18, 1 Lee: Mottled Leaf of Citrus Trees 93 
To the unaided eye there is no evidence of any considerable 
amount of decaying organic matter in the soil. The nurseries 
and orchards are given very much the same treatment that or- 
chards in the United States would receive. In the dry season 
they are clean cultivated and a good dust mulch is maintained. 
In the wet season a small crop of weeds grows up which is plowed 
under at the close of the rains. 
THE USE OF STOCKS IN CONNECTION WITH MOTTLED LEAF 
From a commercial viewpoint the data presented here would 
indicate that the use of pummelo varieties of Citrus maxima 
under certain conditions is to be avoided. In the Philippines 
under the same conditions, where mottled leaf occurred severely 
upon pummelo stock, trees upon mandarin orange and calamondin 
stocks were affected but slightly, or not at all. It is suggested, 
therefore, that in areas where mottled leaf prevails the promising 
types of mandarin orange and calamondin, if given a trial as 
stocks, might give good results. The mandarin orange is used 
as a stock almost exclusively in the Swatow region in China, and 
is found to give very satisfactory results. It should be under- 
stood definitely that the mandarin orange or calamondin stocks 
are not recommended here for orchard practice until they have 
been thoroughly tried out experimentally under the local condi- 
tions in which it is intended to utilize them. They are suggested 
here only as promising for such a trial. 
SUMMARY 
1. The history of the disease has shown that a number of 
careful investigations at different times and in different places 
have pointed to different factors as causing mottled leaf. It 
would seem possible that the symptoms of mottled leaf accom- 
pany several different types of malnutrition, and are not neces- 
sarily the result of excess or lack of any one substance in the soil. 
2. In the Philippines it was observed that trees upon pummelo 
stock were badly affected with mottled leaf. Trees upon man- 
darin orange and calamondin stocks under the same conditions 
did not mottle. The relationship of stocks to mottled leaf in 
this case is not advanced as a cause of the disease, but it would 
seem that certain stocks are conducive to mottled leaf when the 
causal factors are present. The calamondin and mandarin 
orange when used as stocks apparently enable the tree to avoid 
the disease, even though the causal factors are present. 
3. In localities where mottled leaf prevails, trees upon & man- 
darin orange or calamondin stock would possibly offer a solution 
in preventing the disease. 
