90 JOURNAL OF AGRICULTUKE OF P. R. 



Unlike many other plant diseases, citrus scab shows a decided 

 preference for healthy, vigorons trees, and it has been generally 

 observed that, other conditions being equal, the best-kept grove or 

 the most thriving trees fall easiest prey. It is rare indeed to find 

 trees suffering from foot-rot, dieback, or advanced cases of wood rot 

 also attacked by scab. In the many abandoned plantings examined 

 no traces of it have been found, even though the trees are well pro- 

 tected, and infected groves are close by. 



It is not, however, recommended that there be any lessening in 

 the cultivation and fertilization of the grove in an attempt to con- 

 trol scab. 



Control. 



During the years when the grapefruit was free of the disease 

 no steps were taken to check it beyond the budding over of the 

 seedlings in the nursery as speedily as possible. No care was taken 

 in setting out young trees to free them of what little scabby growth 

 might be pi'esent. As this state of affairs occurred in practically 

 every grove, it is not surprising that the disease has been able to 

 spread so rapidly, once the resistance of the grapefruit was destroyed. 

 Since then great efforts have been made to control the disease, par- 

 ticularly by the use of various sprays. 



One very vital fact, that has been largely overlooked, is that all 

 measures must be preventive and that a "cure," once the disease 

 has a hold, is impossible. Once penetration of the host tissues has 

 been effected, any amount of spraying is without avail, other tlian 

 to kill such slight surface growth as there may be, which is readily 

 replaced from within, after the spray material has washed off. To 

 be effective the fungicide applied must reach and kill the spore 

 before it begins its growth. Hence for absolute protection it would 

 be necessary to keep the surface of the leaves and fruit cotnpletely 

 covered during the susceptible period. 



Removal of infected sour-orange and lemon growth. 



This is a step generally recommended and one that should be 

 followed out most carefully. It will be desirable to destroy, not only 

 any wild sour-orange trees that may be present on the finca, but 

 to search for any that may be growing in the vicinity in waste land, 

 windbreak lines, or other holdings, especially small native plantings. 

 In the case of lemon trees, removal is advised if they show any con- 

 siderable amount of scab; but if, as has been observed in several 



