xiii. A, 4 Remking: Philippine Economic-Plant Diseases 185 



canker: pseudomonas citri hasse 



This infectious and destructive disease is widely prevalent in 

 the Islands. On the commonly planted citrus fruit, Citrus nobilis 

 Lour. (Satsuma orange, Canton mandarin), the disease is only 

 slightly prevalent and does little damage. It is, however, severe 

 on certain species in the college plantation at Los Banos, Laguna, 

 where test varieties are grown. These different varieties are 

 affected in the following order, the first-named being the most 

 severely attacked: Citrus maxima (Burm.) Merr. {Citrus decv^ 

 mana Linn.) (large pomelo, bitter pomelo, djersek boli). Citrus 

 sp. (Kusaie lime), Citrus (large orange), Citrus (Lisbon lemon), 

 Citrus (Washington navel). Citrus (rough lemon). Citrus medica 

 Linn, (citron), Citrus nobilis Lour. (Satsuma orange. Canton 

 mandarin). Citrus sp. (small orange). Citrus hystrix DC. (wild 

 lime), Citrus mitis Blanco (calamondin), and Citrus japonica 

 Thunb. 



This list is based on young plantings, and the order of attack 

 will probably change somewhat as observations continue. A 

 great variation occurs in the susceptibility of different varieties 

 in the same species. 



Citrus maxima (Burm.) Merr. {Citrus decumana Linn.) is 

 most severely attacked when young. Older trees of native varie- 

 ties grown by the Filipinos in the neighborhood of the college 

 are attacked, but as with Citrus nobilis Lour, little damage is 

 done. 



Symptoms. — The characteristic appearance of the disease as 

 it occurs on citrus is as follows : 



Spotting is produced on leaves, stems, and fruit. At first the 

 spots on leaves are small, round, watery, slightly raised dots. 

 These dots enlarge, turn brown, extend through the leaf, become 

 raised on one or both surfaces, and have ruptured surfaces. Con- 

 centric, irregular rings may be produced in the brown portion 

 of the spots. A light yellow border is produced around the 

 brown center. Frequently spots run together, producing an 

 elevated, elongated, ruptured, brownish blotch (Plate III, fig. 

 2). In many cases a leaf -mining insect carries the infection 

 through its winding gallery, or mine, in the form of a chain of 

 canker spots (Plate III, fig. 3). This miner is the larva of a 

 small moth, Phyllocnistis citrella Stainton, and is common 

 throughout the Orient, being especially injurious to nursery 

 stock. 



On the twigs the spots are somewhat different. At first they 

 are similar to those on the leaf, but later become irregular, 



