i6o DISEASES OF CROP-PLANTS 



and the combination Diplodia Theobromce would seem to be the 

 soundest to use until a better subdivision of the genus is worked 

 out. 



Control. 



The symptoms of dieback often appear in a group of trees or 

 along the borders of a field, and occasionally in individual trees. 

 It will be obvious from the foregoing account that for purposes of 

 control it is first of all necessary to form some idea as to which 

 of the many causes capable of bringing about this condition is 

 responsible in the particular case. Where a group of trees is 

 affected, especially if the trees around its margin are progressiv^ely 

 involved, search should be made for the usually well-marked 

 evidence of Rosellinia root disease. If this is not found the 

 adequacy of the drainage and aeration on heavy soils should be 

 next questioned. On light soils the conditions as to exposure 

 both lateral and vertical should be considered in relation with the 

 amount of rainfall received and the retention of sufficient moisture 

 in the soil during periods of dry weather. If deficiencies in these 

 latter respects are suggested by the circumstances they may be 

 met by closing in the edges of the cultivation with quick-growing 

 screens, for which purpose Aralia Guilfoylei has proved very 

 suitable, by providing overhead shade, which may be most 

 rapidly effected by putting in large cuttings of Gliricidia maculata 

 (madura) and by the addition of pen manure or heavy vegetable 

 mulch to the soil. A great deal of dieback, especially in peasant 

 holdings, is directly due to neglect of reasonable measures of 

 cultivation and manuring. The same is true of some neglected 

 estates, while another cause is to be found in injudicious removal 

 of shade. Cacao trees are sometimes planted where the soil is 

 too shallow to maintain them, and the subsoil too dense for root 

 penetration, in which case the trees reach in a few years the limits 

 of their development and begin to fail. 



In an affection such as this, which may be due to any of the 

 numerous causes which react unfavourably on a tree which is 

 tolerant only of a distinctly limited range of conditions, it is 

 sometimes very difficult to find the cause of an unthrifty con- 

 dition. The measures suggested above are all along the lines 

 of good agricultural practice, and will not be wasted if they are 

 applied in turn until the directly appropriate measure is dis- 

 covered. 



Diplodia Pod-rot. 



The rot of pods brought about by the fungus Diplodia 

 TheobromcB has commonly been known in the literature referring 

 to the West Indies as the brown pod-rot, an unfortunate choice 

 of name, since the Phytophthora pod-rot in its early stage 

 colours most pods attacked by it a clear brown, and the antbrac- 



