348 MONTHLY BULLETIN. 



are affected, including tlie Washington Navel, Thompson Improved Xavel, and 

 Valencia orange, grapefruit, tangerine and lemon. 



The conclusions of the present paper are based upon a field and laboratory study 

 of 130 orange groves and 45 lemon groves, located mainly in Riverside and San 

 Bernardino counties, California. 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, 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 that groves supplied principally with 

 commercial fertilizers. Groves which for some years had received only the "complete" 

 fertilizers in general use in the areas studied were badly mottled in all cases, so 

 far as observed in these studies. This was also the case where sodium nitrate was 

 used alone or as the principal fertilizer for some years. 



The results of the soil analyses show in the case of oranges a marked inverse 

 correlation between the humus content of the soil and the percentage of mottling, the 

 Jatter tending to diminish as the humus content increases. An impartial statistical 

 study of the data from the individual orange groves shows that approximately one- 

 half the mottling can be accounted for by the low humus content of the soil. 



The humus content of the lemon soils studied is much below that of most of the 

 orange soils, averaging less than 0.1 per cent. This amount of humus is apparently 

 too low to produce a normal foliage growth, all of the lemon groves being badly 

 mottled. 



No correlation was found between the mineral carbonates of the soil and the 

 mottling of orange trees. In lemons the mottling decreased slightly as the mineral 

 carbonates increased, but the correlation is low. The lime content of nearly all the 

 citrus soils studied, is low, and the effect of heavy applications of lime can only be 

 determined by suitably controlled field experiments. The present study indicates 

 that the application of lime would be more likely to benefit lemon trees than orange 

 trees. 



The percentage of mottled leaves on orange trees is definitely correlated with the 

 increase of the ratio of organic carbon to humus, indicating the importance of the 

 organic matter in the soil being well decomposed. 



No relation was found between the i>ercentage of leaves mottled and the total 

 nitrogen content of the soil in either the orange groves or the lemon groves studied. 



The principal conclusion of this investigation is that the mottling of orange trees 

 in the areas studied is definitely correlated with the low humus content of the soil, 

 the mottling diminishing as the humus content increases. A study of the data by 

 statistical methods shows that approximately one-half of the mottling can be accounted 

 for on this basis. The incorporation of organic matter with the soil in such a manner 

 as to be accessible to the roots during its decomposition is indicated as a promising 

 treatment for mottle-leaf. 



Horticultural Laws. — The movement to amend some of the existing 

 horticultural statutes, thereby strengthening them in places where they 

 have been found weak, is commendable, and will no doubt receive the 

 support of all wide-awake horticulturists, fruit growers, nurserymen 



