• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Introduction
 Distribution
 Dissemination of the disease
 Trees susceptible to the disea...
 Symptoms of citrus dieback
 Development of dieback
 Factors suggested as causal...
 Conditions complicating the connection...
 Dieback and fertilizers
 Explanation of the cause of...
 The control of citrus dieback under...
 Summary














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 140
Title: Dieback or exanthema of citrus trees
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027331/00001
 Material Information
Title: Dieback or exanthema of citrus trees
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 31 p. : ill. ; 23 cm.
Language: English
Creator: Floyd, B. F ( Bayard F )
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1917
 Subjects
Subject: Citrus -- Diseases and pests   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by B.F. Floyd.
General Note: Cover title.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00027331
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000922758
oclc - 18162174
notis - AEN3267

Table of Contents
    Title Page
        Page 1
    Credits
        Page 2
    Introduction
        Page 3
    Distribution
        Page 4
    Dissemination of the disease
        Page 4
    Trees susceptible to the disease
        Page 4
    Symptoms of citrus dieback
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Development of dieback
        Page 10
        Page 11
    Factors suggested as causal agents
        Page 12
    Conditions complicating the connection of dieback with soil organic matter
        Page 13
        Page 14
        Page 15
    Dieback and fertilizers
        Page 16
        Page 17
    Explanation of the cause of dieback
        Page 18
    The control of citrus dieback under various conditions
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
    Summary
        Page 30
        Page 31
Full Text

August, 1917


UNIVERSITY OF FLORIDA

Agricultural Experiment Station



DIEBACK, OR EXANTHEMA OF
CITRUS TREES
By B. F. FLOYD


FIG. 1.-Orange tree severely injured by dieback


Bulletins will be sent free upon application to Experiment Station,
GAINESVILLE, FLORIDA


Bulletin 140













BOARD OF CONTROL
JOE L. EARMAN, Chairman, West Palm Beach, Fla.
T. B. KING, Arcadia, Fla.
E. L. WARTMANN, Citra, Fla.
J. B. HODGES, Lake City, Fla.
H. J. BRETT, DeFuniak Springs, Fla.
BRYAN MACK, Secretary, Tallahassee
J. G. KELLUM, Auditor, Tallahassee

STATION STAFF
P. H. ROLFS, M.S., Director
J. M. SCOTT, B.S., Vice-Director and Animal Industrialist
B. F. FLOYD, A.M., Plant Physiologist
J. R. WATSON, A.M., Entomologist
H. E. STEVENS, M.S., Plant Pathologist
C. D. SHERBAKOFF, Ph.D., Associate Plant Pathologist
S. E. COLLISION, M.S., Chemist
ARTHUR M. SMITH, B.S., Assistant Chemist
MILDRED NOTHNAGEL, Ph.D., Assistant Plant Physiologist
F. F. HALMA, B.S., Assistant Horticulturist
JULIUS MATZ, B.S., Laboratory Assistant in Plant Pathology
CLIFTON W. LONG, B.S.A., Laboratory Assistant in Dairying
K. H. GRAHAM, Auditor and Bookkeeper
T. VAN HYNING, Librarian
E. G. SHAW, Secretary
L. T. NEILAND, Farm Foreman










DIEBACK, OR EXANTHEMA OF CITRUS TREES
By B. F. FLOYD

Dieback, or exanthema, is one of the most common and most
important of the diseases of citrus trees. It affects all kinds
and varieties, and occurs on trees planted on all types of soil.
A large percentage of the groves planted on the sandy lands of
peninsular Florida have at some time in their history been af-
fected by it to some extent.
The disease is important on account of its relation to the
organic matter in the soil. Any attempt to feed the trees or to
build up the soil by the addition of any large amount of organic
material is likely to lead to a rank growth in the trees and to a
development of the disease. It is not known whether the organic
matter induces the disease directly, or merely develops a type of
growth that is susceptible to the attack of some obscure organism
which causes the disease. The amount of damage done by the
disease in any one community varies considerably, and is usually
small, but the total in the State as a whole amounts to many thou-
sands of dollars.
Dieback has been studied in Florida and in California by
a number of investigators. Swingle and Webber studied it and
other diseases in Florida from 1892 to 1895. They gave a com-
plete description of the disease, with a discussion of its cause
and methods of treatment, in bulletin No. 8, The Principal
Diseases of Citrous Fruits in Florida," published by the Division
of Vegetable Physiology and Pathology of the United States De-
partment of Agriculture.
Grossenbacher, in a paper entitled Some Bark Diseasea of
Citrus Trees in Florida (Phytopath., Vol. 6, p. 29, 1916), dis-
cussed dieback and classed it as a bark disease that begins with
injuries which occur in the cambium and phloem of the bark of
certain of the plant parts. He does not suggest a possible cause
for the injuries. Dr. C. B. Lipman studied dieback in California.
He suggests (Science, Vol. 39, p. 728, 1914) the hypothesis that
it is caused either by a lack of nitrogen or by ammonia poisoning.
The purpose of this bulletin is to give a popular summary of
the information which is extant at the present time concerning
the disease, its cause and treatment. A technical discussion of
this subject is avoided wherever possible to do so. Our informa-






Florida Agricultural Experiment Station


tion concerning the absolute cause of the disease is still far from
complete.
DISTRIBUTION
Dieback has long been known as a disease of citrus trees in
Florida. It was first reported by Fowler in 1875. (Florida,
its scenery, climate and history. Sydney Lanier, 1876. Appen-
dix, pp. 281-290.) It has probably been present in the State ever
since citrus trees have been grown under culture. It is not
known to affect trees growing in the wild.
The disease is largely confined to trees growing on sandy
soils. While trees growing on clay soils are subject to it to some
extent, it often happens that the same soil treatments that induce
the disease in the trees on sandy soils fail to induce it in those on
clay soils. Consequently, the disease is not common in groves
planted on the heavier soils in the western part of the State, and
in other Gulf States.
Altho the disease is probably most prevalent in Florida, it is
by no means confined here. It is found to a considerable extent
in California, and in many other parts of the world. It is re-
ported to be in Australia, the Philippines, Dutch Guinea, and
Italy. It probably exists to some extent in other countries, but
has not been recognized there as such.
DISSEMINATION OF THE DISEASE
Dieback seems to be neither infectious nor contagious. Ex-
tensive experiments have been carried out to produce the disease
by inoculation. The results have invariably been negative. Bud-
ders in the nursery have been able to bud healthy trees with buds
from dieback-affected trees. They state that the resultant growth
showed dieback. It is very difficult to make such buds "take"
on account of their lack of vitality.
'The writer has repeatedly moved young trees that were se-
verely affected with the disease, to new locations. The trees that
lived showed no dieback, and made a normal growth.
TREES SUSCEPTIBLE TO THE DISEASE
Dieback affects practically all kinds and varieties of citrus
that are grown commercially in Florida. It is a disease of the
growing tissues and reaches its greatest development in the trees
making rapid growth. Therefore, other conditions being equal,
it is likely to be more prevalent in trees budded on stocks that
give a rapid and rank growth. While it is somewhat more
prevalent in trees budded on rough lemon stock, there is not






