June 1, 1920 Decline of Pseudomonas citri in the Soil 223 



(5) The decline is accelerated decidedly by the addition of dilute sul- 

 phuric acid or by a moderate rise in temperature. 



(6) P. citri may easily penetrate the soil to depths ordinarily culti- 

 vated, but the normal decline seems to occur at such depths. 



(7) In water the decline is more rapid than in soil. Previous steriliz- 

 ing of the water has a decided effect in prolonging persistence. 



(B) Certain bacteria found commonly in soils have a marked dele- 

 terious effect on P. citri in artificial culture media both by inhibiting 

 growth and by killing. 



(9) The presence of such deleterious organisms in soils would probably 

 be concerned in producing a decline of P. citri. 



(10) Young roots of grapefruit seedlings seem not to be readily infected 

 by P. citri except through wounds. 



CONCLUSION 



The main question at issue is whether or not P. citri can persist in the 

 soil to a sufficient degree or for a long enough time to be a source of 

 danger in the dissemination or holding over of the citrus-canker disease. 

 The experimental evidence shows clearly that the organism undergoes a 

 rapid and continuous decline in numbers under soil conditions that 

 would obtain in agricultural practice. As a rule, this decline reaches 

 the vanishing point for P. citri in about two weeks by the test methods 

 employed, and it is only reasonable to suppose that the downward trend 

 continues rapidly in such cases to absolute extinction. The potential 

 ability of certain soil organisms to destroy P. citri, as shown in certain 

 artificial culture media, lends weight to this latter supposition. Even 

 where long-time persistence has been induced experimentally, the 

 conditions necessary to bring it about are too extreme to make a dupli- 

 cation probable under natural conditions. Furthermore, the experi- 

 mental methods employed for testing the infectiveness of the soil are 

 many times more severe than would obtain under most favorable natural 

 conditions for the spread of infection from soil to plants. All these con- 

 siderations suggest that agricultural soils probably can not long retain 

 a dangerous possibility of disseminating the citrus-canker organism. 



