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Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 337
Title: Commercial control of citrus scab in Florida
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00015116/00001
 Material Information
Title: Commercial control of citrus scab in Florida
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 47 p. : ill. ; 23 cm.
Language: English
Creator: Ruehle, George D
Thompson, W. L
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1939
 Subjects
Subject: Sour orange scab   ( lcsh )
Citrus -- Diseases and pests -- Control -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 47.
Statement of Responsibility: by Geo. D. Ruehle and W.L. Thompson.
General Note: Cover title.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
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Bibliographic ID: UF00015116
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000924567
oclc - 18214656
notis - AEN5194

Table of Contents
    Front Cover
        Page 1
        Page 2
    Main
        Page 3
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        Page 6
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        Page 17
        Page 18
        Page 19
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        Page 25
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    Bibliography
        Page 47
Full Text



September, 1939


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA
WILMON NEWELL, Director





COMMERCIAL CONTROL

OF CITRUS SCAB

IN FLORIDA


By GEO. D. RUEHLE and W. L. THOMPSON


Fig. 1.-Citrus scab on leaves and fruits of the rough lemon.


Single copies will be sent free to Florida residents upon request to
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA


Bulletin 337









EXECUTIVE STAFF
John J. Tigert, M.A., LL.D., President of
the University3
Wilmon Newell, D.Se., Directors
Harold Mowry, M.S.A., Asst. Dir., Research
V. V. Bowman, M.S.A., Asst. to the Director
J. Francis Cooper, M.S.A., Editor3
Jefferson Thomas, Assistant Editors
Cly.e Beale, A.B.J., Assistant Editors
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Managers
K. H. Graham, Business Managers
Rachel McQuarrie, Acoountant3

MAIN STATION, GAINESVILLE

AGRONOMY
W. E. Stokes, M.S., Agronomist'
W. A. Leukel, Ph.D., Agronomist3
G. E. Ritchey, M.S., Associate2
Fred H. Hull, Ph.D., Associate
W. A. Carver, Ph.D., Associate
John P. Camp, M.S., Assistant
Roy E. Blaser, M.S., Assistant
ANIMAL HUSBANDRY
A. L. Shealy, D.V.M., Animal Husbandman' a
R. B. Becker, Ph.D., Dairy Husbandmans
L. M. Thurston, Ph.D., Dairy Technologists
W. M. Neal, Ph.D., Asso. in An. Nutrition
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M., Veterinarian3
N. R. Mehrhof, M.Agr., Poultry Husbandman3
0. W. Anderson, M.S., Asst. Poultry Husb.3 4
W. G. Kirk, Ph.D., Asso. An. Husbandman3
R. M. Crown, M.S.A., Asst. in An. Husb.3
P. T. Dix Arnold, M.S.A., Assistant Dairy
Husbandman3
L. L. Rusoff, M.S., Asst. in An. Nutrition3
CHEMISTRY AND SOILS
R. V. Allison, Ph.D., Chemist1 3
F. B. Smith, Ph.D., Microbiologists
C. E. Bell, Ph.D., Associate Chemist
H. W. Winsor, B.S.A., Assistant Chemist
J. Russell Henderson, M.S.A., Associate3
L. H. Rogers, M.A., Asso. Biochemist
Richard A. Carrigan, B.S., Asst. Chemist
ECONOMICS, AGRICULTURAL
C. V. Noble, Ph.D., Agricultural Economist'
Bruce McKinley, A.B., B.S.A., Associate
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Assistant
ECONOMICS, HOME
Ouida Davis Abbott, Ph.D., Specialist'
Ruth Overstreet, R.N., Assistant
R. B. French, Ph.D., Associate Chemist
ENTOMOLOGY
J. R. Watson, A.M., Entomologist'
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant

HORTICULTURE
G. H. Blackmon, M.S.A., Horticulturistx
A. L. Stahl, Ph.D., Associate
F. S. Jamison, Ph.D., Truck Horticulturists
R. J. Wilmot, M.S.A., Specialist, Fumigation
Research
R. D. Dickey, B.S.A., Assistant Horticulturist
J. Carlton Cain, B.S.A., Asst. Horticulturist
Victor F. Nettles, M.S.A., Asst. Hort.
PLANT PATHOLOGY
W. B. Tisdale, Ph.D., Plant Pathologist's
George F. Weber, Ph.D., Plant Pathologist3
L. 0. Gratz, Ph.D., Plant Pathologist
Erdman West, M.S., Mycologist
Lillian E. Arnold, M.S., Assistant Botanist