Florida Agricultural Experiment Station


tion concerning the absolute cause of the disease is still far from
complete.
DISTRIBUTION
Dieback has long been known as a disease of citrus trees in
Florida. It was first reported by Fowler in 1875. (Florida,
its scenery, climate and history. Sydney Lanier, 1876. Appen-
dix, pp. 281-290.) It has probably been present in the State ever
since citrus trees have been grown under culture. It is not
known to affect trees growing in the wild.
The disease is largely confined to trees growing on sandy
soils. While trees growing on clay soils are subject to it to some
extent, it often happens that the same soil treatments that induce
the disease in the trees on sandy soils fail to induce it in those on
clay soils. Consequently, the disease is not common in groves
planted on the heavier soils in the western part of the State, and
in other Gulf States.
Altho the disease is probably most prevalent in Florida, it is
by no means confined here. It is found to a considerable extent
in California, and in many other parts of the world. It is re-
ported to be in Australia, the Philippines, Dutch Guinea, and
Italy. It probably exists to some extent in other countries, but
has not been recognized there as such.
DISSEMINATION OF THE DISEASE
Dieback seems to be neither infectious nor contagious. Ex-
tensive experiments have been carried out to produce the disease
by inoculation. The results have invariably been negative. Bud-
ders in the nursery have been able to bud healthy trees with buds
from dieback-affected trees. They state that the resultant growth
showed dieback. It is very difficult to make such buds "take"
on account of their lack of vitality.
'The writer has repeatedly moved young trees that were se-
verely affected with the disease, to new locations. The trees that
lived showed no dieback, and made a normal growth.
TREES SUSCEPTIBLE TO THE DISEASE
Dieback affects practically all kinds and varieties of citrus
that are grown commercially in Florida. It is a disease of the
growing tissues and reaches its greatest development in the trees
making rapid growth. Therefore, other conditions being equal,
it is likely to be more prevalent in trees budded on stocks that
give a rapid and rank growth. While it is somewhat more
prevalent in trees budded on rough lemon stock, there is not






Florida Agricultural Experiment Station


tion concerning the absolute cause of the disease is still far from
complete.
DISTRIBUTION
Dieback has long been known as a disease of citrus trees in
Florida. It was first reported by Fowler in 1875. (Florida,
its scenery, climate and history. Sydney Lanier, 1876. Appen-
dix, pp. 281-290.) It has probably been present in the State ever
since citrus trees have been grown under culture. It is not
known to affect trees growing in the wild.
The disease is largely confined to trees growing on sandy
soils. While trees growing on clay soils are subject to it to some
extent, it often happens that the same soil treatments that induce
the disease in the trees on sandy soils fail to induce it in those on
clay soils. Consequently, the disease is not common in groves
planted on the heavier soils in the western part of the State, and
in other Gulf States.
Altho the disease is probably most prevalent in Florida, it is
by no means confined here. It is found to a considerable extent
in California, and in many other parts of the world. It is re-
ported to be in Australia, the Philippines, Dutch Guinea, and
Italy. It probably exists to some extent in other countries, but
has not been recognized there as such.
DISSEMINATION OF THE DISEASE
Dieback seems to be neither infectious nor contagious. Ex-
tensive experiments have been carried out to produce the disease
by inoculation. The results have invariably been negative. Bud-
ders in the nursery have been able to bud healthy trees with buds
from dieback-affected trees. They state that the resultant growth
showed dieback. It is very difficult to make such buds "take"
on account of their lack of vitality.
'The writer has repeatedly moved young trees that were se-
verely affected with the disease, to new locations. The trees that
lived showed no dieback, and made a normal growth.
TREES SUSCEPTIBLE TO THE DISEASE
Dieback affects practically all kinds and varieties of citrus
that are grown commercially in Florida. It is a disease of the
growing tissues and reaches its greatest development in the trees
making rapid growth. Therefore, other conditions being equal,
it is likely to be more prevalent in trees budded on stocks that
give a rapid and rank growth. While it is somewhat more
prevalent in trees budded on rough lemon stock, there is not





Bulletin 140, Dieback of Citrus Trees


much difference between its prevalence in these and in those
budded on the sweet, grapefruit and sour stocks. On the other
hand, it is much less prevalent in trees budded on the slow-grow-
ing trifoliata stock.
It is stated by growers that the Ruby Blood orange budded on
the rough lemon stock is particularly susceptible to the disease.
SYMPTOMS OF CITRUS DIEBACK
Dieback affects the terminal and subterminal branches, the



Al













FIG. 2.-Dieback in young trees on spruce pineland

leaves and the fruit. It seldom affects the larger branches or the
trunk except in severe cases in young trees. The roots of se-
verely affected trees often show a lack of fibrous roots and
feeding tips. In some cases, brownish, discolored and injured
roots are found. On the other hand, the root system of less se-
verely affected trees may show.no injury whatever.
The name given this disease is misapplied. It suggests that
the dying of the limbs is a distinguishing characteristic. Such
is not the case. The disease does not manifest itself by a mere
dying back of the outer branches, as in the case of withertip. The
death of the limbs is an after effect, and occurs only in severe
cases. The distinguishing characteristic of dieback is the gum
symptoms.
PRIMARY SYMPTOMS
Dieback is a gum disease of citrus trees. In the absence of





Florida Agricultural Experiment Station


gum manifestations, the disease cannot be recognized. There
is a possibility that the disease exists in citrus trees without pro-
ducing gum symptoms, but no means is known at present for
identifying it as such with certainty.
The symptoms of the disease with which the production of
gum in the tissues is associated, are the gum pockets, the bark
excrescences, the stained terminal branches, the marked or am-
moniated fruit, and the multiple buds. Of these, the bark ex-
crescences, the stained terminal branches, and the marked fruit
are the ones that are characteristic of the disease. When any of
these symptoms are found, the disease may be suspected or can
be identified with certainty. The gum pockets and the multiple
buds are not entirely characteristic of the disease. Gum pockets
that are not exactly the same, but are very similar, are produced
in the immature branches by the sting of plant bugs. However,
where the gum pockets occur regularly one to a node, they in-
dicate dieback. Multiple buds have been found accompanying
some severe cases of frenching, but there are indications that
these may be cases of dieback with an absence of the gum
symptoms.
Dieback is a disease of the growing tissues. Mature tissues
are not affected by it. It reaches its greatest development in
rapidly growing trees. It is less likely to develop in slow or
normal growing trees. Each of the gum symptoms is confined
to particular plant parts, and is coincident in development with
certain stages of growth of these parts. The symptoms which
will be produced in the tree by the disease stimulus are entirely
dependent upon the stage and condition of growth of the plant
parts.
Gum Pockets.-The gum pockets develop in the young suc-
culent terminal branches before they have attained full length
growth. They never develop anew in mature branches. Early
stages of their development may occur in connection with each
of the other primary symptoms. They consist of pockets of a
pure, clear or amber colored gum between the wood and bark at
or near the leaf nodes. In the formation of a pocket, the bark
above is raised giving a gross appearance similar to that of a
large blister. (Fig. 3.)
The gum pockets never kill a branch. As the stem grows
older, new wood is formed over the pocket, burying it. It finally
disappears.
Bark Excrescences.-The bark excrescences develop in the






