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COOPERATIVE WORK FOR
ERADICATING CITRUS CANKER
'-IA
BY
KARL F.IELLERMAN, lus
Assocet fBureau of Plant Industry6
CONTENTS
Introduction of the Disease . . . . .
Disease Characteristics and Early Control Efforts .
Drastic Control Measures Necessary . . . .
Thorough Disinfection Practiced . . . .
The Spread of the Disease . . . . . .
Eradication Contingent upon Continued Efficient Work
Separate from Yearbook of the
Department of Agriculture, 1916
No. 711
WASHINGTON : GOVERNMENT PRINTING OFFICE : 10ir
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COOPERATIVE WORK FOR ERADICATING CITRUS
CANKER.
By KARL F. KELLERMAN,
Associate Chief, Bureau of Plant Industry.
INTRODUCTION OF THE DISEASE.
F OR a little more than two years the Federal Govern-
ment and the Gulf States have been engaged in a joint
,campaign for the purpose of eradicating from the United
States the disease of citrus fruit and trees called citrus canker.
This undertaking is unique in character in that it is the first
instance of the use of Federal funds appropriated specifi-
cally for the eradication of a plant disease. It is of over-
whelming importance to the citrus industry, because citrus
canker has been recognized as the most contagious of all
known plant diseases and the most destructive of commer-
cial values.
The origin of the disease is obscure. It appears probable
that it is native in Chosen (Korea) or in south China and
that from China it has been carried to Japan during rather
recent years, but there appears to be no doubt that it has
been introduced into this country direct from Japan. The
first observation regarding a plant disease which pre-
sumably was citrus canker is with reference to nursery.
stock introduced into Texas in 1911. It is not improbable
that earlier shipments of nursery stock were infected, and
it is certain that many later shipments of Citrus trifoliata
orange seedlings from Japan, both into Texas and into other
Gulf States, were infected.
DISEASE CHARACTERISTICS AND EARLY CONTROL
EFFORTS.
Citrus canker is primarily a leaf-spot and fruit-spot, al-
though it also affects twigs and even old bark and wood.
In its early stages, however, it resembles the sour-scab of
citrus trees, a troublesome but not an especially serious dis-
ease that is widely prevalent in the South. Until late in the
year 1913 plant pathologists and nurserymen did not clearly
distinguish between these two diseases, and, therefore, prior
to its recognition and the determination of its' serious char-
acter, the shipment of infected nursery stock was probably
taking place throughout the southern areas where citrus cul-
ture was being extended.
75892-17 1
2 Yearbook of the Department of Agriculture.
During the seasons of 1913 and 1914 special efforts were
made by State nursery inspectors, by nurserymen, and by
citrus growers to check the spread of the disease by complete
defoliation of infected stock followed by immediate and
thorough spraying with strong Bordeaux mixture and by
painting visible infections with Bordeaux paste. These
treatments were ineffectual, however, and citrus growers in
southeastern Florida became so concerned over the rapid and
destructive spread of citrus canker and the failure of the
methods usually employed for controlling plant diseases
that they originated the plan of spraying infected trees with
burning oil, thus completely destroying them. Eradication
work of this character was undertaken immediately and
financed almost entirely by private subscriptions, but the
disease appeared to be gaining upon the forces attempting to
control it. Recognizing that a severe epidemic menaced the
citrus industry and that neither the citrus industry nor the
States concerned were prepared to deal promptly and ade-
quately with this emergency, on December 4, 1915, the Sec-
retary of Agriculture suggested for consideration by the
Congress the desirability of the immediate appropriation of
sufficient funds to cooperate during the winter and spring
-with State officials, organizations of growers, and individ-
uals to continue the eradication campaign under way in
Florida and to organize similar inspection and eradication
campaigns in the other States believed to harbor citrus
canker.1 As a further protection, under date of December
10, 1914, the Secretary of Agriculture promulgated a quar-
antine, effective January 1, 1915, that prevents the introduc-
tion into the United States from all foreign countries of
citrus nursery stock, including buds, scions, and seeds.
An immediate and rapid inspection of the Gulf region by
an agent of the Federal Horticultural Board of the Depart-
ment of Agriculture, supplemented by reports from State
officials, indicated the occurrence of infected citrus trees in
more or less widely separated and sharply defined localities
from southeastern Florida to southern Texas.
The cooperative work of inspecting citrus groves and
nurseries and destroying infected trees was begun in Florida
immediately upon the approval of the urgent deficiency act
1The urgent deficiency act approved Jan. 25, 1915, provided $35,000 for this
purpose.
Cooperative Work for Eradicating Citrus Canker. 3
of January 25, 1915, and shortly afterwards cooperative
agreements for similar work were made with Alabama,
Texas, Louisiana, and Mississippi.