BOARD OF CONTROL
R. P. Terry, Chairman, Miami
Thomas W. Bryant, Lakeland
W. M. Palmer, Ocala
H. P. Adair, Jacksonville
Chas. P. Helfenstein, Live Oak
J. T. Diamond, Secretary, Tallahassee

BRANCH STATIONS

NORTH FLORIDA STATION, QUINCY
J. D. Warner, M.S., Agronomist Acting in
Charge
R. R. Kincaid, Ph.D., Asso. Plant Pathologist
Jesse Reeves, Farm Superintendent

CITRUS STATION, LAKE ALFRED
A. F. Camp, Ph.D., Horticulturist in Charge
John H. Jefferies, Superintendent
Michael Peech, Ph.D., Soils Chemist
B. R. Fudge, Ph.D., Associate Chemist
W. L. Thompson, B.S., Asso. Entomologist
W. W. Lawless, B.S., Asst. Horticulturist
R. K. Voorhees, M.S., Asst. Plant Path.

EVERGLADES STATION, BELLE GLADE
J. R. Neller, Ph.D., Biochemist in Charge
J. W. Wilson, Sc.D., Entomologist
F. D. Stevens, B.S., Sugarcane Agronomist
Thomas Bregger, Ph.D., Sugarcane
Physiologist
Frederick Boyd, Ph.D., Asst. Agronomist
G. R. Townsend, Ph.D., Plant Pathologist
R. W. Kidder, B.S., Asst. An. Husbandman
W. T. Forsee, Ph.D., Asso. Chemist
B. S. Clayton, B.S.C.E., Drainage Engineer2
SUB-TROPICAL STATION, HOMESTEAD
W. M. Fifield, M.S., Horticulturist Acting in
Charge
S. J. Lynch, B.S.A., Asst. Horticulturist
Geo. D. Ruehle, Ph.D., Asso. Plant Pathologist
W. CENTRAL FLA. STA., BROOKSVILLE
W. F. Ward, M.S., Asst. An. Husbandman
in Charge2

FIELD STATIONS
Leesburg
M. N. Walker, Ph.D., Pant Pathologist in
Charge
K. W. Loucks, M.S., Asst. Plant Pathologist
Plant City
A. N. Brooks, Ph.D., Plant Pathologist
Cocoa
A. S. Rhoads, Ph.D., Plant Pathologist
Hastings
A. H. Eddins, Ph.D., Plant Pathologist
Monticello
Samuel 0. Hill, B.S., Asst. Entomologist2
Bradenton
Jos. R. Beckenbach, Ph.D., Truck Horticul-
turist in Charge
David G. Kelbert, Asst. Plant Pathologist
Sanford
R. W. Ruprecht, Ph.D., Chemist in Charge,
Celery Investigations
W. B. Shippy, Ph.D., Asso. Plant Pathologist
Lakeland
E. S. Ellison, Meteorologist2
B. H. Moore, A.B., Asst. Meteorologist2
1Head of Department.
2In cooperation with U.S.D.A.
'Cooperative, other divisions, U. of F.
40n leave.








COMMERCIAL CONTROL OF CITRUS SCAB
IN FLORIDA

By GEO. D. RUEHLE and W. L. THOMPSON

CONTENTS
Page Page
Introduction ..............- ...... ............. .. 3 Effect of added sulfur or oil to copper
Symptoms of scab ............... ................... 4 sprays ...................................................... 23
Control of melanose with scab sprays .... 26
Cause of scab ........... .................................... 6 o ined insect and s cab control .............. 31
Combined insect and scab control .......... 31
Contributing conditions ............................ 7 Scale increase following copper sprays 32
Spraying experiments ................................. 8 Control of scale-insects with combina-
Methods and m materials .............................. 8 tion sprays .............................................. 32
Dormant vs. bloom sprays ............ 10 Concentrations and timing of oil sprays 39
Discussion and conclusions ..................... 40
Sulfur sprays ............................................... 11 cushion and conclusions .......... ............. 4
Sum m ary ................ ... .... ..... ........... 45
M ercury sprays .. .. ..... ........................... 16 Acknowledgm ents .. ...... .. ...... ........ 47
Copper sprays ... .......................................... 18 Literature cited ..... .. -..-.... ... ..... ..- --.... 47