Bulletin 140, Dieback of Citrus Trees


bark of mature or nearly mature terminal or subterminal stems.
(Fig. 10.) In severe cases, they may be formed in the bark of
larger branches, or on the trunk of young trees. They consist of
longitudinal to more or less rounded breaks in the bark thru
which a gummous tissue is forced. This forms a double convex
to rounded pad of crystal-like gum on the stem.
When the excrescences occur on the terminal stems, they are
more commonly formed near the base of the stem. They may be
formed in the leaf axil among the buds. They may also occur
in the bark of the fruit spurs (fig. 11). When this happens, the
fruit will also show markings by the disease. The excrescences
frequently occur in breaks in the bark formed by the bark cam-
bium which gives rise to the true bark of the stem. It is probable
that they have their origin, at least in some cases, in this tissue.
This symptom does not lead to the death of the branch. After
a year or more, new bark is formed below and the diseased bark
is sloughed off.
Stained Terminal Branches. Staining of the terminal
branches occurs while they are still young and somewhat suc-
culent but are approaching maturity. It never develops anew
upon either very young or mature branches. The stained ap-
pearance is due to a brownish, glossy, gum-soaked condition of
the bark in spots and irregular areas (fig. 4). The areas are
only slightly, if at all, raised. The staining is caused by the
closing of the epidermal and subepidermal cells with a gum-like
substance. No corky tissue or other proliferous tissue is devel-
oped beneath the areas. The staining is almost entirely confined
to the bark of the terminal stems; it never occurs on the sub-
terminal stems. It sometimes extends on to the leaves (fig. 5),
in which case it is found mostly on the petioles and the base of
the blades.
In severe cases, the trees make a very promising growth of
terminal stems that later become stained and die. The repeated
growing and dying back of these stems give the trees a character-
istic appearance. This suggested the name Dieback," which
was given to the disease.
It is doubtful, except in extreme cases, whether the staining
of a branch causes sufficient injury to kill it. It is more likely
that it so weakens the branch that secondary factors, such as the
withertip fungus, are able to attack and kill it. In the extreme
cases, however, where the entire area of the bark is largely






Florida Agricultural Experiment Station


stained, the branches may be so injured that they become defoli-
ated and may die.
Marked or Ammoniated Fruit.-The same brownish, glossy,
gum-soaked areas that occur on the terminal branches also occur
on the rind of the fruit. (Figs. 6, 7, 8, 9.) They. may develop
on the fruit at any time after it is a month old and until it is
nearly full size. The areas rarely occur anew on very young
fruits and never on the mature fruits.
The stained fruit is spoken of as die-
back-marked or ammoniated fruit. The
term ammoniated arises from the
fact that organic ammoniates, when
used in excessive amounts as a fertil-
izer, frequently induce the disease. The
term is a misnomer, however, because it
suggests that mineral ammonia fertil-
izers may induce the disease. These
sources are not known to induce it.
When used in limited amounts they are
useful in aiding the trees to recover.
When used in excessive amounts on af-
fected trees they may accentuate the
disease.
The stains or marks occur on the
fruit in the form of spots and irregular
areas. These markings are likely to be
located anywhere on the rind, but are
FIG. 3.-Gum pocket, more commonly found on the side or on
(Indicated by arrow) the lower half of the fruit. The blos-
som scar is often the center of a stained
area. The surfaces of the areas may become cracked in a criss-
cross manner like that of dried mud. (Fig. 9.) The areas form
weak points thru which lines of splitting may occur. Dieback-
marked fruit very commonly splits and drops. In severe cases
this may occur so extensively that by the time the fruit should be
mature there is very little left on the tree. The splitting and
dropping often occurs to the greatest extent in midsummer at
the time the fruit is beginning to take on size. Because the line of
splitting may develop anywhere on the fruit that a stained area
occurs, such split fruit is spoken of as dieback splits." (Fig.
6.) This is to distinguish it from the splitting of normal fruit,
which often occurs in the fall of the year. The splitting






Bulletin 140, Dieback of Citrus Trees


and dropping varies with conditions. It sometimes happens
that a crop of fruit is. badly scarred by the disease, yet
very little of it splits and falls.
The stained areas are often
coincident with thrips marks,
when they occur on dieback
fruits. Melanose markings on
such fruits are usually more
pronounced than those on nor-
mal fruits, and may stimulate
dieback staining in the adjacent
tissues.
Collections of a clear to am-
ber colored gum sometimes show Wb I
in the angles of the segments
and surrounding the seed of die-
back fruits. It may be so plen-
tiful that the seeds are floating in
it. This is not known to occur
in other than dieback fruits.
When found, it furnishes evi-
dence that the fruit is affected
with dieback.
Dieback fruits are often coarse
and may have a thick rind (fig.
9), particularly if the disease f
has been brought on by the use
of excessive amounts of organic
fertilizers. On the other hand,
very prominent staining may oc-
cur on fruits that are normal in
all other respects. Dieback-af-
fected fruits are usually insipid
in flavor. They are lacking in FIG. 4.-Stained terminal branch.
acid and bouquet. The marked Note the gum pockets (blister-
fruits are mostly culls and are olie risi no the bark) near
worthless for shipment.
Multiple Buds.-Multiple buds are clusters of several to
thirty or more buds in a leaf axil where normally two would
occur (figs. 12, 13). Strictly speaking, this symptom is not a
gum symptom. But a multiple bud is rarely found that does
not have either some gum formation in the cambium region,






