June 1, 1520 



Decline of Pseudomonas citri in the Soil 



213 



mental tests. At the same time it is probably as high as would be en- 

 countered in citrus plantings under infected trees; and the whole series 

 may be regarded as representing high, medium, and low degrees of soil 

 infection under natural conditions. 



The differences are apparently not so much in rate of decline as in 

 time required to reach the zero level from the different initial levels of 

 inoculation. 



INFLUENCE OF SOIL TEMPERATURE ON PERSISTENCE 



The test for low temperature effect, series i, which is reported in 

 Table V, was made by exposing the inoculated potting soil to outdoor 

 temperatures, beginning October 11, 191 8. During the test the minimum 

 daily readings ranged from 60° to 23° F., and the maximum daily readings 

 from 83 to 58°. For moderate temperatures, exposure was made in the 

 greenhouse, with daily means averaging about 15° higher than outside. 

 Series 2 was begun October 20, 191 9, using an incubator at 95° for the 

 high range and the greenhouse for the moderate. The actual soil tem- 

 peratures 2 inches below the surface were taken, the high temperature test 

 ranging from 86° to 90° and the moderate from 68° to 72°. Percentages 

 for series i are based on 600 inoculated punctures, and for series 2 on 2,000. 



Table V. — Percentages of infection on grapefruit leaves inoculated with graded dilutions 

 of soil solutions at various intervals after the soil had been inoculated "uvith P. citri and 

 had been held at different temperatures 



o Inoculated by dipping plant top in liquid. 



There is a very evident retardation of the rate of decline at the lower 

 temperatures. A second series of October 23, 191 8, confirms this for a 

 still lower range of temperature. The higher temperatures seem to 

 accelerate the decline, but the unfortunate low initial inoculation of the 

 soil requires a repetition of the test. At the time of handling series i , the 

 influence of soil dryness in prolonging persistence had not been determined, 