INTRODUCTION
The disease variously known in Florida as citrus scab, sour
orange scab, or lemon scab is one of the oldest and most im-
portant of citrus diseases. Known in the Orient since ancient
times, it is believed to have been introduced into the United
States on Satsuma orange trees from Japan. In Florida it was
first observed near Ocala in 1885 on sour orange nursery stock
and gradually spread to all the citrus-producing sections of the
State and later to the Gulf States (11)1.
Bordeaux mixture and bordeaux-oil emulsion have been ex-
tensively used for the control of citrus scab. Lime-sulfur sprays
also have proved to be of value but are less effective than
bordeaux and are ineffective where the disease is of primary
importance.
The use of bordeaux mixture has never proved to be entirely
satisfactory on citrus trees under Florida conditions because
of its pronounced tendency to increase destructive insects,
particularly the citrus scale-insects and rust mites. Beginning
in 1932, considerable experimental work has been done in vari-
ous parts of the citrus-producing section of Florida to develop
improved methods for control of citrus scab in commercial
groves. Started originally as a disease-control project, the work
was extended in 1934 to include studies of the effect of various
spray materials on the scale-insects and rust mites with the
purpose in view of developing spray schedules to control both
the insects and the disease.

1Italic figures in parentheses refer to "Literature Cited" in the back
of this bulletin.







Florida Agricultural Experiment Station


The present paper includes the results obtained in scab con-
trol with bordeaux mixture, various proprietary insoluble copper
sprays, mercury compounds, lime-sulfur and other sulfur sprays.
Data are also presented on scale increases following bordeaux
mixture and proprietary insoluble copper compounds. The effec-
tiveness of various sprays for scale control following copper
sprays for scab control is also discussed. Work on the insect
phase of the scab problem is being continued by the junior
author.
SYMPTOMS OF SCAB
The outstanding characteristic of scab is the formation of
irregular, more or less raised, scabby or wart-like lesions on
infected parts.
On the leaves the lesions first appear as minute, circular,
semitranslucent spots which quickly become raised excrescences
with corky crests on one side of the leaf, often with a conical
depression on the opposite side. The lesions occur on either
surface but are more numerous on the under side. The crests


Fig. 2.-Citrus scab on mature grapefruit.







Commercial Control of Citrus Scab in Florida


of the scabs are variously colored, usually pale yellowish or
buff at first, then changing to pinkish buff or rose and finally
to dull olive drab. Severely infected leaves may be distorted,
wrinkled, or stunted (Fig. 1).
On the fruits the lesions appear either as raised protuberances
tipped by corky crests and accompanied by more or less distor-
tion of the fruit (Fig. 4), or as irregular slightly raised corky
scabs without much distortion (Figs. 1 and 2). When infection
is severe, the lesions may coalesce to form large irregular patches
of scabby tissue. On grapefruit the protuberances appearing
on young fruits may flatten out and the fruit regain its normal
shape when it attains full size. This is frequently accompanied
by a more or less complete flaking off of the corky tissues of
lesions produced on fruits which set in early spring, leaving
scar tissue which cannot always be readily distinguished from
mechanical injuries (Fig. 2). On mature grapefruit the skin
immediately surrounding the scabs has a decided tendency to


Fig. 3.-Citrus scab on grapefruit set from late bloom.







Florida Agricultural Experiment Station


remain green, which is an aid in diagnosis of the disease. Scabs
on lemon, tangelo, or on grapefruit set from a late bloom seldom
flake off (Figs. 1 and 3). The crests of the scabs exhibit the
various color changes noted for leaf lesions.
Lesions on twigs sometimes develop on very succulent growth
of the more susceptible varieties. They have much the same
appearance as on leaves and fruit but are usually smaller.




















Fig. 4.-Citrus scab on half-grown fruits of Samson tangelo.