Florida Agricultural Experiment Station


or some staining or bark excrescence associated among the
buds.
SECONDARY SYMPTOMS
While the foregoing symptoms are the ones that characterize
the disease, they do not constitute all of its manifestations. There
are other symptoms that are largely secondary and are not char-
acterized by any production of gum. They are chiefly growth
disturbances that arise incident to the development of the disease..
These may occur in trees entirely unaffected by dieback.
Deep Green Color of Foliage.-Trees that are chronically af-
fected with and trees that are on the verge of dieback and are
growing under favorable food and moisture conditions, may
show an exceptionally deep green color of the foliage. Where
such occurs, the leaves are usually large and the trees may have
a very promising appearance.
Frenching.-Frenching is a lack of green color in tissues
between veins of the leaves, and often shows in parts of the af-
fected trees (fig. 14). This is not a constant symptom, and oc-
curs in trees entirely unaffected by dieback.
Abnormal Leaves.-In the more severe cases of dieback,
where the disease stimulus has affected the growth of the whole
tree, the foliage sometimes shows a condition that is typical of
the disease. The leaves will be thick and coarse to the touch.
The length of some of them will be so increased, and the breadth
so decreased, that they resemble large peach or mango leaves.
S-shaped Growth of Branches.-Trees making a rapid, rank
growth often develop long, immature, angular branches of a
distorted shape. In some cases they appear to have been bent
over by their own weight, and the tips to have grown upward
again, resulting in an S-shaped appearance of the branches (fig.
15). In other cases they appear to have grown downward and
then upward again, giving the S-shaped appearance. This type
of growth is frequently seen in dieback trees, and often precedes
the development of the disease.
Rosette-like Growth.-In a number of instances some of the
multiple buds will develop into branches, giving a bushy, rosette-
like growth. Branch growth will also be made from an excep-
tionally large number of the nodes. These growths give the
tree a thick, bushy head.
DEVELOPMENT OF DIEBACK
Dieback may be said to exist in two forms-chronic and
acute. The only difference between the two is the persistence of






Bulletin 140, Dieback of Citrus Trees


the disease due to the persistence of the causal agent. In the
chronic form, the disease often attains complete development in
which all of its symptoms are evident. If the conditions con-
tinue, the trees will be badly injured and may die. This form of
the disease usually occurs where the disease has been brought
on by some persistent, improper soil conditions, such as lack of
drainage or a hardpan too near the soil surface.
In the acute form, the disease is only temporary. In this
form the disease does not always attain complete development.
Only one or more of the primary symptoms may be present. For
example, the only evidence of the presence of the disease may be
the gum pockets and stained terminal branches, or it may be
the gum pockets, stained terminal branches, and bark ex-
crescences, etc. It may happen that the fruit is the only part of
the tree affected, and the marking of the fruit may not be ac-
companied by any extensive splitting or dropping. This form of
the disease occurs where it is brought on by some improper soil
treatment, such as overfeeding the trees with organic fertilizers,
or by excessive cultivation; or by some soil condition such as an
irregular moisture content. In these cases, with the changed soil
and seasonal conditions, the disease may in time disappear with-
out treatment.
The development of the disease is the result of the complex
action of many factors, and is dependent not only upon the dis-
ease stimulus but also upon stages of growth in the tree. Con-
sequently, a complete development of the disease is attained only
where the disease stimulus is more or less continuous so that the
two are coincident. It is probable that the partial development
of the disease in which only a few of the symptoms are evident-
which so often occurs-is explained by the temporary or inter-
mittent action or existence of the causal agent.
In'the typical development of a case of dieback, the first indi-
cation of the disease is an exceptionally deep green color of the
foliage and a rather rank growth of the branches. The trees
have a thrifty and promising appearance. Many of the terminal
stems are long, angular, and distorted in shape. Frenching of
the foliage may or may not be present. Gum pockets are the first
of the gum symptoms to appear. With further growth of the
tree and development of the disease, staining of the terminal
branches and the fruit, and the formation of bark excrescences
and multiple buds occur. The formation of these is coincident
with the development of the parts which they affect. In later






Florida Agricultural Experiment Station


stages, the affected terminal branches may die back. New
growth is made from the nodes farther back, and in some cases
from the multiple buds. The result is a tree with a more or less
rounded, thick, bushy head that is typical of the disease. (See
frontispiece.) With successive flushes
of growth, a new development of the
symptoms occurs.
**A complete development of the dis-
ease will not occur in the course of one
Sflush of growth in the tree, but usually
requires several. Such a course of de-
velopment more often occurs in the
chronic cases than in the acute cases of
the disease.

FACTORS SUGGESTED AS CAUSAL
AGENTS
The absolute cause of dieback in cit-
rus trees is not definitely known. It is
apparently very closely associated with
the organic matter in the soil. In prac-
tice it has long been known that the use
of stable manure or organic ammonia
fertilizers in excessive quantities on the
sandy or rocky soils of Florida will of-
ten bring on a dieback condition in the
citrus trees. Groves set out in fields
previously planted to vegetables and
heavily fertilized with the usual vege-
table fertilizers are often subject to the
disease. Groves in which vegetables are
grown between the rows frequently be-
come affected. Trees near stables, out-
houses, cesspools, poultry yards, and
dFc. 5Lef yainieng kitchens where the roots are able to
feed in the manures, garbage, and other
organic refuse, often develop the trouble. Trees about which
urine and night-soil from the home are placed, and trees border-
ing middles where dead animals and other offal are buried often
become affected. The growing and turning under of large quan-
tities of legumes and other crops in the grove may give a rank
type of growth in.the trees, that may develop dieback.





Bulletin 140, Dieback of Citrus Trees


CONDITIONS COMPLICATING THE CONNECTION OF DIEBACK
WITH SOIL ORGANIC MATTER
Dieback occurs in citrus trees planted in locations and grow-
ing under conditions which make it less easy to establish the
connection between the cause of the disease and the organic
material in the soil. Trees subject to a lack of drainage are very
often affected. The fact that organic hardpans are formed in
sandy soils at the level of the water-table indicates an accumula-
tion of these materials at this point. The feeding of the roots
in these materials may be a factor in the development of the
disease. Water-logging of the soil will give rise to anaerobic
decomposition of the residual organic matter in the soil. The





4-~i





FIG. 6-Dieback marked, or ammoniated fruit

feeding of the tree upon these products may be a causal factor.
Trees planted on lands underlaid by hardpans are often af-
fected with dieback. These hardpans may be the organic hard-
pans that occur on the flatwoods land, layers of clay, marl, rock,
or even the layers of compact sand. The hardpans interfere
with the ready movement downward of materials leached from
the more open soils above. The accumulation of soluble organic
materials above these hardpans, and the feeding of the roots
therein is a possible factor in the development of the disease.
Irregularities in the surface of the hardpan may form pockets
deficient in drainage and lead to the development of the disease
in trees planted thereon.
Excessive cultivation of the soil is apparently conducive to
the development of dieback. It has been observed repeatedly
that healthy groves on pineland which has been cultivated vig-
orously to kill out Bermuda grass, have become affected. Clean
cultivation thruout the year is favorable to its development. Any
considerable stirring of the soil in groves planted on shell ham-





Florida Agricultural Experiment Station


mock lands is likely to throw the trees into a dieback condition.
One of the common methods of overcoming dieback on high
pineland and hammock land where cultivation is practiced is to
cease cultivation entirely or to cultivate as little as possible.
Irregular moisture conditions in the soil is sometimes a factor
in the development of the disease. Dieback is more prevalent in
seasons following extreme drouth. Some such cases have been


FIG. 7.-Dieback marked, or ammoniated fruit

traced to the overfeeding of the trees. A drouth was beginning
at the time of application of the fertilizer and continued until
near or after the second application. Owing to the dry condition
of the soil very little of the first application was made available
for use by the tree. When the rains came both applications were
made available at the same time, and overfeeding resulted.
Groves planted on the spruce pineland or dune soils are sub-
ject to dieback. It is probable that the irregular moisture con-
dition in these soils is an important factor in the development of





Bulletin 140, Dieback of Citrus Trees


dieback. The disease is also of frequent occurrence in trees
planted in what are known as sand-soaked" spots, sometimes
found in high pineland groves. The spots are small areas of
pure white sand that are somewhat similar to the dune soils.


