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
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
Bulletin 128 November, 1915
UNIVERSITY OF FLORIDA
Agricultural Experiment Station
H. E. STEVENS
Fig. 1.-Six-months-old canker spots on grapefruit leaf.
The Station Bulletins will be sent free upon application to the Experiment Sta-
tion, Gainesville, Florida.
PeDper Publishing & Printing Co..Gainesville. Fla.
Introduction ...---_ .._ ..... ..------------ 3
History of Citrus Canker .....------------ ....--.------------------------- 4
Distribution in Florida .......----------- ...... -----------.--------- 6
Appearance of Citrus Canker _--...-------. -------.... ------------------ 7
Canker Distinguished from Other Diseases -------------------- 11
Cause of Citrus Canker ___----__--- _----.._.-------------. ------------. 12
Pseudomonas citri Hasse----------...-----.... --.---- ------------ 13
Laboratory Investigations _.----.- ---------------------------------------- 14
Growth in Sterilized Soil----------.. ...------------------ -----------.- 14
Effects of High Temperature ..------------.---------- ---------- 17
Effects of Drying out .--------. ..--.- --------------..------------. 17
Spread of the Disease.-.- --------------------------------.--------------- 18
Control ---------_----------------------------------..-------------------.. 19
1. Citrus Canker is a bacterial disease caused by a species of bacterium known as
Pseudomonas citri Hasse.
2. The disease has proved to be a serious one to most of the varieties of citrus
grown in Florida. All parts of the trees above ground may become infected.
3. High temperatures and high humidity favor a rapid development and spread of
4. The bacteria are capable of growing in sterilized soil, and under favorable cir-
cumstances may retain their vitality for long periods in such soil.
5. They are capable of withstanding considerable drying or dessication, which
capability may also be a factor in spreading the disease.
6. A complete destruction of all infected trees is the only effective method known
of checking the spread of the disease.
BY H. E. STEVENS
Citrus canker is one of the worst of the plant diseases that have
appeared in Florida. The disease is a new one, and three years
ago was not known to science. It threatens the citrus industry
of the State, and has already cost individual citrus-growers
thousands of dollars, in addition to Federal and State funds spent
in the effort to eradicate it.
There are certain peculiar features of the disease that are
not easily explained. Canker seems to be capable of remaining
dormant on a tree, or quiescent in its vicinity, for long periods
without any symptoms of the disease appearing; but eventually
such trees may develop typical cases of canker. This has been
illustrated on a number of occasions when trees have been
removed to new localities from nurseries infected with the
disease. These trees had been defoliated, and they showed no
signs of the disease at the time of removal. To all appearance
they were healthy and free from canker, although they had been
exposed to the disease. Shipments of such trees were made to
different parts of the State. A year later, some of these trees had
developed virulent cases of canker. Thus any tree that has been
exposed to the disease may possibly develop canker at some
The same holds true where the disease appears in the
groves. All healthy trees surrounding the center of infection
must be kept under constant observation for future outbreaks.
Until recently there has been more or less confusion among
different investigators regarding the cause of citrus canker; but
it is now generally conceded that the disease is caused by a
species of bacterium, Pseudomonas citri Hasse. Formerly citrus
canker was considered of fungus origin. Recent investigations,
however, prove beyond a doubt that the above-named species of
bacterium is the cause of citrus canker, and that this organism is
highly injurious to most of the varieties of citrus, especially
those of commercial importance.
4 Florida Agricultural Experiment Station
The object of this bulletin is to bring together the latest and
most important facts concerning citrus canker, and to clear up
any confusion that may result from former publications on this
HISTORY OF CITRUS CANKER
Citrus canker appears to be a new disease, or one of recent
introduction, in all the countries in which it has appeared. The
question of its origin is still obscure. W. T. Swingle stated
before the Citrus Seminar, on October 6, 1915, that he had found
the disease in Japan, China, and the Philippine Islands. Prof.
Swingle made a thorough study of the disease in these countries,
and endeavored to ascertain its place of origin. His investigation
let him to conclude that the disease was not indigenous to any of
these countries, but had been introduced into all three.
The disease was probably brought into the United States
from Japan, according to the report made by Dr. E. W. Berger,
State nursery inspector, who investigated this subject in the
spring of 1914 (see Proc. of Fla. State Hort. Soc. for 1914, pp.