CAUSE OF CITRUS SCAB
Citrus scab is caused by a minute parasitic fungus which
grows and fruits on the surface of the scabby lesions. There
was considerable confusion regarding the identity of the causal
fungus until 1925 when the critical study of this and related
forms by Jenkins (6) established the true classification of the
organism in the form genus Sphaceloma. The name Sphaceloma
fawcetti Jenkins has been generally accepted since that date.
Recently a perfect spore stage for this species was described
in Brazil as Elsinoe fawcetti Bit. & Jenkins (1).
The spores of S. fawcetti are disseminated by such agencies
as winds, rains, dew-drip, and possibly by insects. These spores
are able to infect only the young and tender growing parts of
susceptible species and varieties. Winston has shown (14) by
extensive inoculation experiments that, under Florida condi-







Commercial Control of Citrus Scab in Florida


tions, young grapefruits are extremely susceptible immediately
after the falling of the petals and become progressively resistant
until they reach immunity at about 3/4 of an inch in diameter.
With leaves the susceptibility is greatest as they emerge from
the bud, and they gradually become entirely resistant by the
time they reach 1, inch in width.
The fungus lives over from one season to the next on infected
leaves and to a lesser extent on infected fruits.
Although the leaves, twigs, and fruits of many kinds of citrus
may be attacked, certain varieties within a given species may
be very susceptible to the disease while others are highly re-
sistant or even immune. Among the commercial varieties or
species grown in Florida, the sour orange, rough lemon, com-
mon lemon, and some of the tangelos are extremely susceptible.
Grapefruit varieties (except Royal and Triumph) are also quite
susceptible to infection. Because of the extent of grapefruit
plantings in Florida the greatest commercial injury caused by
scab is to the grapefruit crop. The disease is of importance
also on the King orange and Satsuma orange, and less frequently
on the Persian lime, the tangerine, and the Temple orange. The
Mexican or Key lime, kumquat, citron and Cleopatra mandarin
are apparently immune.
Citrus scab has been reported occasionally on sweet or round
oranges in Florida but such attacks are rare and very mild,
so that for all practical purposes sweet oranges may be con-
sidered immune. A sweet orange fruit scab, caused by a closely
related species, Elsinoe australis Bitan. & Jenkins, has been
described recently, as causing serious damage to sweet oranges
in South America (3, 7).
Avocado scab is caused by a species of Sphaceloma which was
formerly believed to be identical with S. fawcetti. Jenkins has
recently shown (8) that the avocado Sphaceloma is a distinct
species which she has named Sphaceloma perseae.
CONTRIBUTING CONDITIONS
The development of scab may vary greatly from year to year,
depending upon climatic conditions prevailing during the stage
of growth when the foliage and fruits are susceptible to attack
and upon the abundance of the sources of infection at these
critical periods.
Winston (14) has shown that, under average Florida condi-
tions, moisture is the important limiting factor in natural in-






Florida Agricultural Experiment Station


fection. The moisture need not be in the form of rain, since
scab infection is commonly quite severe even during seasons of
minimum rainfall in localities where heavy dews or fogs occur
during critical periods for infection. The disease is of annual
occurrence on susceptible varieties in groves planted in regions
of low hammocks or flatwoods soils, which are usually damp
localities. In the high and drier sand ridge sections scab is
a negligible factor during average years and only becomes seri-
ous during seasons unusually favorable to its development.
Temperature within certain limits is necessary for infection
and this factor has been shown (4, 9) to be of importance in
limiting the world distribution of scab. It has been shown that
the temperature range for severe infection is between 59 and
73 degrees F. Under Florida conditions the mean temperatures
seldom exceed these limits from the time new growth appears
in spring until the fruits are no longer susceptible to infection.
During the warmer months temperatures may drop low enough
for periods sufficiently long to permit infection to occur. Thus
severe infection may occur on nursery trees during the summer
when abundant rains coincide with the presence of unmature
foliage, although the mean monthly temperature may be above
73.4 degrees F.
There is considerable evidence that excessive nitrogenous fer-
tilization favors the development of scab, by inducing a large
amount of lush growth with subsequent slow maturation, espe-
cially if this occurs during periods of optimum moisture con-
ditions.
There is practically no evidence that the use of one rootstock
favors scab infection more than another.

