CITRUS DISEASES. . 53 



Lack of fertilizer becomes readily apparent in the yellowing- of the 

 leaves, followed by a dying back of terminal twigs, and it may even 

 induce a tendency to premature dropping of fruit. Certain recog- 

 nized diseases, or types of diseases, are attributed to excessive amounts 

 of nitrogen supplied in organic form. This point is considered more 

 specifically later. 



It is a surprising but true fact that soils of the majority of citrus 

 groves of the Island are decidedly acid, in spite of the fact that they 

 are in large part surrounded by or adjoin limestone hills, and in 

 many cases are cut up by them into irregular-sized blocks. The use 

 of lime to improve the physical condition of the soil, and to supply 

 the other benefits derived from its use, has always been strongly 

 recommended. It has been said, by those who have studied the 

 matter, that, as a general rule, the lime required per acre to neutralize 

 the soil acidity would amount to a considerable number of tons. ' 



Lime may be applied in various forms, such as live lime, air 

 slacked, or ground limestone, the second form being the one most 

 commonly used in Porto Rico. No reports have been received of 

 injury to Island groves from applications of lime in any form. Its 

 use will be of value in promoting a better tree growth, with the ac- 

 companying result of more satisfactory yields of disease-free fruit. 



A precaution is, however, necessary at tliis point in view of 

 certain results reported from Florida. It has become apparent there 

 that finely ground limestone, a form but little used as yet in Porto 

 Rico, is capable under certain conditions of producing marked injury 

 to the trees. This injury takes the form of a yellowing of the leaves, 

 partial defoliation, multiple buds, bushy terminal growth, and a dying 

 back. Studies by Prof. Floyd of the Florida Experiment Station 

 have made it seem probable that this trouble is most apt to occur 

 on light soils, and particularly on those lacking in humus. While 

 it is considered doubtful that this trouble will appear here under 

 present conditions, ground limestone should be used in moderation, 

 and in conjunction with any such applications means should be 

 taken to supply humus by growing a cover crop of velvet beans, sword 

 beans, or similar legume, or even by light application of manure. 

 Mulching the trees would also be of benefit. 



PRUNING. 



It would be foreign to the subject to enter into any discussion of 

 ])runing, other than to direct attention to the necessity of removing 

 all dead and dying, or fungus-infected, twigs and branches. This 



