Florida Agricultural Experimlent $allon.
SYMPTOMS OF CITRUS DIE-BACK.
BY B. F. FLOYD.
Die-back is one of the common diseases of citrus trees, occurring in
groves where the trees have been weakened by improper treatment or by
other conditions. No locality is exempt from this disease; and any variety of
citrus fruit-trees may be attacked by it. The damage in any one grove is
usually small, but throughout the state the total loss amounts to thousands of
dollars. The symptoms are very characteristic, and are most noticeable in
the spring when the new growth is coming on.
Rounded elevations or swellings, known as gum-pockets, are often found
at the bases of the leaves on the young stems of diseased trees. They may
also be located on the twigs between the leaves. These elevations vary in
size from that of half a pea to a large lima bean. A cross section shows a
cavity in the woody portion of the stem, which is filled with a clear gum-like
Resinous eruptions are present both upon the young and old growth of
affected trees, and may be very numerous. They form during any time of
year, starting as rounded elevations in the bark. The epidermis usually
breaks in a line, exposing a surface from which a light resin-like substance
oozes that hardens and forms a pad which becomes dark brown in color with
age. The bark is the only part of the stem affected by these eruptions, the
woody portion remaining apparently healthy.
The young terminal branches lose their leaves and turn yellow; and later
become stained yellowish to reddish brown, by a deposit of colored sub-
stances in the bark cells. The stain occurs in irregular areas, which some-
times unite so as to cover the whole surface of the branch. This staining of
the terminal branches may be confined to a single part of the tree, or it may
be general. It is usually followed by a dying back of the branch to the ex-
tent of several inches. This manifestation has given the disease the name
The buds on a diseased branch frequently develop in large numbers, in-
stead of as usual singly or in pairs. Resinous eruptions may occur, covering
them with a resinous coat. In some cases, apparently no buds are formed,
but resinous knots occur in their place. The buds may develop into leaves and
PRESS BULLETIN No. 98.
May 28, 1908.
shoots forming dense leaf-clusters or rosettes of shoots, which are character-
istic of die-back. As the disease progresses these new shoots lose their
leaves, become stained, and die back.
SPLITS AND "AMMONIATED FRUITS."
Very young fruit on a diseased tree takes on a pale-green appearance,
and often falls off. Fruits from one inch in diameter upward may turn yel-
lowish. The rind becomes stained reddish brown in irregular areas, accom-
panied by splitting. This form is often spoken of by citrus growers as
"ammoniated", or "fungused" fruit. The latter term is incorrect, since no
fungus has been proved to be concerned in this disease. In this stage die-back
resembles a fruit disease caused by the withertip fungus and known as anth-
racnose. The brown or reddish spots of withertip are always slightly sunken,
while the brownish spots of die-back on the so-called ammoniated fruits are
never depressed. It is in this stage that most of the fruit falls. In severe
cases very little of the fruit will reach maturity. Splitting may also occur at
the bloom end of fruits that are not stairied. In this case, deposits of a clear
gummy substance are usually found in the inner angles of the sections at the-
center of the fruit. The gumming may be so extensive as to surround the
LESS CHARACTERISTIC SYMPTOMS.
The foliage of trees on the verge of dieback shows a very dark-green ap-
pearance. Usually this is the first symptom to be seen, but it is often mis-
leading because it so nearly resembles the healthy color desired by growers.
It may be accompanied by a rank growth of the foliage, and by large, thick-
rinded fruit. These three manifestations indicate that the tree is obtaining
an over-supply of ammonia, which nearly always produces die-back.
Young twigs acquire an s-shaped appearance on account of their first
drooping downward and finally turning upward at the tip during growth.
This is not a certain characteristic, as it is sometimes seen in healthy trees.
The above enumeration of the characteristic symptoms of die-back will
aid in determining its presence in a grove.
State papers please copy.