196 DISEASES OF CROP-PLANTS 



which ensues, and is associated with Diplodia infestation, marks 

 the limitation of the Hfe of the tree under these circumstances. 

 A similar restriction in the citrus cultivations of some districts 

 in Queensland has been the subject of much investigation, without 

 any other conclusion being reached than that the decline is due 

 to a reduction of the natural duration of the tree. A comparable 

 reduction is recognised in the cacao tree when grown under 

 conditions near the margin of suitability in respect of depth of 

 soil, unequally distributed rainfall, and absence of shade or 

 shelter. 



Lime. 



Diplodia Dieback, 



(The Montserrat Type.) 



As indicated above, the result of experience in planting limes 

 in the Lesser Antilles has been to show that while the tree finds 

 permanently congenial conditions in the islands which are 

 mountainous in character and retain a large amount of forest, 

 in the islands which are more open and more completely cleared 

 its duration is restricted to the period during which it retains 

 its first vigour of growth. A comparison may be made with 

 the more familiar difference in the effective duration of sugar- 

 cane stools under similar circumstances. 



The climatic conditions by which the difference is determined 

 are reduced rainfall, lower humidity, more exposure to wind, 

 and a more marked difference between the wet and dry seasons. 

 Montserrat being the only island of this type with a considerable 

 lime industry, affords the best illustrations of the effect, but 

 from such opportunities for observation as occur it is evident 

 that the same effect is produced in the other islands where 

 conditions are similar. 



The sequence of events in its most regular form is as follows : 

 Young trees, after coming through the critical period elsewhere 

 described, may become established and do well for about ten 

 years, attaining to a good size and bearing normal crops. Then 

 in a uniform field of this nature the most exposed trees begin to 

 show signs of dieback, which sooner or later extends widely and 

 in two or three years may involve the whole field or may leave for a 

 time groups of less affected trees here and there. 



To follow out the process of the dieback it is necessary to 

 know the growth form of the tree. This may be described as 

 of the fountain type, i.e., the main shoots, generally several 

 together with no pronounced leader after the first two or three 

 years, grow up long and slender, and sooner or later bend out- 

 wards from the top. As they become inclined new vertical 

 shoots arise from the upper side of the curve so formed, and 

 as the outer end of the original branch is bent lower and lower 