SPRAYING EXPERIMENTS FOR CONTROL OF SCAB
METHOD OF PROCEDURE AND MATERIALS
The experiments were conducted mostly in commercial grape-
fruit groves situated in localities where citrus scab is of annual
occurrence and of major importance. Several tests were carried
out also in the sand ridge section on Samson tangelos and King
oranges where the disease is serious on these varieties but rela-
tively unimportant on grapefruit.
Of necessity, most of the experiments were conducted co-
operatively with certain commercial growers who were vitally
interested in the problem. The cooperative agreement adhered
to was, to the effect that the Experiment Station furnished the







Commercial Control of Citrus Scab in Florida


fungicides and insecticides, supervised their application, and
checked the results, while the grower furnished the machinery
and labor for the application of the sprays and cared for the
experimental plots in the same manner as the remaining portion
of the grove.
Groves selected for the tests were those having a sufficient
number of trees of the same age, variety, and rootstock, and
which showed nearly equal scab infection and scale infestation
as determined by an examination of the leaves. In groves where
marked differences in scab infection were noted in different por-
tions, replications were made to overcome this variation. The
plots were of approximately equal size in individual groves but
were varied in size in the different groves according to the size
or bearing ability of the trees. They were made sufficiently
large that at least 1,000 hanging fruits could be readily and
conveniently examined from each treatment, provided an aver-
age crop was produced. Occasionally adverse growing conditions
reduced the yield below this desired goal.
Fungicides tested included several proprietary copper, mer-
cury, and sulfur preparations, liquid lime-sulfur, bordeaux mix-
tures, and compatible combinations of many of these fungicides
with insecticides. Many of the proprietary products were new
or little known at the time and the recommendations of the
manufacturer or field representative were followed in mixing
and applying these sprays. In the earlier tests bordeaux mix-
ture was made with either equal amounts of rock lime and blue-
stone, or of one-third more hydrated lime than bluestone. In
all the later experiments with bordeaux mixture this spray
was prepared with equal amounts of bluestone and hydrated
lime, since the rock lime gave no better results and was more
difficult to handle, and it was found unnecessary to increase
the amount of lime in the hydrated lime formula. The materials
for making bordeaux mixture were always of the best com-
mercial grade obtainable.
The sprays were all applied with power machinery at 300 or
more pounds pressure, and various types of spray guns supply-
ing a mist type of spray were used during the course of the
tests. Generally one of the non-casein colloidal spreaders was
added to the sprays. However, combinations of oil emulsion
with bordeaux mixture or other copper compounds were equally
satisfactory, as were the combinations of wettable sulfur with
the various copper sprays.







Florida Agricultural Experiment Station


In checking the fruits for severity of scab infection, 1,000
or more from each plot were examined after the danger of fur-
ther infection was definitely past and usually before the fruits
were more than half grown, in order that the scab lesions could
be readily distinguished from other types of blemishes. Three
degrees of infection were arbitrarily distinguished as follows:
slight, or those with one to several small lesions not seriously
affecting the appearance; moderate, several to many small
lesions affecting the appearance enough to exclude from higher
grades; and severe, those seriously blemished or distorted
(Fig. 5).


Fig. 5.-Comparative severity of scab infection. Lower left, no infection; lower right,
slight infection; upper right, moderate; upper left, severe. Fruits about half grown.

DORMANT VS. BLOOM SPRAYS
The logical time for applying protective sprays is shortly
before the spring flush of growth appears. At this time the
causal fungus is relatively inactive on the surface of the old







Commercial Control of Citrus Scab in Florida


scabs on leaves, twigs, and certain fruits. Thorough applica-
tions of effective fungicides to cover these old scabs eliminate
them as sources of infection to the expanding leaves of the
spring flush of growth, and consequently eliminate the main
source of spores for fruit infections later in the season. Some
fruit infection may occur from wind-borne spores from other
groves or from infected rough lemon or sour orange sprouts
or from escape plants. It should be- pointed out, in this con-
nection, that spray coverage is rarely perfect and some scabs
are generally missed by this spray.
In cases where the dormant spray is omitted because scab
was not very abundant on the old foliage but, due to favorable
conditions for infection, built up rapidly on the spring flush
of growth, a protective spray may be applied when most of
the petals are shed.
A comparison of the efficacy of dormant and bloom applica-
tions of bordeaux mixture for the control of scab on grapefruit
was made in 1932 and repeated in 1935 and 1936. The loca-
tion of the tests, the formulas, and the results obtained, are
recorded in Table 1.
The results indicate that under most conditions, better con-
trol will result from the dormant application.
At Vero Beach in 1935 the control was the same with either
application, but that was an unfavorable season for early scab
infection because of dry weather in the early spring months.
Most of the infection that occurred in this year took place after
the petals were shed. In the same locality in 1936 scab infec-
tion was more severe in the control plot and the bloom applica-
tion gave poorer control than the dormant spray. Control of
fruit infection was still less efficient with the bloom spray at
Bradenton in 1932, under conditions of severe scab infection.
The chief objections to spraying in the bloom for scab control
are that the crop may be reduced because of toxicity of the
spray to the small fruits and opened blossoms, and that two or
three days' delay in applying the spray due to unfavorable
weather conditions may allow for considerable infection. The
practice is not applicable, therefore, for large groves where it
may take a week or more to finish the spraying operation.
SULFUR SPRAYS FOR THE CONTROL OF SCAB
In the spraying experiments performed by other workers
(5, 14) prior to 1932, liquid lime-sulfur proved of some value













