72 



Journal of Agricultural Research 



Vol. VI, No. 2 



greenish yellow spots. By continued development the convex surface 

 of the spots comes to be more and more elevated until the epidermis is 

 broken by the increased tension and the subjacent tissues are thus ex- 

 posed to desiccation. The exposed tissues then become corky, darken- 

 ing with age. The ruptured epidermis is turned back irregularly and 

 persists as a lacerated membrane. The margin of the diseased area 

 maintains an oily appearance even after the spots have ceased to increase 

 in size. Mature spots (Pi. XI, fig. i) vary in size from very minute to 

 a quarter of an inch in diameter and are typically circular in outline. 

 They may occur singly; or when they are very numerous, fuse, thus 

 forming large, irregular areas. Cankered areas are typically elevated on 

 both leaf surfaces. In the case of canker on Satsumas (Pi. VIII, fig, 2, 

 and PI. IX, fig. 4), however, there is little or no elevation of the upper 

 leaf surface. Neither is the oily margin so evident on this host, espe- 

 cially in case of old cankers, 



in which diseased tissues have 

 become dark brown, simulat- 

 ing the appearance of mela- 

 nose. The appearance of the 

 disease on leaves of Citrus 

 trifoliata as shown in Plate 

 X, figure I, is very similar to 

 that on grapefruit. Stevens 



Fig. I.— Diagrammatic representation of young open type of (2) rCpOrtS that he haS neVCr 



Citrus canker of half the diameter of the one shown in fo^^d CitrUS Cankcr On trifo- 



figure 2. pp. Palisade parenchyma; ue, upper epidermis; _ 



le, lower epidermis; d, diseased tissues: a, air space aris- Hate OraUgC IcaVCS. The Un- 



ing from tensions due to the enlargement of cells and dis- j^vadcd tisSUCS SUrrOUndiug 



mtegration of tissues. ° 



the cankers are paler green 

 than the normal tissue and gradually form a chlorotic or yellowish zone 

 (PI. VIII, fig. I, and PI. X, fig. 6), which may invade all the tissues 

 not actually occupied by the cankers. At this stage considerable 

 defoHation, especially in the case of grapefruit and trifoliate oranges, 

 may occur. Cankers on the leaf petioles cause defoliation even though 

 the leaves are otherwise uninvaded. 



OCCURRENCE ON THE TWIGS AND BRANCHES 



Limb canker appears more commonly on very young twigs because 

 of the absence of any considerable suberization, but larger branches are 

 subject to infection. Growing cankers have been observed on limbs 

 >^ to K inch in diameter (PI. VIII, fig. 3, 4), The disease has been found 

 on branches of grapefruit, trifoliate oranges, lemons, Satsumas, and 

 certain varieties of round oranges. Cankers on twigs are first apparent 

 as small, circular, watery spots. They rapidly enlarge, become blister- 

 like and the epidermis ruptures, exposing the cankerous tissue below. 

 At this stage they project more or less prominently and are very similar 