FIG. 8.-Dieback marked, or ammoniated fruit. Note coarseness
of rind

Trees located on slopes are often subject to dieback. This
may be due to seepage areas bringing about a lack of drainage,
especially during certain seasons of the year; or it may be due
to the leaching and washing of soluble food materials from the
higher grounds so that the trees are overfed. Not all of the
trees on the slope are likely to be affected. They are more often
confined to particular parts of the slope. In some cases, they





Florida Agricultural Experiment Station


may be located near the brow of the slope, and in other cases,
near the base; whereas the trees on the higher and more nearly
level ground will be wholly unaffected.
DIEBACK AND FERTILIZERS
Perhaps most of the cases of dieback, which have been classed
as acute, can be attributed to the effect of ammonia fertilizers
added to the soil. The use of excessive quantities of ammonia
from organic sources often leads to the development of dieback.


FIG. 9.-Dieback marked, or ammoniated grapefruit


The amounts of these which will prove excessive vary consider-
ably and are dependent on soil conditions and the kind of citrus
tree. These sources can be used much more freely upon clay soils
than upon the sandy soils. They can be used much more freely
upon the dry sandy soil lacking in humus than upon the low
moist soil well supplied with organic matter. In fact, their use
should be avoided almost entirely upon the latter class of soils.
Citrus trees differ in their fertilizer requirements. Grape-
fruit and tangerine trees are heavier feeders than orange trees.
The quantities of ammonia from organic sources which can be





Bulletin 140, Dieback of Citrus Trees


used without injury to a grapefruit or a tangerine tree may in-
duce dieback in an orange tree.
Of the different organic sources of ammonia, cottonseed
meal and tankage are considered in practice to be more active
in bringing on the disease. But tankage is not a uniform product
and may consist of a variety of materials. It is probable that
some tankages are much more active in bringing on dieback than
others. The guanos and dried blood are considered as next in
activity to cottonseed meal and tankage. Tobacco stems, sheep
manure and goat manure, raw bone and steamed bone, are the
least active.
While these fertilizer materials may be active in bringing on
dieback when used in excessive quantities, their use is not to be
avoided under all conditions. When used in limited quantities and
under proper conditions they are excellent fertilizers for citrus
trees. Their effect is to produce growth; when they are used in
excess this growth is likely to be rapid, rank and sappy and
may be followed by the development of dieback. But, when used
in smaller amounts, especially on trees that are backward and
need stimulation, these fertilizers may produce an active but
more nearly normal growth. Experienced growers on the pine-
lands sometimes give their trees a rather liberal application of
cottonseed meal, tankage, or stable manure when they get into a
condition of slow growth, but care is used not to repeat it until
after a long interval.
Many of the best brands of fertilizers on the market contain
limited amounts of ammonia from organic sources. When these
fertilizers are used in average amounts and under average con-
ditions the organic ammonia gives good results. But if the trees
are overfed with them they may be thrown into a dieback con-
dition. The mineral sources of ammonia, such as nitrate of
soda, sulphate of ammonia and nitrate of potash, are not known
to induce dieback even when used in excessive quantities. When
used liberally about trees that are already affected they may
for awhile accentuate the trouble. It is only rarely and for
short periods that ammonia should ever be entirely omitted from
the fertilizers used about dieback trees. In some cases a low
percentage of ammonia is desirable and in others the usual per-
centage should be used. In most cases the ammonia should be
derived entirely from mineral sources. The amount of fertilizer
used about dieback trees should not be large, but it should be
applied regularly-three times each year.





Florida Agricultural Experiment Station


The phosphoric acid in fertilizers is not known to have any
effect, either beneficial or detrimental, upon dieback. The more
common organic sources, such as raw bone and steamed bone,
are excellent for use in fertilizers where the effect to be derived
from a small amount of organic ammonia is desirable.
Potash is believed to have a balancing effect upon the action
of ammonia on growth. When the market permitted, a high
percentage was used in fertilizers for dieback trees. The organic
sources of potash, such as tobacco stems, may be favorable to
dieback when used in quantity, but the amounts that are con-
tained in fertilizer mixtures are usually not sufficient to induce
injury under ordinary conditions. They should be used with care
on low moist soils well supplied with organic matter.
Hardwood ashes and other forms of lime, such as air-slaked
lime and ground limestone, are frequently applied as an antidote
for dieback. In some cases they appear to be beneficial and in
others they are without effect.
EXPLANATION OF THE CAUSE OF DIEBACK
It is seen that dieback may occur in trees growing in a variety
of locations and under many different conditions. In a study of
the affected trees the symptoms appear to be exactly the same
under all these conditions.. The only difference noticed was in
the degree of effect and in the predominance of certain symp-
toms. This suggests that there must be a cause for the disease,
that is common to all. It is probably connected with certain bac-
terial transformations in the soil. Dr. C. B. Lipman of the Uni-
versity of California suggests the hypothesis (Science, Vol. 39,
p. 728, 1916, and Florida Grower, June 3, 1916) that the disease
is caused either by "a lack of available nitrogen in any form
resulting in nitrogen starvation," or a weak nitrifying power
of the soil and a normal ammoniafying power resulting in the
enforced assimilation by the citrus tree of ammonia instead of
nitrate, with the result that the tree will give evidence of poi-
soning."
The results of the writer's studies lead him to believe that
dieback is the result of the action of some chemical agent upon
the growing cells of the plant parts. The results of his experi-
ments do not confirm Lipman's suggestion that it is due to the
injurious action of ammonia, neither does he agree with Lipman
that it can be due to nitrogen starvation. Nitrogen starvation is
well known to be shown by a yellowing of the foliage. The foliage
of dieback trees often exhibits an intense green color that is very





