Dec. 15, 1920 Effect of Temperature and Humidity on Citrus-Canker 489 



Wolf (17), varying the method somewhat, states that: 



The organism seems to exhibit a very considerable resistance to drying. In the 

 desiccation experiments bacteria from vigorous pure cultures on potato plugs were 

 smeared by means of a sterile platinum needle on clean microscopic slides in moist 

 chambers. The moist chambers containing the microscopic slides were sterilized 

 prior to transferring the bacterial smear to the slides. These preparations were made 

 on June 1, and placed in a wall closet in the laboratory. On July 1, August 1, and 

 September 1, several of the microscopic slides were removed from the moist chambers 

 and placed in the sterilized Petri dishes, using proper aseptic precautions in making 

 the transfers. Tubes of melted nutrient agar which had been cooled almost to the 

 point of solidification were poured upon these smeared slides. No growth occurred 

 in the case of those tested on September 1 , but those tested on July 1 and August 1 

 were still alive. From this, it is believed that the organism can retain its viability for 

 about two months. 



Stevens (12) later carried out the following experiment: 



Pieces of sterilized cloth were wetted with suspensions of bacteria (P. citri) from 

 cultures of different ages, from four days old to seventy-five days old . The pieces were 

 then allowed to dry in the air of the laboratory in the dark. Germination tests from 

 these pieces of cloth showed a very large number of the organisms alive after a drying 

 period of five weeks. 



He also states: 



That the bacteria may live for a month or more in the dried canker spots, is shown 

 by the disease having been transferred to healthy citrus tissue fronf dried leaves that 

 had been kept in the laboratory for a month. 



On the other hand, Wolf (77) states that : 



Unsuccessful attempts, however, have been made to recover the organism from the 

 leaves kept in the laboratory from September, 1014, to May, 191 5; nor has recovery 

 been possible in the case of twig cankers kept under laboratory conditions from March 

 to October, 1915. 



Stevens (75) concludes from his experiments with the growth of Pseu- 

 domonas citri in dry sterilized soil that — 



P. citri can propagate and remain alive and virulent when kept in soil for a 

 period of twenty-six months, and that the organisms are capable of surviving long 

 periods of desiccation without complete loss of vitality and with little apparent loss 

 of virulence. 



The following experiments, which are to be considered of a preliminary 

 nature only, were undertaken to determine the viability of the organism 

 at different temperatures and under various humidities. 



The method used was essentially as follows: Eighteen silk threads 2 

 inches long were stretched across an aluminum wire frame 2}i inches 

 square, with legs 1^ inches high , inclosed in glass stockings of the same 

 height. These frames were then placed in ordinary moist chambers 2 

 inches high and 3^ inches wide and sterilized in the autoclave. Larger 

 Koch moist dishes, with ground-glass lids, were then sterilized. Under 

 sterile conditions, the threads were immersed in a 48-hour-old culture of 

 Pseudomonas citri in beef bouillon for 5 minutes. In the meantime, a 