TABLE 1.-COMPARISON OF DORMANT AND BLOOM APPLICATIONS OF BORDEAUX MIXTURE FOR THE CONTROL OF CITRUS SCAB
ON GRAPEFRUIT IN COMMERCIAL GROVES.


Spraying Schedule
Fungicide


bordeaux mixture
bordeaux mixture

bordeaux mixture

check


and
Year



Bradenton
1932





Vero Beach
1935




Vero Beach
1936


6-6-100
6-6-100

6-6-100


mixture 6-6-100

mixture 6-6-100


bordeaux mixture 6-6-100
bordeaux mixture 6-6-100


check


[ Scab Infection of Fruits by Classes
Stage of Growthl None I Slight IModeratel Severe
Percent Percent Percent Percent

dormant 94.2 5.6 0.2 0.0
bloom

bloom 69.6 22.2 7.6 0.6
26.4 42.0 22.0 9.6


dormant 96.6 2.0 0.7 0.7

bloom 96.6 2.4 0.7 0.3

69.6 11.8 7.2 11.4


dormant 96.1 3.2 0.4 0.3

bloom 84.3 7.8 4.8 3.1


bordeaux

bordeaux

check







Commercial Control of Citrus Scab in Florida


for scab control, but was much less effective than bordeaux
mixture. The use of lime-sulfur solution was generally recom-
mended where the disease was not likely to be severe, however,
because the elimination of red spiders and reduction of rust-
mite injury following one or two applications was considered
as money well invested, even if scab did not appear.
During 1932 and 1933 experiments with liquid lime-sulfur and
other sulfur spray solutions were carried out in the laboratory,
greenhouse, and in commercial groves. At this time new forms
of sulfur were being offered for sale in Florida as substitutes
for liquid lime-sulfur, and many of these preparations were
included in either greenhouse or field tests, to determine the
possibility of using them for the control of scab.
In the laboratory the addition of lime-sulfur solution, ben-
tonite sulfur, or colloidal sulfur to culture media in amounts
equal to concentrations used in field spraying failed to inhibit
the growth of S. fawcetti on such media, establishing the fact
that the fungus is quite tolerant to sulfur.
The greenhouse experiments consisted of spraying susceptible
leaves of potted rough lemon seedlings with solutions of fungi-
cides made up to standard concentrations, and 24 hours later
inoculating such leaves with a suspension made from pure cul-
tures of the causal fungus. Unsprayed leaves were inoculated
at the same time to serve as controls. Liquid lime-sulfur 2.5-100
allowed a slight infection to take place, but dry lime-sulfur,
bentonite sulfur and a proprietary liquid sulfur preparation
gave very little reduction in severity of attack. In the same
series of experiments 6-6-100 bordeaux mixture gave nearly
absolute protection to the sprayed foliage.
The spraying experiments in commercial groves in 1932 in-
cluded tests with liquid lime-sulfur, dry lime-sulfur, and ben-
tonite sulfur in comparison with bordeaux mixture. Since liquid
lime-sulfur gave better control of scab than the other forms but
lacked sufficient toxicity to give good control, experiments in
1933 were made with liquid lime-sulfur solution with the sulfur
content increased by the addition of bentonite or wettable sulfurs.
The spray formulas, location of tests, and results obtained
in both seasons are shown in Table 2.
It may be readily seen from Table 2 that liquid lime-sulfur
is more effective than dry lime-sulfur or bentonite sulfur for
the control of scab, but it is very inferior to bordeaux mixture
even when its sulfur content is increased by the addition of













-TABLE 2.-COMPARISON OF BORDEAUX MIXTURE AND SULFUR SPRAYS FOR THE CONTROL OF CITRUS SCAB IN COMMERCIAL
CITRUS GROVES.