Bulletin 140, Dieback of Citrus Trees


indicative of a plentiful to an over-plentiful supply of available
nitrogen. Since he was able to produce dieback in trees growing
under controlled conditions in the greenhouse by feeding them
with stable manure and with cottonseed meal (Florida Agr. Exp.
Sta. Rep. 1912, p. cii) he has adopted the working hypothesis:
First, that the disease is due to the injurious action of some
organic chemical compound or compounds obtained from the soil,
upon the growing cells in certain of the plant parts; second, that
these chemicals do not kill the cells but induce abnormal processes
in the living matter whereby a particular type of gum formation
results; third, that this gum with the secondary disturbances
incident to its formation are the manifestations which character-
ize dieback; fourth, that the organic chemical or chemicals are
formed by the decomposition of the organic matter, either added
to or residual in the soil; and fifth, that they are formed under
certain limited conditions which are common to the different
locations where dieback occurs.
It has been shown by Schreiner and co-workers (U. S. D. A.,
Bureau of Soils, Buls. 83 and 87) that wheat plants can absorb
and make use of certain organic nitrogenous compounds in the
absence of sufficient nitrates. Lipman found lack of nitrification
and increased ammoniafication in the soil about dieback trees in
California. According to Schreiner, these are the conditions that
would be favorable to the absorption of the organic compounds
from the soil.
The writer recognizes the possibility that dieback may be
due to the attack of some obscure organism and that the action
of the organic material in the soil merely develops a type of
growth in the tree that is susceptible to attack. But no such
organism has been found thus far that could be considered a
causal agent.
THE CONTROL OF CITRUS DIEBACK UNDER VARIOUS CONDITIONS
In general, there are two methods used in Florida for the
control of citrus dieback. They are the preventive and the cura-
tive methods. Both are based more upon experience and observa-
tion than upon a definite knowledge of the cause of the disease.
PREVENTIVE METHODS
The use of preventive methods for treatment of dieback re-
quires the exercise of judgment and a knowledge of the condi-
tions under which the affected trees are growing. In this method
the soil conditions under which the disease has developed are





Florida Agricultural Experiment Station


corrected and the trees are given every facility for making nor-
mal growth. Soil and seasonal conditions are always changing,
so it sometimes happens that the conditions
under which the disease is developing become
corrected without any effort on the part of
the grower, and the trees grow out of the
diseased condition. This may happen where
Sthe disease has developed following a drouthy
1 condition in the soil, or from some partic-
S ular overfeeding of the trees which is not
repeated.
:It has already been stated that it usually
requires more than one flush of growth for
the disease to develop. It also requires more
than one flush of growth, even with treat-
ment, for the trees to grow out of the dis-
ease. Neither the preventive nor the curative
methods remove the symptoms already devel-
oped in the tree; both merely prevent their
further development in succeeding flushes of
growth. Other conditions being equal, it is
probable that some of the curative treat-
ments induce an earlier prevention of the de-
velopment of the gum symptoms than the
preventive treatments.
Among the different preventive practices
carried out in the grove to cure dieback,
starving the trees for ammonia and stopping
cultivation entirely, or reducing it to a min-
imum, are the ones more commonly em-
played. The effectiveness of these methods
depends much upon the soil conditions.
FIG. 10. Bark ex- Ammonia Starvation.-Where it is clear-
crescences on a sub-
terminal branch ly evident that the disease is due to overfeed-
ing with the organic ammoniates, ammonia
starvation will be helpful. The extent of starvation necessary
will vary. In the average grove ammonia fertilizer is applied in
three applications each year. The entire omission of ammonia
from the fertilizer is a dangerous practice. If the ammonia is
omitted entirely it is safest to omit it from one application only.
In succeeding applications a low percentage of ammonia should
be used and this practice not continued beyond two or three






Bulletin 140, Dieback of Citrus Trees


applications, after which the normal amount of ammonia should
be used. Ammonia starvation is not necessary in all cases of
dieback. In cases other than those brought on by overfeeding,
it may be harmful. In such cases the normal amount of ammonia
should be used.
The ammonia in fertilizers used on trees where dieback has
been brought on by overfeeding, or where
dieback occurs on soils well supplied with
humus and moisture, should be derived en-
tirely from mineral sources; in the other
case it should be largely from mineral
sources.
In the case of dieback on dry sandy soils
of the high pine and the spruce pinelands,
where it has not been brought on by over-
feeding, the use of stable manure in limited
amounts has in a number of cases proved
helpful in curing the disease. The writer is
unable to explain why, in certain cases, ma-
nure can induce the disease, whereas under
other conditions it appears to be helpful in
curing it. It is probably on account of the
effect which it has upon the organisms in
the soil.
Cultivation and Cover Crops.-The stop-
ping of cultivation is a helpful practice where
the disease occurs under certain conditions. FIG. 11 -Bark ex-
crescences on fruit
The normal system for the cultivation of spurs
lands, where cultivation is possible, is to
plow the grove either in the fall or the early spring, and to main-
tain a dust mulch during the spring months until the rainy
season begins, by the use of the Acme harrow over the whole
grove every ten days or two weeks. The grove is then laid by
for the summer and a crop of grass and weeds, or of legumes, is
allowed to cover the ground. In late summer this is cut. In
some cases it is removed for hay, in others it is allowed to lie
until fall when it is plowed under; or an early cutting may be
made and removed for hay, and a second growth made to be cut
and turned under. In a bearing grove the plow is run shallow,
not to exceed six inches. In young groves the plowing may be
deep in the middles but shallow near the trees.
In cultivated groves affected with dieback the system should





Florida Agricultural Experiment Station


be modified to avoid, as much as possible, any plowing. In most
cases the grass and weeds and legumes should be removed as hay
or left to rot without being turned under. Where necessary to
break up the sod in order to use the Acme harrow, the cut-away
harrow will be better than a plow.
In the early years of the citrus industry clean cultivation was
practiced extensively. By this method the soil was cultivated
thruout the year. No growth of grass or legumes was allowed
to be made. This was found by experience to be a bad practice.
After a time the humus content of the soil was so reduced that
the soil was put into a condition not conducive to growth. The
excessive stirring of soil was found to be favorable to develop-
ment of dieback. This system is seldom used at the present time.
Mulching the trees with a heavy covering of grass, weeds or
legumes is a good substitute for cultivation and should be used
during the dry season to hold the moisture in the soil if a dust
mulch is not maintained. The chief objection to this mulch is
that it is difficult to obtain mulching material in some localities
and the danger during dry periods of fire spreading thru the
grove. Pine straw is not a good material to use as a mulch; it
sometimes causes young trees to turn yellow. When worked into
the soil it does not decay readily and substances in the leaves
put the soil into a poor physical condition.
It sometimes happens that Bermuda grass gets started in the
grove and the grower tries to eliminate it by cultivation. The
deep and continued cultivation necessary to do this will often
throw the trees into a dieback condition. It was formerly thought
that a Bermuda sod was very harmful to the grove. It has been
found by experience that such is not the case.
The repeated growth of corn and other crops that require
deep cultivation may bring on a dieback condition of the trees.
The occasional growth of such crops is probably not harmful.
Care should be taken to use a fertilizer for such crops that is
adapted to the trees.
It is a common practice along the lower East Coast of Florida
to raise pineapples between the rows of grapefruit trees. A
regular organic fertilizer is given the pineapples but no fertilizer
at all is given the trees. The trees make a thrifty growth and
dieback is much less prevalent than might be expected. It is
probably explained by the particular type of soil, the small
amount of cultivation given, and the heavier feeding ability of
the grapefruit trees.






