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Title: Citrus canker, II.
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Title: Citrus canker, II.
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Publication Date: 1914
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Full Text





HISTORIC NOTE



The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida







Bulletin 124 October, 1914

UNIVERSITY OF FLORIDA

Agricultural Experiment Station



CITRUS CANKER---II

HISTORY OF CITRUS CANKER
By E. W. BERGER

STUDIES OF CITRUS CANKER
BY H. E. STEVENS

ERADICATION OF CITRUS CANKER
BY FRANK STIRLING




















Fig. 6-Canker on Grapefruit. About half natural size.

The Station Bulletins will be sent free upon application to the Experi-
ment Station, Gainesville, Fla.
THE E. 0. PAINTER PRINTING CO., DE LAND, FLA.








N IMPORTANT FACTS

i. Citrus Canker is present in all the Gulf States.
2. It was introduced into the United States from Japan, probably within
the last five years.
3. Florida appears to have been the latest State to become infected.
4. Remedial measures are unknown. The usual methods have proved in-
effective. Extermination is the only wise policy.
5. In the most seriously affected locality less than one per cent of the bear-
ing trees showed canker and had to be destroyed.
6. The work of eradication should be done by those who are expert in
handling this disease.
7. Application for advice and assistance in eradication should be made to
Lloyd S. Tenny, Manager, Growers' & Shippers' League, Orlando, Fla.
S. Suspected material should be mailed to Dr. E. W. Berger, Inspector of
Nursery Stock, Gainesville.





CONTENTS

HISTORY OF CITRUS CANKER- PAGE
First Observations in Florida ----------------------------. -------- 27
Canker in Other Gulf States ----------------------- -----------... ---- 28
Campaign Against Citrus Canker ------------------------------ 29
Rule 44 ---------------------------------------- 29
Aid From Growers' and Shippers' League ------------------ 29
Governor Trammel Gives $,ooo -------------------------- 30

STUDIES OF CITRUS CANKER-
Introduction __---------------------------------------- 31
Appearance of Citrus Canker ------------------------------------- 31
Canker Distinguished From Other Diseases ---------------------- 36
Laboratory Investigations ------------------------------------ 36
The Fungus -------------------- --------------------- --- 40
Spread of the Disease -------------------------------------- 41
Control ------------------------------------------------------ 42

ERADICATION OF CITRUS CANKER-
Conditions in Dade County ------------------------------------ 44
The Campaign --------------------------------------- 45
Failure of Cutting Back -------------- ----------------------- 46
The Fire Treatment --------------------------------------- 46
Rapid Spread of Canker -------------- -------------------- --- 47
What Has Been Done ------------------------------------ --- 48
All Florida Threatened ------------------------------------ 48
"V'arieties Affected --------------------------------- --- --- 49
Effect on Trees and Fruit ------------------------------------ 49
How We Do the Work ---- -------------------------------- 50












HISTORY OF CITRUS CANKER
BY E. W. BERGER
(From the shorthand report of a speech at the Citrus Seminar on Septem-
ber 23, 1914).


FIRST OBSERVATIONS IN FLORIDA
It was in July, 1913, when we first realized that we probably
had discovered a new citrus disease. Mr. Blackman, as deputy, had
inspected a nursery at Silver Palms, South Dade County; and had
found the disease in several blocks of grapefruit. He notified me in
a letter dated July 18. I went down on July 28, and studied the sit-
uation. I brought some specimens home, and induced the Experi-
ment Station plant pathologist to undertake an investigation. It ap-
pears that specimens had been received earlier by him (I think, in
April).
As we knew nothing about the disease, certificates were with-
held from this nursery, and permission to sell stock from infected
blocks was not granted. Sales from a block of sweet seedlings near
the Silver Palm school, found free from disease both by Mr. Black-
man and myself, were permitted; with the express understanding
that each lot of trees sold must be defoliated, treated with Bordeaux
or ammoniacal solution of copper carbonate, re-inspected by Mr.
Blackman, and sold only under package certificates which Mr. Black-
man was authorized to issue. Arrangements were also made with
the owner to give remedial treatment. Bordeaux mixture was rec-
ommended. Being busy at the time with the inspection of Ioo other
nurseries (143 nurseries were inspected in 1913-1914, of which the
writer personally visited ioo, and the deputies 43), I could give the
nursery at Silver Palm only a limited amount of attention. It was
visited again in December and AipriI, and very little Canker found.
Specimens of a similar disease had been brought from Monti-
cello about a year earlier and referred to the Experiment Station.
These being the first specimens ever seen, they were supposed to be
an unusual form of citrus scab. The owners of this nur-
sery at Monticello went into bankruptcy, and fortunately the
stock was not sold. The block was still there a year later, in Sep-
tember, 1913, when I again visited Monticello and again found the
27







28 Florida Agricultural Experiment Station

disease. A report was made to the Citrus Seminar in October,
1913, by myself, and another by H. E. Stevens. Later reports were
made by the writer at the Farmers' Institute meeting at Lakeland,
in February, 1914, and by H. E. Stevens in Bulletin 122 of the Flor-
ida Experiment Station, which was mailed in April, 1914.

