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 296 April, 1936
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
WILMON NEWELL, Director
BLIGHT A NON-PARASITIC DISEASE
OF CITRUS TREES
By ARTHUR S. ROADS
Fig. 1.-Blight often strikes the best trees in affected groves, making
it necessary to remove them and giving an uneven and "ragged" appear-
ance to the grove. All trees have been removed from a large area of
the above grove, which is on a hammock phase of Norfolk sand on a slope
on the south side of Lake Harris.
Bulletins will be sent free to Florida residents upon application to
AGRICULTURAL EXPERIMENT STATION
EXECUTIVE STAFF BOARD OF CONTROL
John J. Tigert, M.A., LL.D., President of Geo. H. Baldwin, Chairman, Jacksonville
the University Oliver J. Semmes, Pensacola
Wilmon Newell, D.Sc., Director Harry C. Duncan, Tavares
H. Harold Hume, M.S., Asst. Dir., Research Thomas W. Bryant. Lakeland
Harold Mowry, M.S.A., Asst. Dir., Adm. J. T. Diamond, Secretary, Tallahassee
J. Francis Cooper, M.S.A., Editor
Jefferson Thomas, Assistant Editor
Clyde Beale, A.BJ., Assistant Editor BRANCH STATIONS
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager NORTH FLORIDA STATION, QUINCY
K. H. Graham, Business Manager
Rachel McQuarrie, Accountant L. O. Gratz, Ph.D., Plant Pathologist in
R. R. Kincaid, Ph.D., Asso. Plant Pathologist
MAIN STATION, GAINESVILLE J. D. Warner, M.S., Agronomist
Jesse Reeves, Farm Superintendent
AGRONOMY CITRUS STATION, LAKE ALFRED
W. E. Stokes, M.S., Agronomist**
W. A. Leukel, Ph.D., Agronomist A. F. Camp, Ph.D., Horticulturist in Charge
G. E. Ritchey, M.S.A., Associate* John H. Jefferies, Superintendent
Fred H. Hull, Ph.D., Associate W. A. Kuntz, A.M., Assoc. Plant Pathologist
W. A. Carver, Ph.D., Associate B. R. Fudge, Ph.D., Associate Chemist
John P. Camp, M.S., Assistant W. L. Thompson, B.S., Asst. Entomologist
ANIMAL HUSBANDRY EVERGLADES STATION, BELLE GLADE
A. L. Shealy, D.V.M., Animal Husbandman"* A. Daane. Ph.D., Agronomist in Charge
R. B Becker, Ph.D., Dairy Husbandman R. N. Lobdell, M.S., Entomologist
W. M. Neal, Ph.D., Asso. in An. Nutrition F. D. Stevens, B.S., Sugarcane Agronomist
D. A. Sanders, D.V.M., Veterinarian Thomas Bregger, Ph.D., SugarcanePhysiologist
M. W. Emmel, D.V.M., Veterinarian G. R. Townsend, Ph.D., Assistant Plant
N. R. Mehrhof, M.Agr., Poultry Husbandman Pathologist
W. W. Henley, B.S.A., Asst. An. Hush.* J. R. Neller, Ph.D., Biochemist
W. G. Kirk, Ph.D., Asst. An. Husbandman R. W. Kidder, BS., Assistant Animal
R. M. Crown, M.S.A., Asst. An. Husbandman Husbandman
P. T. Dix Arnold, B.S.A., Assistant Dairy Ross E. Robertson, B.S,, Assistant Chemist'
Husbandman B. S. Clayton, B.S.C.E., Drainage Engineer*
L. L. Rusoff, M.S., Laboratory Assistant SUB-TROPICAL STATION, HOMESTEAD
Jeanette Shaw, M.S., Laboratory Technician
CH Y AD S S H. S. Wolfe, Ph.D., Horticulturist in Charge
CHEMISTRY AND SOILS W. M. Fifield, M.S., Asst. Horticulturist
R. W. Ruprecht, Ph.D., Chemist** Geeo. D. Ruehle, Ph.D.. Associate Plant
R. M. Barnette, Ph.D., Chemist Pathologist
C. E. Bell, Ph.D., Associate W. CENTRAL FLA. STA., BROOKSVILLE
R. B. French, Ph.D., Associate
H. W. Winsor, B.S.A., Assistant W. F. Ward, M.S.A., Asst. An. Husbandman
ECONOMICS, AGRICULTURAL in Charge *
C. V. Noble, Ph.D., Agricultural Economist**
Bruce McKinley, A.B., B.S.A., Associate FIELD STATIONS
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Assistant Leesburg
ECONOMICS, HOME M. N. Walker, Ph.D., Plant Pathologist in
Ouida Davis Abbott, Ph.D., Specialist** W. B. Shippy, Ph.D., Asso. Plant Pathologist
C. F. Ahmann, Ph.D., Physiologist K. W. Loucks, M.S., Asst. Plant Pathologist
ENTOMOLOGY J. W. Wilson, Ph.D., Associate Entomologist
J. R. Watson, A.M., Entomologist** PPlant P Pathol
A. N. Tissot, Ph.D., Associate A. N, Brooks, Ph.D., Plant Pathologist
H. E. Bratley, M.S.A., Assistant Cocoa
HORTICULTURE k. S. Rhoads, Ph.D., Plant Pathologist
A. F. Camp, Ph.D., Horticulturist* Hastings
G. H. Blackmon, M.S.A., Horticulturist and A. H. Eddins, Ph.D., Plant Pathologist
Associate Head of Department
A. L. Stahl, Ph.D., Associate Monticello
F. S. Jamison, Ph.D., Truck Horticulturist G. B. Fairehild, M.S., Asst. Entomologist*
R. J. Wilmot, M.S.A., Specialist, Fumigation
R. D. Dickey, B.S.A., Assistant Horticulturist David G. Kelbert, Asst. Plant Pathologist
PLANT PATHOLOGY C. C. Goff, M.S., Assistant Entomologist
W. B. Tisdale, Ph.D., Plant Pathologist** Sanford
George F. Weber, Ph.D., Plant Pathologist E. R. Purvis, Ph.D., Assistant Chemist,
R. K. Voorhees, M.S., Assistant"** Celery Investigations
Erdman West, M.S., Mycologist
Lillian E. Arnold, M.S., Assistant Botanist Lakeland
Stacy O. Hawkins, M.A., Assistant Plant E. S. Ellison, Ph.D., Meteorologist*
Pathologist B. H. Moore, A.B., Asst. Meteorologist*
SPECTROGRAPHIC LABORATORY In cooperation with U.S.D.A.
L. W. Gaddum, Ph.D., Biochemist **Head of Department.
L. H. Rogers, M.A., Spectroscopic Analyst *** On leave.
INTRODUCTION ----........................-.......-- .......--..... ..---...-.......--- ... --................. 5
THE DISEASE..........-......... .---........-- -................ 6
Distribution and Economic Importance-.....---.......-.. ......-..........................- 6
Description and Symptoms----..........--- ....-------- .--.......... ...... 13
General ................................ ....... ....- ................ ........ 13
Early Symptoms ...................---..--.-...--- --............-16
Effect on Fruit........----...---. ........-- ---... .................. 18
Effect on Blooming................... --... ....---- ..---.....--....19
Effect on Growth ..........................------- ..--.--- .----........... .....-... 19
Distinction Between Blight and Other Forms of Wilting of Citrus Trees 21
Cause of Blight ........................................................ -- -............. 25
Propagation of Blighted Trees..................-- ..---..-- .......... 28
Budding and grafting experiments...............-- ..---.--...-- ..-.....--- 28
Grove tests of budded trees --.... ......------.....----........-- ... 31
Root cuttings .......... .... ........ ----------------....----...... 36
Transplanting blighted trees.......................................... ... .. 36
Correlation of Blight Incidence with Various Factors.--.---................. 38
Soil conditions and grove practices-........................----. -...... 38
Soil types -------.......-- --.................................................. ................... 39
Proximity of rock and other structural defects---....--..........--...- .... 40
Soil moisture ..... ------..... ..-- ........-- ................. 44
Kind of tree and rootstock-................------- -.......--............-- .....54
Use of commercial fertilizers.-.......--....--..----. -----......... 56
Other factors .......----.....- ..---- ...... .....--- ....... 56
CONTROL ... ......................------.................. ..................... 57
SUMMARY AND CONCLUSIONS ---.......----.--........--- --..... ---- ............. 59
ACKNOWLEDGMENTS ................. ------ ......-.. ----..-....--..--....... 61
LITERATURE CITED ............. --.................. -.. --.-.. ...... ...- .......-- .. 61
o 0 oC
(5 0 ( ( D .
o o o
ao o" 0
N o /
"* N N b eg @
no g ad .
stgo of b the wo o a
Sn bee r o ay a
o N N a00 0 0 o o
rersn O oa g r a set out
N @0 G0
N N N N > N
e i 3 1N 1 0 3 1 0
o o o o + o
N j 00
1916 on a poor phase of Gainesville sand.
Mapped in May 1933 to show the high mor-
tality from blight. Legend: indicates trees
not exhibiting blight; X1, X1, and Xs indicate
trees showing early, intermediate and late
stages of blight, those with circles around
them having been removed in July and Octo-
ber; the numbers 0, 1, 2, 3, 4, and 5 in circles
represent Pineapple orange replants set out
respectively in 1933, 1932, 1931, 1930, 1929, and
1928 to replace treesshow there removed on
account of decline from blight.
account of decline from blight.
BLIGHT A NON-PARASITIC DISEASE
OF CITRUS TREES
By ARTHUR S. ROADS
"Blight" is a term applied by Florida growers, at least as
early as 1883, to a peculiar chronic wilt and decline of citrus
trees. Since that time the disease has also been variously
designated as orange blight, citrus blight, limb blight, wilt, dry
wilt, and leaf curl. Within recent years, however, the term
"blight" has been used by growers, who are not especially
familiar with this particular trouble, to refer to other types of
decline of citrus trees of obscure cause, regardless of whether the
trouble is characterized by a chonic wilting of the foliage. As
a result, the term "blight" as used by citrus growers at present
is often misleading. In this bulletin use of the term "blight"
is limited to that chronic wilting and decline of citrus trees which
was clearly described first by Underwood (45)1 in 1891, and
by Swingle and Webber (42) in 1896.
When the investigations reported herein were begun it was
found that, aside from the description of the symptoms and the
inability to treat blighted trees successfully, there were few
points of general concord of opinion and few facts established
concerning the disease. Although there are numerous popular
reports and discussions of blight by growers to be found in
agricultural periodicals, only the most meager information had
been published on the observations and experimental work of
scientific investigators. Since most of this old literature com-
prises only discussions of popular nature, it is of little practical
value, other than to record the occurrence and distribution of
the disease in the State in the early days of the citrus industry
and to show the importance with which it was regarded by the
former generation of growers. Furthermore, many old groves
referred to, as well as many growers who wrote the articles,
could not be located.
Methods of managing groves have been changed greatly within
the last few years and it is now difficult to evaluate reports
made on this disease in former years and to determine exact
conditions existing then. Consequently, it was necessary to fol-
1 Figures (Italic) in parentheses refer to "Literature Cited" in the back
of this bulletin.
6 Florida Agricultural Experiment Station
low the advice given by Webber to Rolfs (32), when he took up
the investigations on blight at the Sub-tropical Laboratory of
the U. S. Department of Agriculture at Miami -in 1901, and begin
at the bottom. However, some of the earlier ideas concerning
the trouble which are recorded in the literature are cited be-
cause of historical interest. The primary purpose is to report
the complete results of experiments and observations conducted
with blight during the past 12 years, progress reports of which
have already been given (30, 31).
DISTRIBUTION AND ECONOMIC IMPORTANCE
Just when and in what part of Florida blight first made its
appearance is not known. The earliest allusion to it that has
been found is that of Manville (23) in 1883. It is evident from
his account that this disease had then been known to older
settlers for a number of years. An earlier reference to "orange
blight" in Florida by Taylor (43) in 1873 proved to relate to
the so-called rust disease, now known as exanthema or dieback.
The reports of Prince (29), J. H. S. (35), Moore (27), West-
lake (51), Hart (17, 18), Williams (52), Bean (3), the anony-
mous account by a reporter of the Leesburg paper (1), Brown
(7, 8), C. S. Brown & Co. (9), and Waite (46, 47) made it
evident that blight was known to occur between 1885 and 1890
in what are now Lake, Orange, Seminole, Volusia, Putnam, Man-
atee and Brevard counties, and that it was beginning to cause
considerable alarm among citrus growers in some sections.
These records, together with published statements by Swingle
and Webber (42) and others, show that blight was of rather
general distribution throughout what was the principal orange
growing region of Florida prior to the disastrous freezes of
1894-95, occurring in the oldest and best groves and on land
that was considered to be best suited for profitable orange cul-
2 Prior to the destructive freezes of 1894-95, the orange (both seedling
and budded trees) and the lemon to a much less extent were the only
commercially grown citrus fruits. The pomelo, or grapefruit, while fre-
quently planted about the houses of the old settlers for shade and orna-
mental purposes, was more of a curiosity than a useful fruit, although it
was considered refreshing and of medicinal value. Only a few commercial
plantings of this tree were made prior to these freezes and it was not until
afterward that this fruit began to come into extensive commercial promi-
nence. The lemon industry, due largely to the severity of citrus scab under
Florida climatic conditions, was never rebuilt after these freezes.