120-127). It was brought into Florida on nursery stock from
Texas, and probably also on trifoliate stock direct from Japan.
The disease was first found in two nurseries in Florida, one at
Monticello, and the other at Silver Palms. From these two
centers it has been spread to various localities in the State,
chiefly through shipments of infected nursery trees.
The first specimens of this disease to come to the writer's
attention were received in April, 1913, from a nursery at Silver
Palms. The disease was recognized as something different from
the usual citrus diseases, but was only given passing attention at
the time. At intervals through the season of 1913, specimens
were collected by Dr. E. W. Berger from Monticello and Silver
Palms, and submitted to the writer. So far, the disease had only
been noted on nursery stock. In the fall of 1913, a considerable
quantity of infected leaves, fruit, and twigs of grapefruit was
received from a citrus-grower in Grand Bay, Ala. He reported
that the disease was attacking his bearing trees severely. The
trees had been sprayed with Bordeaux mixture, but the treatment
had apparently not checked the disease.
In the spring of 1914, Dr. E. W. Berger visited those Gulf
States where citrus is grown, and found the disease in Alabama,
Mississippi, Louisiana and Texas. Previous to this it was realized
Bulletin 128 5
that a new and probably dangerous citrus disease had made its
appearance, and investigations were undertaken to determine its
nature and cause. No reference to this disease or to one with
similar characters, could be found after a search of all the
available literature on citrus diseases; so a preliminary report was
published in March, 1914 (Florida Agr. Exp. Sta. Bul. 122), in
which the disease was described and named.
During the summer and fall of 1914, citrus canker had
assumed a more serious aspect in south Dade County. The
disease was found in other nurseries in that locality and also in
young citrus groves. The usual methods for control by spraying
with Bordeaux mixture or other fungicides failed to check the
disease, and even cutting the young trees back to short stumps
failed to prevent its recurrence.
With the help of Mr. L. S. Tenney, Secretary of the Growers
and Shippers League, the growers of south Dade County were
organized, and an effort was made to stamp the disease out in that
section. A campaign of eradication was begun, which resulted
in the burning of all infected trees in the grove where they stood.
A thorough periodic inspection was made of all infected properties,
and of those exposed to infection, for any new cases of the disease
that might appear. Such infected trees were burned on discovery.
This campaign was continued in the face of many obstacles in
Dade County, and was later extended to include all localities in
the State to which shipments of nursery stock from infected
nurseries had been made.
With the passage of the Crop Pest Bill by the State Legislature
in April, 1915, provisions were made to further extend the
work, with the view of completely eradicating the disease from the
State. Adequate laws were provided, and such an appropriation
was made as was thought necessary to complete the work. The
inspection and eradication forces were increased, and every
locality in the State that was known to have received infected
nursery stock, or stock that was even suspected, was placed
under observation. All localities in which infected trees were
found were placed under strict quarantine, until such a time as
such properties were known to be absolutely free from citrus
The work is being carried on at present by the State Plant
Board under this plan, and there are but few growers in Florida
who fully realize the serious nature of the disease, who are not
in full sympathy with the plan.
6 Florida Agricultural Experiment Station
DISTRIBUTION IN FLORIDA
The accompanying map (Fig. 2) shows the localities in Florida
where citrus canker has been found. These include all infections
that have been discovered since the disease was first introduced,
NOV. I,1915. '*-.. ,^
R QUARANTI NE REMOV
L E VY
BFORE NOV. 1, 915. 5
PRIMARY INFECT ON
S.. .L Y . B A
S.^NTA "RA ^. 0 "s^ -- ,,,^A
NOV. 1,1915 'DADE
Fig. 2.-Distribution of Citrus Canker in Florida.
Bulletin 128 7
up to November 1, 1915. With the exception of Dade, Broward
and Walton Counties, each symbol represents an individual
property (grove or nursery) in which infected trees have been
found. In the above-named three counties it is not possible to
indicate the individual properties in a given locality on so small
a map. Sixteen infected properties have been found in the
vicinity of Santa Rosa, Walton County, and 305 infected properties
in Dade and Broward Counties. In the latter counties most of
the infected properties are grouped around a few centers. In
these sections many of the infected properties have been entirely
destroyed, or released from quarantine; but in most cases such
properties are either surrounded or overlapped by quarantine
areas, which prevents any representation of their true status being
shown on so small a scale.
