

 man also find their way sooner or later into the soil ; many soil-inhabit- 

 ing fungi have a marked antagonistic effect against fungus and bacterial 

 plant pathogens. 



Some fungi that produce plant diseases are able to survive in the soil 

 for only short periods of timcj others become established in the soil 

 saprophytically and remain capable of attacking living plants when 

 proper conditions arise. Some of these fungi are specific, their ability to 

 attack different plants being limited, whereas others can cause diseases 

 of a great variety of plants and many survive in infected soil for long 

 periods. Some plant diseases, as in the case of virus infections, are trans- 

 mitted by specialized means, as by insect carriers. This complicates fur- 

 ther the interrelationship among the different organisms, in relation to 

 plant and animal diseases. 



Microorganisms causing diseases of plants may either reduce the 

 vigor and productivity of the plants or destroy them completely. 

 Plants appear to develop at times a certain degree of resistance to mi- 

 crobial infection. Whether this is in the nature of a phenomenon of im- 

 munization, similar to that of animals, is still a matter of speculation. 

 Whatever the nature of the reaction, the degree of resistance depends 

 to a certain extent upon the imperviousness of the outer layers of the 

 plant tissues to penetration by the parasites, as well as upon the chemi- 

 cal composition of the plants. It is believed that an acid plant reaction, 

 combined with the presence of tannins and lignins, retards the growth of 

 many disease-producing agents. The survival of the pathogens outside 



