186 ^^6 Philippine Journal of Science isis 



raised, spongy brown spots, often with a dark brown border. 

 Spots are cankerous and persistent, but are only formed in the 

 outer layers of the bark tissue. Frequently twigs are entirely 

 encircled by cankers, but do not seem to be killed outright in 

 all cases (Plate IV, figs. 1 and 2) . 



In the first stages of the disease the spots on the fruit are 

 similar to those on the leaf. They may be scattered, but fre- 

 quently run together, forming irregular, raised, brown cankerous 

 blotches. The surface of the fruit is depressed or slightly 

 wrinkled in the neighborhood of large blotches (Plate IV, figs. 

 2 and 3). Cankers do not penetrate deeply below the surface 

 and seem to do little harm other than producing an unsightly 

 appearance. Gumming of the fruit is associated with the disease 

 in some cases, but this is not general. 



Causal organism. — The bacteria causing this disease gain 

 entrance to the host primarily through injuries. Citrus leaves, 

 especially those of the highly spiny species, have many injuries 

 due to the whipping of the leaves against the spines. The or- 

 ganism is spread throughout the tree primarily by rain. The 

 bacteria grow well in pure culture, producing a yellow pigment. 



Control. — The control of citrus canker is rather difficult. The 

 orange, Citrus nobilis Lour., is the most commonly cultivated 

 species in the Philippines and is relatively free from the disease ; 

 so no control measure need be applied for this species. Other 

 species are, however, severely infected. For these control mea- 

 sures should be practiced. In order to obtain an effective control 

 for citrus canker, persistent efforts must be used. This is true 

 of the majority of bacterial diseases of fruit trees. A combina- 

 tion control of spraying and pruning out of the diseased por- 

 tions will produce beneficial results. It is necessary, however, 

 to be on the lookout for new infections, and these must im- 

 mediately be removed. Monthly sprayings with Bordeaux mix- 

 ture, to which a sticker has been added, is the most beneficial. 

 Lime-sulphur must be applied in place of Bordeaux from time 

 to time, in order to combat aphids and scale insects. In severe 

 cases of infection it is advisable first to remove diseased leaves 

 by spraying with a strong formalin spray (0.4 to 0.5 per cent), 

 which will cause them to drop off. 



chlorosis: nonparasitic 



Symptoms. — Frequently leaves on certain trees show a general 

 yellowing in contrast to a definite mottling as produced in mot- 

 tled leaf. In some cases this yellowing may cover the entire 

 leaf, while in others large yellow blotches are produced. A 



