158 DISEASES OF CROP-PLANTS 



When the conditions fall short of any of the requirements 

 outlined the effect is to induce in the tree a more or less severe 

 manifestation of the condition referred to above as dieback. 



The lightest form is shown only in diminished size and harden- 

 ing of the leaves. Next comes the condition in which the out- 

 standing twigs lose their leaves and dry off, new foliage being 

 produced later from short shoots which develop further back. 

 Trees in this state are familiar objects and present a more or less 

 dense head of foliage with numerous twigs projecting above it. 

 In yet more severe cases the dying back extends to the larger 

 branches and may gradually involve them all. At every stage 

 in this retrogression any temporary improvement in the con- 

 ditions stops the process and new shoots are sent out behind the 

 margin of injury, but with the recurrence of unfavourable con 

 ditions the process may continue until the tree is wholly dead. 



Some cases of root disease have occurred, in trees un- 

 favourably circumstanced, in which Diplodia has been the only 

 fungus detected. 



Such failing trees, and trees in which the condition is as yet 

 only incipient, are particularly liable to severe attacks of the 

 cacao thrips, which contribute materially to their decline. There 

 are good grounds for believing that it is only on trees lacking 

 in vigour that infestations of thrips persist and do more than 

 passing harm. 



The part played by the fungus is impossible to define as it 

 varies with every circumstance. It is safe to say that dieback 

 would take place under appropriate conditions if the fungus 

 were entirely absent, for this only means that the range of con- 

 ditions to which the tree can successfully adapt itself is limited ; 

 but with the fungus universally present as it is there is no doubt 

 that it often takes a definite part in the process. This is especially 

 seen when its mycelium coming down a smaller branch which is 

 dead or dying enters and infests the wood of the larger branch 

 to which it joins, cutting off the water supply and causing the 

 death of that part of the branch above the junction. It may be 

 said in general that the effect of the fungus when it has obtained 

 a footing on the dead twigs is to extend the range of the injury, 

 and give it a progressive nature in weakly resisting twigs or 

 branches which might otherwise have remained for the time 

 being alive. Where the conditions which induce the attack 

 remain unameliorated the result in the end is much the same, 

 but where they are temporary in their nature or exceptional the 

 action of the fungus may greatly increase the resulting loss of 

 branches. 



Associated Fungus. 



Diplodia Theobromce (Pat.) : Fructifications consisting of black, 

 carbonaceous, more or less rounded pycnidia immersed in the cortex ; 

 single or combined in groups or sometimes in a stroma ; pilose when grown 



