Florida Agricultural Experiment Station


Leaves affected by greasy spot are the first to fall when the tree
receives a shock of any kind that tends to weaken it, although
this may be in part due to the fact that it is the oldest leaves
that generally exhibit the trouble most abundantly.
 CAUSE OF GREASY SPOT
 The cause of greasy spot is not known. This trouble has been
 considered by some experienced growers to be a sign of faulty
 nutrition or to indicate a more or less weakened condition of the
 tree. This disease bears no relation to true melanose and has
 been called "greasy spot" to avoid confusion. There is no evi-
 dence to indicate that an organism is associated with the trouble
 and its occurrence appears to be purely the result of some psysi-
 ological disorder. Greasy spot is not considered to be of suffi-
 cient importance to warrant the employment of control measures,
 even if any were known, as it rarely causes any particular con-
 cern in well-maintained groves.
 YELLOW SPOT
 A characteristic yellow spotting of citrus foliage, first reported
by Floyd in Florida in 1908 and described in detail by him in
1909, is found occasionally throughout the citrus region of the
state, especially in the ridge section. This trouble was also re-
ported from the Isle of Pines in 1915. It is not confined to any
particular kind or variety of citrus trees but is most conspicuous
on grapefruit and orange and least conspicuous on tangerine trees
by reason of the marked differences in the sizes of the leaves.
 SYMPTOMS OF YELLOW SPOT
 The spots develop on the under sides of the leaves, occurring
characteristically between the main veins on either side of the
midrib. They extend through the leaf tissue and show on the
top as yellowish to golden-colored, more or less circular areas
ranging from insignificant spots to those about 1/2 inch in diam-
eter (Fig. 80). On leaves where the spotting is of frequent oc-
currence, large areas of the leaf surface may become yellow.
On the under surfaces of the leaves the golden color of the spots
is less pronounced and, in well developed cases, the epidermis is
characterized by a more or less irregular brownish eruption,
closely resembling that occurring in the greasy spot trouble pre-
viously described. In some cases the eruptions are irregular and
scattered but in most well-developed cases they result in uni-