No canker-infected trees really exist in any of the localities
indicated, for all infected trees are burned on discovery. In most
cases the entire grove is burned if the trees are small. All
properties where infection has been found are quarantined and
kept under observation until they are considered free from the
The black circles represent properties where canker has
been found in both primary and succeeding infections, and these
are still considered dangerous territories. The-infected trees have
been burned, a quarantine established, and the property is under
periodic inspection. The quarantine area includes a circle with
a mile radius, the infected area being the center of the circle.
The squares represent areas in which only primary infections
have been found, or cases in which the entire grove has been
burned. These areas are under quarantine and inspection.
The triangles represent properties where the disease has been
found, but which have passed through the period of quarantine
and inspection, and are now considered free from canker.
(Acknowledgment is here made to Mr. F. M. O'Byrne, nursery
inspector, and to Mr. Frank Stirling, general inspector of the
State Plant Board, for the data used in the preparation of this
APPEARANCE OF CITRUS CANKER
Citrus canker attacks all varieties of citrus trees of commercial
importance in Florida, except the kumquat. Any part of the tree
above ground may become infected. Grapefruit is apparently
most severely attacked, the infections occurring on leaves, twigs,
branches and fruits, and occasionally in the bark of exposed
8 Florida Agricultural Experiment Station
roots. Citrus trifoliata is probably next in susceptibility and is
followed by some of the varieties of sweet orange. According to
Dr. E. W. Berger, whose statement is based on field observations,
the different varieties of citrus are susceptible to the disease about
in the order named; pomelo (grapefruit), Citrus trifoliata, Key
lime, Navel orange, sweet orange, Satsuma, tangerine, Mandarin,
King orange, and lemon. The writer has observed infections on
all parts of Citrus trifoliata above ground, except on the fruits.
The disease has been observed on leaves, twigs and fruits of the
Navel and some other varieties of sweet orange. Scanty infections
have been found on the leaves and twigs of the Satsuma,
tangerine, and lime. The rough lemon seems to be quite
susceptible. Almost any of the varieties of citrus may be
severely attacked by canker, if all the conditions are favorable
"Fig. 3.-Canker spots on leaves of grapefruit.
Bulletin 128 9
for the development of the disease. Inoculations made on sweet
orange, trifoliate orange, rough lemon, and grapefruit trees
indicate that these varieties show about the same degree of
susceptibility to infection, where growth and moisture conditions
are the same. The young growth is more readily attacked by
canker, but tissue of any age may become infected. Canker has
"been observed to develop in the bark of grapefruit branches that
were two or three years old.
The distinguishing feature of citrus canker as observed in
the field is the characteristic spotting produced on the fruit and
from less than
to one quarter
of an inch in
and may occur
Fig. 4--Canker on grapefruit. About half natural size. T h i s I a s t
on fruits. The spots are raised above the surrounding healthy
Fig. 5. Canker on young twig of Citrus trifoliata, showing the broken
membrane around the spots. (Magnified about three times.)
10 Florida Agricultural Experiment Station
tissue, and are composed of a spongy mass of dead cells, covered
by a thin white or grayish membrane. The membrane finally
ated or ragged
tions first S <
dots are usually
of a darker
green than the
surround i n g"
times, ho\v- -
ever, the sur-
face of the ; ,
spots is broken
as soon as they
may. appear on
of the leaf, but .
they do not at
through the e
increase in size,
change to a 1
light bro w n
visible on both
sides of the
1 e f .: In 'thel Fig. F.--anker on grapefruit nursery stock. Natural size.,
Bulletin 128 11
older spots one or both surfaces may be bulged or raised, and
such spots are commonly surrounded by a narrow yellowish
band or zone. (Figs. 1 and 3.) In the more advanced stages the
surface of the spots becomes white or grayish, and finally
ruptures, exposing a light brown spongy central mass. Old spots
soon becomes 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. 4.) Gumming is sometimes associated with the spots
formed on the fruits. Canker, apparently, does not cause a rot
of the fruits directly, 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 and fruit. (Fig. 5.) On the older twigs
they are more prominent, and more or less irregular in shape.