Bulletin 140, Dieback of Citrus Trees


Since dieback occurs in trees growing under such different
conditions, methods of prevention in one case are not necessarily
applicable in others. Therefore, methods of
treatment for different conditions and locations
will be discussed separately. In all cases the
trees should be pruned to remove the dead and
weak wood. Badly injured and stunted trees
should be replaced with new ones. The trees
'should be fed at regular intervals and sprayed
when necessary.
Overfeeding.-Under this heading are in-
cluded trees overfed with organic fertilizers,
and trees located near outhouses, stables, etc.,
where they are feeding upon organic waste.
Ammonia should be omitted in whole or in part
from the fertilizer used about these trees, dur-
ing a short period. If market conditions will
permit, a fertilizer formula high in potash
should be used. Only mineral sources should
be included in the fertilizer and average to less
amounts of the fertilizer be applied.
If trees have been cultivated, this practice
should be reduced to a minimum. Any plowing
of the land should be avoided. Where possible,
the trees should be heavily mulched. Any
heavy growth of grass, weeds or legumes may
be cut and used as a mulch about the trees, or
be removed as hay.
If the condition is complicated by a lack of
drainage, or the presence of a hardpan too near
the soil surface, these conditions can be reme-
died after the manner suggested under these
headings.
Lack of Drainage.-Where the disease is
brought on by a lack of drainage it is of course
necessary to get rid of the excess water in the
soil. This excess may be present only at certain
periods of the year. This condition must be
avoided.
The lack of drainage may be. due to the
water-table being too near the soil surface on
FIG. 12.-ultiple account of the lowness of the land. Where
buds account of the lowness of the land. Where






Florida Agricultural Experiment Station


drainage is not possible it is sometimes helpful to raise and
mound the trees. It is probable that in many cases the trees
should never have been planted on lands where such a practice
is necessary. On the other hand, lack of drainage may be due
to the presence of a pocket formed by a-hardpan subsoil. This
should be broken thru or drainage established by means of
ditches.
After draining, the land should be put in a good condition of
tilth. Lime should be used in amounts in proportion to the needs
of the soil. If the land is rather heavy and well supplied with
organic matter only mineral fertilizers should be used; the crop
of grass and weeds should be cut and removed for hay or used
as a mulch, and cultivation reduced as much as possible.
If the land is sandy and lacking in humus and the crop of
grass, weeds and legumes does not grow well, a light scattering
of stable manure-one or two tons per acre-worked into the
soil will prove helpful. The application should not be repeated
for a year or more. If possible, the trees should be mulched and
normal cultivation be given to the middles. A leguminous cover
crop should be grown and cut. It should be turned under at
plowing time every other year. Each intervening year it' may
be removed as hay. The average amount of ammonia shofhld be
used in the fertilizer, and the fertilizer not be used in laige
amounts. Regular applications are to be made. A
Hardpan.-The organic hardpans occur on the palmetto flat-
woods lands where the water-table has stood at a certain depth
for a considerable period of time. They are often thin and can be
broken thru by hand tools or with dynamite. They interfere
with the movement of moisture from the lower strata, so that
during times of drouth the soil above them may be insufficiently
.supplied with moisture.
Where possible, the grove should be given the normal cultiva-
tion as previously described. If the cover crop makes a healthy
growth it should be turned under only in alternate years. The
amount of fertilizers used need not be large but it should be
.applied regularly. If there is any lack of drainage this must be
taken care of and the grove practices modified as already de-
:scribed under the heading, Lack of Drainage."
The organic hardpans are sometimes underlaid by a pure
white sand, which is quite unsuitable for the growth of the
-citrus roots. Where such occurs the roots should be encouraged





Bulletin 140, Dieback of Citrus Trees


to grow in the upper layers of soil. Cultivation should be re-
duced to a minimum and the trees be heavily mulched.
Where the hardpan consists of layers of clay or of rock or
marl, it should be broken with dynamite to give the roots plenty
of loose soil in which to grow. Care must be taken that there is
no lack of drainage. Where possible, normal cultivation should
be given; where not, the trees should
be heavily mulched to hold the soil
moisture. The cover crop can be cut
and left on the ground to rot, or be
worked into the soil with a cut-away
Sharrow. Purely mineral fertilizers
^i with the average ammonia content
l may be used at regular intervals, care
being taken not to overfeed the trees.
Excessive Cultivation.-When die-
back is brought on by the excessive
cultivation of pineland, flatwoods or
interior hammock land, cultivation
should be reduced as much as possible.
The grove should not be plowed for a
year or more. The cut-away harrow
may be used instead when it is neces-
sary to break up the sod. The Acme
harrow should be used only where it
is necessary to conserve the soil mois-
ture. The cover crop can be cut, and
worked into the soil with the cut-away
harrow. Mineral fertilizers with a low
ammonia content should be used for
one or two applications, if the grove
has been well fed previously.
FIG. 13.-Maltiple buds Any considerable amount of stirring
of the soil in groves located on the shell
lands, or lands where the coquina rock occurs near the surface,
may be favorable to dieback. When it occurs in trees on such
soils cultivation should not be practiced. The trees should be
mulched and the grass and weeds cut and left on the ground.
Irregular Moisture Conditions in the Soil.-Irregular mois-
ture conditions that may lead to a development of dieback may
occur in either the high or low soils. The disease is more likely
to develop under such conditions where the trees have been well





Florida Agricultural Experiment Station


fed. In such groves extreme dryness of the soil is to be avoided
as much as possible by regular cultivation or mulching. If irri-
gation is possible, the water supply can easily be controlled.
Great care should be exercised in choosing the amount of fer-
tilizer to apply following a long period of drouth. If the previous
application was not well washed into the soil by rains, only a
light application should be made. A formula somewhat low in
ammonia will be preferable. If heavy rains follow so that the
trees may later show the need of ammonia, the application can
be supplemented by an application of a readily available source
of ammonia.
Spruce Pineland.-Dieback is very common in trees planted
on the loose dry soils of spruce pinelands (fig. 2). Such soils are
deficient in humus and become extremely dry at times. It re-
quires much care to grow trees successfully on this type of land.
The practice should be such as to build up these soils. Cultivation
should be shallow and be made only where necessary to conserve
moisture. The space around the trees should be heavily mulched.
Plowing should be avoided as much as possible. Leguminous
crops should be grown between the trees, cut, and worked into
the soil in the fall or winter. If these crops do not grow well,
an application of a ton or two of stable manure to the acre can
be made to advantage. This should be applied and worked into
the soil a short time before the legumes are planted. It will also
be well to inoculate the seed of the legumes before planting. The
fertilizer applied should be largely from mineral sources; small
amounts from organic sources would probably not be harmful.