CANKER IN OTHER GULF STATES
I was informed by the owners of the nursery in Dade County,
that they obtained sour seedlings from Port Arthur, Texas, in Jan-
uary, 1912; and that the blocks of stock for which these were used
first showed the infection. The infection at Monticello developed
in part on stock obtained from Texas, and in part on stock obtained
directly from Japan. The infection was brought to Monticello on
Citrus trifoliata seedlings.
Some time during the fall or winter of 1913, specimens of Can-
ker were received from Alabama. It was thus apparent that we
were exposed on the west to the dangers of a new citrus disease,
though we had been depending in good faith upon the inspection
certificates of the Western States. The obvious course was to go
and see. At the meeting of the Board of Control in March, 1914,
the Board promptly responded to a suggestion to investigate the sit-
uation in the States to the west of us. The inspector was instructed
to make a trip of inspection into the Gulf States. I started on March
14, 1914. (I went by way of Monticello, as I had left particular
instructions for treating some infected stock there.) From Monti-
cello I went to Auburn, Alabama, to get what information I could
from the State inspectors; and then proceeded to view the situation
at Grand Bay and Mobile. Dr. Wolfe had just returned from Mo-
bile and Grand Bay, and stated the conditions he found there. I
went and found just what he had described. I found the nurseries
in Alabama sending out trees with Canker infection. I found this
at Mobile and also at Grand Bay. I then went to Mississippi, and
found the disease at Wiggins. They first discovered it in Mississippi
on Citrus trifoliata from Tapan. I proceeded to Louisiana and found
Canker present at Lake Pontchartrain. I have learned since, through
correspondence, that Louisiana has Citrus Canker in several places,
and is getting alarmed. They are undertaking to organize for its
eradication, but they are a year behind us. At one place in Louis-
iana. $5,000 or $6,000 worth of nursery stock has been burned vol-
untarily. (They are also getting busy in Alabama and Mississippi.







Bulletin 124 29

Meetings of growers have been held at Gulfport, Miss., and at Mo-
bile, Ala.) In Texas I found the disease at Port Arthur and at Al-
vin. About a dozen citrus trees were examined in and about Mat-
amoras, Mexico; but no Canker was found. Canker has since been
reported from Kingsville, South Texas, and from Galveston County,
Texas.
(It is rather remarkable that the State Inspector of Texas still
insists that they have no Canker there. I am glad that the Federal
Government man has informed him that he has found it there.)
As to the origin of Citrus Canker--we got it partly from Tex-
as, and in part, directly from Japan. Men in Alabama, Mississippi,
and Texas, told me they first saw it on trifoliata stock obtained from
Japan. There is no doubt but that they have the disease in Japan,
as specimens were received from there by Prof. Floyd of the Ex-
periment Station, in last May. The specimens from Japan were la-
beled "Scab."

CAMPAIGN AGAINST CITRUS CANKER
RULE 44
When I returned to Florida and made my report before the
Board of Control, Rule No. 44, prohibiting importations of citrus
stock into the State of Florida, was at once adopted. The weak
point with Rule 44 is that we are not able financially to enforce it.
We have no money to pay men to be on the lookout in Western
Florida, and to seize contraband shipments.
AID FROM GROWERS' AND SHIPPERS' LEAGUE
It was at once evident to all that something must be done in
Florida. The matter had been brought to the attention of Mr. Ten-
ny, Secretary-Manager of the Florida Growers' and Shippers'
League. He brought the subject before his Board of Directors,
which resulted in our sending Mr. Stirling as deputy inspector of
nursery stock to the Redlands district to look further into the situa-
tion, and to advise with the people as to the best methods of control.
Through Mr. Tenny, the Florida Growers' and Shippers' League
agreed to raise $2,000 to pay Mr. Stirling's expenses. On May 19,
1914, Mr. Stirling and the writer visited Redlands, when Mr. Stir-
ling was placed in charge of his work. On June 4, a mass meeting
of growers was addressed at Redlands, and interest aroused in the
prospective Canker fight.







30 Florida Agricultural Experiment Station

The subject of Citrus Canker was also discussed before the
State Horticultural Society on April 30, 1914. For further partic-
ulars about the writer's visit to the Gulf Coast States and Mexico
on account of Citrus Canker, the Proceedings of the Society for
1914 should be consulted.
GOVERNOR TRAMMELL GIVES $1,000
I wish to announce that the Governor of Florida has given
$1,ooo toward the Citrus Canker fight. The money will be ex-
pended through the office of Inspector of Nursery Stock. Five hun-
dred dollars will be used in Dade County, and $500 for looking up
other localities in the State suspected of having Canker.







Bulletin 124 31

STUDIES OF CITRUS CANKER
BY H. E. STEVENS
(Paper read at the Citrus Seminar on September 23, 1914.)
During the Citrus Seminar in 1913 I called attention to a new
citrus disease that had recently been found in the State. Since then
we have had more experience with this disease, and have discovered
several things of interest in connection with it. It has been named
Citrus Canker. In March of the present year Bulletin 122 was is-
sued as a preliminary report, in which Citrus Canker was described
as a new disease of citrus. From the data collected and the ex-
perience of the past season, Citrus Canker appears to be the worst
fungus disease of grapefruit in Florida. It is a disease with which
the grower should not temporize. Every effort should be made to
stamp out infections as soon as they appear, and extra precautions
should be taken to keep any more of the disease from coming into
the State.
Our investigations have been confined chiefly to laboratory
studies of the causes of the disease and the conditions under which
the infection occurs and spreads. I have made no field studies of
Citrus Canker beyond the artificial infections made in the Experi-
ment Station Grove. However, through the kindness of Dr. Berger
and his assistants, I have been kept informed in regard to its dis-
tribution and the field conditions associated with it in the infected
areas. Numerous specimens have been received and studied from
the infected localities in Florida, and also from other States.

APPEARANCE OF CITRUS CANKER

Citrus Canker attacks the leaves, young shoots, branches and
fruit, forming a characteristic spot, which is similar in general ap-
pearance on whatever portion of the tree it occurs. The disease has
been found on all varieties of citrus except the kumquat. Grapefruit
is apparently most severely attacked, the infection occurring on
leaves, twigs, branches, and fruits. (Fig. 7.) Citrus trifoliata is
probably next in susceptibility, followed by some of the sweet orange
varieties. Infections have been found on the stems, twigs, and pet-
ioles of Citrus trifoliata, but not on the leaves. The disease has
been observed on leaves, twigs, and fruit of the navel and some other
sweet orange varieties. Scattered infections have been found on the
leaves and twigs of the Satsuma, tangerine, lime, and rough lemon.








