306 CONTROL OF SOIL-BORNE PLANT DISEASES 



The role of microbiological antagonism in the natural control of 

 soil-borne fungus diseases of plants has been well emphasized (408, 

 821 ). Methods for combating plant pathogenic fungi by the use of bac- 

 teria and other antagonists have been suggested by various investigators 

 (46,869). 



The principles underlying the biological control of soil-borne plant 

 diseases were outlined by Garrett (316) in terms of the soil population 

 in a state of dynamic equilibrium. When a given crop is grown continu- 

 ously in the same soil, the parasitic organisms capable of attacking the 

 roots of that crop multiply. Organic manures stimulate the develop- 

 ment of saprophytic organisms in the soil, and are thus able to check 

 the activity of the pathogens, which are destroyed by the saprophytes. 

 Either the metabolic processes of the saprophytes check the growth of 

 the pathogens, or the saprophytes actually attack and destroy the myce- 

 lium of the pathogens. The microbiological control of plant diseases 

 was said to be most effective against those organisms which have be- 

 come highly adapted to a parasitic form of life. The pathogenic O-phiob- 

 olusy when present in the form of mycelium inside the infected wheat 

 stubble buried in the soil, is able to tolerate adverse physical soil con- 

 ditions. Those soil treatments which favor increased activities of the 

 microbiological population, such as addition of organic matter, partial 

 sterilization followed by reinoculation with fresh soil, and improvement 

 in soil aeration, favored loss of viability of the pathogen. 



Van Luijk (930) recommended the control of plant parasites by 

 inoculating the soil with specific microorganisms selected for their an- 

 tagonistic capacity, or by the addition of the growth products of these 

 microorganisms. Living soil fungi, including Trkhoderma viridis and 

 Absidia s-pinosa, exerted an adverse influence upon Rhizoctonia solani 

 and reduced its pathogenicity to cabbage seedlings (458). Broadfoot 

 (87) and others (247), however, emphasized that the antagonism of 

 a saprophyte to a plant pathogen, determined on artificial culture 

 media, is not a reliable measure of the actual control of the parasite in 

 the soil. A lack of specific microorganisms in the soil is not a sufficient 

 factor limiting biological control under natural conditions. Therefore, 

 no inoculation of soil with an antagonistic organism, such as T. lig- 

 norum, can have more than a temporary effect in changing the micro- 