This is especially true of old spots. (Fig. 6.) They show the
same spongy tissue as is found in the spots on the leaves, but
assume a cankerous appearance and the surface membrane
completely disappears. These spots or cankers 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 readily spread. This is
confirmed by observations on infections produced on potted trees
in the greenhouse and in the grove, by artificial infection. Some
of these spots were under observation for over a year, and
showed 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,
readily be distinguished from any of these by noting the following
1. 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
wartlike projections. Canker occurs on older wood, scab does
2. Canker differs from scaly bark in the size of the spots,
which are much smaller and more circular than those of scaly
%12 Florida Agricultural Experiment Station
bark; and the spongy nature of the spots-scaly bark spots are
hard and glazed. Canker is common on grapefruit, scaly bark is
not. Canker 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
CAUSE OF CITRUS CANKER
Citrus canker is a bacterial disease. Recent investigations
have demonstrated this conclusively. Miss Hasse of the Bureau
of Plant Industry, U. S. Dept. of Agr., was the first to observe
the relationship of bacteria to citrus canker. A report of her
work was published last April, in which she described and named
Pseudomonas citri as the cause of citrus canker. (Clara H.
Hasse. Pseudomonas citri, the cause of citrus canker. U. S.
Dept. of Agr., Journ. of Agr. Research, April, 1915.) Her inves-
tigation demonstrates rather conclusively that this particular
organism is the cause of the disease.
Previous to this, canker was considered of fungus origin,*
and all the data at hand regarding the disease at that time seemed
to warrant this conclusion. The general appearance of the dis-
ease, its reactions to certain external influences, the constant
association of a certain species of fungus with canker infections,
and the fact that infections were produced from supposedly pure
cultures of this fungus on healthy citrus tissue: all appeared to form
a fairly sound basis for concluding that the disease was of fungus
A few days before receiving notice of Miss Hasse's investiga-
tion, I had come to the conclusion that bacteria were concerned
in the development of citrus canker, and that they were probably
the direct cause of the disease. My experiments had not pro-
gressed far enough to warrant publishing any positive conclu-
sions before Miss Hasse's report was published.
"*Stevens, H. E. Citrus Canker. Florida Agr. Exp. Sta., Bul. 122, 1914.
Stevens, H. E. Citrus Canker. Ann. Rept., Fla. Agr. Exp. Sta. for 1914. 1915.
Stevens, H. E. Citrus Canker II. Florida Agr. Exp. Sta., Bul. 124, 1914.
Wolf, F. A., and Massey, A. B. Citrus Canker. Alabama Agr. Exp. Sta.,
Circ. 27, 1914.
Edgerton, C. W. Citrus Canker. Louisiana Agr. Exp. Sta., Bul. 150, 1914.
Bulletin 128 13,
Later, tests made with pure cultures of Pseudomonas citri fully
demonstrated that this organism was the cause of citrus canker,
and the results corresponded closely with those obtained by Miss
A number of inoculations have been made with pure cultures
of the bacteria during the past few months. Sixty-seven citrus
seedlings and small trees have been inoculated with pure cultures
of Pseudomonas citri, and 57 of these trees have developed typi-
cal canker spots. The inoculations have been made at different
dates with cultures from different sources, and in several series
one hundred per cent of the inoculations gave positive results. A
majority of these inoculations were made by spraying young
citrus foliage with bacteria from pure cultures which had been
shaken up in sterile water. In a few instances the surfaces of
leaves and stems were punctured before applying the bacteria.
This was soon found unnecessary, as the organisms are capable
of penetrating the unbroken surfaces of the tissue. After inocu-
lation, the trees were kept in a moist chamber for three or four
Young and succulent growth under humid conditions is very
susceptible, and the spots begin to appear in about two weeks
from the date of inoculation. The organisms are capable of
penetrating either surface of the leaf, and the presence of wounds
or abrasions is not necessary in order that infection may take
place on the younger tissue.
The results obtained thus far leave no doubt that the bacte-
rial organism under study is the cause of citrus canker.
