212 



Journal of Agricultural Research 



Vol. XIX, No. s 



There is a marked decline preceding the tenth day in all of the soils 

 of Table III. But scattering infections are apparent over a much longer 

 period, and no one of these soils could be safely declared free of P. 

 citri at the times of discontinuance of the respective tests. It is a fact 

 that regular watering of the pots was overlooked during the latter part 

 of the longer tests, and the dry condition probably contributed to the 

 long persistence. Special evidence on this point is given later in this 

 paper. 



Florida soils were also used in a number of other special tests, ac- 

 counts of which follow throughout this paper. 



Samples of soil from citrus plantings at Biloxi and Big Point, Miss., 

 were artificially inoculated and tested at 6-day intervals for persistence 

 of P. citri. The results were negative on the twelfth day and afterwards. 



INFLUENCE OF DEGREE OF INITIAL SOIL INOCULATION 



A series was set up September i6, 191 8, using three degrees of inocu- 

 lum, one five times and another one-fifth the usual medium degree. Un- 

 fortunately this series was discontinued on the twelfth day, just when 

 the decline from the heavy inoculation was beginning to be pronounced. 

 A second test was begun October 20, 191 9. Greenhouse potting soil was 

 used. The medium inoculation consisted of 0.4 of the washings from 

 a potato cylinder culture for each of the duplicate pots. The heavy 

 inoculation was 10 times this, and the light inoculation one-tenth. The 

 pots were kept in the greenhouse, were shaded, and were given ordi- 

 nary watering. Each percentage given in Table IV is based on 2,000 

 inoculated punctures. 



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

 tions of soil solution at various intervals after the soil had been inoculated with P. citri 

 in different degrees 



It is not understood why all the initial soil inoculations in this series turned 

 out to be so far below the expected degree. What was intended for heavy 

 soil inoculation ran considerably below that ordinarily used in other experi- 



