98 



.TOI^RXAL OF AGRICULTURE OF P. R. 



operations tend to keep it in cheek. The removal of affected, ma- 

 ture gandnl plants will also doubtless aid in checking- the twig type. 



FRENCHING, MOTTLED LEAF. 



The yellowing of leaves of orange and grapefruit trees is a very 

 common phenomenon in Porto Rican groves, and may be due to any 

 one of several causes. Various specific diseases, due to both fungi 

 and unknown causes, are marked in part by a yellowing of the foli- 

 age, and this state of affairs is especially prevalent in abandoned 

 blocks of trees, or those suffering from neglect. Trees which have 

 borne a heavy crop of fruit will show considerable yellowing be- 

 fore the spring application of fertilizer is given, resuming normal 

 color very quickly after this operation. 



Yellowing of this nature is generally easily diagnosed and cor- 

 rectives can be applied, but there is a distinct type, commonly known 



as frenching or mottled leaf, the 



cause of whicli is obscure. In 

 this case the knaves show irregu- 

 hu' yellow spots, with definite 

 margins, tlie background re- 

 remaining green (Fig. 17.) 

 Very often isolated trees only 

 are subject to this spot, oi" a 

 few limbs only in a given tree. 

 Studies made in California seem 

 to indicate that this trouble is 

 due to a lack of humus in the 

 soil, which is being supplied 

 there by a syslein of mulching. 

 The disease is hardly of sufficient importance to wa riant any de- 

 tailed attention here. 



Yellow spotting, a similar trouble of the leaves occuri'iiig in 

 Florida, has not been noted here. 



Fig. 17. — Fienching or mottled leaf of 

 grapefruit. 



SOOTY MOLD. 



The black, sooty layers of growth so couunonly seen in the groves, 

 and known to all as sooty mold, are fungus growths, Init not of a 

 parasitic nature. They live on the honey-dew or seci-etions of cer- 

 tain insects, in particular of the hemispherical and turtle-back scales 

 and the wooly white fly. 