Meantime the disease has been critically studied both
in the laboratories of the Department of Agriculture and
the State experiment stations, and its cause, though twice
erroneously reported to be a fungus, has been definitely
proved to be a bacillus new to science, which is apparently
unable to infect plants other than the species of the genus
Citrus or its close relatives. This new bacillus has been
named Pseudomonas citri. The spots, or cankers, occur-
ring on leaves, twigs, or fruit are not especially difficult to
recognize. As shown in Plates I and II, the cankers may
vary from less than one-eighth to about one-fourth of an
inch in diameter, and they may occur on green fruit (espe-
cially of lemon, grapefruit, and orange), on the bark (espe-
cially of young twigs), and upon leaves. By far the
greater number of cases are found at first affecting only the
leaves. The spots are reddish brown, raised slightly above
the level of the healthy surface, and frequently are sur-
rounded by a rather indistinct narrow yellowish zone.
Leaves are especially characteristic, for, as shown in Plates
III and IV, the spots go clear through the leaf and are
almost equally prominent on the upper and lower surfaces.
They may be slightly mottled, and usually the older spots
at least have broken through the thin outer layer or epider-
mis of the leaf. Before breaking through the leaf surface
the cankers are smooth and almost waxy, but they after-
wards have a corky appearance.
DRASTIC CONTROL MEASURES NECESSARY.
If spots or cankers fitting this description are found in
any citrus grove, the owner of the property should at once
take all possible precautions to avoid spreading the disease,
lest it prove to be citrus canker. He should prohibit
all visiting to that or adjacent groves, stop all cultivating or
other work, and should send for a State specialist to defi-
nitely determine the disease. If this is not possible, a few
infected leaves may be picked off, wrapped in paper, sealed
1 Aid may be requested from Wilmon Newell, Plant Commissioner, Gaines-
ville, Fla.; G. C. Starcher, State Horticulturist, Auburn, Ala.; R. W. Harned,
State Entomologist, Agricultural College, Miss.; E. Lee Worsham, State Ento-
mologist, Atlanta, Ga.; J. B. Barrett, State Entomologist, Agricultural Experi-
ment Station, Baton Rouge, La. ; Ed. L. Ayers, Chief Nursery Inspector, Hous-
ton, Tex.
4 Yearbook of the Department of Agriculture.
in a heavy envelope, and forwarded to State authorities or to
the United States Department of Agriculture. After pick-
ing these infected leaves, the owner should thoroughly wash
his hands in a disinfecting solution.
If a tree is found to be actually infected with citrus canker
the wisest course to pursue is to burn it at once. Although
this treatment is extremely severe, it is the only one which
has been found practicable and effective in checking the
spread of the disease. This method of eradication, first used
in Florida in the autumn of 1914, has been adopted through-
out the entire region where citrus canker has been found,
and additional safeguards not at first recognized as neces-
sary are now employed.
THOROUGH DISINFECTION PRACTICED.
Inspectors for citrus canker are required to wear suits
similar to the overalls worn by thrashing crews or by garage
mechanics, completely covering their usual clothes. Leggins
and canvas hats are also required. These must be completely
and thoroughly disinfected in a 1 to 1,000 solution of bi-
chlorid of mercury before entering citrus properties and
upon leaving citrus properties, and at the same time the in-
spectors must thoroughly disinfect their shoes, hands, and
faces. The inspectors are instructed to avoid touching any
citrus trees when inspecting them, though if it becomes
necessary to move a limb in order to thoroughly inspect a
tree, this can be done with especial precautions to avoid as
far as possible the chances of spreading contagion from dis-
eased spots as yet not visible to the naked eye. All appa-
ratus taken into citrus groves, such as oil cans and special
pumps for spraying oil or formalin, etc., must be thoroughly
sprayed with disinfectants before being taken from the
orchard. Upon finding infected trees, the ground under the
tree should be sprayed thoroughly with a 5 per cent solution
of formalin, and especially in properties where grove trees
are of considerable size the spraying- with a 1 per cent
formalin solution of all apparently healthy trees adjacent
to infected trees which are to be destroyed is advised.
These precautions are more extreme than have been found
necessary in fighting any other plant disease, yet they are
necessary on account of the extreme infectiousness or con-
tagiousness of citrus canker; and partly because of this the
success of the campaign for the eradication of citrus canker
Yearbook U. S. Dept. of Agriculture, 1916.
CITRUS-CANKER INFECTIONS ON THE LEAVES, YOUNG WOOD, AND
FRUIT OF GRAPEFRUIT.
PLATE 1.
Yearbook U. S. Dept. of Agriculture, 1916.
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CITRUS-CANKER INFECTIONS, SHOWING THE LIGHT-BROWN, SPONGY,
RAISED, CHARACTER FOUND CHIEFLY IN WARM MOIST WEATHER
AND SHOWING THE MORE FLATTENED DARK SPOTS FREQUENTLY
FOUND DURING THE WINTER OR COOL, DRY WEATHER.
