JOURNAL OF AGRICDLTDRAL RESEARCH 



DEPARTMENT OF AGRICULTURE 

 Vol. VI Washington, D. C, August 7, 1916 No. 19 



MOTTLE-LEAF OF CITRUS TREES IN RELATION TO 

 SOIL CONDITIONS 



By Lyman J. Briggs, Biophysicist in Charge, C. A. Jensen, Assistant in Plant Mal- 

 nutrition, and J. W. McLanE, Laboratory Assistant, Biophysical Investigations, 

 Bureau of Plant Industry 1 



INTRODUCTION 



"Mottle-leaf" is a term applied in California to a mottled or spotted 

 condition of the leaves of Citrus trees (PI. XCVI). The affected portions 

 of the leaf appear to be nearly or quite devoid of chlorophyll and are 

 light yellow in color. In the first stages of the disease irregular spots 

 several millimeters in diameter appear between the larger veins, usually 

 midway between the midrib and the margin. The half of the leaf next 

 to the tip is often first affected. In the more advanced stages, the spots 

 are larger and more numerous, until finally the only chlorophyll remain- 

 ing is confined to the midrib and the larger veins. The various stages are 

 illustrated in Plate H. The condition is distinguished from what is gen- 

 erally termed "chlorosis" by the fact that the areas surrounding the yel- 

 lowish spots retain their normal green color, at least until the spots 

 embrace a large proportion of the leaf. The term "mottle-leaf" as here 

 used is also to be understood as not including that type of functional 

 disturbance sometimes found in Citrus leaves in which the midrib and 

 veins are lighter in color than the surrounding tissue. 



Mottle-leaf in its advanced stages is accompanied by a serious reduction 

 in the yield and in the size and quality of the fruit. The foliage 

 becomes thin and weak, with many very small leaves (Pi. XCVII) ; and 

 the ends of the branches have a brushy appearance, owing to the 

 development of numerous small weak twigs. 



DISTRIBUTION OF MOTTLE-LEAF 



Mottle-leaf is at present quite widely distributed through the Citrus 

 areas of California, but is, as a whole, worse in the southern sections of the 



1 The writers are indebted to the University of California Citrus Experiment Station and Graduate 

 School of Tropical Agriculture at Riverside for many courtesies and facilities extended during the course 

 of this work, and to Dr. H. L: Shantz and Mr. R. L. Piemeisel, of the Office of Alkali and Drought Resistant 

 Plant Investigations, for their cooperation in the work preliminary to this investigation. 



The writers also wish to express their obligation to the Citrus growers of the sections studied for their 

 cooperation in supplying information regarding the field treatments of their groves and assistance in other 

 ways. 



Journal of Agricultural Research, Vol. VI, No. i > 



Dept. of Agriculture, Washington, D. C. Aug. 7, 1916 



ev G— 90 



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