DISEASES OF LIME AND OTHER CITRUS TREES 199 



in the dead bark. Evidence of its inability to attack tissue in a 

 state of activity is given above, but it is undoubtedly parasitic 

 when growth is checked or vigour is reduced. It would be possible 

 and legitimate to describe the disease as due to this fungus, as 

 has been done by F. S. Earle and J. M. Rogers, in treating of 

 grapefruit dieback in the Isle of Pines, but these authors agree 

 that " no tissue seems to be attacked until it is fully matured 

 or until enfeebled from unfavourable conditions." In the present 

 account the predisposing causes have been regarded as more 

 important, since it is upon the modification of these that control 

 must depend. 



Relation of Scale Insects to Dieback. 



Scale insects are so commonly associated with the dieback 

 of lime trees that they are very often credited with a very con- 

 siderable part or even the whole of its causation. While it is not 

 denied that the presence of the insects intensifies and accelerates 

 the failure of the trees, the writer is in agreement with that 

 section of entomological opinion which in this and similar cases 

 attributes the severity of the infestation to the impairment of the 

 resistance of. the tree, or, what comes to the same thing, the 

 special suitability of trees in this condition for the rapid increase 

 of the insects. Numerous cases have been seen in which the 

 dieback has followed its ordinary course in the absence of any 

 significant infestation, and it has always done the same in the 

 face of attempts at prevention by controlling the insects with 

 sprays. 



Control. 



The underlying cause of the decline appears to be one of 

 insufficiency and irregularity of water supply, and the duration 

 and completeness of the dry season seems to have more effect 

 than the intensity of the wet. Close shelter to conserve humidity 

 and hinder the soil from drying out has most influence in pro- 

 longing the life of the trees. The effect seems mainly to depend 

 on the protection given to the fibrous roots. It is admitted that 

 individual lime trees can and do exist under conditions of low 

 rainfall and considerable exposure, though the writer has no 

 evidence as to the age to which they attain. It is possible that 

 such trees continue to flourish in the open because they have 

 developed a type of growth suitable to their position, but it is 

 none the less certain that, in lime cultivations, the provision of 

 adequate shelter is necessary, first of aU to establish a regular 

 stand, and later to delay the incidence of dieback in the fully 

 matured trees. Between these periods it is advantageous but 

 not essential. 



The provision of windbreaks in all exposed positions has 

 come to be regarded as a cardinal principle of lime-planting in 



