June I, 1920 



Decline of Pseudomonas citri in the Soil 



209 



SENSITIVENESS OF DILUTION METHOD OF TESTING 



In various tests involving several thousand plants, the standard 

 testing method, which employs graded dilutions of the soil washing for 

 inoculation on punctured grapefruit leaves, has proved reasonably 

 sensitive in detecting the presence of viable P. citri in the soil. It is 

 satisfactory for securing a rather definite idea of the relative numbers 

 of this organism at the various times of sampling. 



To test the efficiency of the method, dilutions in decimal series were 

 made from a loopful of potato cylinder culture of P. citri distributed in 

 the requisite number of cubic centimeters of sterile distilled water and 

 were carried well beyond the vanishing point. One-cc. portions from 

 each dilution were plated in beef agar for P. citri counts. Cotton swabs 

 were dipped in the remainder of each dilution and applied to grape- 

 fruit leaves having 100 punctures each. Measurement showed these 

 swabs to carry an average of 0.7 cc. of the liquid. The results of two 

 independent tests are given in Table I. 



Table I. — Comparison between number of infections on grapefruit lea'ves and counts on 

 poured plates, using graded dilutions of P. citri 



TEST A 



Average number of infections, 



20 leaves tested 



Average count for i cc. inoculum, 



5 plates 



Average number of organisms 



applied per leaf 



Average number of organisms 

 per infection 



Average number of infections, 



16 leaves tested 



Average count for i cc. inoculum, 



6 plates 



Average number of organisms 



applied per leaf 



Average number of organisms 

 per infection 



76 



22, 300 



15, 600 



205 



14 

 2,500 



1.750 

 125 



300 

 210 



105 



TEST B 



69 

 25, 000 

 17. 500 



9 

 2,500 



1.750 

 194 



I. 4 

 277 

 194 

 139 



o- 13 

 28 

 20 

 154 



2-3 



0-5 



The method apparently gives evidence of something like 30 organisms 

 per cubic centimeter of inoculum, provided as many as 20 test leaves 

 with 100 punctures each are used. The upper limit of sensitiveness 

 would evidently be reached when numbers of bacteria are sufficient to 

 infect practically all punctures, and diminution of sensitiveness would 

 appear earlier. The ratio between infections per leaf and bacteria 



