724 Journal of Agricultural Research voi.v1.No.19 



orange stock. In some instances also, tops on grapefruit stock were 

 found to be more mottled than tops on sweet-orange seedlings. One 

 specific instance of two adjoining navel-orange groves under the same 

 ownership illustrates the case. One grove is on lemon stock, the other 

 on sweet-orange stock. The trees on lemon stock showed 70 per cent 

 of their leaves mottled, and those on the sweet-orange stock 50 per cent. 

 Also, instances were found where individual orange trees on lemon stock 

 were much more mottled than the surrounding trees on sweet-orange 

 stock. As it is frequently impossible to obtain definite information 

 about the stock used, this factor complicates the investigation. The 

 physiological behavior of the buds on various stocks would be an inter- 

 esting study in this connection. 



Frosts and severe winds tend to increase mottling. After a strong, 

 dry, north wind late in the fall of 191 2 the leaves on the north side of the 

 trees were more mottled than those on the opposite side. The new leaves 

 put out later on the north side of the tree, however, were less mottled; 

 and during the summer of 19 14, the period covered by the field survey, 

 the leaves on the south side of the trees were generally more mottled 

 than those on the north side. Even in very severe cases of mottling, 

 large healthy leaves are also often found in the center of the trees. This 

 suggests that strong sunlight may increase mottling; and if so, the effect- 

 iveness of this agency would vary with the size of the trees and the close- 

 ness of planting. 



Badly mottled trees cut back and rebudded on the stumps produce 

 badly mottled new top growth, and the mottling persists unless the soil 

 treatment is changed. 



FERTILIZERS IN RELATION TO MOTTLING 



The results of the field survey showed that groves which were plenti- 

 fully supplied with organic material, either in the form of manures or 

 °reen cover crops, were less mottled than those that had been fertilized 

 entirely with commercial fertilizers. Several growers stated that they 

 had cured mottle-leaf in limited areas by a liberal application of barn- 

 yard manure. It was also found in the case of all the groves included in 

 this field study that each grove that had been fertilized with commercial 

 fertilizers alone and kept under clean cultivation was badly mottled. 

 This condition was especially marked in groves in which sodium nitrate 

 had been employed for a number of years as the principal or only fertilizer. 

 On the other hand, some groves that had received organic fertilizer were 

 also badly mottled. This latter fact has discouraged many growers from 

 using such material, especially manures, which usually have to be pur- 

 chased and shipped in at high cost. 



It was also observed that plowsole (an incipient hardpan just below the 

 cultivated layer) frequently accompanied a badly mottled condition of the 

 tree. Numerous observations have shown that the plowsole is a serious 



