738 Journal of Agricultural Research vol. vi.No. 19 



groves which are low in this constituent, but lime experiments with the 

 lemon would appear to be more promising than with the orange, other 

 conditions being the same. 



No correlation was found to exist between organic carbon or total nitro- 

 gen and mottling, the correlation coefficient in each case being no greater 

 than the probable error. 



SUMMARY 



Mottle-leaf of Citrus trees is characterized by the disappearance of 

 chlorophyll from parts of the leaf, the portions farthest removed from 

 the midrib and larger veins being first affected. As the disturbance 

 progresses, the yellowish spots increase in size until the only remaining 

 chlorophyll is confined to narrow areas along the midrib and the larger 

 veins. The advanced stages are accompanied by a marked decrease in 

 the size, quality, and yield of fruit. No organism has yet been proved 

 to be causally associated with mottle-leaf, but the Citrus-root nematode 

 has been found by Thomas to be widely distributed in mottled districts. 



Mottle-leaf is found in most Citrus-fruit sections of California, but is 

 more prevalent in some districts than in others. All the Citrus fruits 

 grown in California are affected, including the Washington Navel, Thomp- 

 son Improved Navel, and Valencia orange, grapefruit, tangerine, and 

 lemon. 



The conclusions of the present paper are based upon a field and labora- 

 tory study of 130 orange groves and 45 lemon groves, located mainly in 

 Riverside and San Bernardino Counties, Cal. The percentage of mottled 

 leaves was determined by examining 10 to 12 typical trees in each grove. 

 A soil sample 3 feet in depth was taken near each tree, each foot sample 

 being kept separately. These samples were analyzed for humus, organic 

 carbon, mineral carbonates, bicarbonates, and total nitrogen. 



During the earlier stages of mottling no serious reduction in yield was 

 observed. The fruit yield was apparently not seriously reduced on 

 either orange or lemon trees which had about 20 per cent of their leaves 

 mottled. Sour-orange stock was found to induce more severe mottling 

 in orange trees than sweet-orange stock, other conditions being the same. 

 A mixed grove of Washington Navel and Valencia oranges showed no 

 difference in the amount of mottling of these two varieties. 



Badly mottled orange trees cut back and rebudded on the stumps 

 produce badly mottled new top growth; and unless the soil treatment 

 of such groves is changed, the mottling persists. 



There was no noticeable difference in the amount of leaf mottling in 

 groves on different soil types, other conditions being the same. 



Orchards fertilized with organic substances, such as stable manure or 

 cover crops plowed under, usually showed less mottling than groves sup- 

 plied principally with commercial fertilizers. Groves which for some 

 years had received only the "complete" fertilizers in general use in the 



