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Copyright 2005, Board of Trustees, University
PRESS BULLETIN 147
Florlda Agricultural Experiment Stallon
By H. S. Fawcett
Scaly bark, or "nail-head rust," as it is often called, is a fungus disease
affecting sweet orange trees, especially in Hillsboro county. Although it
is only found as yet in a few places outside this county, it is likely to spread
to other parts of the State, since it has been proved to be an infectious trou-
ble. The grower should therefore be on the watch for its appearance. Only
rarely does it seriously affect any other variety of citrus than the sweet
orange. Quite often cases of gummosis are mistaken for this disease. If
the grower is in doubt, he should send specimens to the Experiment Station
Those who did not head back their diseased trees and treat them with
carbolineum in January, or did not top-work them to other varieties as sug-
gested in bulletin 98, may use the Bordeaux treatment in April with a fair
degree of success.
Spraying With Bordeaux Mixture
In a series of experiments on the treatment of scaly bark, extending
over three years, it was found that spraying with Bordeaux mixture, at al-
most any time of the year, was quite effective in keeping the spots from
the branches and fruit. This spraying was attended with the best results
when the dead branches and badly diseased limbs had been previously
pruned out. The month of April appeared to be a good time to spray;.
As the diseased spots and scabs are located almost entirely on the branches,
large limbs, and trunk, and not on the leaves, care should be taken to
spray the interior of the tree thoroughly. Spraying should be done so as
to cover the trunk and larger limbs, and also the entire surface of the
wood out to the smallest twigs. If the spray is thrown on from the outside,
most of it will be caught by the leaves, and but little good will result.
Effects of Bordeaux Mixture
It is taken for granted in suggesting this treatment that the grower
already knows quite well the effect of this mixture in allowing an increase
of'scale insects, and that he uses it with a full knowledge of this danger. No
one is advised to use the Bordeaux, unless the progress of the disease makes
it necessary. Such increase of scale insects is due to the parasitic fungi that
April 2, 19 10
keep them in control being killed by the Bordeaux mixture. This increase
may be prevented in part, either by the use of a good insecticide, or by
hanging into the sprayed trees, two or three weeks after spraying, pieces
of branches bearing the beneficial fungi.
Description of the Disease
The bark of the trunk and the bark of branches of all sizes becomes
spotted and scabby, while on the fruit there appear tinged or circular spots
known as nail-head rust spots. The spots on the small limbs and branches
are more or less circular or oval. They vary in size from one-sixth to one
inch in diameter, are raised above the surface of the bark, rusty or brown
in color, and have well defined margins. The bark becomes brittle, cracks,
and forms flakes or scales. These spots, which are at first scattered, may
increase in number to such an extent as to become joined together, and
present the characteristic scaly appearance. On the rind of the fruit, the
spots start either as rings or as round yellowish areas, In July or August.
As the fruit approaches maturity, the rings become sunken and brown.
Fruits spotted with the nail-head rust color prematurely, and drop before
the picking season.
Distinction From Gummosis
The name scaly bark is often used erroneously for the disease of gum-
mosis, one stage of which looks almost identical with scaly bark. Scaly bark
develops on all parts of the tree, from the trunk to the smallest twigs, while
gummosis affects only the trunk and larger limbs. Scaly bark appears as
round or oval "nail-head rust" spots on the small branches and fruit, while
gumniosis does not. Scaly bark often has the same scaly appearance on
the trunk and larger limbs as does gummosis, but it lacks the large amount
Cause of Scaly Bark
This disease has been found, after a long investigation, to be due to
the combined effects of two different fungi: one a minute fungus growth
that makes its way through the epidermis and grows only into the outer-
most cells of the bark; and the other, the withertip fungus, which enters
at the wounds and does the greater part of the damage.
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