Apr. lo, 19 16 



Citrus Canker 



73 



in appearance to the spots on the foHage. Isolated cankers remain 

 circular in outline. When the spots originate close together, however, 

 large irregular, variously cracked or fissured cankers are developed, 

 which may involve an area several inches in length. The epidermis 

 persists as a grayish broken membrane at the margin of these cankers 

 (PI. VIII, fig. 5). Twigs and larger branches may be completely girdled, 

 resulting in the death of the distal parts. Affected trees exhibit a 

 stunted growth, and numerous branches may be developed below the 

 d5dng tips. 



The disease is very severe upon stems of grapefruit and trifoliate 

 oranges. On the latter host the thorns are abundantly cankered and the 

 base of the thorns appears commonly to be the initial seat of im'ection. 



Fig. 2. — Diagrammatic representation of canker on old Citrus leaf: pp. Palisade parenchyma; uc. upper 

 epidermis; le, lower epidermis; p, pycnidium of Pkoma socia; d, diseased tissues; a, air space arising 

 from tensions due to the enlargement of cells and disintegration of tissues. 



lyimb cankers on trifoliate oranges oftentimes are zonate with different 

 shades of brown, especially if the outer membranes have not yet been 

 ruptured. 



OCCURRENCE ON THE FRUIT 



The cankerous areas on the fruits are quite similar in appearance to the 

 leaf cankers, differing mainly in the larger size of the former. They are 

 scurfy elevations, for the most part circular in outline and surrounded by 

 a zone of chlorotic rind tissues. The corky diseased tissues are quite 

 superficial; and if the spots unite, large scaly areas are formed (PI. X, 

 fig. 2). In this case the fruits may crack open because of their increase 

 in size owing to the growth of the fruits and may become prematurely 

 yellow and drop. Fruits which are badly cankered and have burst open 

 are, of course, subject to invasion by various organisms of decay. Even 

 if they remain on the tree, they are rendered very unsightly and are 

 unsalable. 



OCCURRENCE ON THE BUDS 



Nurserymen experience considerable losses from failure of Citrus buds 

 to unite with the stock. In some cases when Citrus irifoliala seedUngs 

 affected with canker are used as stock, losses of over 50 per cent have 