Blight-A Non-Parasitic Disease of Citrus Trees 7
In his account of blight in 1891, Underwood (45) briefly
described the situation as follows:
"Bad cases of this disease are found as yet in only a few local-
ities where the orange groves have long been established. While
it is evidently not a new disease its ravages have only recently
extended sufficiently to give alarm to cultivators. All things
considered, this disease is the most dangerous that has yet ap-
peared among the orange groves, and a study of its causes and
cure demands immediate attention."
In the account by the Leesburg paper in 1891 (1), it is stated
that some groves in that section were said to have been damaged
by blight alone to the extent of $1,500 and upwards.
Remarks during the discussion of blight at the meeting of
the Florida State Horticultural Society in 1891 reflected the
increasing alarm with which blight was viewed by growers
familiar with it. It was brought out that blight was becoming
more frequent in its occurrence throughout the orange regions
of the State and that it had proved fatal in every instance. Since
from 50 to 75 percent of the older trees in certain groves were
cited by Williams (53) as having died from blight some growers
believed that if this disease was not checked it would attack
all the groves in the State.
Swingle and Webber (42), after making a detailed study of
blight, estimated the annual loss to citrus growers from the
ravages of this disease prior to 1895 at from $100,000 to $200,000
and the total loss of the 10-year period preceding that time at
several million dollars. They stated that in some localities from
1 to 10 percent of the trees blighted annually. Since many of
the orange groves in which blight was very prevalent were killed
by the freezes of 1894-95, the disease caused little concern among
growers for a period of several years. However, after the old
trees had renewed their growth, blight not only reappeared in
these old groves, but also made its appearance in many of the
newer grove plantings after they came into heavy bearing.
In 1902 Waite (47) stated that about 100 trees had been re-
moved in the past two years on account of blight from groves
containing 1,200 large seedling orange trees located on Terra
Ceia Island in Manatee County. Porcher (28) in 1903 reported
that blight was becoming quite prevalent again in Brevard
County. In 1907 Bessey (4) mentioned that "blight" or "wilt"
had continued to spread slowly through the State and added that
it had now taken hold firmly in the lower end of Merritt's Island
and was doing great damage to the groves there. He also men-
8 Florida Agricultural Experiment Station
tioned that in a few localities he had found the disease attacking
not only the orange but also pomelo (grapefruit) and tangerine
as plantings of these trees increased in age, although the most
serious injury was done to the orange. As late as 1908 Hart
(19), a citrus grower, considered blight the most serious disease
with which citrus growers had to contend. Fawcett (15) pointed
out that the annual loss in 1909 probably exceeded the estimate
quoted by Swingle and Webber in 1896.
With the passage of time, however, and the development of
thousands of new groves farther south, general opinion in regard
to the increasing prevalence and destructiveness of blight evi-
dently changed considerably, for no further publicity was given
to this trouble until 1923. While the disease continued to be
destructive in a number of localities where it had occurred
formerly, it evidently did not prove troublesome in general to
citrus growers. In his account of blight, given in his bulletin
on Florida citrus diseases in 1918, Stevens (38) stated that it
was much less in evidence than formerly, although it still oc-
curred in certain localities.
In 1923 Burger (11) stated that blight was serious in certain
areas of the State and that in several groves along the East
Coast a third of the trees remained unproductive. In the same
year Bragdon (6) gave specific figures in regard to tree losses
from blight in groves in the vicinity of Cocoa, Brevard County,
and emphasized the importance of this disease and the need for
its investigation. He stated that in one grove of about 5,000
trees, in which the trees developing blight were removed
promptly after the disease made its appearance, approximately
2 percent of the total were affected by blight each year. In an-
other grove where no consistent attempt was made to remove
blighted trees promptly, the manager found that of a total of
6,600 trees, 1,500 or nearly 25 percent were dead or dying from
blight when he assumed charge. In one particular block of 990
trees, he found 333 or 33 percent in this condition. On a basis
of these observations, Bragdon pointed out the possibility of
blight becoming more general in comparatively young groves
in other parts of the State as these groves became older and
more susceptible to the trouble and stated that growers might
perhaps again be losing from 1 to 10 percent of their trees
annually, as was the case previous to 1895.
It was by reason of the great prevalence and economic im-
portance of blight in the central part of the Indian River section
that Cocoa, in Brevard County, was selected as headquarters for
Blight-A Non-Parasitic Disease of Citrus Trees 9
the writer's investigations on this disease. In the groves of this
county this trouble has been a serious factor in grove manage-
ment since 1890 and still continues to be.
Soon after the work was inaugurated; a careful survey was
made of the prevalence of blight in several groves in Brevard
County. The results obtained in these cases may be regarded
as indicative of the severe losses sustained under the most out-
standing examples of blight coming to the attention of the
writer and are by no means exceptional cases. Figures obtained
in the summer of 1924 on the worst half of a grove at Bona-
venture on Gainesville sand more or less closely but irregularly
underlaid by coquina rock are shown in Table 1.
TABLE 1.-PREVALENCE OF BLIGHT IN 1924 IN A BLOCK OF GROVE AT BONA-
VENTURE, WHERE A LARGE NUMBER OF BLIGHTED TREES WERE REMOVED
Classification of Trees Orange Grapefruit Tangerine Kumquat
Blighted trees removed during 81 0 0 0
the summer of 1924 and not (mostly
yet replanted orange)
Trees so badly affected by
blight that they should have 10 0 0 0
Trees showing more or less
evidence of blight. I 65 9 3 2
Trees not exhibiting blight. 1 340 | 157 6 8
Replants where blighted trees
(mostly orange) had been 238 0 0 0
removed in the past. These
trees too young to develop
Total number of trees tabu-
lated in area inspected-919 734 166 9 10
Of the 919 trees tabulated in the area inspected, 408 or 44.4
percent were either affected by blight or had been removed on
account of this trouble. The percentage of orange trees affected
by blight obviously would be still greater. It was the policy of
the manager of this grove to remove each summer those trees
that had declined to the point where they were worthless com-
mercially. This grove was well fertilized and was being cared
for as well as almost any grove in this section of the State. With
the exception of a few scattered seedling orange trees and the
kumquat trees, which were on rough lemon stock, the trees
were all on sour orange stock.
10 Florida Agricultural Experiment Station
A similar high rate of mortality from blight was found in
a grove studied at Tropic, on the lower end of Merritt's Island,
where virtually no effort had been made to remove declining
trees as long as any life remained in them. This grove likewise
occurred on Gainesville sand closely underlaid by coquina rock.
Practically all trees on one-half of the grove had died. Figures
were secured from a survey of the better half during the summer
of 1924 and are shown in Table 2.
TABLE 2.-PREVALENCE OF BLIGHT IN A BLOCK OF GROVE AT TROPIC, MER-
RITT'S ISLAND, WHERE BLIGHTED TREES WERE NOT REMOVED AS THEY
DEVELOPED THE DISEASE.
Classification of Trees Orange Grapefruit Tangerine
Trees removed on account of blight and
not yet replanted 2 0 0
Trees so badly affected by blight that
they should have been removed 65 0 0
Trees showing more or less evidence of
decline from blight. Many of these 54 21 4
became worthless by the next year. _
Trees not exhibiting blight I 93 136 2
Replants where blighted trees (mostly
orange) had been removed. These trees 16 4 0
too young to develop blight yet. I
Total number of trees tabulated in area
inspected-397 230 161 6
Of the 397 trees 166, or 41.8 percent, either were affected by
blight or had been removed on account of it. Here again the
percentage of orange trees obviously would be still greater.
The trees were all on sour orange stock. Apparently the grove
was being fairly well fertilized but received no other care except
flathoeing once or twice a year. During the succeeding nine
years the trees in both, but especially the one at Tropic, con-
tinued to die rapidly from blight. After about 1927 the grove
at Tropic received little and eventually no fertilization and no
cultivation whatever and had mostly died out by 1929. The
grove immediately adjoining on the south was neglected at the
time and was dying rapidly but was taken in hand by a purchaser
in 1926 and given good care and attention. Many of the older
trees that had developed blight were removed and others of the
older trees have developed this trouble subsequently. However,
this grove, which formerly looked quite forlorn in comparison,
with the moderately well-cared-for grove adjoining it on the
north, is now in good condition.
Blight-A Non-Parasitic Disease of Citrus Trees 11
A much higher rate of mortality from blight was found later
in a much younger grove block at Bonaventure, where trees
rendered worthless by this disease were removed each year. This
grove was on a very light phase of Gainesville sand where coquina
limestone occurred within 21/2 feet of the surface under a few
trees and at depths of three or more feet under a large percentage
of the trees examined. This irregular block was cleared about
1916 and planted to Lue Gim Gong orange trees on sour orange
stock. The freeze of 1917 damaged this planting extensively
and many of the trees had to be rebudded or replanted. Despite
fairly good care, the trees were rather small for their age, owing
to the general droughtiness of the soil.
A detailed survey and analysis of this block of grove was
made early in May 1933, when the trees were about 16 to 17
years from planting. A total of 248 trees was tabulated and
69, or 27.8 percent, had been removed on account of decline
from blight and were replanted with Pineapple orange trees;
78, or 31.5 percent, were in various stages of decline from
blight; and 101, or 40.7 percent, were apparently free from blight.
This makes a total of 147 trees, or 59.3 percent of the planting,
that developed blight. Forty-three of the worst blighted trees
remaining at the time of the survey were removed during July
and 23 more in October. Judging from the size of the replants,
which were examined in company with the grove manager, one
was made in 1928, seven in 1929, eight in 1930, 11 in 1931, 19
in 1932, and 23 in 1933. Since it was customary to replant
the next year after removing the trees, it is evident that the first
tree was removed in 1927, when the grove was about 10 to 11
years old, and that trees continued to develop blight with in-
creasing rapidity each year. A diagrammatic plot of this grove
block is given in Figure 2, which depicts the distribution of
the trees that have developed blight. This affords striking evi-
dence that blight is quite uniformly distributed throughout this
block of the grove. The mortality from blight in this planting
is the highest observed.
The block of grove dovetailing irregularly into the east side
of this block was an old grove on sour orange stock which had
virtually died out from blight and had been replanted at various
times with Pineapple orange trees on the same rootstock, many
of them being set in 1916. This block exhibited a heavy mortal-
ity from blight among the second crop of trees.
Blight not only destroys hundreds of fine bearing citrus trees
annually in Brevard County alone, but the necessity for re-
12 Florida Agricultural Experiment Station
moving and replacing declining trees rapidly destroys the uni-
formity and beauty of the groves (Fig. 1) and results in having
trees of mixed ages, all of which greatly detracts from the
market value of the groves. One grower at City Point, who
purchased a fairly old 12-acre grove, stated that he removed
293 blighted trees upon acquiring it and that, during the nine
years that he has owned it, he has removed a total of approxi-
mately 800 trees. Another grower on an adjoining property is
reported (2) to have found that blight is taking almost 100
percent toll of trees reaching 25 years of age. The acute loss
in fruit production entailed by the replacement of trees at a
comparatively young age can readily be imagined. The trees in
these groves are on sour orange stock, as are practically all trees
in this section.
Extensive observations during the past 12 years indicate that
although blight occurs sporadically throughout a large part of
the citrus region of Florida, apparently it continues to be of
major importance in only a few sections of the State. Blight
occurs most abundantly at present in certain sections of the
central portion of the East Coast, especially in the eastern, or
coastal, section of Brevard and Volusia counties on those types
of soils more or less closely underlaid by coquina rock. Formerly,
it also occurred frequently on the limestone lands of Dade County
but, as a result of the rapidly increasing demand for home-sites
in this section during the past several years, practically all of
the older groves have been cut up into subdivisions. In the
central part of the peninsula, blight is undoubtedly of most fre-
quent occurrence in Lake County, where it has caused heavy
losses for many years (Fig. 1). It has been observed to occur
in one isolated grove as far north as seven miles south of
Jacksonville. In the ridge section, where rough lemon stock
is used almost exclusively for budded trees, blight is very rare.3
As a matter of fact, the majority of citrus growers of the State
today are totally unfamiliar with this trouble.
3 A distinctly different form of decline of citrus trees on rough lemon
stock, which has become quite widespread and prevalent in parts of the
ridge section of the State during the last few years, is erroneously termed
"blight" by many growers who are not familiar with the latter trouble.
Unlike typical blighted trees, these declining trees do not exhibit a chronic
wilting of the foliage, but, instead, a very pronounced mottle-leaf or frenched
and small-leaved condition, especially on the ends of the branches. A vigor-
ous growth of sprouts develops from the interior of the trees as the decline
progresses. This trouble also often appears to spread to trees immediately
adjacent to those first developing it.
Blight-A Non-Parasitic Disease of Citrus Trees 13
Blight appears to be confined to Florida. Fawcett (16) stated
that it does not occur in California. Outside of the United States,
a number of different types of wilting and dying of citrus trees
have been reported from several citrus producing countries, but
all appear quite distinct from blight. However, some of them
appear somewhat similar in certain characters. The most im-
portant of these is "mal secco", a disease of lemon, citron and
sour orange trees occurring in Sicily, Greece and Palestine.
Other cases of wilting and dying back of citrus trees reported
have been caused by the withertip and anthracnose fungus,
Colletotrichum gloeosporioides, and still others have been in-
duced by water-logging of the soil, drought injury, and by wind-
burning or scorching as a result of warm desiccating winds.
The orange blight mentioned by Savastano (36) in 1888 proved
to relate to a rind spotting of citrus fruits in Europe. In 1908
Cook and Horne (13) reported from Cuba six cases of what
they considered blight. In 1918 Stevenson (39) reported a de-
cline of grapefruit trees in Puerto Rico which he stated has been
regarded by some observers as being identical with Florida
citrus blight. Matz (24) referred to what is evidently this same
disease of grapefruit trees near Camp Alegre in Puerto Rico.