PSEUDOMONAS CITRI HASSE
Pseudomonas citri belongs to a genus of bacteria which in-
cludes several species that are responsible for some of the serious
diseases of farm and vegetable crops. Some fifteen species of
this genus have been reported as parasitic on higher plants, and
a majority of these attack plants of economic importance.
The canker organism is a short thick motile rod with rounded
ends. These rods are about three times as long as broad, and
vary from one to two twenty-five thousandths of an inch in
length. They move about in liquid with a free swimming mo-
tion, by means of a hair-like thread called a flagellum, which
protrudes from one end. A characteristic yellowish growth is
produced on standard agar, and there are other cultural charac-
14 Florida Agricultural Experiment Station
ters that readily distinguish this organism from other members of
The organism is parasitic on most of the varieties of citrus,
and in the presence of moisture can penetrate and readily infect
young citrus tissue in the absence of any wounds or abrasions of
the epidermis. At the present time the organism is not known
to be parasitic on any other plants but citrus.
GROWTH IN STERILIZED SOIL
In the work of eradicating citrus canker, many problems
have come up regarding the disease. Preliminary experiments
with the canker organism have been conducted during the past
few months with a view of solving some of these problems as
far as possible.
One question that has an important bearing on the eradica-
tion work and also on the dissemination of the disease, is whether
or not the canker bacteria can live for any length of time in the
Some results obtained recently in the laboratory with soil
cultures of the canker bacteria indicate that they are capable of
remaining alive in sterilized soil for a long time. The organisms
multiply rapidly and penetrate to a considerable depth in such
soil when a favorable environment is provided. Soil cultures
have been kept under observation for a period of.six months, and
the bacteria are at present alive and active in these cultures.
Large test-tubes containing sterilized garden soil saturated
with water were inoculated with bacteria from a pure culture on
April 30, 1915. Six tubes were prepared, and were kept in the
laboratory at room temperature. Cultures have been made from
some of these tubes at intervals during this period to test the
vitality of the organism in this soil. Plates of melted agar were
inoculated with small amounts of soil from each tube and in-
cubated for several days. The presence or absence of bacterial
colonies in these plates indicates the presence or absence of bacte-
ria in the tube from which the test was made.
The following table summarizes the results of these tests.
Bulletin 128 15
Date of Test Tubes Tested Presence of Pseudomonas citri
May 8 ----------- 4 Abundant
May 15 ...-----. 4 Abundant
June 3------------ 4 Numerous
July 10 --------_ 4 Fairly numerous
Aug. 24 ..---.. 4 Few in dry soil; abundant in moist
Sept. 2 ----------- 1 Few in dry soil; abundant in moist
Sept. 30 ----__ 3 Few in dry soil; abundant in moist
Oct. 19 ----------. 1 Scanty in dry soil; abundant in moist
Oct. 28 ..--...--- 1 Abundant in moist soil.
At first, cultures were made only from the surface soil in the
tubes; but after two or three months the top soil became dry, and
only a small number of the organisms survived in it. In every
case it has been possible to recover a few organisms from the
dry soil at the top of the tubes, even after the soil had remained
in this air-dried condition for three or four months. In the moist
soil in the tubes the bacteria have always been found abundantly
in these tests. When dry soil from these tubes was again satur-
ated with sterilized water, a rapid multiplication of Pseudomonas
citri took place, and test cultures showed the bacteria to be as
numerous as they were before the soil dried out. A test of soil
cultures also showed that the bacteria can penetrate through six
or eight inches of soil down to the bottom of tubes in a few days,
even where there is no drainage or movement of water.
Bacteria grown in sterilized soil for four months still retained
their virulence, and readily infected healthy citrus tissue when
applied to it. One of the tubes in the above series was used for
this purpose. The young foliage on seven small citrus trees was
sprinkled with the dry soil from the top of this tube, and seven
other trees were treated in a similar manner with moist soil from
the bottom of the tube. Sixteen check trees were treated in a
like manner with sterile soil. All trees were thoroughly sprayed
with sterile water and kept in a moist chamber for several days.
Six of the trees treated with the dry top soil developed canker,
and all trees treated with the moist soil showed canker infection.
No canker developed on any of the check trees.
