722 Journal of Agricultural Research vol. vi.No. i 9 



State. It is not a new trouble in California, having been observed by 

 some of the growers at least 15 years ago. During the last three or four 

 years the mottling has become much more pronounced in the groves first 

 affected. Other groves which are reported to have' been relatively free 

 from the trouble a few years ago are now badly mottled. There are 

 many groves in the affected districts, however, that show little or no 

 mottling at present. 



FACTORS SUGGESTED AS CAUSAL AGENTS IN MOTTLING 



The cause of mottling is a much disputed point. Various factors have 

 been assigned as causal agents, such as an excess of lime, magnesium, or 

 organic matter; a deficiency of lime, iron, or organic matter; frost; poor 

 drainage, etc. Smith and Smith * conclude from their observations that 

 the most prevalent and typical form of mottle-leaf is due to an irregular 

 supply of moisture and plant food. No fungus or bacterium has yet been 

 proved to be causally associated with mottle-leaf. Thomas, 2 however, 

 has shown that the Citrus-root nematode (Tylenchulus semipenetrans 

 Cobb) is widely distributed in districts in which mottle-leaf occurs, but 

 is not invariably found on the roots of affected trees, and the extent to 

 which mottling can be directly induced by such parasitism has not yet 

 been determined. 



One of the most striking features of mottle-leaf is the fact that the de- 

 ficiency in chlorophyll is first in evidence in those portions of the leaf 

 farthest removed from the midrib and largest veins; in other words, 

 farthest from the main conducting channels of the leaf (Pi. H). This 

 suggests a deficiency in the available supply of some substance essential 

 in the formation of chlorophyll. The entire supply of this substance is 

 apparently used by those portions of the leaf near the conducting chan- 

 nels, the supply being insufficient to reach the more remote portions of 

 the leaf. That the disappearance of the chlorophyll is due to the ab- 

 sence of some essential constituent in the leaf rather than to the presence 

 of some deleterious substance is also indicated by the fact that the 

 chlorophyll next to the midrib and larger veins is the last to disappear. 

 If the plant were absorbing something which reacted unfavorably on the 

 chlorophyll, the effect of such absorption might be expected to be first 

 in evidence nearest the veins. This analysis of the problem is to be 

 considered simply as a working hypothesis which up to the present 

 appears to accord with the observations. 



The marked reduction in the yield of marketable fruit from badly 

 mottled trees and the decrease in vigor led to the undertaking of a sys- 

 tematic survey of a number of groves in districts in which mottle-leaf 



1 Smith, R. E., and Smith, Elizabeth H. California plant diseases. Cal. Agr. Exp. Sta., Bui. 218, p. 

 1139. 1911. 



2 Thomas, E. E. A preliminary report of a nematode observed on citrus roots and its possible relation 

 with the mottled appearance of citrus trees. Cal. Agr. Exp. Sta., Cir. 85, 14 P-. 8 fig. 1913- 



