DECLINE OF PSEUDOMONAS CITRI IN THE SOIL 



By H. R. Fulton 



Pathologist, Fruit'Disease Investigations, Bureau of Plant Industry, United States 

 Department of Agriculture 



This investigation was undertaken primarily to determine whether or 

 not the citrus-canker organism, Pseudonwnas citri Hasse, is capable of 

 persisting in the soil to such an extent as to make the soil an important 

 medium in holding over or disseminating the organism. 



EXPERIMENTAL METHODS 



The work has been conducted in an isolated greenhouse near Washing- 

 ton, D. C. During the tests the soils were kept in ordinary 4- or 6-inch 

 earthenware flower pots in duplicate, triplicate, or quadruplicate sets for 

 each test. For the original inoculation of the soil it was found most 

 satisfactory to use washings from potato cylinder cultures 2 to 10 days 

 old. One such culture tube diluted with 200 cc. of water would give 

 heavy inoculation in a 4-inch pot. The bacterial suspension was well 

 mixed with the upper 3 inches of the soil, and samples were taken 

 from this portion from at least three different points. 



Because of the preponderance of more rapidly growing soil organisms, 

 ordinary plating methods are inadequate for determining the abundance of 

 P. citri in soil samples, and recourse was had to inoculation of punctured 

 mature grapefruit leaves with graded dilutions of washings from the soil 

 to be tested. The procedure was as follows: A sample of about 20 gm. 

 of the soil was removed with a sterile spoon to a sterile Petri dish, and 

 enough sterile distilled water was added to give an excess of about 10 cc. 

 beyond saturation. This was well stirred, and i cc. of the soil solution 

 was transferred to another Petri dish in which 9 cc. of sterile distilled 

 water had been previously placed. The first washing described above 

 is referred to as the i/i dilution in this paper, and the second as the i/io 

 dilution. In a similar way dilutions of i/ioo, 1/1,000 or beyond were 

 made from the original soil wash water. Small wefts of sterile absorbent 

 cotton were placed in each dish, one for each leaf to be inoculated. The 

 grapefruit seedlings used were grown in 2, 2X. or 3 inch pots. They 

 averaged 6 or 8 inches in height and had as a rule 8 to 12 leaves. 

 Usually 5 leaves per plant were used for inoculation, and each leaf was 

 punctured at 100 points. A simple device for making these punctures 

 rapidly and accurately was improvised by inserting 10 sewing needles 

 through a small cork stopper. This "punch" was readily sterilized by 

 flaming the needles, was convenient to handle, made the punctures in a 

 uniform [^roup pattern, and thus contributed materially to the rapidity 



Journal of Agricultural Research, Vol. XIX, No. s 



Washington, D. C. June i, igao 



uh Key No. G-193 



(207) 



