74 Journal of Agricultural Research voi. vi, no. a 



been sustained. The operation of budding either directly conveys the 

 organisms into the wounded tissues or they are subsequently washed into 

 them from cankers above the insertion of the bud before union has been 

 effected. 



ETIOIvOGY OF THE DISEASE 



The primary cause of Citrus canker is a bacterial parasite, Pseudomonas 

 ciiri Hasse (6). Hasse isolated this organism from cankers on grapefruit 

 and proved it to be pathogenic to grapefruit seedlings. This claim was 

 established at a time when the disease was regarded as of fungus origin. 

 Hasse further pointed out the fact that a number of fungi were isolated 

 from old Citrus cankers. The writer had found a fungus, as had also 

 Prof. H. E. Stevens, of the Florida Agricultural Experiment Station, 

 belonging to the form genus Phoma, commonly associated with cankerous 

 tissues. The writer's initial inoculations were made not with pure 

 cultures of Phoma, as has subsequently been learned, but with cultures 

 which had overrun the bacterial parasite. Successful infections reported 

 in the previous publication (19) are thus accounted for. Consideration will 

 be given in another part of the present report to the part which Phoma 

 spp. and certain other fungi play in the production of Citrus canker. 



PATHOGENICITY 



Pseudomonas ciiri has repeatedly been isolated during the past season 

 from cankers on grapefruit, trifoliate orange, lemon, and Satsuma oranges. 

 The strains from these different hosts present the same cultural charac- 

 ters. Because of this, together with the added fact that no difficulty has 

 been experienced in making cross inoculations, the strains are regarded 

 as identical. 



The plants used in making the inoculation experiments were grown in 

 the greenhouse at Auburn, Ala. Typical cankers have been produced on 

 McCarty and seedling grapefruits (PI. IX, figs. 1,2), pineapple oranges, 

 Satsuma oranges, and seedling trifoliate oranges. Infections on all these 

 species were as readily secured, whether the organism had been isolated 

 from Citrus trifoliata, Satsuma, grapefruit, or lemon. Neither was there 

 any evident difference in virulence of any of the strains. A suspension 

 of the organism taken from pure cultures grown either on potato cylinders 

 or in bouillon was used in making the inoculations. This suspension 

 when applied with an atomizer resulted in a high percentage of successful 

 inoculations. A greater number of successful inoculations were secured, 

 as would be expected, when the plants were covered with bell jars to pre- 

 vent the too rapid evaporation of the moisture. When the inoculum was 

 introduced into the tissues of leaves, stems, or fruits through needle punc- 

 tures, cankers developed in all cases. In some cases the suspension was 

 applied to leaves with the fingers. They were dipped into the suspension 



