i62 DISEASES OF CROP-PLANTS 



penetrated by an abundant dark slaty- or brownish-grey mycel- 

 ium, which envelops and quickly destroys the beans. 



Control. 



The main points in the prevention of loss from this affection 

 are indicated by the conditions of attack, i.e., avoidance of 

 wounds (most of which are made by the pickers) and prompt 

 collection of ripe or injured pods. The fungus is so abundant 

 and the materials for its development so varied that attempts 

 to avoid infection, once a favourable opening exists, seem to a 

 mvcologist rather hopeless. Against this has to be set, however, 

 the fact that experienced agricultural officers in close touch 

 with local conditions attribute visibly good results to sanitary 

 disposal of husks and deplore any looseness in this matter. A 

 further means of reducing the amount of infective material 

 lies in the regular removal of dead and sickly branches from the 

 trees. 



Phytophthora Pod-rot and Canker. 



It is regarded as established that the common types of 

 pod-rot and stem canker are due to the attacks of the same 

 fungus, and that there is in many cases a connection between 

 them; they may therefore be most conveniently discussed 

 together. 



Distribution. 



Apparently these diseases occur in all cacao-growing coun- 

 tries, though there are very considerable differences in their 

 incidence. In the Western Tropics and in Ceylon both canker 

 and pod-rot are prevalent, but pod-rot is the more prominent 

 trouble, while in Java canker is a very serious disease and pod-rot 

 relatively uncommon. These differences have not been ex- 

 plained, but may possibly be accounted for by the varying 

 resistance of different types of tree and by the close dependence 

 of the fungus on humidity. The loss of ripe pods during the 

 rainy season in Trinidad is estimated by J. B. Rorer at 30 to 60 

 per cent. There are also heavy losses of young pods, as is shown 

 by the increase obtained from spraying, and, further, the injuries 

 to the trees themselves are a matter of serious consideration. 



History. 



The existence in Ceylon of a disease of the nature of canker 

 was recorded in 1833. At a much earlier date, in 1727, the 

 plantations in Trinidad were crippled by an epidemic of blight 

 on the pods, which more likely than not was due to the affection 

 under notice. The industry did not recover until the Criollo 

 then grown was replaced by the more hardy Forastero 30 years 

 later. In recent times J. B. Harrison called attention to pod-rot 



