222 Journal of A gricultural Research voi. xix, No. s 



inoculated. After two months the seedlings in all had made good growth, 

 and there was no evidence of citrus-canker lesions on any part of the 

 plants. In the light of present knowledge it would not have been ex- 

 pected that a single soil inoculation at the time of planting would per- 

 sist long enough to become very effective. 



On July ID, 1 91 8, a series of pots was planted with grapefruit seed and 

 given frequent waterings with P. citri suspension, on July 10, 13, 17, 20, 24, 

 27, and August 2. By this time the seedlings had emerged above ground, 

 and before each subsequent watering several cuts were made through 

 the soil with a knife to produce root wounds. Further applications of 

 P. citri suspension were made August 5, 8, 10, 14, and 16. On August 

 31 the seedlings were removed, washed, and examined with a hand 

 magnifier. No canker lesions were apparent on any part of the 40 plants 

 thus examined. A test performed on grapefruit leaves with soil from 

 these pots which had received 12 applications of heavy inoculum at 

 close intervals gave negative results. 



Direct inoculation of the roots of potted grapefruit seedlings was 

 made as follows: On July 27, 191 8, potted plants were selected with 

 vigorous roots of about ^/^g inch diameter extending J^ to 3 inches 

 through the drainage holes of the pots. These roots were punctured at 

 10 points each, wrapped in cotton wet v\^ith P. citri suspension, and later 

 placed in flats of moist, clean sand. Two weeks later infection was 40 per 

 cent. Microscopic sections showed typical canker lesions involving the 

 cortex. Pure cultures of P. citri were readily obtained by plating, and 

 grapefruit leaves were infected therefrom. Four months later no ex- 

 tension of infection was apparent on the roots, most of them having 

 continued their growth to all appearance normally. In several, however, 

 the roots were broken at old lesions apparently following secondary decay. 

 The plants as a whole had not suffered. 



The indications are that young grapefruit roots are not readily infected 

 except through direct wound inoculation and that the plants do not suffer 

 from a moderate number of lesions so produced. 



SUMMARY 



(i) The method of using graded dilutions of soil washings for inocu- 

 lating punctured grapefruit leaves proved satisfactory for indicating 

 the relative abundance of P. citri in the soil at times of sampling. 



(2) Tests on many types of soil, including representative ones from 

 citrus regions, show a very rapid decline of P. citri in all. 



(3) This decline was retarded slightly by rendering the soil alkaline 

 with lime water or by lowering its temperature, and more decidedly by 

 withholding water or by previous sterilizing \vith steam. 



(4) An extremely long persistence, in very small numbers, is noted in 

 soil held in air-dry condition ; but the organism seemingly suffers prompt 

 extinction when water is again added. 



