304 CONTROL OF SOIL-BORNE PLANT DISEASES 



their fungal symbionts. These antagonists are distributed by the insects, 

 thus spreading agents that are destructive to themselves and to their 

 fungus gardens. 



GENERAL METHODS OF CONTROL OF 

 SOIL-BORNE DISEASES 



Soil sterilization by heat and chemicals has long been practiced as a 

 method of controlling soil-borne fungus diseases. This phenomenon is 

 usually designated as partial sterilization of soil, since not all micro- 

 organisms are killed by these treatments. However, once a soil thus 

 treated becomes reinfected with a disease-producing organism, the in- 

 fection may become much more severe. It has, therefore, been sug- 

 gested (383) that partially sterilized soil be reinoculated with a mix- 

 ture of saprophytic microorganisms before it is used as a seedbed, so as 

 to counterbalance the injurious effect of the parasites (Table 51 ). 



The importance of the soil microflora in modifying plant diseases 

 caused by soil-borne pathogens is being realized more and more clearly. 

 One of the earliest attempts to control a plant disease by microbiological 

 agents was made in 1 908 by Potter (731). He found that Pseudomonas 

 destructansy the cause of rot of turnip, produces a potent, heat-resistant 

 toxin. The bacteria failed to grow in the presence of this toxin, and were 

 completely killed by the substance. By spraying turnips with this mate- 



TABLE 51. EFFECT OF A BACTERIAL ANTAGONIST ON DAMPING-OFF 

 OF PLANTS IN THE SOIL 



From Cordon and Haenseler (154). 



