292 Disease-Producing Microorganisms 



In addition to organic matter, the use of artificial fertilizers, espe- 

 cially nitrogen and phosphorus, and the nature of the soil have a 

 considerable influence on the development of different organisms 

 causing plant diseases. The other most important treatments which 

 have a significant effect are those leading to changes in soil reaction. 



Organisms Antagonistic to Plant-Disease-Producing Microbes 



The antagonistic action of soil organisms to fungi causing plant 

 diseases results in a modification of the virulence of the pathogens 

 that find a temporary or permanent habitat in the soil. Various bac- 

 teria capable of inhibiting the development of the corn smut have 

 been isolated. These bacteria are able to destroy the smut fungi. 

 The widespread distribution of such bacteria in the soil was believed 

 to check the multiplication of the pathogen. Some of the antago- 

 nistic bacteria produce enzymes which have the capacity to dissolve 

 the chemical constituents of the cell walls of the fungi. Chudiakov 

 isolated two bacteria which lysed different species of Fusariiim and 

 other pathogenic fungi. These bacteria were widely distributed in 

 the soil, but were absent in certain flax-sick soils. When the pathogen 

 was introduced into soils containing active antagonistic bacteria, it 

 did not develop, and no disease was produced. 



The virulence of the root rot of cereals caused by Ophiobolus 

 graminis received special attention. It was believed that this or- 

 ganism could be completely controlled by the activities of various 

 soil-inhabiting microorganisms; filtrates of these organisms were 

 nearly as effective in repressing the pathogen as were the living 

 organisms. The fact that potato scab can be reduced by plowing 

 under a green rye crop has also been explained by the development 

 of other organisms, such as saprophytic actinomycetes, which sup- 

 press the growth of the pathogenic S. scabies. 



The "biological control" of various soil-borne diseases was thus 

 suggested. This method consists in modifying the soil so as to 

 encourage the maximum development of the antagonizing sapro- 

 phytic soil population. Inoculation of sterilized soil with the sapro- 

 phytic fungus Trichoderma, for example, was found to prevent in- 

 fection of citrus seedlings by the pathogenic Rhizoctonia (Table 64). 



The spoilage of fruits can also be suppressed by inoculation with 

 mixtures of known organisms. According to Potter, the organism 

 causing rot of turnips produces a potent, heat-resistant toxin, which 

 is also destructive to the pathogen itself. Spraying turnips with this 