32 Florida Agricultural Experiment Station








































Fig. 7.-Grapefruit tree infected with canker from a pure
culture, showing defoliation and canker spots.

(I have had no opportunity yet to study just how susceptible the
different varieties of citrus may be to the disease, and the above
statements are based on examinations of specimens received from
various localities.)








Bulletin 124 33















































Fig. 8.-Twig from tree shown in Fig 7. Canker spots on leaves.



Fig. 8.-Twig from tree shown in Fig 7. Canker spots on leaves.








34 Florida Agricultural Experiment Station

The distinguishing feature of Citrus Canker as observed in the
field is the characteristic spotting produced on the fruit and foliage.
As usually seen, the infection appears as small light-brown spots,
from less than one-sixteenth to one quarter of an inch in diameter.
The spots are usually round, and may occur singly, or several may
run together forming an irregular area. This last usually occurs on
fruits. The spots project above the surrounding healthy tissue, and
are composed of a spongy mass of dead cells, covered by a thin white
or grayish membrane. The membrane finally ruptures and turns out-
ward, forming a lacerated or ragged margin around the spot.
On the leaves, infections first appear as small, watery dots, with
raised convex surfaces. These dots are usually of a darker green
than the surrounding tissue. Sometimes, however, the surface of the
spots is broken as soon as they appear. Spots may appear on either
surface of the leaf, but they do not at first penetrate through the
leaf tissue. They gradually increase in size, change to a light brown,
and become visible on both sides of the leaf. In the older spots one
or both surfaces may be bulged or raised, and such spots are com-
monly surrounded by a narrow yellowish band or zone. (Fig. 8.)
In the more advanced stages the surface of the spots becomes white
or grayish, and finally ruptures, exposing a light brown spongy cen-
tral mass. Old spots soon become overgrown by saprophytic fungi,
and may appear pink or black on account of these fungus growths.
On the fruits the spots are very similar to those formed on the
leaves. They project and retain a circular outline. .They do not
penetrate far into the rind. They may be scattered over the surface,
or several may occur together forming an irregular mass. (Fig. 6.)
Gumming is sometimes associated with the spots formed on the
fruits. Canker, apparently, does not cause a rot of the fruits di-
rectly, but opens the way for other fungi to enter and cause infected
fruits to rot. The spots on young twigs are like those on the leaves









Fig. 9.-Canker on young twig of Citrus trifoliata, showing the broken
membrane around the spots. (Magnified about three times.)








Bulletin 124 35

and fruit. (Fig. 9.) On the older twigs they are more prominent,
and more or less irregular in shape. This is especially true of old
spots. (Fig. Io.) They show the same spongy tissue as is found










































Fig. io.-Canker on grapefruit nursery stock. Natural size.








36 Florida Agricultural Experiment Station

in the spots on the leaves, but assume a cankerous appearance and
the surface membrane completely disappears. These spots or can-
kers are formed in the outer layers of the bark tissue, and do not
penetrate to or kill the wood. The spots once formed in the bark
are persistent, and are not readily sloughed off. They may remain
for a long time, and form centers from which infections may read-
ily spread. This is confirmed by observations on infections produced
on potted trees in the greenhouse, and in the grove by artificial in-
fection. Some of these spots have been under observation for over
a year, and show no tendency to slough off.

CANKER DISTINGUISHED FROM OTHER DISEASES
Other citrus diseases with which Canker may be confused are
Scab, Scaly Bark, and possibly Anthracnose. It can, however, read-
ily be distinguished from any of these by noting the following
points:
I. It differs from Scab in the typically round spots produced;
the size of the spots, and the fact that the spots penetrate through
the leaf tissue. It does not distort the leaves. There are no wart-
like projections. Canker occurs on older wood, Scab does not.
2. Canker differs from Scaly Bark in the size of the spots,
which are much smaller and more circular than those of Scaly Bark:
and the spongy nature of the spots-Scaly Bark spots are hard and
glazed. Canker is common on grapefruit, Scaly Bark is not. Can-
ker forms spots on leaves, Scaly Bark does not.
3. Canker differs materially from Anthracnose in the size of
the spots, which are much smaller than those of Anthracnose.
Canker spots are raised, Anthracnose spots are sunken. Canker
has spots of spongy character, those of Anthracnose are hard.
Canker occurs on young shoots and older twigs, Anthracnose
does not.
LABORATORY INVESTIGATIONS
The first specimens of Citrus Canker that were recognized as
showing a new and distinct disease were received from near Miami
in the spring of 1913. The infections were on leaves and young
twigs of grapefruit, and the characteristic spots indicated plainly
that the disease was different from Scab or any other citrus trouble
with which we were familiar. Some specimens of what was prob-
ably this disease were collected by Dr. Berger in the fall of 1912,
and submitted for examination. These were finally considered to be







