Florida Agricultural Experiment Station


 A number of observations in various groves with marly sub-
 soil, where sour orange rootstock is used, demonstrates that
 grapefruit trees often develop more or less satisfactorily when
 orange and tangerine trees develop but poorly or die out. This
 was so striking in a portion of one such grove that the orange
 trees were topworked to grapefruit. Tangelo trees also often
 thrive on such soil types when orange and tangerine trees de-
 velop an acute form of mottle-leaf and decline, although tan-
 gerine trees seem to be less seriously affected than oranges.

 CAUSE OF MOTTLE-LEAF
 Mottle-leaf and other forms of chlorosis of citrus appear to
 be the result of a deranged nutritional condition, the exact
 causes of which are still involved in much uncertainty but appear
 to be associated with the complex r61e of the mineral require-
 ments in the life processes of the tree. Some forms appear to
 be due to the inability of the tree to get a sufficient supply of
 certain elements, such as iron, calcium, magnesium, potassium
 or nitrogen to satisfy their requirements, while others are re-
 lated to excessive amounts of certain elements taken up by the
 tree. Mottle-leaf in particular has provoked extensive discus-
 sion among growers, pathologists, physiologists and chemists.
 As a result of investigations at different times and in different
 places, several rather diverse theories have been advanced to
 explain the cause of this trouble.
 The exact cause of mottle-leaf, however, is less understood
than the diverse array of conditions and practices which con-
tribute indirectly to its development. Probably the most con-
stant condition with which this nutritional disorder is associated
in Florida is a deficiency of organic matter and moisture in many
of the soils devoted to citrus culture and the tendency for grow-
ers to rely largely upon commercial fertilizers alone to supply
the requirements of the trees. In soils deficient in organic mat-
ter and subject to deficits of moisture the trees are prevented
from making the most effective use of the commercial fertilizers
applied and the low biological activity of such soils is often un-
favorable for a good growth of citrus trees. Such physical and
biological conditions of the soil, with which the availability and
utilization of fertilizers is intimately tied up, naturally tend to
derange the nutritional processes of citrus trees. A proper un-
derstanding of the situation involves a consideration of the in-