In a subsequent report on this disease, Matz (25) considered it
to be a root disease favored by adverse soil conditions. Tucker
(44) reported briefly upon this root trouble, which he found to
be very prevalent along the north coast of Puerto Rico. Sirag el
Din (37) mentioned as one of the common but minor troubles
of citrus in Egypt a blight disease which attacks the branches,
twigs and leaves, and gave an illustration of this trouble on a
young tree. Upon his return to Florida from Brazil, Rolfs
informed the writer in 1934 that he did not see a single case
of citrus tree decline that he regarded as blight during the 13
years that he spent in that country.
DESCRIPTION AND SYMPTOMS
Blight is characterized by a chronic wilting of the foliage of
part or all of the affected tree. It attacks vigorous and pro-
ductive trees and renders them worthless for fruit production
within a period varying from a few months to a few years.
Florida citrus growers have complained for many years that
blight frequently takes their finest trees that have borne the
14 Florida Agricultural Experiment Station
largest crops of fruit. The trouble ordinarily does not develop
in trees until they have attained an age4 of 12 to 14 years and
older and are bearing large crops of fruit. The minimum age
at which blight develops has been placed generally at from six
to seven years by various growers and investigators, although
Swingle (41) stated in 1893 that no cases had come under his
observation where the disease had been contracted by trees less
than five years old. This is probably an extreme minimum, for
Rolfs (34) in 1905 stated that blight rarely or never occurred
on trees under six years of age. It is extremely seldom that
trees develop blight at such early age.
Fig. 3.-A close-up view of side of orange tree (sour orange stock) in
which blight developed first on the lower limbs. Photographed January 31,
1924. By March 15, the affected branches had died and the leaves had dried
up and dropped.
Typical blight in trees that have been well cared for may be
recognized readily by a person familiar with the disease. When
associated with a dying back of trees from insufficient fertiliza-
tion or other causes, however, the symptoms are much less con-
In healthy, productive trees blight usually starts very grad-
4 The age of citrus trees in Florida is expressed by number of years the
budded trees have been set in the grove.
Blight-A Non-Parasitic Disease of Citrus Trees 15
ually with definite symptoms which may be overlooked by the
casual observer or confused with the temporary wilting so com-
mon during dry weather. Many growers have repeatedly claimed
that blight attacks their trees literally over night, but this
is a sure sign that they have not had their trees under very
frequent, or at least very close, observation. Trees may exhibit
Fig. 4.-Valencia orange tree (sour orange stock) showing development
of blight in sector on one side of crown. Photographed on May 18, 1926.
By February 25, 1927, the entire crown exhibited a general wilting of the
16 Florida Agricultural Experiment Station
a more or less decided wilting and curling of the foliage during
dry periods for at least several months or a year before chronic
wilting becomes apparent. Often the foliage on the first few
affected branches dries up and falls, the branches die, and then
no further signs of blight develop for a few months. In such
temporarily arrested cases the trouble may escape detection for
a time. To appreciate properly and follow all stages of the de-
velopment of blight, one must inspect the same trees repeatedly
over a period of weeks, months, and sometimes even a few years.
In these studies numerous photographic records of blighted trees
have been made at timely intervals, accompanied by adequate
field notes, and these have proved an extremely valuable aid in
depicting the development and progress of this disease on
The first visible indication of blight is a gradual wilting and
curling of the foliage which usually starts on the lower branches
of one side of the tree (Fig. 3) and progresses more or less
rapidly until the entire tree is involved.6 Frequently a long
time elapses after the first branches show signs of the disease
before other parts become affected (Fig. 4), although occasion-
ally all portions of the top may wilt simultaneously. This initial
stage of blight is indistinguishable from wilt exhibited by trees
suffering temporarily from drought, except that usually it is
localized on one part of the tree. This wilting may be noticeable
at first only during the middle of hot days, the foliage reviving at
night for several weeks. Ultimately the wilted leaves lose their
normal sheen and become dull and lifeless. In this respect blighted
trees behave in a manner similar to trees transplanted without
cutting back the tops sufficiently. With the citrus tree develop-
ing blight, however, irrigation or generous rainfall merely serves
to arrest the progress of the wilting temporarily (Fig. 5).
Eventually, rain failsto revive the foliage appreciably and finally
the tightly curled leaves become pallid, dry up and fall off, the
blade of the leaf falling first and the petiole, or base, soon there-
after. The twigs and smaller branches lose their leaves first.
The normal greenish color of the defoliated twigs is soon lost
5 In some instances trees bear large crops of fruit the year before they
begin to decline. This, however, appears to be merely a contributing cause
to the development of blight rather than an early symptom of its develop-
ment, as some have regarded it.
Blight-A Non-Parasitic Disease of Citrus Trees 17
and they turn brown, shrivel and die, drying out progressively
backward from the terminal portions (Fig. 6). In cases of
rapid wilting, affected branches may dry up so quickly that
the leaves will curl up tightly, become pallid, and remain more
or less strongly attached for a time, giving these branches the
appearance of having been scorched by heat from fire. This
manifestation doubtless was responsible for the term "white
wilt" having been applied to the disease by growers several
Fig. 5.-Early stage of blight on Pineapple orange tree (sour orange
stock) on Gainesville sand. Tree has shown a progressive wilting and
curling of the foliage throughout the crown for several weeks. Photo-
graphed November 28, 1924.
18 Florida Agricultural Experiment Station
Fig. 6.-Photograph of blighted orange tree shown in Fig. 5, made on
August 3, 1925, to show marked progress of blight despite an abundance
EFFECT ON FRUIT
If blight develops sufficiently early in the season, affected
branches die and the fruit withers and drops before attaining
maturity. Even though the affected branches may not wither
until in January or February the fruit, having attained maturity
by this time, is inclined to drop easily after it begins to lose
its turgor. Occasionally the fruit on declining trees becomes
more or less pyriform in shape. The same phenomenon has
been noted also in trees not exhibiting blight but suffering from
drought. Branches that appeared perfectly healthy in summer
Blight-A Non-Parasitic Disease of Citrus Trees 19
often become completely defoliated and shed the fruit by October
or November and the wood dies. When the affected branches
do not die too early in the season, the fruit borne on them may
attain maturity but a large percentage of it frequently remains
below commercial size and strongly acid in taste, while the un-
affected parts of a blighted tree may continue to bear a fair
amount of good fruit until they become affected.
EFFECT ON BLOOMING
Blooming of blighted trees or parts of them is greatly delayed,
starting from a month to six weeks or sometimes even longer
after the normal blooming season. Many such trees continue to
bloom more or less irregularly over a period of about a month
and even develop a small amount of bloom on individual branches
at intervals during the early summer months. In rare instances
trees that were largely dead except for a mass of watersprouts
have been observed to develop a small amount of bloom in Decem-
ber, the time of year when girdled trees or branches are com-
monly stimulated to blossom production.
Trees that have died back extensively from blight and are
in a greatly devitalized condition often exhibit a strong tendency
to develop an abnormally heavy bloom. In such cases nearly all
of the buds have been stimulated to develop into fruit buds.
This heavy bloom on the devitalized branches usually is char-
acterized by abnormally small and weak blossoms, which set
very little fruit. The overtaxed branches, which seem to have
expended their last reserve in developing the bloom, usually
wither and die soon after blooming.
EFFECT ON GROWTH
The spring flush of growth6 of blighted trees is delayed and
usually is marked by reduction in amount. This growth usually
develops at or about the time the adjacent healthy trees are in
bloom. The amount of new growth produced depends upon the
extent to which the tree has been devitalized. It is usually scanty
and confined to the more vigorous branches, but may be entirely
lacking. This delayed growth is particularly striking in the
spring on trees which have not declined to the point where
6 The mention of new growth does not include the watersprouts developed
at the base of the crown, which continue to grow vigorously until the
20 Florida Agricultural Experiment Station
defoliation and death of the branches has occurred. Even though
such trees may not exhibit any particular evidence of wilting
and curling of the foliage at the time, the fact that they remain
dormant for several weeks after adjoining healthy trees of the
same kind and age have developed their spring flush of growth,
together with the general dullness and lack of sheen of the
foliage, indicates that such trees are in the incipient stages of
blight. This point has been confirmed by repeated observations.
In cases where blighted trees decline slowly, the larger
branches on which the foliage wilts and drops off in the fall or
winter may not die after defoliation, but put forth a sparse new
growth of shoots and foliage late in the spring. This new
foliage, except that of the watersprouts developed from the
upper trunk and lower main limbs, is always small and under-
sized. As they grow older, the leaves remain small and acquire
a dull, dingy green color. This abnormal foliage presents a
marked contrast with the vigorous foliage of the watersprouts,
which constitutes one of the characteristic symptoms of blight
in its less rapid form. These small leaves likewise wilt and curl
during succeeding drought periods.
Fig. 7.-Blighted orange tree (sour orange stock) which continued to
decline despite repeated cutting back of the top. On a hammock phase of
Norfolk sand in part of a grove on slope on south side of Lake Harris.
Blight-A Non-Parasitic Disease of Citrus Trees 21
Watersprouts begin to develop from the trunk and main limbs
of trees that are declining slowly after from half to two-thirds
of the crown has died (Fig. 7). Contrary to the opinion com-
monly expressed by growers and others familiar with blight, the
development of water sprouts, considered by itself, should not
be regarded necessarily as an indication of blight, as their de-
velopment appears to be a normal tendency of citrus trees when
they have been greatly injured or have had their tops reduced
from various causes. The watersprouts on blighted trees develop
vigorously for a time, but during prolonged dry periods their
foliage wilts like that on other parts of the tree, although they
are the last part of the crown to die. When the trees have
reached the stage where they consist largely of a growth of
watersprouts, it is often difficult to say whether they have died
from blight or some other cause, unless one is well acquainted
with local conditions.
DISTINCTION BETWEEN BLIGHT AND OTHER FORMS OF
WILTING ON CITRUS TREES
Since all wilting of citrus trees, regardless of cause, looks more
or less alike when each case is considered separately, the ques-
tion naturally arises as to how the different types may be dis-
tinguished. Wilting of the foliage is by no means always the
forerunner of blight. Trees in active growth may wilt tempo-
rarily from exposure to hot sun, hot winds and prolonged periods
of drought. However, growers familiar with blight commonly
distinguish between what they term "drought wilt" and "blight
wilt". In the former case, even though the foliage may wilt
and curl to the point of extensive defoliation, the trees usually
recover following ample rainfall or irrigation. In the case of
blight wilt, however, the trees or parts of trees first affected
continue to wilt and decline until they are virtually dead or
worthless commercially. This form of wilting on the higher,
well-drained soils likewise has its inception, as a rule, during
the dry season of the year when affected trees decline most
Trees which were in good healthy condition prior to periods
of drought commonly develop a chronic wilt and decline after-
ward. On the other hand, cases of blight frequently develop in
groves where the trees did not exhibit any extensive wilting as
a result of drought, or at least not for some time prior to blight
development. It may be that, while the citrus tree is able to
withstand a certain amount of drought, there is a limit of toler-
22 Florida Agricultural Experiment Station
ance dependent upon the kind of rootstock, which must be
reached before a Healthy tree lapses into a blighted condition.
This appears probable, because the initial stages of blight may
be in the process of development for several months or some-
times a year or more before becoming sufficiently pronounced
to be recognized by a casual observer. Consequently, the fol-
lowing criteria should be observed to enable one to differentiate
between the two:
Fig. 8.-Orange tree (sour orange stock) growing in soil underlaid by
coquina limestone, showing pronounced wilting of the foliage of the entire
crown which developed a few weeks after prolonged waterlogging of the
soil. Photographed November 13, 1924. Condition of this tree 8 months
later is shown in Fig. 9.
Blight-A Non-Parasitic Disease of Citrus Trees 23
1. In incipient stages of drought wilt, all trees in the grove
or at least in one particular part of the grove usually exhibit
about the same degree of wilting, while in the early stages of
blight wilt only certain trees, usually scattered, wilt.
2. In incipient cases of blight, wilting of the foliage is usually,
although by no means always, most pronounced on only a portion
of the affected tree or trees, whereas drought wilt usually is
rather general throughout the tree. The incipient wilting from
Fig. 9.-Orange tree illustrated in Fig. 8, showing progress of the
chronic wilting and decline. This tree began blooming irregularly after
the regular blooming period was over and continued to bloom sporadically
until August. Photographed July 11, 1925.
24 Florida Agricultural Experiment Station
blight may also occur generally throughout the tree and in such
cases it is not always possible to determine at once from the
manner of wilting which form is involved.
3. Trees affected with drought wilt recover after rainfall or
irrigation, while with blight wilt the wilting becomes chronic
and the tree declines rather rapidly.
Fig. 10.-A 6 or 7 year old Lue Gim Gong orange tree (sour orange
stock) on muck soil, showing a delayed wilting and decline in May 1925
following root injury resulting from flooding in the fall of 1924.
The problem is further complicated, however, by the fact that
trees located in areas where the soil becomes waterlogged follow-
ing heavy rainfall may also exhibit an acute wilting (Figs. 8, 9
and 10), which may become chronic and indistinguishable from
that of blighted trees on high land that never becomes too wet.
This wilting results from injury to the root systems occasioned
by prolonged saturation of the soil. If water injury is less
severe, wilting and decline of the affected tree may not become
Blight-A Non-Parasitic Disease of Citrus Trees 25
apparent until the subsequent occurrence of drought. In cases
of this kind, the affected trees may recover in time, provided the
injury is not repeated too frequently.
