June 1, 1920 



Decline of Pseudomonas citri in the Soil 



219 



While testing experimental methods, it was found that the organism 

 is readily carried in the capillary current along an absorbent cotton wick 

 at least i o inches. 



The indication that P. citri may readily penetrate deep into the soil 

 raises the question of whether conditions deep in the soil may influence 

 the persistence of the organism differently from those near the surface. 

 A test was made by burying 4-inch pots of inoculated potting soil in large 

 containers, so that the pots were completely surrounded by 8 inches of 

 soil. Samplings were made at approximately 5-day intervals. No 

 decided difference was noted between the buried pots and similarly 

 inoculated ones held on the greenhouse bench. 



PERSISTENCE IN AUTOCLAVED SOIL 



Greenhouse potting soil in 4-inch pots was autoclaved July 15, 191 8, 

 for one hour with steam at 1 2 pounds pressure. When the soil was cold 

 four autoclaved pots were inoculated, as well as four others containing 

 similar soil not autoclaved. The percentage results in Table X are based 

 on infection out of 1,200 inoculated punctures. 



Table X. — Percentages of infection on grapefruit leaves inoculated with graded dilu- 

 tions of solutions from unauioclaved and autoclaved soils at various intervals after the 

 soils had been inoculated with P. citri 



Number of days 

 between in- 

 oculation and 

 sampling. 



Unautoclaved soil. 



i/i 



i/io 



i/ioo 



1/1,000 



Autoclaved soil. 



i/i 



i/io 



l/l,C 



4 

 9 

 14 

 18 

 24. 

 29 



35 

 44 



73 

 60 

 0.9 



73 



35 



o. 



16 



7- 



o 

 o 

 o 



44 

 60 

 28 



9-5 

 9.2 



3- 

 68 



17 



4- 



13 



6.2 



13 

 75 

 .8 



9-4 



2. I 



•3 

 o 



15 

 40 



The pots were kept on the greenhouse bench, each covered with paper. 

 No special precautions were adopted to insure continued sterility in the 

 autoclaved pots, if indeed the original steaming was sufficient for com- 

 plete sterilization. Platings on agar at the end of the test showed mis- 

 cellaneous bacteria in these pots in apparently as great numbers as in the 

 unautoclaved ones. The autoclaved soil shows a decided lag in the 

 decline of P. citri. A second series run two months later confirms this 



result. 



PERSISTENCE IN WATER 



Water was held in cotton-stoppered flasks in 200-cc. quantities. Water 

 from a local spring was used in comparison with distilled water. Unfor- 

 tunately the flasks of autoclaved distilled water became contaminated, 



175343°— 20 3 



