or waliTV (lots, most commonly on the lowei' leal' sur- 

 face. They are slightly convex and within a lew days 

 will liave extended entii'ely through the leal". The 

 spots gra(hially increase in size, the convex surlace 

 comes to l)e more and more elevated on one or both 

 leaf surfaces until the outer mend)ranes of the leaf 

 are broken, whereupon the exposed canker tissues be- 

 comes light brown in color (Plate 2, figs. 1, 4 and 6.) 

 An oilv border marks the margin of the cankers which 

 come to be surrounded by a yellowish zone. All of 

 the tissues not occupied by the lesions may become 

 yellow, in which case the leaves fall, especially when 

 grapefruits and trifoliate oranges are affected. 



Clankers on twigs and larger lind^s do not differ ma- 

 terially in appearance from lesions on the leaves. (Plate 

 1. figs. 1 and 2.) They arc larger, project more or less 

 prominently and become variously cracked or fissured. 

 Their presence on the twigs may result in the stunted 

 growth or death of the distal parts. The cankerous 

 areas on the fruits are also similar in appearance to 

 the leaf cankers. Scurfy, elevated spots surrounded by 

 a yellowish zone are formed. When large areas are 

 involved the fruits crack open, thus permitting organ- 

 isms which cause decay to enter. Affected fruits 

 usually fall, or if they remain on the tree they are 

 rendered very unattractive. 



Cause of Citrus Canker. 



The primary cause of citrus canker is a bacterial 

 parasite, Pseudomonas citri. Announcement of this 

 fact was first made by Miss Hasse (3) as a result of iso- 

 lation of the organism from grapefruit and reinocula- 

 tion with pure cultures upon grapefruit seedlings. The 

 disease had previously been regarded as of fungous 

 origin but the successful artificial inoculations from 

 which this conclusion was drawn were produced by 

 the use of mixed cultures containing Pseudomonas 

 citri. The bacterial organism has repeatedly been isolat- 

 ed during the past summer from cankers on grape- 

 fruits, Satsumas, trifoliate oranges and lemons. No 

 difficulty has been experienced in effecting cross inocu- 

 lations from any of these hosts upon either grapefruit, 



(3 1 Hasse, Clara H. Pseudomonas citri, the cause of citrus 

 canker, Jour, Agr. Researcli 4. No. 1, pp. 97-101, Pis. IV and 

 X, 1915. 



