78 JOURNAL OF AGRICULTURE OF P. K. 



Control. 



The absence of any extensive infections makes unnecessary such 

 drastic measures as painting the trunk or limbs with Bordeaux mix- 

 ture. Very effective control is possible by pi-ompt removal of all 

 diseased liml)s, making sure to cut well back of any infected areas. 

 All wounds should be immediately treated with- gas tar or other 

 wound dressing. If taken in time areas on the trunk or main limbs 

 (ian be cut as described for gummosis, observing all precautions. 



It is desirable that attention be given to the possibility of its 

 occurrence on the gandul, or other plants in and about the grove. 

 Any such plants which come under suspicion should be destroyed. 



DIEBACK OR EXANTHEMA. 



Dieback or exanthema, a common disease in Florida and to a 

 less extent in other citrus-growing regions, is of very limited occur- 

 rence in Porto Rico. In fact in but one instance has it been found 

 to be present to a serious extent. This was in a block of orange 

 trees, several of which were already dead, and others dying or in 

 advanced stages of the disease. 



The symptoms of true dieback are very distinct and have been 

 worked out in detail by Swingle and Webber. Briefly they are as fol- 

 lows: Growing shoots turn yellow and become stained reddish-brown, 

 finally dying back. On new unhardened growth distinct swellings, 

 due to an accumulation of gum, appear. In serious cases the bark 

 on both old and jiew twigs, and even the smaller branches, bursts 

 and reddish-brown stained, corky ridges form. Young shoots very 

 often droop in a striking manner, described as S-shaped. One of 

 the most marked signs is the production of multiple buds in the 

 axils of the leavefe on young twigs. This results in a bushy terminal 

 growth, most of which finally dies back. Many of the larger limbs 

 eventually succumbing, a crop of water sprouts is produced from 

 the lower part of the tree, giving a most ragged appearance. The 

 foliage is said to take on a deep green color, although individual 

 leaves sometimes show stained areas. The fruit loses its deep green 

 color, and there is a tendency to split. Brown irregular stains appear 

 on the surface as well as gum exudations, and a large percentage 

 falls. Finally there is present an infiltration of gum in the angles 

 of the segments at the center of many of the fruit. 



It is only rarely in Porto Rico that any number of these symptoms 

 are found together. Multiple buds and dieback are common enough, 



