494 Journal of Agricultural Research volxx.no. 6 



EFFECT OF HUMIDITY ON INFECTION AND DEVELOPMENT OF THE 



DISEASE 



The literature on this subject has already been discussed thoroughly, 

 and the consensus of opinion has been that citrus-canker developed best 

 and spread most rapidly in a warm, humid climate. It has also been 

 pointed out that the host plants themselves thrive best under these 

 influences. It has likewise been shown that the greatest number of 

 plants are infected at 30 C. in a saturated atmosphere, while even at 

 20 infection takes place, particularly on grapefruit and citrange plants. 



Just before the plants were placed in the humidity cases already 

 reported on, they were thoroughly sprayed with a 48-hour-old culture 

 of Pseudomonas ciiri in beef bouillon, which was almost allowed to dry 

 on the foliage before they were placed in the cases. 



No infections of any kind occurred on the plants listed in Table XIX 

 during a period of 18 days. 



In the second experiment (Table XX), only two infections occurred 

 during the 15 days the plants were in the cases. Both of these occurred 

 at the higher humidity. In one case, one spot developed on a young 

 leaf of a calamondin plant, and several corky spots were found on the 

 tip leaf of one grapefruit plant. No doubt, in these instances, the 

 organism was able to enter before the plants had adjusted themselves 

 to the humidity of the case. On January 31, 1919, the plants in both 

 cases were removed to a saturated atmosphere and approximately the 

 same temperature. Within eight days, one plant of the Rusk citrange, 

 two of the calamondin, and one of the grapefruit became infected as 

 shown in Table XXI. Only two spots on two mature leaves of one 

 of the grapefruit plants developed on those held at the lower humidity 

 before being transferred to a saturated atmosphere. 



