1919.\ BOOT DISEASE OF CACAO IN TRINIDAD. 185 



case. The observed facts as they regularly present themselves are 

 these : that one or more of the cacao trees immediately adjacent to a 

 sickly or a dead shade tree, or shade tree stump, begui to show 

 symptoms of root disease, and removal of the soil reveals the fungus 

 advancing along one or more of the cacao roots from the neighbourhood 

 of roots of the shade tree, which are found to be infested, and usually 

 in an advanced stage of infestation. Sometimes actual contact has 

 been proved, sometimes not. In view of the difficulties of digging 

 among roots the negative evidence has not much weight. The trees 

 most often concerned in connexion with the the disease on cacao are 

 avocado, breadfruit, and pois doux {Inga spp.). Immortel {Erythrina sp.) 

 and mango are sometimes but not often involved. 



The fact should be mentioned here, though its discussion belongs 

 rather to the next section, that the disease can occur on cacao trees in 

 the absence of stumps or any obvious dead wood. Cases of this nature 

 are as a rule uncommon, but in only one badly infested area examined a 

 fair number were seen. 



Methods of Infection. 

 Several of the authors cited in the first part of this paper refer to 

 the spread of the fungus through the upper layers of the soil and in 

 accumulations of decaying vegetable matter on its surface, resulting in 

 the infection of trees at those levels. The usual mode of occurrence 

 of the disease in the West Indian cacao plantations does not bring out 

 this feature at all clearly. With the exceptions alluded to above, the 

 disease is generally clearly traceable to dead roots, which are often 

 deep underground, and its course is along one or more of the cacao 

 roots in the direction of the stem. All stages of the process have been 

 seen, and quite commonly in the less advanced cases the crown is found 

 still unattacked. 



The cases sometimes met with of cacao trees becoming diseased in 

 the absence of stumps can be accounted for by infection of the surface 

 type. A livmg root was found which had been injured where it came to 

 the surface, and from two points on the margin of the injuries white 

 fans recognizable as the myceUum of B. Pepo were found radiating 

 under the bark. This example is illustrated in Fig. 5. The cases noted 

 have been in the wettest districts, where production of organic matter 

 is at a maximum, and where cloudy days, the depth of shade produced 

 by luxuriant growth, and the saturation of the soil with water, all 

 reduce the rate of its destruction. Where rainfall and shade are not 

 excessive, the addition of organic matter to the soil does not keep pace 

 with its decay, and the general run of soil on the ordinary cacao estate 

 is not particularly rich in humus, nor is there much, if anything, in the 

 way of leaf mould on its surface. 



The spread of the disease along closely planted hedges and wind- 

 breaks, of which some striking instances have occurred in St. Lucia, is 

 characterized and probably mainly effected by infestation of the surface 

 soil. The leaves and twigs which accumulate along the base of the wind- 

 break, and decaying, enrich the soil with humus, and the shelter 

 afforded by the trees and by the vegetation which grows up under their 