Bulletin 124 37

an unusual manifestation of Scab, and no further attention was giv-
en to the matter at that time. The specimens from Miami were col-
lected from nursery stock, and the sender wrote that he had seen this
same disease or something similar to it on trifoliata stock in Texas.
These specimens were carefully examined, but they were so overrun
with different fungi that it was impossible to tell from a mere ex-
amination of the material what was the cause of the disease. In-
vestigations were begun later to determine, if possible, the cause of
the trouble.
An examination of all available literature on citrus diseases
failed to show any mention of a disease similar to this on citrus
fruits. Specimens were sent to the plant pathologist of Texas with
a request for any available information on the nature and distribu-
tion of the disease in that State. He replied that the disease was
Scab, and was more or less common where citrus was grown in
Texas. More specimens of the disease were sent him together with
some specimens of Scab for comparison, and his attention was called
to the difference between the two diseases. He admitted that the
disease was different from Scab, and that it was something new to
him. It had not been reported in Texas, and so far as he knew, it
did not exist there. We learned later through Dr. Berger's inspec-
tion, that the disease did exist in Texas, and was rather prevalent
in different sections of the State. Specimens of the disease were re-
ceived at different intervals throughout the season from near Miami
and Monticello, and one lot came from Grand Bay, Alabama.
Experiments were tried to see if the disease was infectious. In
August, young succulent growth was available on some small grape-
fruit trees in the open. Some leaves and twigs showing canker spots
were pinned to young healthy foliage, and allowed to remain for a
week or ten days. The infecting material was dry, as it had been
kept in the laboratory for nearly a month before using. Shoots from
three different trees were treated as stated. One month later, small
watery spots were noticed on the leaves and shoots in the vicinity of
the diseased material. In six weeks to two months, these infections
had developed into the typical spots of Citrus Canker. This experi-
ment showed that the disease could be communicated from diseased
tc healthy tissues by contact, and indicated that the disease was
most likely of fungus or bacterial origin. The disease was trans-
ferred from infected to healthy tissue in this manner on several oc-
casions during the winter, in the greenhouse. In these experiments
it was found that considerable moisture must be present before in-







38 Florida Agricultural Experiment Station

fection took place, and in many cases the small trees thus treated
had to be kept drenched and under bell-jars for two or three days.
Infections developed slowly under greenhouse conditions, and were
fewer in number than those obtained in the open. Cultures of fungi
and bacteria were isolated from canker spots at different times dur-
ing the season. Several series of infection experiments were tried
with these cultures in the open and in the greenhouse. All
gave negative results.
Cross-sections of young diseased spots were prepared and stud-
ied under the compound microscope. In many of these could be
traced small mycelial threads of fungus penetrating the tissue cells.
However, there was no method of determining whether this mycel-
ium belonged to a parasitic fungus, or whether it was the mycelium
of some saprophyte that had later gained entrance to the spot.
Some of the leaves that were infected in August in the open
were allowed to remain over winter on the trees. The spots on these
had developed to their normal size by fall and were apparently in a
dormant condition. In early spring it was noticed that small watery
dots, typical of first infections, were forming on the leaf surface
surrounding the old spots. The disease was spreading from these






















Fig. II.-Portion of grapefruit leaf enlarged. The cross shows the
point of infection. The disease has spread to the other parts.







Bulletin 124 39

old spots, and the new infections developed later into characteristic
canker spots. (Fig. II.)
The old spots were carefully examined, but the fungi which
were present did not show any great difference from those in former
specimens that had been examined. Cultures were carefully made
from several small watery spots surrounding an old spot. A fungus
appeared in these cultures that had not been observed before, and it
was more or less constant in all the cultures obtained. Pure cultures
were isolated and grown until the fungus fruited. It proved to be
a pycnidial form belonging to the group of Phomatales. The same
fungus was isolated again from similar spots and from pycnidia
found in the old spot. After obtaining the pycnidia and spores, it
was an easy matter to identify this fungus. Many of the early speci-
mens received were examined again, and this fungus was constantly
found associated with the canker spots on leaves and twigs. All
canker spots do not show pycnidia of this fungus, however, and other
pycnidial forms appear that are confusing. Pycnidia are not found
in young spots, and even in some of the older spots it is a difficult
matter to find them. They are formed down in the spongy tissue,
or under the ragged and lacerated edges of the spots, and easily es-
cape detection. After the spores are exuded the pycnidia collapse,
and, being about the same color as the spongy -dead mass of the spot,
they cannot be distinguished unless the specimens are soaked in wa-
ter or caustic potash solution. This swells the pycnidia to their nor-
mal size, and they may be more easily seen. The minute size of the
pycnidia also makes them difficult to find.
Several experiments have been tried during the present spring
and summer to produce canker infections from pure cultures of this
fungus. These experiments have been conducted mainly in the
greenhouse. Small seedling grapefruit trees in pots were used. The
young growth on such trees was sprayed with a suspension of
spores from pure cultures, and the trees were covered with bell-jars
to conserve the moisture. Most of these experiments have given
negative results; however, in two instances I have succeeded in get-
ting unmistakable infections on young grapefruit foliage. One case
of infection occurred in the greenhouse, and the other on leaves of
grapefruit in the orchard. The culture from which this infection
was obtained was isolated from a single spore from a pycnidium
taken from one of the diseased spots.







40 Florida Agricultural Experiment Station

THE FUNGUS

The fungus causing Citrus Canker belongs to a group of fungi
that are responsible for a number of plant diseases. It is closely re-
lated to the Stem-End Rot fungus, Phomopsis citri, but is appar-
ently a much more virulent organism. I have classed it temporarily
among the Phyllostictas, since it seems to me, from the studies I
have made of it, to belong in this group rather than any other. The
name or position of the fungus, however, is of secondary importance
from a practical standpoint, and further study will be required be-
fore the true position of the organism can be definitely determined.
The mature fruiting bodies of the fungus are found in the spots
or cankers, and they may be easily overlooked unless one knows just
what to look for. (This happened with the first specimens exam-
ined.) These bodies are called pycnidia, and they contain the spores
with which the fungus is propagated and the disease is spread. The
pycnidia are small, round, globe-shaped bodies, about one hundred
and fiftieth of an inch in diameter. They are hollow within, with a
thin, fragile wall, and a small neck or opening at the top. They are
brown to dark brown in color, and in many cases are hardly distin-
guishable from the brown spongy cells of the diseased spots. They
may be formed on both sides of the spot, at the center or near the
edges. In spots that have ruptured, pycnidia may be found imbedded
in the spongy cell mass or just under the edge of the upturned
ragged membrane. In the presence of moisture the spores are ex-
uded in thread-like tendrils or strings. The spores are very minute,
4,100oo to an inch, colorless, thin-walled, and oval to elliptical in shape.
They are produced in enormous numbers in each pycnidium. Being
so small and light they may be carried a great distance by the wind,
by insects, or even on the feet of birds. The spores germinate and
grow readily on artificial media under laboratory conditions. They
germinate within 12 to 24 hours, forming a thin, white mycelial
growth on agar. Pycnidia and spores are formed in from one week
to ten days.
Experiments are now in progress to determine the vitality of the
spores (that is how long they will endure dessication, and how long
they will live in the soil), and also the effect of different germicides.
It was found that hydrocyanic acid gas used in the strength recom-
mended for fumigating trees does not kill the spores.