CAUSE OF BLIGHT
A survey of the literature on blight indicates that there have
been nearly as many theories and opinions advanced as to the
cause and nature of the disease as there were men writing or
talking about it. Some of the theories mentioned by growers
were: overbearing, too close planting, cold, scale-insects, fungi
attacking the bark and wood of the smaller branches, decay of
the taproot or the taproot coming into contact with the under-
lying hardpan or rock formation, a peculiar oiliness of the soil,
and nematodes. Many of the early growers believed that blight
was a contagious disease, advancing from one tree to another
through the branches of the trees or through the soil. Some
of the earlier scientific investigators also believed that the dis-
ease was contagious. Underwood (45) stated that from all evi-
dence he had been able to gather in three months spent in Florida,
he was inclined to consider blight bacterial in nature.
Swingle and Webber (42) and Webber (48) stated in 1896
that observations which they had been able to make strength-
ened the belief that the disease is of a contagious nature. This
conclusion was based on the fact that in groves where blight
developed it frequently attacked trees in groups and that a year
or two after a tree developed this trouble-it-was a common thing
to see the adjacent trees contract it, often first on the limbs
next to the diseased tree, thereby giving the appearance of
SSwingle and Webber (42) further stated that experience has
taught that it was not only useless but dangerous to attempt
to cure blighted trees, because a diseased tree may infect sur-
rounding healthy ones. They also stated that, in many instances,
digging out and burning blighted trees had decidedly reduced
the number of new cases as compared with adjoining groves
similar in all respects, but where the diseased trees were allowed
to remain. However, they stated that trees planted where
blighted ones had stood were not more likely to contract the
malady than others in the vicinity and, in any event, not before
they begin to bear. Swingle (41) and Webber (50) also gave
evidence to substantiate this statement. Although they were
unable to detect, the presence of any causal organism during their
26 Florida Agricultural Experiment Station
investigations, Webber (48) concluded that, since the disease
again made its appearance in sprouts that developed from trees
killed to the ground after the freezes of 1894-95, it was at least
in the lower portion of the trees, if indeed not systemic.
In 1900 Hume (20) stated that his observations confirmed
those of others in that the disease was likely to spread from
one tree to another. In 1915 he stated (21) that blight appeared
to be the result of internal derangement of the functional pro-
cesses carried on within the tree and suggested that it may be
caused by an enzyme or by some poisonous substance present
in the soil. He also suggested that the causal agent might move
through the soil in the ground water.
In his discussion of blight in 1903 Rolfs (32) regarded it as
a contagious disease. A year later he (33) suggested the pos-
sibility of the disease being disseminated by grove implements.
In 1909 Swingle (40) stated that it was even suspected that
trees were infected through the blossoms.
Following the termination of Bessey's work at the Sub-tropical
Laboratory of the U. S. Department of Agriculture in Miami
in 1908, no further investigations of blight were made until 1923.
At this time the principal point generally conceded by scientific
investigators and growers alike was that it was useless to at-
tempt to effect a cure of trees that had developed blight.
Since no definite information had been obtained on the cause
of blight, early investigations by the writer after the work was
started in 1923 were conducted from a number of different
angles. Careful observations and attempted isolations revealed
no indication of a parasitic organism being associated with twigs,
branches or trunks of blighted trees and a careful study was
made of the root systems of such trees for evidence of the pres-
ence of a causal organism or possible relation of the trouble to
In the majority of blighted trees removed for study the roots
exhibited no discoloration or decay whatever, except perhaps
for occasional dead small roots and root-tips. In well advanced
cases the roots, as a rule, were strikingly deficient in the fibrous
feeding rootlets which are essential to maintenance of general
health and vigor of citrus trees. Also, where trees had exhibited
pronounced blight for a long time, the wood showed some evi-
dence of reduced growth, was often somewhat drier than it
normally should be and appeared as though partially seasoned
or cured. Trees that had been declining from blight for a few
years often showed decay on the ends of some of the larger
Blight-A Non-Parasitic Disease of Citrus Trees 27
roots, but this apparently developed after they had died back.
Two types of rot were found; one a friable, light brown rot and
the other a white, delignifying type in which the growth layers
separated readily from one another. A species of Fusarium was
isolated from what was thought to be the incipient stage of the
former, but this appeared to be a secondary organism rather than
a primary one. Aside from the occasional occurrence of rotted
roots, which is most evident in trees with long-standing cases
of blight, there was no indication of the presence of a causal
organism. Neither was there any evidence of decay or discolora-
tion in the sapwood of either the dying twigs or branches of any
of numerous blighted trees examined, or anything in the macro-
scopic appearance of these trees to indicate the presence of an
organism that might be responsible for the trouble. Moreover,
neither microscopic examination of the roots, trunks, branches
and twigs of blighted trees nor cultures made from different
parts of such trees have revealed any organism that could be
construed as causal. Such organisms as were obtained in cul-
tures in some cases were obviously superficial ones occurring
on the bark, or contaminations. Results of these studies indi-
cate that affected trees die as a result of failure of part of the
roots to support the top rather than from any initial dying of
the top itself. This is further supported by the fact that removal
of affected limbs or even of entire trunks does not arrest the
progress of the disease after it has become well-developed, al-
though it may stimulate temporary growth for two or three
years by greatly lessening the transpiration demand upon the
Swingle and Webber (42) and other scientific workers who
have made a thorough examination of the various parts of blight-
ed trees were unable to detect the presence of a causal organism
or any abnormality in any part of blighted trees. In the same
year, however, Webber (48) mentioned that Swingle had found
certain characteristic pathological features in the leaves of
blighted trees. Rolfs7 obtained a Fusarium from blighted trees
a number of times in widely separated localities, but was unable
to secure direct evidence that this was a causal agent.
In 1904 Rolfs (33) stated that blight is manifected by (a)
a matted root condition and (b) destruction of the smaller feed-
ing roots. Webber (49) described a mat-root condition of citrus
trees in 1897 but did not mention its occurrence in connection
7 Letter to writer, dated September 8, 1927.
28 Florida Agricultural Experiment Station
with blight. The writer has never observed this abnormal root
development in connection with blighted trees.
Dufrenoy (14) briefly alluded to blight in his notes on citrus
diseases, based on a brief visit to Florida early in 1928. He
stated that this disease shows a certain resemblance to the "mal
secco" of citrus studied by Petri in Sicily, and that he was able
to demonstrate the presence of the withertip fungus, Colletotri-
chum gloeosporioides, which was formerly considered to be the
cause of the latter disease. Dufr6noy's statement was based on
the finding of yellowish discolorations in the region of the vessels
in the twigs of blighted trees. As a matter of fact, the wither-
tip fungus is ubiquitous in Florida. The writer has made many
examinations of twigs and branches of blighted trees without
finding the discoloration described by Dufr6noy associated with
the disease. In all the numerous sectional preparations of the
roots, trunks, branches, and twigs of various blighted trees
examined, the tissues appeared perfectly normal, the lumina
of the wood vessels being clear and not occluded by either my-
celium or gum deposits.
PROPAGATION OF BLIGHTED TREES
Budding and Grafting Experiments: Inasmuch as the trees
budded from blighted trees by Swingle and Webber in 1892
grew for only about 21/2 years before they were killed by freezes,
it was contended by some that this was not conclusive proof
that the disease could not be transmitted by budwood. In view
of this it was considered especially desirable to repeat their
work on a much larger scale. Accordingly, extensive experi-
ments in budding and grafting were started in the spring of
1924 and additional trees were budded and grafted during the
two succeeding years.
Trees considered by experienced citrus growers to show typical
blight were selected for budwood and marked by painting num-
bers on the trunks. In each case the budwood was selected from
branches on which the foliage exhibited a pronounced wilting
and curling or had dropped off completely. All the trees selected
as sources of budwood for these experiments were photographed
and notes were made of their condition.
Budwood was propagated successfully from trees Nos. 1 and
2 in the Young and Metzner grove No. 1 at Rockledge and trees
Nos. 2, 3, 4, 6, 8, 9, 10, 11, 12, 13, 16, 21, 27, 30, 31, and 32 in
the American Fruit Growers grove at Bonaventure. All of these
trees were Pineapple oranges except No. 16, which was Valencia.
Blight-A Non-Parasitic Disease of Citrus Trees 29
These trees were all growing on well drained, drought Gaines-
ville sand more or less closely underlaid by coquina limestone
of irregular formation.
Budding work was initiated in March 1924 in the grove of
the Deerfield Grove Company at Bonaventure, where over 100
two-year old sour orange trees were donated for the experiment.
About half of the trees were budded on March 21 with budwood
from trees Nos. 1 and 2 in the Young and Metzner grove and
from trees Nos. 2, 3, and 12 in the AFG grove. Trees in which
the buds failed to live were rebudded later with budwood from
the same trees, except that No. 16 was substituted for No. 12
which had been dug up by that time. Remaining trees were
budded on March 22 with budwood from selected healthy Pine-
apple orange trees in what was then Chase & Company's Nea-
mathla grove, near Audubon on Merritt's Island. A few check
trees in which the buds failed to live were rebudded later with
budwood selected from healthy Pineapple orange trees in the
Young and Metzner grove. Numbered aluminum tags were
wired to budded trees.
On March 24, 25, 26, and 27, 110 sour orange and four seedling
grapefruit trees in an old nursery in the American Fruit Growers
grove at Bonaventure were budded with budwood from various
typical blighted trees. The nursery trees were several years
old but the tops had been killed when a barn had been burned
about two years previously, after which they were cut off and
the trees allowed to sprout again. Prior to budding these sprouts
were thinned to from one to five per tree. Each tree in which
a bud failed to grow was rebudded subsequently during the
spring and summer of both 1924 and 1925 with budwood from
the same or other blighted trees. By persistent effort, buds were
induced to grow on virtually all of these trees that were in any-
thing like suitable condition. In addition, a large sour orange
tree fully 12 years of age in the same grove was topworked with
budwood from tree No. 16. Since the tree used as a source of
budwood died before enough grafts were successfully made to
complete the working over of the sour orange tree, a few grafts
were put in subsequently from a healthy Temple orange tree.
A considerable number of other old sour orange trees in another
nearby nursery block in this grove were subsequently budded
with budwood secured from nearby healthy Pineapple orange
trees, so that the resulting trees might serve as checks to com-
pare with those previously budded from various typical blighted
trees in the first nursery block.
30 Florida Agricultural Experiment Station
On March 28, 25 old sour orange nursery trees in the Deerfield
Grove Company's (Sunset) grove near Courtenay on Merritt's
Island were budded with budwood from a number of selected
blighted trees in the AFG grove at Bonaventure. The ones that
failed were rebudded in May, thus propagating on this lot of
sour orange trees blighted trees Nos. 4, 6, 9, 10, 11, 12, 13, 16,
Very poor success was obtained in many cases with both buds
and grafts from blighted trees. In general, however, success
varying from poor to fair was obtained with the use of budwood
from blighted trees, depending upon the degree to which the
vitality had become exhausted and whether or not the trees
budded or grafted were in active growth. In practically all cases
the successfully budded or grafted trees developed into a good
lot of trees, compared favorably with check trees, and anyone
unfamiliar with their pedigree would have been glad to plant
them. In the nursery the 50 trees developing from budwood
from blighted trees compared favorably with the 50 from buds
from healthy trees.
In the American Fruit Growers grove at Bonaventure the
sour sprouts on two trees that already had a sweet orange shoot
developed from previous budding were successfully budded with
budwood from different blighted trees. On each the growth
from buds on a sour sprout was allowed to grow in competition
with the sweet orange shoot already developed, and all other
sprouts were cut off. On both, the growth that developed from
budwood taken from blighted trees equaled in size and vigor,
within a year and a half, the original sweet orange shoot and
compared very favorably with it in every way after 12 years.
Aside from much higher mortality of buds from blighted trees
and their greater tardiness in developing shoots, the only dif-
ference noted between them and the buds from the healthy
trees was that a few of the buds from blighted trees proved to
be fruit buds, developed a blossom and, in two instances, set
fruit. This appears to be merely a concrete illustration that
blighted trees tend to differentiate enormous numbers of fruit
buds, a response which is not uncommon in trees injured by
water, girdling, bending of limbs and root-pruning.
On August 15, 1926, a proximal slot graft was made in each
of the three main limbs of an old sour orange tree three inches
in diameter, using budwood from healthy trees of Marsh Seed-
less grapefruit, Valencia orange, and cultivated lemon, respec-
tively. The lemon graft failed to live and this limb was re-
Blight-A Non-Parasitic Disease of Citrus Trees 31
grafted with budwood from an old Mandarin orange tree nearby
that had exhibited blight for several years. This bud not only
lived but was first of the three grafts to put out growth. All
three grafts developed fine tops and bore fruit during the winter
of 1928-29. After nearly 10 years the top of the tree developed
from the three grafts is perfectly healthy, the grapefruit part
being dominant, as would be expected.
Grove Tests of Budded Trees: Budded trees were set in grove
formation where they could be maintained for several years.
In January 1925 39 trees budded the preceding year in the AFG
grove at Bonaventure with budwood from various typical blight-
ed trees were removed from the nursery and set out in this grove.
