86 The Philippine Journal of Science 1921 
plained that the proportion of magnesium to calcium was too 
great in areas where mottled leaf was found. This explanation 
was apparently not so well supported by evidence as was 
Hilgard’s. 
It was shown by Thomas‘ that, in a large number of cases, 
ncmatodes were to be found upon the roots of trees with mottled 
leaf. These observations by Thomas were taken in representa- 
tive districts of California. The conclusion that nematodes were 
a direct cause of mottled leaf was abandoned, however, by the 
subsequent finding of mottled leaf trees, upon the roots of which 
no nematodes could be found. 
Smith and Smith*® found mottled leaf to be associated with 
poorly drained soils, and with soils of a poor physical texture. 
Mottled leaf was also recorded by them as accompanying excesses 
of manure or as occurring on trees situated near stables or 
corrals where there was an excess of organic matter. They 
concluded that mottled leaf was a result of an irregular supply 
of food and moisture. Kellerman and Wright,’ in 1914, claimed 
to have correlated the occurrence of mottled leaf with soils in 
which there was an excess of nitrate nitrogen. They stated that - 
the turning under of straw and other substances high in cellulose 
increased the amount of mottled leaf; the explanation apparently 
advanced was that the cellulose-dissolving bacteria, which in- 
creased when straw was turned under, used up at the same time 
large amounts of nitrate nitrogen. They claimed to have induced 
mottling, artificially, by introducing into the soil straw and other 
substances high in cellulose, and suggested the turning under of 
green cover crops aS a means of preventing mottled leaf. Lip- 
man,’ in 1915, advanced an interesting theory in explanation of 
the cause of mottled leaf; his conclusion was that the disease 
was caused by a lack of nitrogen in an available form, and was 
based upon field observations and pot experiments. He sugges- 
ted that, although the total nitrogen content of the soil might 
be high, such nitrogen was not necessarily in an available form, 
* Thomas, E. C., A preliminary report of a nematode observed on citrus 
roots and its possible relation with the mottled appearance of citrus, 
Calif. Agr. Exp. Sta. Circular 85 (1913). 
. *Smith, R. E., and Smith, E. H., California plant diseases, Calif. Agr. 
Exp. Sta. Bull. 213 (1911) 1137-1159. 
* Kellerman, K. F., and Wright, R. C., Relation of bacterial transforma- 
tions of soil nitrogen to nutrition of citrous plants, Journ. Agr. Res. 2 
(1914) 101. 
‘Lipman, Chas. B., A suggestion of a new phase of the problem of 
physiological diseases of plants, Phytopathology 5 (1915) 111. 
