198 DISEASES OF CROP-PLANTS 



are in connection with functioning leafy branches is regularly 

 shown by the persistence in a healthy condition of a sector so 

 connected embedded in a branch or stem otherwise dead and 

 infested. Such living sectors may produce considerable growths 

 of callus along their margins, but in the gradually waning tree 

 are not effective in making a permanent repair. The infestation 

 of the smaller branches becomes self-limited at their base. 



In Montserrat, and probably elsewhere, a dieback comparable 

 with that of the branches takes place in the root system. The 

 two do not seem to be directly connected, but are presumably 

 related in their origin. Here again Diplodia, and probably 

 other fungi, infest the failing roots, and the same type of re- 

 placement by lateral branches and the same persistence side 

 by side of living and dead sectors are seen. The damage to the 

 roots is often complicated by the long groove-like wounds made 

 by the larvae of weevils of the genera Diaprepes and Exophthal- 

 mus. These are quickly healed in actively growing trees, but 

 in trees in failing vigour remain open and afford access to weakly 

 parasitic fungi which infest the wood. A similar slowing-down 

 of the process of repair is seen in the upper parts of the tree 

 where pruning wounds have been made. 



A characteristic early symptom of the condition described 

 is the selective loss of shaded branches. This in its origin is a 

 natural process, seen in most trees however healthy, but in the 

 failing trees sensitiveness to shade is so much increased thcit it 

 leads to persistent reduction of the total amount of foliage. 



Behind the whole process there appears to be a decline in 

 vigour such as we know to be universal in animals which have 

 passed their prime, but do not so readily recognise with regard to 

 plants. The conclusion reached is that, under the conditions 

 described at the outset, the natural duration of the tree is reduced 

 to the period to which the initial vigour extends, and this would 

 seem to be in general somewhere about 15 years. The ensuing 

 failure may be either relatively rapid or long drawn out, according 

 to local circumstances fairly easy to follow. 



The history of lime-growing in Montserrat, where to maintain 

 production it has been regularly necessary to establish new fields 

 from time to time, accords well with this interpretation. There 

 is no evidence whatever of the occurrence there of any pest or 

 disease not existing in Dominica or St. Lucia. 



Relation of Diplodia to Dieback. 



The fungus accompanying this form of dieback is very 

 regularly a species of Diplodia. It is frequently regarded as 

 D. natalensis, but the writer has seen no evidence produced of 

 essential difference from D. Theobyomce, the ordinary dieback 

 fungus of cacao and many other trees. Infested lime wood is 

 recognisable by its slaty grey colour, and the fungus fruits freely 