These trees were planted in places where old blighted trees had
been removed the preceding summer and the underlying coquina
rock broken up by dynamite and removed preparatory to re-
planting. A few of the larger trees were left growing where
they were, since they were favored by removal of surrounding
trees. Approximately the same number of trees that were bud-
ded at the same time from various healthy trees in this grove
were also set out in a similar manner. A special effort was
made to plant most of the trees in these respective lots in ad-
joining blocks so that it would be easier to keep track of them,
since the grove was not set out in regular rows. A few of the
older trees budded from the blighted ones had a decidedly poor
root system when dug from the nursery, due to closeness of
underlying rock, and on others it was impossible to preserve
much of the root system on account of rock interfering with
the digging. Despite this drawback, however, and the occur-
rence of a severe drought that spring, all but two lived. Five
other trees budded in the nursery of this grove with budwood
from blighted trees were set out in a newly cleared area of the
Deerfield Grove Company's (Sunset) grove near Courtenay on
Other trees were set out on a two-acre tract of new land,
obtained for this purpose through the kindness of A. B. Michael.
The plot was located on the Deerfield Grove Company's property
south of Courtenay on Merritt Island. It was covered with a
mixed growth of scrubby evergreen oaks and miscellaneous
shrubs, saw palmetto and scattered slash pine. The soil is
Norfolk fine sand and consists of a layer of grayish, fine sand
underlaid by a brownish-yellow sand extending to a depth of
several feet. With the exception of the better types of low
hammock soils, it is the best type of citrus, land on Merritt
32 Florida Agricultural Experiment Station
Island. This land was cleared in the spring and early summer
of 1925 and planted to velvet beans, which were disked under
in the fall. The trees were planted 22 by 22 feet in January 1926.
All available budded trees in the nursery of the Deerfield
Grove Company were used in this planting, together with 10
that had been set out in the American Fruit Growers grove at
Bonaventure the preceding year, the five that had been moved
from this grove and set out in the Deerfield Grove Company's
grove near Courtenay at the same time, 15 that had been budded
in the latter grove, and one that had been budded on a vacant
lot in Cocoa.
So far as possible all trees budded from any particular blighted
tree were grouped in one row in this grove. Beginning at the
south boundary, trees budded from healthy trees were planted
in odd-numbered rows, while those budded from various typical
blighted trees were planted in even-numbered rows. In all, 15
rows of 11 trees each were set out originally, comprising eight
rows of check trees and seven rows of trees budded from blighted
trees. About a year later an additional row of trees budded
from blighted trees was set out in the sixteenth row. Unfor-
tunately all the trees in this row except the third and the elev-
enth were killed by fire. The first, second, fourth, and fifth
trees in this row were replaced with experimental trees in the
spring of 1927 and the grove manager later replaced numbers
6-10, inclusive, with commercial trees. The majority of the check
trees were budded by the writer, while the remainder were pur-
chased from a commercial nursery. Despite the fact that the
trees which were grown on low hammock land closely underlaid
by marl had very poorly developed root systems, they all lived
when transplanted. Six trees from the commercial nursery died
back extensively and were replaced by others purchased locally.
One Valencia orange tree budded from blighted tree No. 16 also
died and was replaced in the spring of 1927 by No. 288 budded
from the same parent tree.
The planting plan of this experimental grove is illustrated in
Fig. 11. The check trees are indicated by C. Numbers in the
even rows alternating with the odd-numbered check rows are
the tag numbers borne by the trees at time of budding. The
numbers over the brackets connecting parts or all of these rows
indicate the numbers of the parent blighted trees from which
these trees were successfully budded. Where not specified by
"Young & Metzner", the parent blighted trees are those in the
American Fruit Growers grove at Bonaventure. Sixteen differ-
Blight-A Non-Parasitic Disease of Citrus Trees 33
ent typical blighted trees from-these two groves were used as
sources of budwood.
P-9 (IplaemPt) P-14 P-21 p-8 P-30 P-32
259 x I I I 2 55I 271 106 295 16
o C C C C C C C C C C 15
P-10 P-9 p-6
155 138 137 136 262 135 13 133 1 1 14
C C C C C C C C C C C 13
233 227 131 217 122 120 119 118 45 44 42 12
C C C C C C C C C C C 11
290 276 261 273 251 288 121 48 47 4' 41 10
C C C C C C C C C C C 9
221 '20 213 39 37 36 35 34 33 32 31 8
C C C C C C C C C C 7
225 128 S2 30 29 2S 2 26 25 2 2 21 6
C C C C C C C C C C C 5
2-2 Tou- & 'etaner
202 20 19 18 17 13 15 14 13 12 11 4
C C C C C C a C C C C 3
P-1 Ton *& :etBner
206 203 10 9 8 T 6 5 4 3 2 2
C c C C c C C C I
11 10 9 1 7 6 5 4 3 2 1 tr ot.
South bounday a
Fig. 11.-Planting plan of trees budded from a variety of blighted orange
trees in 1924 and set out in 1926, in rows alternating with trees budded
from healthy ones (C).
This grove planting has met with a number of reverses, but
in general trees budded from blighted trees have made as good
growth, have been as fruitful, and appear as healthy as check
trees budded from healthy trees. Without knowing the planting
plan, one could not distinguish the rows of the one from those
of the other. As a matter of fact, the trees in row No. 10, which
34 Florida Agricultural Experiment Station
were budded from a blighted Valencia orange tree, have made
more vigorous growth, as a whole, than any other row in the
Fig. 12.-Pineapple orange tree (No. 261) grown from young sour
orange.tree budded on March 26, 1924, with budwood from blighted tree
P-9. Tree set in grove in January 1925, in place where a blighted tree
had been removed. Photographed December 1, 1932.
In the American Fruit Growers grove at Bonaventure, where
an additional lot of trees budded from blighted trees are growing
in comparison with the check trees budded from healthy trees
and set out at the same time, the one lot in general is indis-
tinguishable from the other (Fig. 12). It is now fully 12 years
since a majority of these trees were budded and 10 years since
they were set out in grove formation and none .show the slightest
Blight-A Non-Parasitic Disease of Citrus Trees 35
evidence that the disease has been transmitted. Even the sour
orange tree in this grove, which must have been fully 12 years
Fig. 13.-Good producing tree (No. 116) developed by grafting in May
1924 a large 12-year-old sour orange tree with budwood from a blighted
Valencia orange tree that died with unusual rapidity. Growth from
additional grafts from a healthy Temple orange tree made late in 1924
and early in 1925 is shown in the lower portion of the crown in the center.
Photographed December 1, 1932.
36 Florida Agricultural Experiment Station
old when topworked in 1924 with budwood from a blighted
Valencia orange tree, does not show any evidence that blight
can be transmitted through budwood from diseased trees (Fig.
13). Neither does the one at Tropic, on Merritt's Island, which
was grafted with budwood from an old blighted Mandarin orange
The successful propagation of blighted trees through top-
working large bearing orange trees is of particular interest,
since some growers contended that trees might become inoculated
with blight at an early age but the disease might not become
manifest until the tree reached bearing age. With this in mind
special effort was made to topwork old trees with budwood from
blighted trees, because if blight should be transmissible by bud-
wood results should be hastened materially by topworking bear-
All experimental trees set at Bonaventure were planted in
places where blighted trees were removed, while those in the
experimental grove on Merritt's Island were planted on virgin
land. This affords a future opportunity to compare the longevity
of trees on these respective soils, which are of different types.
Since these experimental trees have been maintained for 10
to 12 years and since two sour orange trees of bearing age have
been topworked successfully with budwood from blighted trees,
it seems quite justifiable to conclude that blight is not trans-
missible by the use of budwood.
Root Cuttings: An attempt was made to propagate cuttings
taken from roots of blighted trees, to see whether the disease
could be transmitted in this manner. In the fall of 1929, 55
root cuttings were made from a large blighted Lue Gim Gong
orange tree on sour orange stock in a grove at Bonaventure.
These cuttings were sent to the Citrus Experiment Station at
Lake Alfred where they were placed in the nursery, mulched,
and kept watered, so that a good percentage should have rooted,
but all died within a short time.
Transplanting Blighted Trees: Various attempts have been
made by both growers and investigators to restore blighted trees
to a healthy productive condition by transplanting. In all in-
stances, however, these attempts have failed.
In 1900 Rolfs8 had 52 trees transplanted in groves of the
Manatee Fruit Company near Palmetto under the direction of
the late F. D. Waite, a skilled citrus grower and former nursery-
SPersonal letter dated September 8, 1927 and subsequent conversation.
Blight-A Non-Parasitic Disease of Citrus Trees 37
man. These trees, exhibiting the early stages of blight, were
dehorned, moved about a half mile, and set out in a different type
of soil. According to Rolfs, none regained health and vigor,
although a few lived for some years but there was no permanent
recovery. Apparently the last of the surviving ones were killed
by the freeze of 1905-06.
Fig. 14.-Blighted Valencia orange tree illustrated in Fig. 4, showing
removal of rock preparatory to transplanting after dehorning top. The
soil is made up of Dade fine sand over rock.
The writer also has tried transplanting on a limited scale to
rejuvenate blighted trees. A 12-year-old blighted Valencia on
sour orange stock was transplanted at Lotus on Merritt's Island.
On May 18, 1926, this tree showed blight in a sector on one
side of the crown but the remainder of the crown appeared
perfectly healthy (Fig. 4). The dying limbs were cut off later,
but the disease progressed until February 25, 1927, when the
tree was dehorned for transplanting. At that time the entire
crown, except for a few exceptionally vigorous shoots in the
interior, exhibited a general wilted condition of the foliage but
carried a box of good-sized fruit. The tree was growing in Dade
fine sand underlaid by a platform of coquina limestone about
six feet in diameter. The tree was carefully removed to con-
serve the root system and placed in shade until the hole was
made ready for replanting. The central rock platform was then
38 Florida Agricultural Experiment Station
dynamited (Fig. 14) and removed down to the water table, which
was encountered at a depth of about 41/2 feet. The hole was
then partially filled with soil and the tree reset and given three
barrels of water. Despite subsequent watering only a few
sprouts developed from the lower main limbs and the entire tree
was dead by July. The grower tried the same thing with another
blighted orange tree with similar results. It is admittedly dif-
ficult to understand why trees with only a moderately developed
case of blight, and which still have an apparently good root
system, cannot be made to live when transplanted carefully,
since they appear to have considerable vitality left.
CORRELATION OF BLIGHT INCIDENCE WITH VARIOUS FACTORS
Soil Conditions and Grove Practices: From careful study of
published discussions and experiences of Florida growers dating
back to 1885, it is quite evident that local soil and soil moisture
conditions and the often crude methods of grove management
during the early years of the Florida citrus industry undoubtedly
were conducive to the development of blight in the older citrus
sections. Considerable evidence has been presented by past
generation of growers to indicate that a large portion of the
orange groves were planted on lands which we now consider
unfit for successful grove production. Groves were frequently
set out with merely a cursory examination of the land, and many
sites planted proved unfavorable later. In many instances the
trees were set improperly in soil closely underlaid by hardpan
formation or rock, lacked adequate drainage, or suffered period-
ically from prolonged droughts and the trees were short-lived.
Moreover, in many earlier grove plantings the trees were set
too close together and after a few years the limbs would inter-
lock and the soil would become filled with an interlocking net-
work of roots.
After the groves were established the belief was held by many
of the earlier growers that deep plowing was indispensable and
many of them plowed to such a depth that large woody roots
were torn up. In fact deep plowing and tearing up the roots
was commonly recommended formerly, as the proper way to
rejuvenate old neglected groves. The deep plowing and other
excessive cultivation so generally practiced even on the lighter
sandy soils in former years, greatly expedited the depletion of
the scanty supply of organic matter and were of questionable
value in conserving soil moisture except as they retarded the
growth of weeds and grass.
Blight-A Non-Parasitic Disease of Citrus Trees 39
In discussing the occurrence of blight in groves on Terra Ceia
Island, Waite (47) stated that blighted trees were very peculiar
in that the root system was perfect in nearly every case, al-
though most of the trees were situated on low flat land only
about two feet above tidewater. Many groves and truck lands
of this island have had tile drains installed to improve the poor
drainage conditions and even then trees have been injured peri-
odically as a result of waterlogging. In a previous discussion
of blight, Waite (46) stated that, while laying their under-drains,
they had found a layer of rock about three or four inches thick
over an area of about 40 acres and that they obtained only 25
boxes of fruit from these trees the previous season. Consider-
able chronic wilting and decline of citrus trees as a result of
inadequate drainage hasbeen observed in groves on this island,
so that tree losses from this cause undoubtedly occurred in earlier
Soil Types: Extensive preliminary field observations indicated
that blight is not uniformly distributed, even in one locality.
Instead it was found to be directly correlated with the capacity
of the soil to hold moisture during the dry season; that is, blight
occurs most frequently on soils of low-moisture-holding capacity.
Such soils include the drought dune and high hammock soils,
which are frequently closely, underlaid by rock formations.
Blight occurs less frequently on typical low hammock land, which
is generally underlaid by marl or other calcareous formation,
and occurs least frequently in groves on low hammock or prairie
soils underlaid by clay.
After making these observations, more detailed studies were
made in which different soil types on which blight was found
to occur were checked against the classification of the soils of
Florida by Bryan and Stoutamire (10). This comparison showed
that blight occurs to some extent on practically all soil types
planted to citrus. However, it occurs most frequently on soils
of the Palm Beach, St. Lucie, Lakewood, Dade and Gainesville
series, especially in those areas where rock formations occur
near the surface. These, except the Gainesville series, are soils
of sand dune origin. Blight occurs with moderate frequency
on well drained upland soils of the Norfolk and Eustis series.
It occurs with least frequency on soils of the Parkwood, Fellow-
ship, Blanton, Orlando, Bladen, Hernando, Portsmouth and St.
Johns series, which comprise medium to poorly drained soils.
