92 The Philippine Journal of Science 1921 
ever exhibited the symptoms, led to the conclusion at first that 
this species was in some way incompatible to the sweet orange 
and mandarin orange species. That is, it was thought that 
possibly the mandarin orange and sweet orange species were 
not closely enough related to the pummelo to bud well. However, 
this theory was abandoned since the converse, pummelo budded 
on mandarin orange in near-by rows under identical conditions, 
did not result in mottled leaf but made perfectly normal growth. 
The correlation of the pummelo stocks to mottled leaf pre- 
sented in this paper is not advanced as a cause of the disease 
but as a factor conducive to mottled leaf if the conditions are 
favorable. That is to say, it is believed that trees upon pum- 
melo stocks mottle only when the other contributing factors 
are present, while if such factors are absent the trees will re- 
main perfectly normal. The use of pummelo stock therefore 
is conducive to mottled leaf only when the original contributing 
cause or causes are present. Trees upon mandarin orange stock, 
however, have but little mottled leaf, even in the presence of 
one of the series of contributing causes. 
A plausible assumption for the different reactions of the 
different species on various stocks would seem to be the compara- 
tive resistance and susceptibility of such stocks. Thus it would 
seem that the mandarin orange and the calamondin as stocks are 
resistant to the peculiar environmental conditions at Lamao 
which are conducive to mottled leaf; the pummelo as a stock 
would seem to be extremely sensitive. No analysis as to the 
reasons for such resistance and susceptibility is possible from 
the data at present available. However, the practical application ~ 
is immediately apparent; that is, it would seem possible in such 
regions where the soils have been shown to be favorable to mot- 
tled leaf to minimize injury from this disease by the use of the 
proper stocks. 
THE SOIL AT LAMAO 
Soil conditions have been so closely connected with mottled 
leaf in previous literature that such a brief description as is 
possible is presented here for the use of others interested in this 
disease. The soil at Lamao is fairly uniform in physical texture 
throughout all the citrus orchards. It is probably of alluvial 
origin and may be classed as a sandy loam; this sandy loam 
extends to a depth of from 2.5 to 3 meters before gravel is 
reached. The drainage throughout the orchards is very good 
and water never stands upon the surface of the soil. During the 
dry season it is possible that irrigation is sometimes irregular. 
