120 DISEASES OF TROPICAL PLANTS 



CH. 



has a much wider distribution. It is much worse in 

 moist than in dry climates. It is most common on the 

 leaves of the lemon which become very much roughened 

 by corky elevations on the upper surface, accompanied 

 by corresponding depressions on the lower surface. So 

 abundant does the fungus become that the leaves are 

 frequently distorted and prevented from fully perform- 

 ing their normal functions. When the fruit is attacked 

 it becomes very much roughened, and its market value 

 is very greatly reduced. 



It is sometimes quite serious on nursery stock, 

 stunting the seedlings before they can be budded. 



The disease can be controlled by spraying with 

 Bordeaux mixture. 



Gum Disease ; Gummosis ; Mai Di Gomma ; Foot Rot ; 

 Collar Rot. This is the most common and widespread 

 disease of the orange. It was first reported from the 

 Azores in 1832, but is now known in the orange-growing 

 districts of southern Europe, West Indies, Bahamas, 

 United States, Hawaii, Australia, and probably in all 

 other orange - growing regions. Of course, its wide 

 distribution gives rise to a very large number of names. 

 In 1878 Professor Giovonni Briozi expressed an opinion 

 that it was the result of the fungus Fusarium limonii, 

 but did not produce positive proof. Borg, as a result of 

 his studies in Malta, expressed the opinion that it was 

 due to Agaricus citri growing on the roots, but there 

 is nothing in other countries to confirm this view. 

 Savastano and Maymone say that it is due to (l) a 

 bacterium (probably B. gummi), (2) a breaking down 

 of cellular tissue due to excessive formation of sap, and 

 (3) to external causes such as wounds. For the present 

 at least, we must consider it a physiological disease 

 which may arise from one or more of several causes, 

 such as unfavourable soil, poor drainage, improper 

 fertilization, and mechanical injury. 



It is possible, and in fact probable, that the above 

 names may represent several diseases, which for want of 

 definite information have been classed as one. It is also 