Bulletin 124 41

SPREAD OF THE DISEASE

Citrus Canker may be spread in various ways, but probably the
wind and the rain are the principal agents in scattering the infection.
Being so small and light, the spores may be blown a great distance
by the wind. Insects, birds, and other agents may carry the disease
for short distances. Rains and heavy dews undoubtedly aid in
spreading the disease through individual trees, and if infection oc-
curs on a single tree in a grove it is an easy matter for it to spread
to other trees. The disease seems to develop and spread rapidly
during rainy weather, but it is more or less retarded during periods
of drought or in the dry season. This was clearly demonstrated
during the past season by some infections that occurred on small
grapefruit trees in the open. Last October, diseased dry leaves were
pinned on healthy young shoots of grapefruit. An early frost killed
most of the growth of this tree, but one leaf became infected and a
single typical canker spot developed. This leaf was allowed to re-
main through the winter. In the following spring the disease spread
slightly to neighboring leaves. The weather was dry through the
spring and summer, and the disease spread very little, so that on the
first of July there were few noticeable infections on this tree. The
tree was not observed again until the first of August. Then a heavy
infection was found on leaves near the old leaf. All infections were
removed a few days later, and the results tabulated as follows:

Leaves, Twi.zs and Fruit Collected on August 7.
Old leaves showing old infections only ------------------------------ 114
Old leaves showing old and new infections ------------------------- 36
Old leaves showing only new infections ---------------------------- 45
New leaves (June flush) new infections --------------------------- 231
Infected twigs --------------------------------------------------- 20
Infected fruit ------------------ ------------------------- - 14
August 17.
Old and young leaves collected --------------------------------- 35
September i.
"Old and young leaves collected ------------------------------------- go
September zo
Old and young leaves collected --------------------------------- 1o
Total leaves collected from tree ------------------------ 561

The infection on all these leaves and fruit came primarily from
a single spot on the old leaf that had held over through the winter.
This illustrates the rapidity with which the disease is spread under
favorable conditions.
The disease is persistent, and is held over through unfavorable







42 Florida Agricultural Experiment Station

periods, in the old spots and cankers on leaves and twigs. As soon
as conditions are favorable the infection spreads from these to the
new foliage.
Young, tender growth is more readily attacked, but there seems
to be no limit as to the age of tissue attacked by the fungus. I have
found the early stages of infection on the oldest leaves on the trees,
and have seen branches that were three or four years old when in-
fection occurred. This makes the disease all the more serious and
more difficult to control, for spores falling on any part of the tree
have a greater number of chances to cause infection. Spots or can-
kers that form on the leaves and fruit or smaller twigs are easily
seen; but small cankers formed in the bark of large branches would
be readily overlooked and remain as centers from which the disease
would spread later.
CONTROL

The problem of controlling Citrus Canker is going to be a difficult
one, and a greater undertaking than the average grower can attempt.
What I mean by to control is to thoroughly eradicate the disease
from infected trees and still be able to save the trees. With the pres-
ent distribution of the disease, confined as it is to a comparatively
small area in Florida, I would advise the complete destruction of all
infected trees just as soon as the infection appears, whether such
trees are in nurseries or groves.
The next important step is to prevent the introduction of any
more of the disease into the State. Citrus Canker has been found in
all of the States bordering Florida, and has been found abundantly
in parts of Texas; and, while the disease may not prove so serious
in those States, it is undoubtedly a menace to the citrus industry of
Florida.
I am not certain just how effective the use of fungicides will be
in controlling this disease. So far I have carried on no spraying ex-
periments in regard to this, but the results obtained by some who
have used Bordeaux mixture are not encouraging. The disease may
be kept from the foliage and fruit by keeping them thoroughly cov-
ered with some fungicide throughout the season, but this would re-
quire a number of sprayings at intervals of ten days or two weeks,
and a continual fight against scale-insects. Cankers formed on the
twigs and larger branches would probably not be affected by the
fungicide, and would remain as centers from which the infection
would spread again as soon as the spraying was discontinued.







Bulletin 124 43

It would be almost impossible to eliminate all infections from a
badly infected tree of any size by defoliating and pruning, for on the
larger branches minute cankers, hardly distinguishable without the
aid of a lens, would be almost certainly overlooked. I should say
that only an expert would be capable of eradicating the disease from
an infected tree, and then it would require continual vigilance on his
part. Uninfected trees or groves in infected districts may be pro-
tected by spraying with some fungicide, but the spraying must be
thorough and often repeated. It would be better, however, to re-
move and destroy the infected trees.







