9H Problem.'^ of Economic Biology in East Africa 



VI. Citrus. 



The lime is found wild along the coast, probably introduced from the 

 east by the early Portuguese settlers, while the excellent greenish orange 

 of Zanzibar was probably planted by the early Arab traders. 



At the present time all varieties of citrus are grown, generally from 

 budded stocks imported mostly from South Africa, and at one time it 

 was thought possible to export the fruit to England. Oranges, lemons, 

 grape-fruit, etc. might be placed on the English market at a time when 

 no citrus fruits from other countries were available. Partly because of 

 heavy sea freights via the Cape, and still more heavy Suez Canal dues, 

 and also because the comparatively young plantations of the Highlands 

 are still producing rather thick-skinned and not very juicy fruits, the 

 attempt has not yet been made. Those plantations which have been 

 raised from seed bear very much better fruit but take longer to reach 

 this stage, namely, eight years as against three years from budded stocks. 

 Very much better fruit is produced at lower elevations such as 3000 to 

 4000 feet. A few factories have been started for the extraction of citric 

 acid. 



Perhaps the most serious disease, if not the most common, is the 

 foot-rot or mal-di-goma, generally ascribed to the attack of Fusarium 

 limonis Briozi, but which is more likely due to bacteria in the first place. 

 The Fusarium is probably secondary and gains entrance through the 

 cracks of the bark brought about by the activity of the bacteria. 



Various bacterial leaf spots, but not the Citrus Canker of the Gulf 

 States and South Africa, are common and usually make their appearance 

 in the dry season, causing considerable damage by defohation. This is 

 particularly the case in a neglected grove or one in an unsuitable situa- 

 tion, such as a stiff soil apt to be water-logged in the rains. The most 

 common form which the spot disease takes is large concentric rings of 

 small bhsters, hard in texture and brown in colour. 



As regards insect pests, citrus in East Africa is an outstanding example 

 of the introduction of pests into a country on the imported hosts. Both 

 the Australian bug, or fluted scale, Icerya purchasi Mask., and the Cali- 

 fornia red scale, Aspidiotus aurantii Mask., have been introduced in 

 this manner. Considerable loss has been caused by both, but more 

 particularly by the former, which does not confine itself to citrus and 

 will infest almost all other woody plants, as for instance coffee, roses, 

 black wattle (Acacia). No insecticide has yet been devised which will 

 control this scale effectively. Fortunately, however, there is an African 



