DISEASES OF CACAO 157 



While the writer believes that the retention of humidity is 

 an important contributory effect, he is disposed to agree with 

 van Hall in the view that it is the protection of the soil, and not of 

 the tree, that is the most important function of shade trees. 

 The most pressing anxiety of the cacao planter— be his fields 

 with or without shade — is to fill up with quick-growing leafy 

 plants any gaps which may occur in the canopy. The rapid 

 loss of humus which under tropical conditions takes place in 

 exposed soil is well known. This is avoided over a large part of 

 the year by the provision of shade, and a longer period of activity 

 is permitted to the biological processes in the soil which lead to 

 the nutrition of the plant. It is noteworthy that where no shade 

 trees are grown, as on many estates in Grenada, their absence is 

 compensated for by close planting and the use of large quantities 

 of pen manure. 



There is another well-recognised requirement of cacao, that 

 of shelter from wind. Its effects are related to those of shade, 

 and shade trees to some extent perform its function. It cannot 

 be dispensed with, and if it is not provided by natural conditions 

 must be artificially supplied. Its influence, without much 

 question, comes from the preservation of a humid atmosphere 

 about the trees. 



DiEBACK. 



The term dieback in connection with cacao has been generally 

 used in connection with the effects ascribed to the fungus Diplodia 

 theohromce, which there is little room for doubt often kills back 

 the twigs and not infrequently large branches of trees when these 

 are for some reason not in a vigorous condition. Since in practice 

 it is impossible to draw a line between the cases of failure directly 

 brought about by adverse conditions of growth, in which this 

 fungus almost invariably appears sooner or later, and those cases 

 in which the fungus plays a more active part, the term may be 

 usefully extended to cover the general type of failure it implies. 



The cacao tree is by its nature suited to humid conditions, 

 to a considerable depth of soil, and a fairly abundant supply of 

 humus. To secure these conditions a cacao field is usually so 

 managed as to approximate to forest conditions in respect of 

 atmospheric humidity, shading of the ground, and supply of 

 organic material. This is usually accomplished by interplanting 

 with shade trees or, where trees are dispensed with, by taking 

 advantage of natural shelter, by close planting, and by the 

 liberal use of mulch and pen manure. A fairly high and regular 

 rainfall is required to maintain the necessary humidity, and where 

 the rainfall is on the margin of adequacy the means of conserva- 

 tion referred to require to be the more perfect. On light, well- 

 drained soil, again, more protection is required than on soils which 

 are heavy and retentive. 