44 Florida Agricultural Experiment Station

ERADICATION OF CITRUS CANKER
By FRANK STIRLING

CONDITIONS IN DADE COUNTY
There is hardly a grower of citrus fruits in the State of Flor-
ida, but has learned of the newest and by far the most dreaded dis-
ease of citrus-Citrus Canker. Bulletin 122 of the Experiment Sta-
tion, issued in March of this year, by Prof. H. E. Stevens, treats
of this disease in a preliminary manner, and tells of its serious na-
ture. Dr. E. W. Berger, in an address to the Florida State Horti-
cultural Society last April, told of the Citrus Canker in the Gulf
coast country, in Alabama, Mississippi, Louisiana, Texas and North-
eastern Mexico. Bulletin 122, Circular 8, and Dr. Berger's report
of investigations along the Gulf coast, give all the known informa-
tion concerning the disease. The present paper treats entirely of my
observations and experiences in combating the disease in South Dade
County, where the Canker was more prevalent than anywhere else
in the State.
It is well known that Mr. Lloyd S. Tenny, of the Florida Grow-
ers' and Shippers' League, took up the matter of financing a cam-
paign against the Citrus Canker (there being no other funds avail-
able at the present time for the work), and of making an effort to
completely eradicate the disease in the State. Along the middle of
May of this year, I was employed by Mr. Tenny to undertake the
work under Dr. Berger's instructions; and with that end in view
came to the southern part of Dade County, where the canker was at
that time beginning to cause trouble. It was thought at first that
there would not be more than ten or a dozen properties having any
Canker in them, and that it would be but a matter of a few weeks, or
possibly two months, to make a clean-up in that part. I found, how-
ever, that there were in all, in Dade County, 95 properties having
Citrus Canker infection, extending from Fort Lauderdale on the
north, to Detroit on the south, a district 55 miles long and in width
from ocean to everglade. Fully 80 per cent of the disease was, how-
ever, in the southern part of this district, in a section about three and
a half miles across, having the little hamlet of Silver Palm for its
center.
In this section, of which little is known except by those living
there, the citrus industry has developed with remarkable rapidity.
In the Redlands territory, which is different from anything else in







Bulletin 124 45

the United States, the geologic formation is chiefly oolitic limestone
which has weathered into angular shapes producing extremely rough
surfaces, with only small quantities of soil in the crevices of the lime-
stone. The soil is frequently red. In this section, while the saw-
mills go ahead and clear away the timber, dynamite follows, not only
blowing out the stumps, but blasting holes in the porous rocks for
the young trees. It may be somewhat taxing to the credulity of
those who have not witnessed it, to be told of digging holes with
dynamite and cultivating the trees with a pick; but it is none the less
true. The amount of clearing and planting is almost unbelievable.
In Dade County south of Miami there are now some eight hundred
thousand trees in groves; almost three hundred thousand of these
trees are in the Redlands territory, and it is safe to say that fully 80
per cent of the whole are grapefruit.

THE CAMPAIGN

When the Growers' and Shippers' League first took up the mat-
ter of Canker eradication in this section, fully 99 per cent of the
growers were ignorant of its presence in the locality. They all, how-
ever, soon learned of it; and wishing to assist in the work of eradica-
tion, organized a branch of the Growers' and Shippers' League,
which at the present time has a paid-up membership of nearly 250.
We knew that this new disease attacked the leaves and young
shoots and fruit; and that when the tree became infected its growth
was stunted, and all of the fruit were reduced to "culls." It was
learned that Canker is not, like other diseases, amenable to treat-
ment; and that spraying did no good whatever. At the outset, we
deemed it necessary that all infected groves and nursery stock should
be cut back, defoliated, and the trunks painted with Bordeaux or
carbolineum. This was a radical treatment; but there seemed
nothing else to do, and we went at it bravely. We cut back and
treated in this manner over two hundred thousand nursery trees and
over five hundred acres of grove trees. This was in May, June, and
the first part of July. At this juncture everyone began to breathe a
little easier. People went on clearing land and planting out new
groves. Several weeks passed; the inspection went on, and the
treatment went on. More and more infection was found, and more
and more was treated. What was at first thought to be a quite lo-
cal infection, proved widespread. Instead of a dozen diseased prop-
erties, a score were found, and then fifty, and then almost a hundred.







46 Florida Agricultural Experiment Station

Still we kept fighting on, and cutting back, and treating with Bor-
deaux and carbolineum.

FAILURE OF CUTTING BACK

But the worst was yet to follow. The groves- cut back and
painted and left for clean, began to put out new growth. Imagine
our consternation when it was found that this new growth still
showed the infection, and that all of our treatment had been useless.
We found that we had been actually spreading the disease. In one
grove, for instance, there were twelve trees originally infected when
it was treated; when the new growth appeared there were forty in-
fected. What was to be done? Positive advice was not to be had.
There was no precedent to follow.
In the absence of knowledge as to what else to do, we redoubled
our efforts; but the disease, every time it was felled, arose ten-fold
stronger. In short we were "up against it" in two different ways;
lack of funds and lack of knowledge. The growers went into their
own pockets as they had never done before. The growers, the busi-
ness men, the banks, the Board of Trade of Miami, professional
men, even the women, did their share. So the money was raised,
but we still lacked the knowledge of what to do. We had followed
the only known remedies, and been defeated. Washington was ap-
pealed to, and a man, we were advised, was detailed to our assist-
ance; but even he had to first make a tour of Alabama, Mississippi,
Louisiana and Texas, where the disease is of longer standing, before
he could come to us.

THE FIRE TREATMENT

In the meantime a number of the growers and others in the
district began trying out methods that might prove successful, and
experimenting in other ways. At this juncture Dr. Hiram Byrd
proposed a method of treatment, namely, applying a forced flame to
the diseased trees. If cutting the trees back and painting them was
a radical measure, what was to be said of turning a spray of burning
oil upon the infected trees, and about reducing them to ashes.
The rationale of the method is this. The disease is of a fungus
nature; it spreads by dissemination of spores; these spores are
washed off the infected trees and get on the ground, the grass, and
rocks beneath the trees. When the tree has been treated, the tree







Bulletin 124 47

itself may be clean, but it has the infection all around it, from which
it becomes re-infected as soon as the new growth puts out. In cut-
ting back the trees, the tools, clothing, hands, etc., of the workmen
become infected; and when other trees are handled, they likewise
become infected. No method of treatment can succeed that does not
take into account all these factors.
The method of burning the trees is to spray a flaming mixture
of kerosene and crude oil upon the tree, the grass, and soil beneath,
till the tree is charred. (See Fig. 12.) In this way nothing comes
into contact with the disease except the forced flame, and there is ab-
solutely no danger of carrying the disease to other and healthy trees.




