1919.] BOOT DISEASE OF CACAO IN TBINIDAD. 187 



Another type of situation continuously capable for a long number of 

 years of becoming infested is presented by the soil on the shaded under- 

 side of logs, the disintegration of which, especially that due to the 

 tunnellings of insects, maintains a plentiful supplj' of organic matter. 



Once the disease has got a footing however, the greatest amount of 

 loss is caused by the spread of the fungus along the roots (as it com- 

 monly appears) or over the surface (where conditions of shade permit) 

 from one cultivated tree to others about it in an ever widening circle. 

 In one case the loss of about 150 trees appeared to be clearly 

 traceable to two original centres of infection. 



Wherever a tree has died, unless strict measures have been taken 

 to control the fungus, the adjacent trees almost inevitably contract the 

 disease sooner or later. Such cases may arise years after the original 

 tree has been removed and the cause of its death forgotten, so that they 

 have the appearance of being sporadic. Examination of the position 

 and age of supplies, and the keeping of records of surveyed areas 

 enable them to be linked up with considerable certainty to previous 

 losses. How far such belated cases are evidence of the slow progress of 

 the fungus along the roots, or how far they are due to delayed infection 

 has not been ascertained, but the general evidence points to the conclu- 

 sion that the process of investment of mature trees is a distinctly slow 

 one. In a case definitely recorded, a full}' infested dead lime tree was 

 removed in October 1914, and the fungus {R. bunodes) was just coming 

 up round the collar of the next tree in the row, a very large and vigoi'ous 

 specimen, in August 1916. The variety of circumstances must produce 

 wide differences, but I should judge that the two years taken in this 

 case is not an unusual period, and is in some cases considerably exceeded. 



Typically an infested clearing in which the disease is of several 

 years' standing shows a few large open patches, each representing 

 perhaps a score of trees, with usually two or three around its margin 

 dead or dying, and several more on which the fungus may be found. 

 Sometimes two or three such patches have coalesced. Scattered about 

 are fresh centres in various stages : a single tree, a gap of two or three 

 trees in a row, with another going, or a group of two or three in diflferent 

 stages about a large stump. Of the supplies put in, some are several 

 years old and still thriving, others are dead within a few months of 

 being planted. 



Appearance of Diseased Trees. 



As the disease is seen on lime and cacao trees there are two types of 

 failure of the top. In the one, where the roots become more or less 

 generally infested before the fungus gets fully hold of the collar, as 

 commonly happens where its approach along the roots is checked by 

 drier conditions around the crown, defoliation is gradual and is preceded 

 by yellowing of the leaves and a general sickly appearance of the tree. 

 lu cacao especially, this is the common form of failure and resembles a 

 severe type of die-back, such as is brought about by poverty of soil or 

 exposure. The development of vigorous suckers excludes R. Pepo as 

 the cause of such a condition but their non-development on failing trees 

 does not necessarily indicate its presence. 



