THE RELATION OF STOCKS TO MOTTLED LEAF OF 
CITRUS TREES * 
By H.. ATHERTON LEE 
Pathologist, Office of Fruit Disease Inuestigations, United States 
Department of Agriculture 
THREE PLATES 
INTRODUCTION 
Mottled leaf of citrus trees has become increasingly important 
within the last few years in the citrus-growing regions of the 
United States and has been the subject of much discussion among 
growers, physiologists, and pathologists. The present paper 
submits definite data concerning the relation of stocks to mottled 
leaf which may contribute slightly to our knowledge of the 
disease. The data also make possible a suggestion for its 
future control. 
BRIEF HISTORY OF THE DISEASE 
A number of hypotheses have been advanced to explain the 
cause of this disease, based usually on substantial investigations 
in some one area where trees were badly affected. Thus 
Hilgard,? in 1906, advanced the theory of an excess of lime as a 
cause of mottled leaf in the Porterville region in California. His 
conclusions were based upon his observations of the occurrence 
of the disease in areas where lime was considerably in excess. 
Another theory, somewhat in opposition to that of an excess of 
lime, was advanced by Snowden,’ and depended on an improper 
relationship of calcium to magnesium in the soil, it being ex- 
*The writer is greatly indebted and herein expresses gratitude to Mr. 
S. Apostol, chief of the Division of Plant Industry of the Philippine Bu- 
reau of Agriculture, for the use of the citrus orchards at Lamao. The 
use of the orchards, as well as living accommodations at Lamao, were 
made available through the efforts of Mr. Apostol; without this unusual 
opportunity the work would have been impossible. ; 
* Hilgard, E. W., Marly subsoils and chlorosis or yellowing of citrus 
trees, Univ. Calif. Agr. Exp. Sta. Circular 27 (1906). 
* Snowden, R. R., The magnesia lime trouble in citrus orchards, Cali- 
fornia Cultivator (1910) 124. 
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