V ..







Fig. 12.-Burning an infected grapefruit tree.

RAPID SPREAD OF CANKER
Canker is without doubt the most infectious of any known
disease; and during a time when the atmosphere is humid, in the
rainy season, it spreads rapidly. I have found that during the early
part of the season it requires two or three months for the Canker
to infect and mature so as to reproduce itself, owing,no doubt, to
the dryness and coolness of the weather. Under favorable condi-
tions, however, the Canker will infect and mature in much less time;
and it is certainly a good thing that we began work when we did, or
we never would have caught up with it.
As I have said before, this disease is by far the worst which has
ever yet affected the citrus industry. The leaves, twigs, and fruit
become covered with a cankerous growth. The fruit itself seems to
be especially susceptible to the disease, and drops soon after becom-
ing diseased. Canker is so deadly that when the tree first becomes








48 Florida Agricultural Experiment Station

infected, in this territory, it is worthless inside of two or three
months.
WHAT HAS BEEN DONE

For the past two months we have been passing through the
worst of the fight. We have destroyed every tree showing the slight-
est infection. We have done this by applying the torch, which re-
sembles a plumber's blow torch, only a hundred times larger. The
diseased trees were burned to a crisp. The torch was also applied
to the surrounding ground. The cost has exceeded one hundred dol-
lars per day (this does not include the $2,000 which the Growers'
and Shippers' League is spending). We have burned, in the Red-
lands district alone, 1,933 grove trees, and 101,300 nursery trees
(see Fig. 13). Over 200 grove trees, and a good many thousand













Fig. 13.-Burning infected nursery trees.

nursery trees were burned in the Miami district. We have in the
two districts some fifty odd men employed in the work of canker
eradication (Fig. 14). Most of these men are being paid by local
subscription, and some few are volunteering their services.

ALL FLORIDA THREATENED
If such a thing were possible that we fail to entirely eradicate
the disease here, then the entire State is in jeopardy. It would be
merely a matter of months before the Canker would be entirely over
the orange belt. If every grower in the State of Florida knew of
the deadliness of this disease as we do here, and of the rapidity with
which it spreads, not one of them would rest a moment as long as








Bulletin 124 49



.. 1.

S''i -I l








Fig. 14.-Canker eradication squad.

there was left a trace of it in Florida. Another thing-if this dis-
ease had been permitted to have gone on six months longer before
taking steps for its eradication, I doubt if one hundred thousand
dollars would have effected a clean-up.

VARIETIES AFFECTED

We have found the disease upon all of the varieties of citrus,
with the exception of the kumquat; although the grapefruit seems to
be by far the most susceptible of them all. The order, according
to the degree of infection observed, is as follows: Grapefruit, Cit-
rus trifoliata, Persian lime, Key lime, navel orange, sweet orange,
Satsuma, tangerine, mandarin, King orange and lemon. The seed-
ling grapefruit and the Triumph seem to be the most susceptible of
the pomelos, and I have never yet seen the disease upon Castalo tan-
gerine. The Canker has never yet been observed upon any non-cit-
rus plant.
EFFECT ON TREES AND FRUIT

The great danger to South Dade County, lies in the fact that it
is principally a grapefruit district. As was stated in articles by Dr.
Berger and others, this disease attacks the twigs and leaves of the
pomelo virulently, resulting in a putting out of more twigs, and thus
overloading the tree with small branches. Canker is also virulent in
the manner in which it affects the leaves, spotting them and caus-
ii g them to turn yellow and drop prematurely. The worst of it is
its effect on the fruit. With oranges, the disease is not often seen







50 Florida Agricultural Experiment Station

upon the leaves and twigs, but upon the fruit itself, causing spotting
to a considerable extent.
In one grove in the vicinity of Silver Palm, known as the
Walker and White grove, there are about four acres of grapefruit
trees about seven years old, bearing from two to twenty boxes of
fruit per tree. During the first week in June these trees were in-
spected, and were found to all appearance free from Citrus Canker.
Three weeks later, one tree began to show a slight infection upon one
of the limbs. Four days later I made a careful inspection of the
grove and found that the disease had spread to five trees. The own-
er was away at the time, and when he returned, a week later, the dis-
ease had spread to twenty-seven trees. I would have had no trouble
whatever in picking fifty boxes of diseased fruit-fruit that was all
covered with the Canker, and some of which was beginning to rot
and fall off. By the time we could get the consent of the owner to
burn them, three days later, the disease had spread further, and we
burned over fifty trees.
Fortunately, there were not many large bearing groves in the sec-
tion (it being a new country), and consequently, the loss per tree
has not been so great as it would have been had the disease broken
out in an older country. Most of the trees are under three years old.
However, I want to say that this disease attacks a large bearing tree
just as readily as it does a young tree.

HOW WE DO THE WORK

When we learned that this disease was so terribly infectious, we
began to take every precaution in regard to disinfecting ourselves,
tools, etc., to prevent carrying infection from tree to tree and from
grove to grove. The men on the job are supplied with suits similar
to the harvesters' suits used in the West. We secured 72 of these
suits. The men use them in the groves. When they leave one grove
they take off the suit they have on, dip it into a solution of mercury
bichloride (strength one to a thousand), and put on another suit to
use in the next grove while this one is drying. No one is ever per-
mitted to touch a tree, whether diseased or not. So particular and
careful have the growers become, that anyone trespassing, inno-
cently or otherwise, is apt to get into serious trouble.
The growers get together once a week to receive reports on the
progress of the Canker eradication work, and to discuss ways and
means for keeping on with the work; as we deem it necessary to