A similar experiment with unsterilized field soil was not
conclusive. It was found impossible to recover the canker
16 Florida Agricultural Experiment Station
organism in cultures from such soil two weeks after the inocu-
lation, owing to the rapid growth of the many fungi and other
bacteria that were present. Citrus tissue inoculated with some
of this unsterilized soil four months later failed to develop can-
ker. These results do not prove, however, that the canker
bacteria were not present, or that they were unable to live in
this unsterilized soil. There are many factors under natural con-
ditions that would probably tend to keep the number of canker
bacteria in soil down to a minimum, and which in certain situa-
tions may entirely inhibit their growth. Still there may be other
situations of a more favorable nature where the canker organ-
isms would rapidly increase so that their presence could be easily
detected by ordinary dilution cultures. (This has not been done
It must be remembered in regard to the cultures on sterilized
soil made in the laboratory that all the conditions were most
favorable to the bacteria, and such pronounced results could
hardly be expected under natural conditions.
The experiment indicates the following conclusions. First
that the canker bacteria can live and grow in sterilized soil,
where conditions are favorable. This may perhaps explain in a
measure the tardy development of the disease on trees coming
from infected nurseries. Small amounts of infected soil may
perhaps be carried on the roots of a few trees. After such trees
are planted the bacteria may multiply in the soil beneath them,
and may later be carried into the trees by insects, thus causing
an outbreak of the disease.
Second, that the virulence of the bacteria is not soon lost by
being grown on sterilized soil. If ordinary soil once becomes
infected with the canker organisms, it would be unsafe to plant
citrus trees in such soil until it is absolutely free from these
Third, the organisms are capable of withstanding long
periods of dessication in sterilized soil. This would probably
enable them to survive long periods of drought in the field and
still be present in the soil and become active as soon as favorable
conditions occurred. The fact that they do not lose their vitality
in dry sterilized soil would suggest the ease with which infection
may perhaps be carried in the dry soil on the roots of trees
coming from infected nurseries.
Bulletin 128 17
EFFECTS OF HIGH TEMPERATURE
Spore production in many kinds of bacteria is usually indi-
cated when such organisms are able to survive very high tem-
peratures. The spores of some bacteria can withstand a boiling
temperature for several hours and not have their germinating
power destroyed; while the normal organism itself, the vegetative
portion, can not endure a temperature of more than 50 degrees
C. (122 degrees F.), if exposed for five or ten minutes. As a rule
the vegetative cells of most bacteria are killed if exposed for a
few minutes to temperatures above 50 degrees C.
Cultures of Pseudomonas citri grown on different media
have been subjected to different degrees of temperature for
periods of five minutes. Cultures of different age have been
treated in the same manner. In all cases the bacteria have been
killed by temperatures ranging from 55 to 60 degrees C.. (132 to
140 degrees F.) when exposed for a period of five minutes. The
results of these tests are summarized below.
Bacteria from an agar slant culture four days old were all killed when ex-
posed for five minutes to a temperature of 60 degrees C. (140 degrees F.).
Bacteria from an agar slant culture seventy-five days old were all killed at 60
degrees C. when exposed for five minutes.
Bacteria from young and old agar cultures were all killed when introduced
into tubes of hot melted agar and kept for three minutes at temperatures ranging
from 87 to 79 degrees C.
Bacteria in soil from soil cultures were all killed when exposed for five min-
utes to a temperature of 60 degrees C.
Bacteria in pieces of dry cloth that had been subjected to the drying air of
the laboratory for twenty-four days were all killed at a temperature of 55 degrees
C. when exposed for five minutes.
Bacteria in the tissue from active canker spots were all killed when exposed
for five minutes at a temperature of 60 degrees C.
EFFECTS OF DRYING-OUT
Bacteria from young and old cultures exposed for two weeks
on glass slips to dry in the air of the laboratory failed to
Pieces of sterilized cloth were wetted with suspensions of
bacteria from cultures of different ages, from four days old to
seventy-five days old. The pieces were then allowed to dry in
the air of the laboratory in the dark. Germination tests from
these pieces of cloth showed a very large number of the organ-
isms alive after a drying period of five weeks. The experi-
ment is still in progress.
