DISEASES OF LIME AND OTHER CITRUS TREES 195 



disability is seen in cacao plantations in the island, but the 

 deeper root system of cacao enables the trees to maintain them- 

 selves, though not, in general, to flourish. It affords a curious 

 study in crop ecology to find such opposite types as cotton and 

 cacao growing in contiguous fields, while limes, which will grow 

 in many places too dry for cacao, fail to thrive. A contributory 

 reason is no doubt to be found in the more serious and lasting 

 effects produced by the scale insects which infest the lime at 

 periods of low vitality than by the thrips which affect cacao under 

 like circumstances. 



The behaviour of the lime tree under cultivation suggests 

 that its natural habitat would be in bush or thicket growth on 

 ground unoccupied by large trees. It profits greatly from the 

 shelter of surrounding vegetation, but is somewhat intolerant of 

 shade. Even in most favourable circumstances, as in the climate 

 of Dominica and St. Lucia, the plants in their first few years 

 benefit enormously from being enclosed in any kind of sheltering 

 growth, and in poorish soils, even in wet districts in those islands, 

 may during the critical period which precedes their becoming 

 " established," die back several times or be killed outright by 

 scale insects unless some such protection is afforded them. Even 

 when the scale insects are kept down by regular spraying growth 

 in the open is nothing like so good. From this has arisen the 

 practice, which is desirable anywhere on previously cultivated 

 land, but especially where the rainfall is somewhat low, of 

 nursing young lime trees in sugar-cane, pigeon pea, or Tephrosia, 

 until they become well established. In fields where this has not 

 been provided for it may be seen how those plants which have 

 the advantage of some bit of natural shelter are the first to take 

 on the shape of trees and how their influence gradually brings on 

 in succession the rest of the field. Manuring is also of great 

 assistance. 



Once well established and come to fruiting age the trees 

 generally do fairly well even under rather hard conditions for a 

 number of years. In Dominica unless the soil becomes unduly 

 impoverished or is allowed to get into bad condition they will 

 bear for 20 or 30 years and then be far from worn out. The 

 effect of neglect of the kind mentioned is the death of upper 

 branches, and the extension of the rot caused by the bracket 

 fungi which come to infest them down the stem of the tree, 

 which has not vigour enough to resist. This type of dieback, 

 common in some places in Dominica, can be arrested by appro- 

 priate measures as described below, and the tree restored to 

 another period of usefulness. 



Such restoration is not possible, or at best is much more 

 temporary, in the dieback which sets in after some 10 to 15 

 years, without obvious neglect as a predisposing cause, in lime 

 fields established under conditions of smaller rainfall and more 

 marked dry seasons, as in Montserrat. The progressive decline 



