Front Cover
 Table of Contents

Title: Citrus canker
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00027149/00001
 Material Information
Title: Citrus canker
Physical Description: Book
Creator: Stevens, H. E.
Publisher: University of Florida Agricultural Experiment Station
Publication Date: 1914
Copyright Date: 1914
 Record Information
Bibliographic ID: UF00027149
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aen2352 - LTUF
18161372 - OCLC
000921884 - AlephBibNum

Table of Contents
    Front Cover
        Front Cover
    Table of Contents
        Table of Contents
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
Full Text


Agricultural Experiment Station


Fig. 43-Canker spots on leaves of grapefruit.
The Station Bulletins will be sent free upon application to the Experiment
Station, Gainesville, Fla.

Introduction ------------ ------------------------ 113
Nature of the Disease -----------_ -- --------- ---- 113
Appearance --------- --------------------------- 114
Distinction from Other Diseases ----------------------------------115
Cause of the Disease -------------------------------------- -- 117
Control ------____----_-----------_. ----------------- 117


i. A new and serious citrus disease has been introduced into two localities in
2. It is confined mainly to grapefruit. The sweet orange is apparently not
3. The disease appears as spots on the leaves and fruit, and forms cankers on
older twigs.
4. It is infectious, and the climatic conditions of Florida may favor a rapid
5. Precautions should be taken NOW to prevent the spread of this disease.



During the past season in Florida there has come to our atten-
tion a new citrus disease, which, if it is once well established in
the State, may become a serious menace to the grapefruit industry.
The purpose of this bulletin is to warn the citrus grower of the
presence of this disease in the State, and to urge that a careful
watch be kept for it. Every precaution should be taken to prevent
its further spread; and if the disease is found, immediate measures
should be taken to eradicate it.
The first specimens of the disease were collected by Dr. E. W.
Berger, State Nursery Inspector, in the fall of 1912; but the trouble
was thought to be an unusual manifestation of Scab, and no further
attention was given to it at that time. In April, 1913, leaves and\
twigs from grapefruit nursery stock, badly infected with this dis-
ease, were received from the vicinity of Miami. A careful exam-
ination of these specimens proved beyond a doubt that the disease
was different from Scab, and was apparently one that had not been
reported before in Florida. At different intervals through the
season, specimens of the disease were collected from two widely
separated localities in the State, and submitted to the writer by
'Dr. Berger, to whom due credit is here given for his valuable
assistance in furnishing specimens and making field observations.
Through the kindness of Prof. H. H. Hume, specimens of the same
disease, on leaves, twigs, and fruit of grapefruit, were received
from one locality in Alabama. The same disease is said to occur
in Texas, but the writer has been unable to confirm this report. At
present the disease is known to exist in Florida at Monticello and
near Miami. Specimens from both places have been received and
Investigations are being carried on in the laboratory and in
the field. The cause of the disease has not been determined, but
sufficient data have been collected to show that the malady is infec-
tious and probably more injurious to grapefruit than any fungus
disease known at present. So far, in Florida, the disease has only
been found on nursery stock, and is confined mainly to grapefruit.

114 Florida Agricultural Experiment Station

Infections have been observed on Citrus trifoliata, and on the Sat-
suma; but the latter seems to be fairly resistant. The sweet orange
is apparently not affected. The leaves, young shoots, twigs, and
fruit of grapefruit are all attacked.

The disease appears as small, circular spots, from less than
one-sixteenth to one-quarter of an inch across. They may occur
singly, or several together may form an irregular area. They are
raised above the surrounding tissue, are light brown, and com-
posed of a spongy mass of dead cells covered by a thin (white to
grayish) membrane that finally ruptures and turns outward, form-
ing a ragged margin around the spot. The general appearance of
the spots is much the same whether they are found on the leaves,
fruit, or twigs. The older spots often become overgrown with
saprophytic fungi, and may be pink or black on account of second-
ary infection by species of Fusarium or Cladosporium.
The infections on the leaves (Fig. 43) appear first as small
watery bulging dots, which are usually of a darker green than the
surrounding tissue. They may appear on either surface of the
leaf, but do not penetrate through the leaf tissue at this stage. The
spots gradually increase in size, change to a light brown color, and

Fig. 44-Canker on grapefruit (from Alabama). About half natural size.

Bulletin 122 115

Fig. 45-Canker on young twig of Citrus trifoliata, showing the broken
membrane around the spots. (Magnified about three times.)

become visible on both sides of the leaf. The spot may project
from the surface on one or both sides of the leaf. Each spot is
surrounded by a narrow yellowish band oi zone. Later the sur-
face of the spot becomes white to grayish, and finally ruptures, ex-
posing a light-brown spongy central mass.
The spots on the fruit (Fig. 44) are similar to those on the
leaves. They project from the surface and retain a circular outline.
They do not penetrate far into the rind; and may be scattered
singly over the surface, or several may occur together, forming ir-
regular masses.
The spots on the older twigs are more prominent and usually
larger and more irregular in shape. They show the same spongy
tissue and the same color as those on the leaves. On growth more
than a year old, the spots assume a cankerous appearance (Figs.
45 and 46), and the membrane covering the surface disappears.
The spots do not penetrate to the wood, but are confined to the
cuter tissues of the bark.

