252 CONTROL OF SOIL-BORNE PLANT DISEASES 



398, 783, 784). Methods for combating plant pathogenic fungi by the 

 use of bacteria and other antagonists have been suggested by various in- 

 vestigators (46, 503, 841). 



The principles underlying the biological control of soil-borne plant 

 diseases were outlined by Garrett (312) in terms of the soil population 

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

 ously in the sam.e soil, the parasitic organisms capable of attacking the 

 roots of that crop multiply (72). Organic manures stimulate the de- 

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

 check the activity of the pathogens, which are destroyed by the sapro- 

 phytes. Either the metabolic processes of the saprophytes check the 

 growth of the pathogens, or the saprophytes actually attack and destroy 

 the mycelium of the pathogens. The microbiological control of plant 

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

 become highly adapted to a parasitic form of life. The pathogenic 

 Ofhiobolus, when present in the form of mycelium inside the infected 

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

 conditions. 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 (906) 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 Trichoderma viridis and 

 Absidia sfinosa, exerted an adverse influence upon Rhizoctonia {Cor- 

 ticium) solanl and reduced its pathogenicity to cabbage seedlings (449). 

 Broadfoot (86) and others (248), however, emphasized that the an- 

 tagonism of a saprophyte to a plant pathogen, determined on artificial 

 culture media, is not a reliable measure of the actual control of the para- 

 site in the soil. A lack of specific microorganisms in the soil is not a suffi- 

 cient factor limiting biological control under natural conditions. There- 

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

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

 biological balance of the soil population. Similar results have been ob- 