PLATE I1I.
Yearbook U. S. Dept. of Agriculture, 1916.
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UPPER SURFACE OF A PORTION OF A GRAPEFRUIT LEAF ENLARGED
2.5 DIAMETERS TO SHOW THE RAISED CHARACTER OF CITRUS-
CANKER INFECTIONS.
PLATE III.
Yearbook U. S. Dept. of Agriculture, 1915.
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LOWER SURFACE OF A PORTION OF A GRAPEFRUIT LEAF ENLARGED 2.5
DIAMETERS TO SHOW THE RAISED CHARACTER OF CITRUS-CANKER
INFECTIONS.
PLATE IV.
Cooperative Work for Eradicating Citrus Canker. 5
may be expected to establish a new era in preventive and
control work in dealing with plant diseases.
THE SPREAD OF THE DISEASE.
There have been periods during which it has been impos-
sible to continue work in all States because of occasional lack
of funds.1 As a whole, however, the campaign has been
practically continuous up to the present time, and the dis-
tribution of the disease is not greatly different from what an
experienced pathologist should have been able to predict
from the circumstantial evidence more than a year ago.
Knowing that infected nursery stock had been shipped into
Texas in 1911, that suspected stock from Japan had at sub-
sequent periods been shipped into other States, and that
citrus stock exposed to infection in nurseries in each of
the States concerned had been distributed, it is obvious
that the disease must have been widely spread throughout
the citrus-growing territory. As might have been expected,
therefore, the more and more thorough inspections of the
southern citrus territory showed that citrus canker was more
widely distributed than the preliminary observations had
indicated. Severe tropical storms during the two past sea-
sons, in addition to thd usual means of spreading the con-
tagion, considerably increased the number of properties in-
fected. Even at the worst, however, but, a very small frac-
tion of the citrus properties of the South have been in-
fected, and those in California have escaped completely.
Furthermore, the infected properties usually can be cleansed
of the disease before many trees are lost.
The grapefruit, the orange, the lime, and the lemon are
so readily infected with citrus canker that it does not appear
probable that any method except that of complete de-
struction of all infected trees will serve to check the dis-
ease in any locality. With other varieties of citrus, and
especially Satsuma oranges, it appears probable that the
burning off of diseased leaves and branches, immediately
followed by thorough spraying with strong disinfecting
solutions, may arrest the spread of the disease, and careful
Since the autumn of 1914 three Federal appropriations have been made for
cooperative work for the eradication of citrus canker : $35,000 in the urgent
deficiency act of Jan. 25,.1915; $300,000 in the urgent deficiency act of Feb. 28,
1916; and $250,000 in the agricultural appropriation act of Aug. 11, 1016.
The State contributions, being often personal and local in their nature, are not
in all cases completely recorded, but they are estimated to be $3,500 from Ala-
bama, $300,000 from Florida, $2,000 from Georgia, $30,000 from Louisiana,
and $11,000 from Texas; a grand total, from all sources, of $931,500.
6-*. 'Y.-earbook of the Department of Agriculture.
Sand extensive tests of this modified plan are now under way.
Even with Satsuma oranges it appears that the disease is
almost uncontrollable if infected orange, grapefruit, or
trifoliate orange plants are near by. Satsuma trees, on the
other hand, frequently show the disease so slightly at first
that the injuries are almost indistinguishable. Because of
these facts, it is becoming obvious that in regions where
citrus canker is appearing the attempt to grow these differ-
ent varieties of citrus plants on the same property or even
in the same general locality seriously jeopardizes the suc-
cess of either variety.
ERADICATION CONTINGENT UPON CONTINUED EFFICIENT
WORK.
The progress of the inspection and eradication work has
been sufficiently encouraging during the past two seasons to
give rise to the confident expectation of completely eradicat-
ing citrus canker from this country, provided effective work
may be maintained constantly for a period of at least two or
three years longer. All or practically all of the infected and
suspected areas are known, and though it is impossible to
find at once all dormant or undeveloped cases of canker,
even in groves or nurseries where occasional diseased trees
are found and burned, the number of infected trees appear-
ing from month to month is decreasing, and the total number
of infected trees during the past season was smaller than
during the season before, especially in the commercially im-
portant orange and grapefruit regions.
The cost of conducting work of this character is very
heavy; but, in view of the magnitude of the industry in-
volved, the total sums expended to the present time represent
but a small fraction of 1 per cent of the capitalized values
that are threatened, and the continuation of the work
appears to be both essential and well justified.
There remains always the hope that some less drastic
method of combating citrus canker may be discovered, and
therefore experiments are under way in different portions of
the South carefully to test different methods of spraying and
other treatments; it should at least be possible to develop
spraying as an auxiliary method to a point where losses of
trees from secondary infections may be negligible instead
of forming as they now do a very considerable part of the
loss caused by the spread of citrus canker.
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