These results are significant. The question has been raised
as to whether or not the canker organism could be carried for
18 Florida Agricultural Experiment Station
any length of time in the clothing of workmen employed in
infected nurseries or groves, and if the disease could be spread in
this manner to healthy trees. The above experiment indicates
that these bacteria can be carried for some time in the clothing,
even if they have become thoroughly air-dried, and such clothing
would undoubtedly be a means for spreading infections, under
That the bacteria may live for a month or more in the dried
canker spots, is shown by the disease having been transferred to
healthy citrus tissue from dried leaves that had been kept in the
laboratory a month.
SPREAD OF THE DISEASE
Citrus canker is spread mainly by some carrier. Insects,
birds, animals, or man may be agents in carrying the disease from
infected to healthy trees. Rains and heavy dews aid in spreading
the disease through individual trees. When infected foliage is.
drenched with dew or rain it is in the best condition possible for
disseminating the canker bacteria. Bacteria ooze out in multi-
tudes from cankers or spots that are thoroughly wet, and the
droplets of water on the surface of an infected leaf may be
teeming with the canker organisms. It is an easy matter for the
bacteria to be carried from leaves in this condition by any object
that comes in contact with them. When the foliage becomes
dry, the bacteria stick firmly to the surface by means of a
gelatinous substance with which they are invested.
The disease develops and spreads rapidly during rainy
weather, but it is more or less retarded during periods of drought
or in dry seasons. This was clearly demonstrated by some
infections that occurred on a small grapefruit tree. In October
some canker infected 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 remain 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.
Bulletin 128 19
All infections were removed a few days later, and the results
tabulated as follows.
Leaves, Twigs 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
Old and young leaves collected ---...-----.---------------.-----. ----- 35
Old and young leaves collected ..------------------- -... ---------------- 90
Old and young leaves collected---------..-----------.... --------.....---- --- 10
Total leaves collected from tree ..---------------------- 561
The infection on all these leaves and fruit came apparently
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 prompt and complete destruction of all infected trees is
the only practical method that has yet been found for checking
the spread of citrus canker. Little hope can be entertained that
any other method of treatment will be found in the near future
by which the disease can be kept under control, especially under
the conditions that are found in Florida. High temperatures and
high humidity favor a rapid development and spread of citrus
canker, and these are prevailing factors of the Florida climate.
Bacterial diseases of plants do not yield to treatment with
fungicides, and citrus canker is no exception to the rule. The
standard fungicides and many other mixtures have been given a
trial by growers and nurserymen in the State; but so far all have
failed to completely control the disease. This is not because the
canker bacteria are especially resistant to fungicides, but rather
because it is not possible to keep every part of the citrus tree
constantly protected with the fungicide. Cankers or spots on
the foliage may be kept covered with a coating of Bordeaux or
other fungicide, but this will not affect the bacteria within such
20 Florida Agricultural Experiment Station
spots, and they will continue to grow and spread just the same.
The bactericides or antiseptics at present known can not be used
for controlling this disease in a practical way, owing to their
caustic nature and the serious injury that would result from their
use on the citrus tree.
The following are the usual methods employed against
1. Removal and destruction of all infected plants or infected
parts of plants.
2. A rotation of crops where the organisms infest the soil.
3. The production of varieties that are resistant or
immune to the particular bacterium. Rotation of crops is out of
the question when applied to the citrus grove. The production
of citrus varieties immune or resistant to canker may require
years of effort, and such immunity may prove only temporary.
A complete destruction of all infected trees is apparently the
only course open to follow in order to eradicate the disease.
Since the canker bacteria are perhaps capable of living and
growing in unsterilized soil, for a long period, the problem of
control is complicated. Infected soil beneath a tree might be a
source from which the disease could be easily carried back into
the tree by insects. Spraying such soil with fungicides would
not help matters, and there is no method of soil treatment
available for eradicating the bacteria without seriously injuring
the growing tree.
Thorough eradication of the disease from every locality in
which it has appeared seems to be the only method to pursue in
this State in order to protect the citrus industry. All infected
trees should be immediately destroyed by burning, and even
trees that have been exposed to the disease may be destroyed
with advantage. Such trees should be burned in the grove as
they stand, in order to avoid any handling or contact that might
result in spreading the disease to new localities. Since the
canker organisms possibly infects the soil, it is a dangerous
policy to plant citrus trees on land where canker has been found.