The frequency of occurrence of blight on any of these soils ap-
pears to vary with their physical characters, fluctuation of the
40 Florida Agricultural Experiment Station
water table, methods of grove management and other local
Proximity of Rock and Other Structural Defects: Investiga-
tions have shown close correlation between blight incidence and
the proximity of rock under trees, regardless of the type of soil.
Many blighted trees removed for study in the Cocoa section
were found to be closely underlaid by a more or less irregular
formation of coquina limestone. Frequently, the trees were
found to be set on top of layers of rock or planted over potholes
occurring in the rock. Although blight unquestionably occurs
with greatest frequency on the lighter, droughtier soils closely
underlaid by rock, it commonly occurs also on soils where no
rock occurs within the first few feet. Consequently, rock, in
itself, cannot be regarded as the direct cause of this trouble,
although it unquestionably is an important contributing factor.
At Bonaventure a great difference in blight incidence has
been noted in two adjoining parts of a grove on sour orange
stock on Gainesville sand. Coquina limestone is equally abundant
within the root zone of trees all over the grove, but soil moisture
conditions vary in different parts. On the low-lying portion
the trees suffered from inadequate drainage from time to time
during prolonged rainy periods, some being badly injured before
the installation of a pump to remove the excess water. How-
ever, very few trees in this portion of the grove developed blight.
In another portion of the grove where the land is somewhat
higher, contains less organic matter and never becomes too wet,
a considerable number of the older trees developed blight an-
Another striking difference in blight incidence was observed
on very dissimilar soils in two adjoining parts of a grove on
sour orange stock at Pineda. In the part of the grove on the
Gainesville sand closely underlaid by coquina limestone the
original trees were found to have died out largely from blight.
In an adjoining part of the grove the trees were on low hammock-
land (Parkwood fine sandy loam) underlaid with marl at a depth
of about 21/2 feet. On this soil trees of apparently the same
age were entirely free from blight, except for two on the margin
between the two soil types. The transition between these soil
types was very abrupt. The marked differences in behavior of
the trees on these two adjoining soil types as they appeared in
1926 are shown in Fig. 15. This grove was visited from time to
time in subsequent years and at the end of 1932 no other trees
on the low hammock soil had developed blight.
Blight-A Non-Parasitic Disease of Citrus Trees 41
Fig. 15.-Comparison of blight incidence in citnus trees in adjoining
parts of the same grove on very dissimilar soil types. The upper picture
shows part on Gainesville sand closely underlaid by coquina limestone.
The lower picture shows the part on low hammock land (Parkwood fine
sandy loam) underlaid by marl.
42 Florida Agricultural Experiment Station
Many citrus groves in the Indian River section along the
middle East Coast of Florida are planted on Gainesville sand
more or less closely underlaid by coquina limestone. The depth
and extent of this rock formation varies greatly in different
localities, ranging from surface outcroppings to depths of a few
feet, and from scattered or variously interrupted formations
(Fig. 16) to more or less solid ledges (Fig. 17). When the early
groves were set on hammock lands along the Indian River, little
and often no effort was made to remove the rock prior to plant-
ing. Trees were frequently set in pothole formations in the
rock (Fig. 18) and where the taproots have grown down into
such potholes, dynamiting frequently is necessary to remove
old trees which have become worthless as a result of blight
Fig. 16.-Soil profile of citrus grove on Gainesville sand, showing strata
and irregular distribution of the underlying coquina rock.
The occurrence of rock within the root zone of citrus trees
naturally accentuates the tendency for the moisture content of
the soil to fluctuate greatly at different radii and depths under
the trees. Such formations within the root zone not only ob-
struct the downward percolation of water, but also its lateral
movement. Trees closely underlaid by rock naturally have a
very limited root distribution and the soil above the rock dries
Blight-A Non-Parasitic Disease of Citrus Trees 43
readily. Moreover, even the rather limited capillary rise of
moisture from the deeper layers of soil is prohibited by the
SOIL MFlru n ft 10m o I
16 13 12 1 10 S T 3 I U 2 1 0 1 2 3 6 5 6 7 9 9 10 11 12 13 1,
SOIL TrinOLI no 0 mnn TO Omtnu
l2 13 12 11 13 9 7 6 5 3 2 1 0 1 2 3 5 6 7 1 9 10 U 1 12 14
SOIL PUS I I w IWT I I
iI I i I
II Im U I
1> 1] 12 11 1: s I ; 6 ) Ik 3 ; 1 0 I 3 > C I L I H; 1a 12 1 It
11 13 12 11 10 9 T7 6 5 4 3 2 1 0 1 I 1 b 3 I U 1A 1 3 it
p It-nc fs eT onter o t It ree t
Fig. 17.-Diagrammatic representation of the distribution of coquina
limestone under blighted grapefruit tree (sour orange stock) about 38
years old on Gainesville sand, showing depths to rock in soundings
taken every 6 inches throughout the average spread of the crown on
radii coinciding with the eight principal points of the compass. Plotted
to scale on one-foot squares.
Pothole formations occurring in the coquina limestone vary
from slight depressions to those several feet in depth. The
water that seeps into the deeper potholes is frequently lost to
the tree roots. The more shallow depressions tend to serve as
holding basins, where the water collects until lost by slow per-
colation. This may result in injury to larger roots of citrus
44 Florida Agricultural Experiment Station
trees developed in potholes. As a result, one or more limbs,
dependent upon the root or roots, may drop fruit and foliage
suddenly and die within a short time, while the remainder of
the tree may wilt and decline much more slowly.
Fig. 18.-Blighted tree (sour orange stock) which was found to have
been planted in a pothole formation in coquina limestone.
Soil Moisture: Investigations have shown a definite correla-
tion between the development of new cases of blight and the
moisture content of the soil. Apparently, blight has its origin
in either a deficit or an excess of soil moisture, or a combination
of the two. Virtually all new cases of blight on high, well
drained land have their inception during, or shortly after, the
dry season, which usually extends from early winter to early
Blight-A Non-Parasitic Disease of Citrus Trees 45
The citrus belt of Florida, according to Mitchell and Ensign
(26), has an average annual rainfall of 50 inches, with 55 to 60
inches in the section between Lake Okeechobee and the lower
East Coast. Unfortunately, the distribution of this generous
rainfall usually is extremely uneven, resulting in fairly well-
defined wet and dry seasons. The period of major precipitation
begins, as a rule, with June and ends with September, or in
October on the lower East Coast.
Fig. 19.-Coquina limestone blasted to remove old citrus tree that declined
from blight. In this part of the grove a more or less solid ledge of rock
occurred at about 2 to 3 feet below the surface of the ground.
As a result of uneven distribution of rainfall, many of the
groves on lighter sandy soils with good to excessive drainage
frequently become too dry to maintain citrus trees adequately,
especially when heavily laden with fruit. On the other hand,
groves on the heavier soils in the lower situations, where drain-
age is naturally poor, soon become injured from waterlogging
of the soil after long continued rainfall. Groves in situations
intermediate between these two extremes often suffer from the
soil alternately becoming too wet and then too dry. Citrus trees,
however, suffer more often from deficits than from excesses
of soil moisture. The adverse effects of droughts on tree growth
46 Florida Agricultural Experiment Station
and fruit production have been alluded to repeatedly in various
Florida agricultural periodicals since 1889. However, the rela-
tion of neither deficits nor excesses of soil moisture to blight
development appears to have been considered seriously by the
earlier workers on this problem.
There is probably not a year that groves in some part of the
State do not suffer from drought at one time or another and
a large percentage of the groves suffer from lack of moisture
during the spring or winter on an average of five years out of
seven. Observations from 1923 to the present indicate that
many groves in central Brevard County have suffered from
drought at some time during every year. It is by no means
uncommon for Florida citrus groves to suffer from lack of moist-
ure for from three to six months at a time, with very disastrous
effects upon the quantity, size and quality of the fruit. Droughts
of such prolonged duration undoubtedly also exert a profound
effect upon the vitality and average life of citrus trees.
When the soil in citrus groves becomes so dry that the trees
wilt, even to the point of defoliation, one naturally speculates
as to the role of inadequate soil moisture in causing the older
bearing trees to develop blight. Even though wilted to the point
of complete defoliation, younger trees invariably recover with
the advent of rain, whereas older trees may recover or may lapse
into a chronic wilt and decline. The limit of tolerance of citrus
trees to drought appears to decrease with age. Bearing trees
carry large crops of fruit, with heavy demand upon available
soil moisture and plant nutrients. During the rainy season of
summer and early fall citrus trees make good vegetative growth
and develop fruit, and then as the fruit approaches maturity
the trees are confronted with the dry season during the winter
and spring months.
It has been observed repeatedly in digging trees that have
declined from blight on well drained, lighter sandy soils, that
the subsoil into which the taproot penetrates is frequently so
dry as to be incoherent and it runs like sugar in a barrel during
the digging. This condition has been known to exist under
trees for weeks or even months at a time. It has been found
by actual test that the sand in this "running dry stage" contains
less than 3 percent of moisture and has already given up about
all moisture that tree roots can get from it. A much higher
moisture content is required to keep trees from wilting in heavier
soils, especially those with a high content of organic matter.
Blight-A Non-Parasitic Disease of Citrus Trees 47
Kay (22) obtained soil moisture data in Brevard County
which disclosed wide variations in the amount of moisture in
the soil under blighted citrus trees, not only at different depths
but also at different points and radii from the trunks. This con-
dition was found to occur regardless of whether the subsoil was
deep or shallow and underlaid by rock. He also found that
variation in moisture content at different locations under the
tree was as great as that in any vertical section. In these
studies, Kay took 12 sets of samples with six samples in each
set per tree. The samples were taken at distances from 4 to
10 feet from the trunk, and at six different depths, making a
total of 72 samples in all from under the trees. Where under-
lying rock occurred the samples were taken to this point. The
data obtained from soil samples taken under a 20-year-old seed-
ling orange tree on June 23, 1924, are shown in Table 3.
TABLE 3.-PERCENTAGE OF MOISTURE IN NORFOLK FINE SAND UNDER A
BLIGHTED 20-YEAR-OLD SEEDLING ORANGE TREE.
Percentage of moisture at different points from tree on
Depth different radii
Depth ----------------^ ^
in I [ 8.5 ft. 1 10 ft.
Inches 5 ft. from tree I 7.5 ft. from tree from tree from tree
N E IS I W NE I SE SW NW N I S IE W
0-6 ........ 4.08 2.38 4.28 3.55 5.25 2.74 1.78 2.64 7.26 2.92 8.47 2.81
6-12......... 2.33 2.17 2.70 2.37 5.01 2.14 2.24 2.25 4.14 2.21 3.51 2.62
12-24.......... 1.97 1.84 2.78 1.99 3.62 2.19 2.03 1.92 1.92 3.12 2.92 2.35
24-36.......... 1.69 1.70 1.59 2.06 4.70 1.68 1.99 1.95 1.79 4.92 1.87 1.78
36-48.......... 1.78 1.79 1.60 2.09 4.87 1.58 1.97 4.61 2.25 2.63 1.83 1.77
48-57.......... 3.21 1.86 1.63 2.90 5.07 2.35 3.01 5.31 3.57 1.92 1.601 1.77
_____ I I I
Minimum moisture content in the vertical sections ranged
from 1.58 to 3.62 percent, while the maximum ranged from 2.38
to 8.47 percent.
Kay found even greater variations in soil moisture of Gaines-
ville sand under a blighted orange tree about 12 years old on
sour orange stock. The foliage was wilted and curled almost
throughout the crown, but mostly on the southeast side. No
new growth had developed. The surface soil was dry when the
samples were taken on July 1, 1924, although there were three
inches of rain in May and 4.54 inches in June. Results of the
determinations are shown in Table 4.
In this set of samples the minimum moisture content in the
vertical sections ranged from 1.35 to 3.14 percent, while the
maximum ranged from 2.20 to 11.55 percent. It may be seen
that moisture content of the soil does not always increase with
48 Florida Agricultural Experiment Station
depth, as is generally assumed. After finding such great varia-
tions in moisture content of the Norfolk and Gainesville sands
at different radii and depths under citrus trees during dry sea-
sons, Kay concluded that they were the cause of blight. He
stated that he had never found the extreme local variations
of moisture in the flatwoods and prairie soils that he had found
in the Norfolk and Gainesville soils.
TABLE 4.-PERCENTAGE OF MOISTURE IN GAINESVILLE SAND UNDER BLIGHTED
ORANGE TREE ABOUT 12 YEARS OLD ON SOUR ORANGE STOCK.
Percentage of moisture at different points from tree on
Depth different radii
Inches 5 ft. from tree | 7.5 ft. from tree | 9 ft. from tree
N IE I S W INEI SE I SWNWI N I E I S I W
0-6 ........ 3.04 2.13 2.24 4.41 2.69 2.33 2.41 2.51 1.98 1.88 4.53 2.96
6-12........ 4.56 2.53 2.87 3.24 2.66 1.69 2.17 1.74 1.41 2.67 1.68 2.76
12-24........ 2.95 2.78 3.27 3.48 3.15 2.00 2.96 2.75 1.59 3.07 2.08 2.69
24-36........ 2.11 3.37 3.55 3.22 3.95 3.69 3.28 3.72 2.20 3.66 3.34 3.58
36-48...... 2.05 1.48 2.41 .3.14 6.36 ........ 2.89 2.44 1.35 2.38 2.89 5.18
48-57........ 3.88 2.80 93 3.91 11.55 3.98 3.13 3.87 1.64 2.18 6.19Rock
Samples taken 4.5 feet from the tree in this case.
"b Rock encountered at 43 inches below surface.
