PRESS BULLETIN No. 104.
Florida Agrlcullural ExpecrlmeCl $SaIon.
YELLOW SPOTTING OF CITRUS LEAVES.
BY B. F. FLOYD.
During this summer and fall, specimens of citrus leaves affected by a
greasy yellow spotting have been received at the Experiment Station from
several localities in the citrus region. This disorder has been noticed in a few
places in previous years, but it has evidently been more prevalent during the'
present season. Some of the places from which it has been reported are
Tropic, DeLand, Winterhaven, Eustis, and Clearwater. Several affected
groves have been carefully studied. In one orange grove nearly every tree
was affected, and in some trees a healthy leaf was hard to find. In another
grove which was shedded and contained both oranges and grapefruit, from
fifty to seventy-five per cent. of all the trees were more or less attacked. It
was equally bad on hammock and high pine land. No relation to any particu-
lar brand of fertilizer was noticed.
Grapefruit appears to be more frequently and severely affected than or-
anges, although the disorder seems not to be confined to any particular variety
of citrus. Among oranges, the King and Enterprise Seedless were found to
be very susceptible.
The symptoms of this malady seem to be confined to the leaves; although
a spotting was noticed on some fruits from a grove near Eustis, that very
much resembled the spots on the leaves. The appearances on the leaves are
yellowish to golden-colored spots, a fraction of an inch or more in diameter.
They usually occur between the main veins, and extend through the substance
of the leaf. The spots differ in appearance on the upper and under surfaces of
the leaf. On the upper surface, the yellowish or golden color is more notice-
able, and the central portion of the colored area is often darker in color than
the margin. The spot is usually not raised above the upper surface of the
leaf. Occasionally, dark-colored elevations, from the size of a pin-head up-
wards, may appear within the yellow spot. On the under surface, the spot
nearly always projects beyond the surrounding surface of the leaf, and usually
has a dull, greasy look, and a yellowish-brown or olive-green color. The sur-
face of old spots may become rough and dark-colored. Cross-sections of the
leaf, seen under the microscope, show a swelling of the cells in the spongy
November 21, 1908.
part of the leaf next the lower surface, with scattered brownish deposits. In
some cases, the cells adjoining the upper surface may also be affected.
A definite cause of this disease is not known. It is perhaps a physiological
trouble. At times, it is associated with die-back, of which it may perhaps be
a form. The microscopical structure of the diseased tissue is similar to that of
die-back. We cannot as yet recommend any treatment for this trouble. One
grower reports having obtained favorable results by applying lime and mulch-
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