121 



The " Canker" Fungus of the Cacao Stem. 



General Characters. — Soon after my arrival in Grenada I discovered 

 a bark disease of the cacao tree which was subsequently noted in all 

 the districts of the island I visited. In all probability it is responsible 

 for most of the trees which die off somewhat suddenly and from no 

 apparent cause. The first well-defined symptom of this stem disease 

 is a reddish gummy liquid which oozes out of the bark of the stem and 

 which when dry gives a rusty appearance to the bark. This may be 

 termed the " bleeding" stage and even now the disease is well estab- 

 lished. On cutting into the tree at these points it is found that the 

 bark is deep claret red in colour and moist and soapy to the feel. The 

 discolouration extends to the young wood and on removing all the 

 diseased bark it can be seen that the darkening of the wood extends 

 for some distance under the still healthy bark and that the diseased 

 patch increases in size from beneath outwards rather than from the 

 surface. The disease may start from any point on the stem and in 

 many cases there are two or three points of attack on the same tree. 

 The spread of the diseased area varies a good deal. Sometimes when 

 the patch is only three or four inches in diameter it rapidty extends 

 on either side and " rings" the tree completely. When this is com- 

 plete the tree dies off suddenly often bearing a full complement of 

 leaves and pods in all stages of growth. In other esses the diseased 

 area extends slowly in all directions and " ringing" does not take 

 place for some time. In such cases the death of the tree is much more 

 gradual than before, the branches immediately above the diseased 

 patch dying off first of all followed by the rest as the diseased patch 

 extends. In some cases, and these are apparently rare, the tree re- 

 covers from an attack by cutting off the diseased area by the growth 

 of fresh bark from below. In these cases the disease had not reached 

 the wood. In general diseased trees rarely occurred singly, but 

 several were noted close together indicating that the disease had 

 spread from one tree to several of its neighbours. 



Nature of the disease. — Microscopic examination of the discoloured 

 bark shewed that the tissues were penetrated in all directions b} r the 

 " roots" of a fungus and there were other evidences of fungoid attack 

 in the still healthy tissues surrounding the diseased bark. 



In the cracks of the discoloured ba k several different kinds of spore 

 were noticed which are probably connected with one or at the most 

 two fungi, but the time at my disposal did not permit even of an at- 

 tempt to work out the life history of these forms. Some of these 

 spores are found in small red or yellowish lemon-shaped flask-like 

 bodies about the size of a pin's head which appear in colonies on the 

 diseased bark in the later phases of the disease when the tree is either 

 dead or nearly so. The other spores occur on whitish cushions in the 

 cracks of the bark of the bleeding patches at an earlier stage. 



Infection experiments were performed at the Botanic Station on 

 healthy cacao trees by introducing portions of the diseased bark from 

 bleeding patches into healthy trees and binding up the wounds with 

 water-tight bandages to prevent drying up. In all cases the " roots" 

 of the fungus in the diseased bark made their way into the surround- 

 ing healthy bark for about half an inch all round, thus indicating that 

 the fungus was parasitic. Unfortunately, however, the length of my 