Unfortunately, Kay's report did not show a comparison of
the soil moisture conditions under blighted trees with those
under adjacent healthy ones. It is quite evident that pronounced
deficits of soil moisture also occur under numerous trees that
wilt severely from drought but recover without lapsing into the
chronic blight. It is possible, however, that blight may not
develop until such deficits of moisture have been unusually pro-
longed or frequently repeated. Moreover, it has been observed
repeatedly that trees carrying large crops of fruit during pro-
longed dry periods often wilt and die rapidly, while adjacent
trees with little or no fruit usually survive the ordeal.
It has been pointed out that during dry periods the sandy
soils of Florida lose water very rapidly. This condition is greatly
aggravated by the slowness with which dry soils take up water
when rain occurs. Such soils are frequently spoken of by grow-
ers as being "oily". This peculiarity was noticed by Underwood
(45) as early as 1891, although its effect upon citrus trees is
still not recognized by many growers. Even after five inches
of rainfall on two consecutive days, small areas of dry sand
have been uncovered near the surface in Gainesville sand at
Cocoa. Furthermore, even after being flooded for a half hour,
perfectly dry sand can be found immediately under the water
Blight-A Non-Parasitic Disease of Citrus Trees 49
in some places. Such soils also permit little lateral movement
of water. In one case noted, where a grower had been running
water down a ditch between the tree rows for 24 hours, soil
samples taken two feet away from each side of the ditch showed
that the water had not reached that far. Even in cases where
water from a flowing artesian well had been running down a
ditch for several months, the deep sandy soil under the trees
immediately adjacent to the ditch was found to be extremely
dry, thus clearly demonstrating how little lateral movement
of water there is through sandy soils when not underlaid by
an impervious layer, such as hardpan or clay.
As was pointed out by Kay (22) it has long been known that
the surface soil, when dry, is repellant to water and difficult
to moisten, but the slowness of rewetting the subsoil is a prob-
lem that is far less appreciated. Moisture determinations made
by Kay show that extensive rainfall may be required to wet the
deeper layers of the soil with a good degree of uniformity after
they have dried out. Samples for soil moisture determinations
were taken under a four-year-old orange tree, which had been
set out to replace a large seedling orange tree that was removed
on account of blight, and the results are shown in Table 5.
TABLE 5.-PERCENTAGES OF SOIL MOISTURE FOUND AT DIFFERENT LOCATIONS
UNDER A FOUR-YEAR-OLD ORANGE TREE OVER A PERIOD OF THREE AND
Percentage of Percentage of
moisture in upper moisture in 5th
Date and previous rainfall 6 inches foot
North side South side North side ] South side
August 1, 1924, after 8.5
inches of rain in July 9 8 2 6
September 12, 1924, after 2.2
inches of rain from 8/1 to 4 5 1.8 2.2
October 13, 1924, after 7.5
inches of rain from 9/12 9.7 8.5 6.3 6.3
These data show that although 8.5 inches of rain fell in July
the moisture content under the tree varied greatly until 18
inches of rain had fallen during a period of 31/2 months. It
appears probable that the dried-out soil under a large bearing
tree would be wet much more slowly, especially since the crown
of a large tree sheds a considerable amount of rainfall. In years
of lighter rainfall, it appears quite probable that the deeper
layers of soil within the root zone would remain dry much longer
and that the trees might suffer extensively.
50 Florida Agricultural Experiment Station
A few cases of blight have been observed in irrigated groves,
where, if prolonged deficits of soil moisture were the cause or a
contributing factor, it would not be expected to occur. However,
many groves in Florida
Shave been irrigated for
& o years but not sufficiently
to wet the deeper layers of
o soil thoroughly. Since soil
moisture conditions in these
S groves were not investigat-
0o D ed over any extensive peri-
0 /od, it can only be stated
o o D that, in general, blight is
S much less likely to occur in
S oo irrigated groves than in un-
o .. irrigated ones, other condi-
o. . 0 0 tions being equal.
0 o0 o A close correlation be-
Se 0 0 ,0 o tween blight incidence and
So \. oi 0 soil moisture conditions has
S0 0 *0 been observed in a 19-year-
00 0. old block of Lue Gim Gong
S0 0 0 0 0 0 orange trees on sour orange
@ ( 0 0 0 to o 0 stock at Bonaventure. This
S: D block is planted on two
S o o :: types of land with highly
o oo o @ D o o o a
S00 . .,-' Fig. 20.-Diagram of 19-year-
/ 0- old block of Lue Gim Gong or-
"" o \ ange grove on sour orange stock
g@ o 0 o \ planted on two adjoining soil
o 0 o types with widely differing soil
moisture conditions at Bonaven-
So G@ o 0 I o I ture. Mapped in July 1933 to
S ( o show marked contrast in blight
S I. incidence. Legend: 0 indicates
G .0 O. o o I i healthy trees; XI, X. and Xa in-
o o o i dicate trees showing early, in-
o o :D o blight respectively, those with
o\ !o circles around them having been
o o removed in July; the numbers
I from 0 to 9 in circles represent
--- ..* ;.- the ages of Pineapple orange re-
-.u.............-...... plants that were set out between
.... .u q........I....i ............ 1924 and 1933 to replace trees
00mo>"" that were removed on account
Blight-A Non-Parasitic Disease of Citrus Trees 51
dissimilar soil and moisture conditions, depicted diagrammatic-
ally in Fig. 20. The undulating line drawn through the block
demarks the abrupt transition from one soil type to the other.
The soil on the left comprises the marginal slope of a low ridge
of Gainesville sand closely underlaid in general by coquina lime-
stone. When the rock occurred close to the surface at the points
where the trees were to be set, it was dynamited and removed to
some extent prior to planting. This portion of the grove is in-
clined to be drought during the dry periods. The soil on the
right of the line comprises a nearly level area of Parkwood fine
sandy loam closely underlaid by a compact layer of marl. The
marl limits the development of the roots to the shallow surface
soil. Because of poor natural drainage an extensive drainage
ditch was cut along the right boundary of this portion of the
grove. In addition a broad, shallow furrow leading to this ditch
has been worked between each two rows of trees, giving the tree
rows at right angles to the ditch a mounded effect. The general
profile of the soil types is shown at the bottom of the figure.
Trees on Gainesville sand in this grove were observed to begin
dying from blight about 10 years ago, and are continuing to
develop this trouble despite good care and the removal of blighted
trees. A survey of this portion of the grove on July 11, 1933,
showed that of 192 trees 121, or 63.0 percent, appeared to be free
from blight; 57, or 29.7 percent, had been removed on account
of blight; and 14, or 7.3 percent, were in various stages of
decline from blight. The oldest replants-Pineapple orange trees
on sour orange stock-are about nine years old and none have
contracted blight yet.
In marked contrast to this situation, only two trees out of 130
wholly within the Parkwood fine sandy loam side of this grove
block have shown any evidence of blight. One developed chronic
wilt after the unusually wet period in the fall of 1925. However,
growth of many trees has been retarded at various times by soil
wetness. The elliptical area outlined by the dotted line indicates
the lowest area in the main part of the Parkwood soil, where the
trees in general are poorest. Those in the middle of the three
rows included in this low area are the poorest of all. The mar-
ginal row of trees, outlined by the dotted line at the south end
of the area, also have suffered from excessively wet soil.
Groves on poorly drained low hammock soils often present a
difficult problem. These soils contain considerable organic mat-
ter in the top soil but are underlaid closely by soft marl, hard
marl, or occasionally clay. Since roots do not ordinarily penetrate
52 Florida Agricultural Experiment Station
either marly or compact clayey subsoils, the root systems are
confined to the shallow surface soil. By reason of the high
organic content of the surface soil and the resulting greater
affinity for water, such soils require a much higher moisture con-
tent than the light sandy soils to keep trees from wilting. In
other words, the wilting coefficient of the former soils is much
higher than that of the latter. Owing to the structural makeup
of low hammock soils, shallow root systems of the trees, and
the relatively high position of the water table throughout the
year, citrus trees on such soils are much more readily affected
by fluctuations of the water table than are those on the sandy
soils. As a rule, however, such soils remain much more uni-
formly moist than the sandy soils. Although general wilting
of citrus trees on low hammock soils usually occurs only during
the most prolonged dry periods, blight occurs with more or less
frequency, especially in areas with pronounced fluctuation of
the water table, due to the marl or clay being near the surface.
Moreover, in both the low hammock and prairie soils, and to
a lesser extent in the flatwoods soils, undulations in the contour
of the more or less impervious subsoil may result in basin-like
formations where water tends to accumulate occasionally and
injure trees in local areas. Such depressions in compact sub-
soils may occur even in the clayey subsoil on hillsides. As a
result, individual trees or small groups of trees may be damaged
by waterlogging of the soil and yet the inadequate drainage
would not be revealed by casual inspection. Such erratic local
soil conditions present an extremely difficult problem for the
Furthermore, low areas or local depressions in upland soils
and in intermediate soils where the drainage is not sufficient
to take care of heavy rains or prolonged wet periods often offer
difficult problems for the citrus grower because he does not
realize that the water table may rise high enough to cause
injury to the tree roots. Such areas may become not only ex-
cessively wet during prolonged rainy periods but also excessively
dry during prolonged drought periods. As a result the depth
to which the root system may extend varies with the average
height of the water table over a period of years. It frequently
happens that during a period of moderate rainfall the roots
develop deeper into the subsoil, but when the inevitable period
of unusually heavy or prolonged rainfall occurs the water table
rises and the root system becomes injured in proportion. Thus,
the deeper portions of the root system may be killed back re-
Blight-A Non-Parasitic Disease of Citrus Trees 53
peatedly, and the final result depends upon the frequency and
the extent to which the roots are injured each time.
In the more severe cases of such root injury the trees usually
wilt rapidly and die within a few weeks. In less severe cases
a number of the limbs may become defoliated and die rapidly
soon after the injury, while the remainder of the crown may
appear to be little affected at the time, but usually develops a
chronic wilt and declines later. This frequently is accompanied
by yellowing and symptoms of starvation akin to those developed
after girdling, together with defoliation and heavy blooming
out of the regular season.
Trees that have the root system injured only slightly may
show no ill effects immediately, but may begin to wilt extensively
with the onset of dry periods, which may be several weeks or
even months after the injury occurred. This condition has been
termed "physiological drought". Despite the relatively high
moisture content of the soil, tops of injured trees suffer from
lack of water because so many of the roots have been injured or
killed that the remainder are incapable of satisfying the require-
ments of the trees. As the soil continues to dry out the effect
of injury is intensified and the wilting may become more pro-
nounced. It has been observed that trees injured by a high
water table on muck soils make a better recovery than those
on sandy soils. Although all of these degrees of water injury
do not develop in every area, it is probable that some of the
more rapid spread of blight reported by growers in former years
may be accounted for by some of these forms of water injury.
Where the balance between the root systems and the tops
is not too greatly upset, the injured trees may show wilting
for a time but eventually recover, provided the injury is not
repeated and favorable soil moisture conditions are maintained.
However, if this balance is upset more completely, the trees
lapse into a chronic wilt and decline (Figs. 8, 9, and 10) that
frequently is indistinguishable from that exhibited by trees de-
veloping blight on high, well-drained land that never becomes
Some of the first typical cases of blight which were photo-
graphed and kept under close observation after the work was
inaugurated at Cocoa were found subsequently to have developed
a chronic wilt and decline following unusual rises of the water
table. As a result of keeping both these and other trees under
close observation over a period of several years, opportunity
was provided to see the effects of fluctuating soil water condi-
54 Florida Agricultural Experiment Station
tions upon the development of blight in all its stages in numerous
Kind of Tree and Rootstock: Blight has been known to affect
practically all kinds and varieties of citrus trees grown com-
mercially within the State. Although some varieties of trees
are affected much more frequently than others, none appear
to be exempt. In summing up their work on blight in 1896,
Swingle and Webber (42) stated that oranges, tangerines, man-
darins, and grapefruit, in about the order named, are most sus-
ceptible, and that lemons blight less, while limes and especially
sour oranges are almost exempt from this disease. They stated
further that the sort of rootstock appeared to have no influence
in increasing or diminishing susceptibility of trees to blight.
Since 1923 blight has not been observed on either limes or the
cultivated lemon. Although blight has been noted occasionally
on unbudded sour orange trees on neglected city lots in Cocoa
and a single case was observed on a neglected rough lemon tree
in the margin of a grove at Bonaventure, it clearly appears to
be of rare occurrence on unbudded trees of these kinds. Tangelo
trees have been found to be quite susceptible to blight and a
few kumquat trees declining from this trouble have been ob-
However, very little can be said concerning the relative sus-
ceptibility of the different kinds of citrus trees or rootstocks
to blight, since it is only rarely that enough of them occur to-
gether on the same type of soil and under the same soil moisture
conditions for a comparison to be made. Thus far there has been
opportunity to compare relative susceptibility only between or-
ange and grapefruit trees on sour orange stock. This comparison
was made in two groves on Gainesville sand closely underlaid
by coquina rock in Brevard County. In one block of a grove at
Bonaventure, where large numbers of blighted trees were re-
moved annually, out of a total of 496 orange trees old enough
"to develop blight 156, or 31 percent, were either virtually worth-
less or showed more or less decline from blight. In the same
block, of 166 grapefruit trees, most of which were of good bear-
ing size, only 9, or 5 percent, exhibited symptoms of blight. In
a block of a grove at Tropic where it was not the custom to
remove trees as they declined from blight, of 212 full-bearing
orange trees 119, or 56 percent, showed more or less decline
from blight, while in the same block of 157 full-bearing grape-
fruit trees 21, or 13 percent, showed some evidence of decline
from blight. These figures, together with other comparative
Blight-A Non-Parasitic Disease of Citrus Trees 55
observations on blight incidence in orange and grapefruit trees
on the same soil type and under the same soil moisture condi-
tions, demonstrate that orange trees are much more susceptible
to this trouble than grapefruit trees. Further comparisons have
shown that the Pineapple orange is more susceptible than the
Valencia when both are on sour orange rootstock. This appears
to be due to the fact that the Pineapple variety is a heavy bearer.
