115 



in the presence of moisture, fungus spores, as a rule, germinate rapidly 

 and send out a minute tube or " root" which under suitable conditions 

 penetrates the tissues of the host plant and sets up disease. 



" Brown-Rot" Disease of the Cacao Pod. 



General Characters. — In every district in Grenada Cacao pods may 

 be found which show distinct brown patches which gradually extend 

 over the pod. This must not be confused with the rusty or " mahog- 

 any" pods which result from " Thrips" where the whole of the pod 

 takes on a rusty colour, but where the rind is not diseased. The 

 brown patches in question generally commence either at the insertion 

 of the stalk or at the free end of the pod, but they may occur at other 

 points, especially when the rind has been injured or where the pod 

 comes in contact with a branch. These diseased pods are particularly 

 numerous near the " breaking grounds" where the beans are extracted 

 by the pickers. If one of these attacked pods is carefully examined, 

 it will be found that the brown area is rotten and that the discoloura- 

 tion extends to and spreads round the shell of the pod to a much 

 greater distance than would be supposed from a surface examination. 

 Decay soon spreads to the beans which are speedily covered with a 

 greyish mould-like substance which quickly destroys them. The 

 beans of nearly ripe pods instead of being covered with the usual moist 

 and sweet mucilage are dry and the contents of the pod have a sour smell. 



Microscopic Characters- — Microscopic examination of the tissues of 

 the brown patches shews that they are filled with the " roots" of a 

 fungus which exhibits all the characters of rapid growth. These roots 

 extend round the shell, penetrate it and reach the mucilage surround- 

 ing the seeds where they grow with enormous rapidity and luxuriance, 

 and, after penetrating the skin of the seeds, attack the beans them- 

 selves which become discoloured and ultimately rotten. When the 

 brown patches have extended to two or three square inches on the 

 surface of the pod, small circular mounds can be seen near the centre 

 of the discoloured area from which a greyish-white powdery dust is 

 expelled which turns black in a short time. This dust is composed of 

 fungus spores which are at first colourless but which rapidly darken 

 on exposure to light. They are formed in spherical bags just under 

 the skin of the pod from which they are squeezed out through a small 

 opening when ripe. It was therefore highly probable that the disease 

 in question is caused by this fungus and to place the matter beyond 

 any doubt whatsoever the following infection experiments were per- 

 formed : — 



Infection Experiments. — The fungus in question was identical with 

 that found on the specimens of diseased cacao branches and pods sent 

 from Grenada last year. It is referred to in the supplement to Mr. 

 Lefroy's report (G.3745 of December 24th, 1900) which was published 

 in the Grenada Official Gazette as probably doing some damage to the 

 cacao in certain localities in that island. Fortunately I had been able 

 to follow out the development of this fungus from a single spore and 

 to prepare many pure* cultivations with which the most trustworthy 



* A pure cultivation of a fungus is a growth of the mycelium of that fungus, un«* 

 accompanied by that [of other forms, in a suitably prepared food substance. It 

 therefore denotes an artificial crrowth of a single fungus unmixed with other 

 forms. 



