CHAPTER XXI 



DISEASES OF HEVEA. 



The South American Leaf Disease. 



A FUNGOID leaf-disease which occurs on the wild Hevea species 

 in the forests of Brazil, Peru, British Guiana and Surinam has 

 attacked the plantations of Hevea brasiliensis made in the two 

 countries last mentioned, and has assumed such severity that 

 there seems no likelihood of the rubber industry becoming 

 permanently established. The disease also occurs in Trinidad, 

 where although young plants are severely attacked the mature 

 trees in plantations have so far almost entirely escaped injury. 

 It has not appeared in the rubber cultivations of the Eastern 

 tropics. 



The history of the disease is instructive as illustrating the 

 difference between the incidence of disease on plants scattered 

 in a natural way and collected in cultivations, and shows also 

 the advantage that may sometimes be gained by growing a 

 crop plant away from its native country and thus outside the 

 range of its specific parasites. 



Nature of the Disease. 



A detailed investigation of the disease has been made in 

 Surinam by G. Stahel, from whose paper the following account 

 is abstracted. 



The attack on the leaves begins when they are very young, 

 and, as they develop, yellowish spots with a grey layer of conidia 

 on the under side become apparent. The central part of the 

 larger spots dries up and falls away, leaving a ragged perforation. 

 On full-grown leaves the surface becomes thickly sown with black 

 dots, and on old leaves these have developed into rounded black 

 spots interspersed with several or many ragged holes surrounded 

 by a black ring. 



The fungus attacks the petioles and young internodes, but 

 much less frequently than the leaves. On the petioles the check 

 to growth on one side may cause curvature or spiral twisting ; 

 on the internodes swollen canker-like patches are produced. 

 The inflorescences and fruits may also be infested. 



The Fungus. 



The causative fungus Melanopsammopsis Ulei (Henn.) Stahel, 

 has three forms of fructification : free conidiophores, pycnidia, 

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