The other citrus diseases with which this one is likely to be
confused are Scab, Scaly Bark, and, possibly, Anthracnose. It can
be distinguished from any of these by the following points of
(1) It differs from Scab in the roundness of the typical spots,
in the larger size of the spots, and in their white or grayish color.
It does not distort the leaves, nor cause the wart-like projections
that are common in infections of Scab. Canker is found on the
older wood, while infections of Scab never occur on older twigs
or branches.
(2) It differs from Scaly Bark in producing much smaller
spots which are more circular in outline. Scaly Bark spots usually

"116 Florida Agricultural Experiment Station

show a hard glazed surface, while Canker is more spongy. Canker
is common on the grapefruit, and forms spots on the leaves; while
Scaly Bark rarely attacks grapefruit or causes spots on the leaves.




Bulletin 122 117

(3) Canker differs materially from Anthracnose spotting.
Anthracnose spots are sunken, usually many times larger, and
much firmer and more compact. Anthracnose occurs only on
fruit, and does not attack young shoots or twigs.

The organism causing the disease has not been determined, but
it is probably a fungus. Several different fungi have been found
associated with the spots, among which a species of Phyllosticta
occurs most frequently. This fungus is suspected of being the
cause of the trouble, and experiments are now in progress to de-
termine this.
The disease is infectious, as is shown by the results of some
experiments in which it was transferred from diseased material to
healthy leaves and shoots of grapefruit. Diseased leaves were
pinned to young shoots of grapefruit in the field. One month later
infection was noted on a number of leaves thus treated; and two
months after treatment these infections had developed into the
characteristic brownish spots of the disease. Similar experiments
were tried with small potted grapefruit trees in the greenhouse,
and characteristic spots of the disease developed in about three
months. Infection was produced from diseased material received
from Monticello, Fla., and from Miami, Fla., and also from speci-
mens received from Alabama. The disease develops rather slowly;
but when a spot once forms, it becomes a center from which infec-
tion spreads to the surrounding tissue. This was especially noted
on the leaves. New infections on the leaf appeared around an old
spot that had developed some three months before. This would
seem to indicate that the older leaf tissue, as well as the younger,
is subject to the attack of the disease.

This disease is much more serious than Scab, and no chances
should be taken with it. Since it has been found in two widely
separated districts, the northern and southern part of the State,
it is apparently not influenced by climatic differences in Florida.
It may be expected to spread rapidly in any locality when once in-
troduced. It is of the first importance to guard against the intro-
duction of this disease into a grove or nursery. Prevention is much
easier and more economical than any cure that can be discovered.
The timely discovery of the disease, and the measures taken to

118 Florida Agricultural Experiment Station

eradicate it, will greatly reduce the chances of its being dissemi-
nated throughout the State, if proper precautions are taken by the
growers and nurserymen. The disease was evidently introduced
into Florida, and probably recently. Just where it came from is not
certain. It is known to occur in Alabama, and may exist in Texas.
!The first step is to avoid the introduction of infected nursery
stock, either in new or old groves. Only young trees that are
known to be free from disease should be planted. Young groves
should be inspected from time to time, and watched for the ap-
pearance of infections. These will develop in early spring and
throughout the summer, but will be more prominent in late spring
and autumn. If one is certain that he has found the disease or
any symptoms of it, there should be no delay in the treatment.
W\'here small trees are affected, these should be removed and
burned. Trees two or three years old may be cut back severely,
leaving just enough to save the bud. All diseased material taken
from such trees should be carefully collected and burned immedi-
ately. Where large trees are infected, all diseased parts should
be pruned off and burned, even if the greater part or all of the top
is sacrificed. What remains of such trees, and also the neighbor-
ing trees, should receive an application of Bordeaux mixture every
two weeks until all further development of this disease ceases. Any
portion of the tree that shows further development of the disease
should be destroyed immediately.
Just how far the use of Bordeaux alone will control the dis-
ease is questionable, and it is folly to rely on it for a cure. It will
probably prevent initial infections if the leaf surface is continually
protected, but this would require continuous spraying every two
or three weeks through the season. Simply covering the diseased
spots with Bordeaux does not seem to prevent infection from
spreading. This was brought out in some of the infection experi-
ments. Pieces of diseased leaves covered with Bordeaux were
pinned to healthy leaves, and infection was obtained from these
as readily as from diseased leaves that had not been sprayed.


Any grower finding a disease with symptoms that suggest this
disease, will greatly aid our investigation by reporting the same
and sending specimens to the Plant Pathologist of the Florida
Agricultural Experiment Station. He will thus obtain reliable in-
formation as to whether it is this disease, or one apparently like it.

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