250 CONTROL OF SOIL-BORNE PLANT DISEASES 



GENEP.AL METHODS OF CONTROL OF 

 SOIL-BORNE DISEASES 



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

 method of control of 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 (380, 682) that partially sterilized soil be reinoculated with a 

 mixture of saprophytic microorganisms before it is used as a seedbed, 

 so as to counterbalance the injurious effect of the parasites (Table 47). 



TABLE 47. EFFECT OF A BACTERIAL ANTAGONIST ON DAMPING-OFF 

 OF PLANTS IN THE SOIL 



PERCENTAGE OF SEEDS PLANTED 

 PRODUCING NORMAL SEED- 

 TREATMENT OF SOIL LINGS IN 2 WEEKS 



Cucumbers Peas 



From Cordon and Haenseler (149). 



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 1908 by Potter (697). 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- 

 rial, the disease could be checked j the toxin was more or less specific for 

 the particular organism. Certain bacteria commonly found in soils were 

 shown (304) to have a deleterious effect on the growth, in artificial 



