Dec. is, 1920 Effect of Temperature and Humidity on Citrus-Canker 45 7 



No visible growth occurred at 5 C. or at 38 to 40 . Growth was 

 visible at io° the third day, at 15 the second day, and at 20 , 25 , 30 , 

 and 33 to 35 at the end of 24 hours. However, at 33 to 35 no increase 

 in growth was noted after the first day. 



Growth proceeded at a regular rate at 20 C, 25 , and 30 , each of the 

 lower temperatures being about one day behind. Growth at 15 never 

 approached that produced at 20 . The growth of Pseudomonas citri in 

 beef bouillon at the different temperatures proceeded at the same relative 

 rate as on agar and potato plugs. 



Cultures placed in the 5 C. and the 33 to 35 cases were viable 

 at the end of 8 days, while no organisms were viable in tubes held 

 at 38 to 40 for 24 hours. In constant water baths, the organisms 

 were viable at 37. 6° to 38 at the end of 24 hours and at 41 ° to 41. 5 

 at the end of 2 hours. At 43. 2 to 43. 6° all organisms were dead at the 

 end of 2 hours. The interesting point brought out here is that no organ- 

 isms were viable in the incubators run at 38 ° to 40 C. at the end of 24 

 hours, but at 37. 6° to 38 in the water bath they were viable at the end 

 of 24 hours. Thus, the point at which growth is completely inhibited 

 at the higher temperatures is very sharp when the time factor is constant. 



In determining the thermal death point by the usual laboratory 

 methods, all the organisms were killed at a temperature of 52 C, while 

 all the trials at 49 yielded positive results. Thus, the thermal death 

 point of the citrus-canker organism is above 49 and below 52 . 



Distilled water. — To determine the length of life of Pseudomonas 

 citri in water, ordinary distilled water, supplied by the Department of 

 Chemistry, University of Illinois, was used, as it was impossible at the 

 time to obtain good well or rain water. The distilled water contained 

 traces of organic matter, but no mineral substances were present. No 

 attempts were made to determine the conductivity of this water. 



The sterilized water blanks were placed in the cases at the various 

 temperatures overnight and were inoculated with a loop from a 48- 

 hour-old culture of the organism in beef bouillon. At the end of each 

 day, two tubes were withdrawn from each case and were reinoculated 

 into beef bouillon to test for growth. In Table V the results of the 

 experiment are given. 



Table V. — Viability of Pseudomonas citri in sterilized distilled water at various temper- 

 atures 



