DISEASES OF LIME AND OTHER CITRUS TREES 201 



Deficiency Dieback. 

 A type of decline quite independent of water shortage, as 

 may be seen by its occurrence on favourably situated estates in 

 Dominica, becomes prominent where the trees are old and 

 cultivation and manuring have been neglected. It is also brought 

 about when growth and heavy bearing are stimulated year after 

 year by transient chemical manures without the condition of the 

 son being maintained by organic manures of a more lasting 

 nature. The resulting dieback can be very severe. The trees 

 lose their healthy colour and some or many of the upper branches 

 die. That lack of tilth and the exhaustion of the soil, rather 

 than age or parasites, are the cause of the condition, is shown 

 by the large degree of restoration possible by the addition of 

 organic matter to the soil and general good handling. 



Wood Rot. 



The dead and failing branches become conspicuously infested 

 with numerous species of fungi, many of them of the bracket type. 

 The bark is usually found to be infested with smaller fungi, of 

 which several species of Nectria and Stilbum are the most 

 common. 



The damage done arises almost entirely from the destruction 

 of the woody framework of the tree, and not from any direct 

 attack on the active tissues of the bark and young wood. The 

 destruction is probably largely due to the mycelium of the 

 bracket fungi in question. 



These fungi are able to establish themselves on any dead stub 

 left from the failure of a branch or from careless pruning, no matter 

 how healthy the tree may be, but the subsequent progress of the 

 infestation depends on the condition of the adjoining parts. In 

 vigorous trees the decay is arrested when the living branch is 

 reached, and the wood shows considerable powers of resistance. 

 It rots slowly where it is exposed, and , unless the wound is trimmed 

 so as to permit the bark to grow over it, there is produced in time an 

 unsightly cavity, but the process is no more than goes on in any 

 tree not carefully trimmed, and, if the wound is not a particularly 

 large one, it does not produce serious results for a long time. 



The less vigorous the tree, the less resistant is its wood to 

 decay, and the extreme cases of fungus infestation are to be seen 

 in fields where, after the trees have attained to a good development 

 from the original humus content of the soil, or from the practice 

 of a good system of manuring, the fertility of the soil has then 

 been allowed to run out, and the trees are no longer sufficiently 

 nourished. The policy of substituting sulphate of ammonia, 

 without any backing of humus-building material, for pen manure, 

 mulch, and other organic manures is clearly seen in some cases to 

 have produced this effect. In other cases it has come about from 

 complete neglect of manuring, combined with careless treatment of 