Observations over a number of years indicate that tangerine
trees are less susceptible than orange trees but more so than
In most instances where the rootstock was specifically men-
tioned in literature, or is evident from a knowledge of the grow-
er, the trees were either sweet seedling orange trees or trees
budded on sour orange stock. These were the most popular
varieties in the earlier years. Rolfs (32) stated that the trouble
occurred on nearly all kinds of stock and nearly all types and
conditions of soil.
Blight appears to occur with equal frequency in trees on sour
orange and bitter-sweet orange stocks, although trees on the
latter stock are now of comparatively infrequent occurrence. It
also occurs frequently on grapefruit stock, which has been used
extensively in only a few sections of the State. It also occurs
frequently in old seedling trees, particularly in the sweet seed-
ling orange formerly widely planted in many parts of the State,
and in trees budded on sweet seedling orange stock. As a rule,
seedling trees are longer-lived than budded trees regardless of
the stock used. It is probable that this is why seedling orange
trees are decidedly more resistant to blight in general than
budded trees. Seedling grapefruit trees, which have been grown
extensively in only a few localities of the State, appear to de-
velop blight much less frequently than sweet seedling orange
No reference to the occurrence of blight on rough lemon stock
has been found in literature. This is probably due to the fact
that it was not used widely as a stock until after the freezes
of 1894-95. Very little blight is found at the present time in the
ridge section where rough lemon is used extensively as a root-
stock, although heavy losses frequently occur from a different
form of tree decline (see footnote 3, p. 12).
General observations throughout the citrus region of Florida
indicate that blight develops less rapidly in trees on rough lemon
stock than in those on sour orange stock. It is also evident'
that lemon trees and trees budded on rough lemon stock are
56 Florida Agricultural Experiment Station
more resistant to drought than sour orange trees and trees bud-
ded on sour orange stock. In this connection it is of interest
to note the conclusions of Boeuf and Genet (5), who contended
that the practice of budding on sour orange stock produces a
tree in which there is a minimum of channels for the conduction
of water and that, although the union with this stock gives the
orange a superior flavor, it lowers its resistance to drought con-
ditions. Susceptibility of the seedling orange tree to drought
appears to be due to its rather shallow root system. As a whole,
the majority of cases of blight observed within the State un-
questionably have been on sweet seedling orange trees and trees
budded on sweet orange and sour orange stocks.
Use of Commercial Fertilizers: Observations have shown that
blight develops regardless of kind and amount of fertilizers ap-
plied. Applications of certain readily soluble fertilizer salts have
injured the roots of citrus trees sufficiently to cause a heavy
defoliation and dropping of fruit, but the trees put out new
foliage later and were not permanently injured. It is quite
conceivable, however, that injury to the fibrous feeding roots
and even the smaller woody roots, resulting from prolonged
periods of drought, may be intensified by the application of
soluble fertilizer salts, which would greatly increase osmotic
concentration of the soil solution, especially when poorly dis-
tributed. This may happen as a result of the second of two
seasonal applications of fertilizer being made before sufficient
rain has fallen to render the first available to the trees. On
the other hand it has been observed that blight incidence in-
creases markedly with inadequate fertilization, deficits of organic
matter and general neglect in groves where it is prone to occur.
Other Factors: A number of other factors appear to be more
or less important in the development of blight. However, most
of them are operative by reason of their influence on the soil
moisture supply and the general vitality of the trees. Among
these may be mentioned close planting, topography of the soil,
deficits of organic matter, and general neglect. Close planting
appears to be an important factor in the development of blight
in soils subject to deficits of soil moisture. When trees are
planted too close together they compete with each other for
water and nutrients and under such conditions develop blight
at an early age.
In groves where soil topography is variable, it has been ob-
served that blight often occurs with greater frequency on knolls,
ridges, and slopes than on more nearly level land. In the central
Blight-A Non-Parasitic Disease of Citrus Trees 57
peninsula, where there is a greater variation in topography than
in the coastal sections, blight is encountered most frequently on
the slopes around lakes. The only apparent reason why blight
incidence should be higher on elevations and slopes is that water
from heavy rains frequently runs off without wetting the soil
very thoroughly. This is especially true where the surface soil
tends to become compact or where it has not been left furrowed
by cultivation along the contour lines.
Growers have made many and varied attempts to save blighted
trees and restore them to health, but without success. Both
trees and the ground under them have been treated with a large
assortment of fungicides, insecticides, and miscellaneous chemi-
cals. Blighted trees have been given various kinds and extra
applications of fertilizer. Often the first limb or individual
trunk to exhibit the symptoms of blight has been cut off, and
in more advanced cases trees have been dehorned more or less
severely, both with and without root pruning, in attempts to
stimulate healthy growth. Also, affected trees have been cut
back and topworked to other varieties and kinds of Citrus. They
have been treated by digging circular trenches around the trees
and filling these with manure and water. Blighted trees have
been raised, when it was thought that they had been set too low,
or were underlaid by hardpan, and have had the bark split with
a knife when they were thought to be "hidebound". Growers
and others have even tried to rejuvenate blighted trees by taking
them up and resetting them, both in the same soil and on other
land. None of these treatments has brought recovery.
Treatments involving cutting back the tops have proved most
encouraging to growers, since this stimulates vigorous growth.
However, this stimulation is only temporary because such treated
trees develop chronic wilt again when the water requirements
of the newly developed tops begin to exceed the capacity of the
root systems to supply them.
When a citrus tree once develops a true case of blight, no
amount of water supplied to it, even systematically over a period
of a year or more, restores it. In his work at Cocoa, Kay at-
tempted to rejuvenate a number of blighted trees in three dif-
ferent groves by irrigation, water being applied judiciously as
the soil tended to become dry, over a period of more than a year.
In addition, the trees were carefully pruned of dead wood and,
in some instances, were given extra applications of fertilizer.
58 Florida Agricultural Experiment Station
While the irrigation encouraged the development of a certain
amount of new growth for a time, and unquestionably prolonged
the life of the blighted trees by retarding the rate of decline,
all trees eventually became worthless from a commercial stand-
point and were removed by the grove managers. Consequently,
it is evident that irrigation offers no possibilities whatever as
a cure for blight. By the time affected trees exhibit a charac-
teristic blighted appearance the vitality of the root systems is
so depleted that no amount of water will restore them to their
former healthy, productive condition.
Results of investigations reported herein have shown that
blight must be prevented, since it cannot be controlled. Once the
disease has developed, the only recourse found thus far is to
remove the affected trees as soon as they cease to be profitable
Therefore, the following suggestions are offered for reducing
losses from blight.
1. In planting new groves, avoid poorly drained or drought
land, land closely underlaid by a large amount of rock, marl or
other undesirable structural characters, and land subject to
extreme fluctuations in the level of the water table. If such
lands must be used, the underlying rock and hardpan should
be broken and the rock removed before the trees are planted.
Also, adequate drainage or irrigation, or both on land subject
to extreme fluctuations in the level of the water table, should
2. Use rootstocks that are best adapted to the soils to be
3. Use irrigation, where a good system can be installed and
operated economically, as a safeguard against inadequate rain-
fall, especially on types of soils that do not retain moisture
well during the dry season.
4. In the absence of irrigation, practice mowing cover crops
during the dry season to conserve soil moisture.
5. Build up and maintain an adequate supply of organic mat-
ter in the soil by the growth of cover crops during the rainy
season, or by hauling in such organic matter as muck, leaf mold
and other vegetable debris from the woods, or green crops grown
on other land.
6. Leave the ground rough or furrowed in the direction of
the contour lines on slopes in the final cultivation before the
rainy season to retard the surface run-off of rainfall.
Blight-A Non-Parasitic Disease of Citrus Trees 59
7. In groves already established on the less desirable types
of soils the loss from blight can be greatly diminished, although
by no means entirely prevented, by employing the best recog-
nized cultural practices.
SUMMARY AND CONCLUSIONS
The results of 12 years' study and observations on blight in
bearing citrus trees are presented. The history of the disease,
theories in regard to its nature and cause, its distribution and
economic importance are discussed, and the symptoms are de-
scribed in detail.
Blight is a chronic wilt and decline which usually starts on
one side of a tree and progresses until the entire top is involved.
It is essentially a disease of older groves, rarely developing until
the trees have attained an age of 12 to 14 years. Although it
causes heavy losses in some localities, when considered from
the standpoint of the entire citrus industry of Florida blight
must be regarded as a comparatively unimportant trouble.
Blighted trees obviously die back as a result of failure of
the root systems to support the tops, rather than from any local
attack upon the top itself. This explains why it is not possible
to arrest its progress by cutting off the first limb or trunk to
become affected, or by transplanting, and why dehorning the
tops of affected trees only delays the progress of the trouble.
No organism that could be regarded as the cause of the
trouble has ever been found in any part of affected trees. More-
over, it has been clearly demonstrated that the disease is not
transmitted in budwood. Numerous trees propagated by buds
from wilted portions of typical blighted trees have been growing
for 12 years without showing any signs of the disease. These
results also indicate that blight is not caused by a virus.
Trees set out to replace blighted ones live just about as long
as did the original trees and do not appear to be any more sub-
ject to blight than those set out on virgin land under similar
conditions. If blight were a contagious disease it seems that
trees set in holes from which affected trees were removed would
contract the disease at an earlier age than the original ones.
Moreover, more or less serious losses from blight have occurred
in many groves since the early 1890's and yet these groves,
where still maintained, continue to be commercially productive.
The progressive dying out of trees in local areas-of groves
can be explained on the basis of the local soil and soil moisture
conditions, as well as on the theory of contagion. The non-
60 Florida Agricultural Experiment Station
parasitic nature of the disease is further emphasized with little
or no relation to the location of the trees which contracted the
The incidence of blight appears to be definitely correlated with
the soil and soil moisture conditions, including both deficits of
moisture on the lighter, drought soils and excesses on the lower,
poorly drained ones, and combinations of these two extremes.
It occurs most frequently on the lighter, droughtier types of
high hammock soils closely underlaid with rock.
Although it has not been possible to prove experimentally
that blight is caused by extreme fluctuations of soil moisture,
all the available evidence and observations secured over 12 years
appear to confirm this conclusion. Of course, it may be contended
that it is difficult to account for all cases of blight upon this
basis, as the disease is most prevalent on high, well-drained
land. If blight were induced to develop in trees on such land
as a result of deficits of soil moisture alone, it would be of wide-
spread occurrence throughout the citrus region of the State,
since periods of drought are of extremely frequent occurrence.
Extended observations have indicated that the physical struc-
ture of the soil, kind of rootseck and grove management are
contributing factors in development of the disease. The diffi-
culty of determining the effect of any one condition or combina-
tion of conditions as possible contributing factors in the devel-
opment of blight can be appreciated readily when it is consid-
ered that the investigator has little or no control over these
conditions or opportunity to segregate them. It has not even
been possible to determine experimentally the effect of deficits
of soil moisture on blight incidence under controlled conditions.
A less frequent but more or less identical chronic wilt and
decline of citrus trees, which is likewise regarded as blight by
growers, has been attributable to injury of the roots following
prolonged saturation of the soil during unusual rises of the
water table, not only in lowland soils of poor drainage but also
in low places in upland soils where drainage is fairly good ordi-
narily but not sufficient to take care of heavy rains or prolonged
wet periods which occur from time to time. In such locations
the trouble has been intensified by the occurrence of drought
periods following unusual rises of the water table, and conversely.
Blight, therefore must be regarded as a composite trouble.
The control of blight must be effected by the application of
preventive measures rather than of remedial ones. Despite
years of trial, no known method of treatment, including irriga-
Blight-A Non-Parasitic Disease of Citrus Trees 61
tion, has succeeded in effecting a permanent recovery of citrus
"trees that have once developed a genuine case of blight. After
this trouble develops, the only recourse available to growers is
to remove the affected trees as soon as they cease to become
profitable and replant.
Grateful acknowledgment is hereby made to Mr. A. B. Michael, regional
"vice-president of the American Fruit Growers, Inc., for his interest and
initiative in providing funds to undertake a special study of the citrus blight
problem, and for his subsequent cooperation in leasing a 2-acre tract of
land on which trees budded from blighted trees could be planted in com-
parison with those budded from healthy ones and maintained for a period
of years sufficient to justify the experiment. The funds contributed by
this organization, supplemented by those appropriated by the county com-
missioners of Brevard County, which was deemed the most logical place
to begin the work, made it possible to undertake this project. Since then
the work has been supported entirely by State funds appropriated to the
Florida Agricultural Experiment Station.
Acknowledgment is likewise made to Messrs. K. E. Bragdon and P. M.
Childers, former county agents of Brevard County, who were instrumental
in perfecting the arrangements for conducting the investigational work in
The writer also desires to express his appreciation to the various citrus
growers who have cooperated with him in the conduction of observations
and experimental work in their groves. He is also deeply appreciative of
the kindness of Drs. H. J. Webber and H. S. Fawcett of the Citrus Experi-
ment Station, Riverside, California, for reading the manuscript of this
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