CURATIVE METHODS
The curative methods which are employed in the control of
citrus dieback consist in the use of bluestone or copper sulphate
in various ways. It may be spread on the soil, placed beneath
the bark, or sprayed onto the tree in the form of bordeaux mix-
ture. While these methods are effective under certain condi-
tions they are not known to be effective under all conditions.
Therefore, to be safe it will be well when using these to use also
the preventive methods which have already been discussed.
Bluestone on Soil.-The most popular method has been to
apply bluestone to the soil. But the price of the chemical has
recently become so great that its use in this manner is almost
prohibited. By this method the crystals are spread over and
worked into the soil in the same manner as fertilizer. The usual





Bulletin 140, Dieback of Citrus Trees


practice has been to use from one-fourth to one pound about each
tree, the amount used depending on the size of the tree. Grossen-
bacher made some tests with this chemical (Cure for Dieback of


FIG. 14.-Orange tree affected with dieback. Note frenching of
the foliage in the lower half of the tree

Citrus Trees, Florida Grower, Jan. 29, 1916) about several
affected grapefruit trees from four to six years old and
found that better results were obtained when it was used in
quantity. He found the bluestone to be most effective as a
preventive when a total of not less than four pounds per tree





Florida Agricultural Experiment Station


was used and it was given in two applications. The affected
trees were located in a grove on high pineland in the vicinity
of Orlando and the chemicals were applied in March and May.
During the years of 1915 and 1916 experiments were con-
ducted by the Experiment Station with severely affected trees on
high pineland in Pinellas County. Two pounds per tree was
found to be an effective preventive on budded and seedling trees
more than twenty years old, whereas one pound was not nearly
so effective. The chemical was given in one application in April.
Hardwood ashes and air-slaked lime used separately on other
plots in the same grove were without effect. During the follow-
ing year the whole grove was subjected to ammonia starvation
by omitting one application of the fertilizer and using a low am-
monia content in the others. In the fall there was very little
evidence of any new development of the disease. The fruit for
the first time in years was largely free from dieback markings.
Bluestone is a strong poison. A number of cases of injury to
citrus trees by its use have been reported. On the other hand,
it has been used about citrus trees in rather large quantities
without injury. Grossenbacher applied as high as 16 pounds
in two applications of 8 pounds each about two months apart, and
12 pounds at a single application on another occasion without
any injury to the trees. It should probably be used with greatest
care on moist soil where it will be put into solution rapidly after
application.
While bluestone is evidently a preventive for dieback, it is
not known how it prevents.
Bluestone Beneath the Bark.-Another method of using
bluestone for dieback that is commonly employed by growers is
to insert a crystal varying in size from a grain of wheat to that
of a pea, beneath the bark of the tree. If the tree is large two
or three crystals are inserted at different points in the circum-
ference of the tree. It is sometimes inserted into the crown roots,
or the larger branches, but it is more often placed beneath the
bark of the trunk.
While it appears to be an effective preventive of dieback in
some cases, especially of young trees, it is more often not effec-
tive. In experiments conducted by the Experiment Station with
affected trees on high pineland at St. Leo, Pasco County, this
method gave negative results. The chief objection to the method






Bulletin 140, Dieback of Citrus Trees


is that it produces a trunk wound and may split the bark upward
a considerable distance. The use of this method is not recom-
mended.
Bordeaux Mixture.-Bordeaux mixture sprayed upon the
fruit and foliage of dieback trees has been found to be an efficient
preventive for dieback. Grove experience and experiments car-
ried out by the Experiment Station (Florida Agr. Exp. Sta. Rep.
1913, p. xxvii) have demonstrated its effectiveness. To be most
effective it should be applied just prior to the flush of growth in






I'i









FIG. 15.-Distorted S-shaped growth of terminal branches

which the dieback may develop. Therefore, it may be applied in
the late winter, late spring, or late summer. To prevent the
marking and dropping of the fruit the spraying should be done
in March or April after the fruit has set and begun to grow
rapidly.
A 3-3-40 bordeaux has been found to beof sufficient strength
for the purpose. It is not known how the bordeaux prevents
the disease. It is supposed to be on account of the stimulating
effect which the copper of the spray has upon the leaf and stem
tissues.
The objection to the use of bordeaux mixture is that it kills
the fungus enemies of the scale insects and whitefly and allows
these insects to become plentiful. When numerous, they will
defoliate the trees and kill the branches back quickly. Therefore,
a nozzle should be used that will direct the spray downward. The
spray should be confined as much as possible to the upper surface






Florida Agricultural Experiment Station


of the leaves and to the fruit. By keeping it from the large
branches and the under surface of the leaves some of the fungi
will be preserved and serve as a source of inoculation for the
scale that may develop on the sprayed parts.
After spraying, close watch should be kept for any consider-
able increase in numbers of the scale. If they become numerous
spray the trees with some of the oil-emulsion sprays to control
them. Do not wait until the trees are severely infested before
applying the spray.




SUMMARY

1. Dieback is a disease of citrus trees, that is characterized
by the production of gum in different tissues of the develop-
ing parts.
2. It is a disease of the growing tissues of the citrus tree.
3. Its five primary symptoms are the gum pockets, the
stained terminal branches, the marked or ammoniated fruits,
the bark excrescences, and the multiple buds.
4. The mere dying of the limbs backward is not dieback.
The disease can be identified only in the presence of one or more
of its primary symptoms.
5. The secondary symptoms of dieback are an exceptionally
deep green color of the foliage, a distorted S-shaped growth of
the immature angular terminal branches, frenching of the foli-
age, and thick, coarse and somewhat peach-leaf shaped leaves.
6. The absolute cause of dieback is not known. It is thought
to be in some way connected with organic matter which has been
added to the soil, or which is residual therein.
7. The soil conditions known to be favorable for the de-
velopment of dieback in citrus trees are the presence of excessive
quantities of organic ammoniates, a lack of drainage, hardpans
too near the soil surface, excessive cultivation, and irregular
moisture conditions.
8. The disease is more prevalent in trees planted in certain
locations, such as near stables, outhouses and cess-pools; on






Bulletin 140, Dieback of Citrus Trees 31

slopes, and "sand-soaked" areas; and on spruce pine, shell,
coquina and rocky lands.
9. The disease is controlled by certain preventive and cur-
ative methods. The preventive methods consist in correcting
the soil conditions that are favorable for the development of the
disease. The curative methods consist in the use of bluestone
(copper sulphate) on the soil and beneath the bark of the trees;
and of spraying the trees with bordeaux mixture.




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