Bulletin 124 51

keep on inspecting for several months after the last Canker has been
seen.
Lately, practically no new infections have been found, although
there are recurrences of the disease in those groves where heretofore
Canker was found; but even these are growing less week by week,
and this being the rainy season and the time when the disease is more
apt to spread, conditions are certainly favorable. The growers are
not by any means out of the woods yet, but we have come to the
turning of the road, with a straight way ahead, and we but need the
vehicle-the finances to go on.
As I said before, the expense of the Canker fight is carried on
mostly by the growers themselves, and in the County the expenses
exceed $Ioo per day. We have some forty inspectors in the field,
men who do nothing but look out for Canker, and several men who
follow up with the eradication outfits and do the burning. The trees
that are found diseased today are burned tomorrow. If there is but
one leaf showing any Canker found on a tree, we burn the tree just
the same.
In the Redlands section there are some 75 properties which
have shown Canker. In the section in and around Miami and north
of there, there are some 20 properties infected. Consequently, we
are having most of the fight in the Redlands section. In almost
every case where we find Canker, we can trace it to some source,
showing that the disease is carried by actual contact, and is not wind-
borne. In many properties outside of the Silver Palm; section, the
disease has been carried on nursery trees which came from diseased
nurseries. In some places the disease has been brought by men
working in a diseased nursery or grove, and then going to a healthy
property. Of course, in some cases, the spread of the disease can be
charged to birds, and many kinds of insects; but this way of carry-
ing has never prevailed to any extent, and only in the nurseries and
near-by groves.
We have already made a tree-to-tree inspection of everything in
the section, and are now on the second inspection. The thorough in-
spection of nearly a million trees, taking a look at about every leaf
on those trees, is, as you see, quite an undertaking. Then we make a
re-inspection every week of the groves in which the disease has been
found. This means that we have to inspect some 45,000 trees every
week. We use for this work 12 special inspectors who do nothing
but re-inspect. It is in this re-inspection that we now find nearly all
of the Canker, and, as I said before, this is becoming less and less.







52 Florida Agricultural Experiment Station

When we first began this burning campaign, the number of trees in-
fected of those inspected was 15 in every thousand, or one and a half
per cent. Now that percentage has fallen to a quarter of one per
cent. Up to the present time one tree in every 167 trees in the en-
tire section has been burned. We believe that by holding down the
disease and gradually decreasing the percentage through the wet sea-
son, by the time that the dry, cool weather sets in there will be no
trouble in making a complete eradication.
Any and every precaution for the prevention of the spread of
the Canker has been urged. The State Inspector of Nursery Stock
has revoked all certificates in the diseased district. This alone has
proved to be of great value to us, as certain recent developments
have shown. Nurseries that three months ago were apparently
healthy and clean, have recently developed Canker. If the buying
and planting of trees had gone on as before, it is hard to tell how
much farther the disease would have been spread. At any rate our
work would have been made much harder in locating and eradicat-
ing these new infections.
The success of our work in Canker eradication, has depended
entirely upon the splendid organization and co-operation of the
growers themselves. In the first place, we had, for the President of
the Local Branch of the Florida Growers' and Shippers' League, one
of the best men obtainable for that office, who is vitally interested
in grove culture in that part of the State. Another thing was that
we had only men who are grove-owners and property holders en-
gaged upon the work of fighting the Canker. All of these men are
vitally interested in the welfare of the country, and most of them
have given up their own private interests in the meantime to help
combat the common enemy.
Experiments and investigations in regard to the nature of the
disease have been, and are being carried on by Prof. Stevens of the
Experiment Station at Gainesville, and by Dr. Wolfe of Alabama.
However, a committee has been appointed in the Redlands section
to carry on some experiments in the diseased territory there. There
we have learned some things which have proved quite valuable to us
in our work. We know that where we have burned the tree and sur-
rounding ground, and where the new shoots have sprung up from
the roots, in no case has the infection returned. This shows conclu-
sively that the roots were not infected. However, only a few of the
trees have sent forth sprouts where they were burned. In many
cases the trees are entirely dead.







Bulletin 124 53

We know that, during the warm, wet periods, the disease in-
fects quickly and matures in a few days. I took a leaf having two
pustules of the Canker on it, soaked it in water one minute, drew it
through my thumb and fore-finger once, then drew a leaf which was
on a healthy potted grapefruit seedling through my thumb and fore-
finger once; with the result that, in eight days, on that leaf I counted
over fifty tiny pustules of the disease. In four days more, these pus-
tules had developed to maturity; that is, they had burst open, and
were infecting the other leaves upon the seedling. To learn if the
disinfectant bichloridee of mercury) which we were using, was ef-
fective, I repeated the experiment, using a solution of one to a thou-
sand in which to soak the diseased leaf; with the result that no in-
fection occurred upon the healthy leaf. We have also noticed that
where groves have been repeatedly sprayed with Bordeaux, even
though they are close to an infected grove, the chance of their be-
coming infected is considerably lessened, although the Bordeaux
does no good after the tree is once infected.
Now the question arises, is it worth while to resort to this most
drastic method of eradicAtion-that of burning the diseased tree. It
certainly is heart-rending to burn beautiful trees, which in some
cases have ten to twenty boxes of fruit on them. As I have said be-
fore, we have burned in the Redlands section one tree in every 167
trees, or something like a half of one per cent. We consider that if
we could make an entire eradication by sacrificing ten per cent of the
whole number of trees in the section, it would be worth while; and I
feel confident that we will not lose more than one per cent of the to-
tal. While there may be found some remedy to effect a control of
the disease, I doubt it very much; and if there is, our knowledge of
other diseases shows us that eradication at almost any price is pref-
erable to any remedy for control. Take Citrus Scab, for instance, I
am told by good authority that that disease will cause something like
a hundred thousand dollars loss to the citrus growers of the State
this year. One-tenth of that amount, if applied properly at the start,
would have stamped it out completely; as could have also been the
case with the whitefly in 1896. So with this new disease, which is
by far the most terrible yet known to affect the citrus industry in
Florida, we deem that the only way to handle it is by complete and
absolute annihilation.





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