Spray Schedule


Materials and Concentrations I Stage of


Locality
and
Year




Lake Alfred
1932








Bradenton
1932


Scab Infection of Fruits by Classes


Growth None I


dormant
bloom

dormant
bloom




dormant
bloom

dormant
bloom

dormant
bloom


Percent

96.5

11.6

3.6


94.2

30.4

29.8


26.4


Slight IModeratel Severe


Percent Percent

2.7 0.3


20.0 23.6

13.7 15.8


5.6 0.2


Variety




Samson
tangelo








Seedling
grapefruit


Percent

0.5


44.8

66.9


0.0


6.6

9.6


9.6


bordeaux mixture 6-6-100
bordeaux mixture 6-6-100

liquid lime-sulfur 4-100
liquid lime-sulfur 2%-100

none-check


bordeaux mixture 6-6-100
bordeaux mixture 6-6-100

liquid lime-sulfur 4-100
liquid lime-sulfur 21-100

bentonite sulfur 6-100
bentonite sulfur 4-100

none-check


20.2


22.0


: '"




TABLE 2.-COMPARISON OF BORDEAUX MIXTURE AND SULFUR SPRAYS FOR THE CONTROL OF CITRUS SCAB IN COMMERCIAL
CITRUS GROVES-Concluded.


Locality
and
Year





Vero Beach
1932








Bradenton
1933


Variety






Marsh
grapefruit








Seedling
grapefruit


Spray Schedule
Materials and Concentrations

bordeaux mixture 6-6-100
bordeaux mixture 6-6-100
liquid lime-sulfur 4-100
liquid lime-sulfur 2%-100
dry lime-sulfur 8-100
dry lime-sulfur 5-100
bentonite sulfur 6-100
bentonite sulfur 4-100
none-check

bordeaux mixture 4-6-100
bordeaux mixture 4-6-100
liquid lime-sulfur 4-100,
wettable sulfur 5-100
liquid lime-sulfur 2%2-100,
wettable sulfur 5-100
none-check

bordeaux mixture 6-8-100
bordeaux mixture 6-8-100
liquid lime-sulfur 4-100,
bentonite sulfur 7%-100
liquid lime-sulfur 2Y2-100,
bentonite sulfur 5-100


46.4

19.8-

87.5


39.1


19.2

97.2


62.6


* 27.4

26.4

8.0


15.5


12.1

2.1


8.2


_I none-check I _19.2 1 18.8 ] 18.2 1 43.8


I Scab Infection of Fruits by Classes
Stage of Growthl None I Slight |Moderatel Severe
Percent Percent Percent Percent
dormant 88.8 7.6 2.0 1.6
bloom
dormant 55.2 25.4 12.0 7.4
bloom
dormant 29.2 .34.0 23.4 13.4


Vero Beach
1933


bloom
dormant
bloom


dormant
bloom

dormant
bloom



dormant
bloom

dormant
bloom


Marsh
grapefruit


0
o


11.4

24.2 o

1.5 Z


27.6


54.4

0.2 -.
9'







Florida Agricultural Experiment Station


wettable or bentonite sulfurs. In these tests it was used at
concentrations close to the limit for safety from burning of
foliage. The higher concentration for the dormant application
frequently causes some defoliation and dropping of mature
fruits. A severe dropping of mature fruits and some defolia-
tion followed the use of this spray in test plots at Bradenton
in 1933.
Since the lime-sulfur-wettable sulfur schedule gives partial
control of scab under conditions of moderate to severe infection,
it may be expected to prevent the disease from increasing in
severity in trees that have been kept relatively free of infection
the previous season by the use of copper sprays. This point
was checked in a grapefruit grove near Vero Beach in 1934.
Trees sprayed twice in 1933 with bordeaux mixture were
sprayed in 1934 with liquid lime-sulfur with bentonite sulfur
added, before the spring flush of growth appeared and again in
the last of the bloom. Experiments with copper fungicides were
carried out in portions of the same grove that were not sprayed
the previous season and where scab had been severe.
Since the desirability of controlling melanose must not be lost
sight of in spraying for scab, fruits from the various plots were
examined for melanose blemish as they neared maturity. The
spray schedules, severity of scab infection and of melanose
blemish on the fruits, are shown in Table 3.
The results indicate that in trees protected from scab infec-
tions one season to the extent that the old foliage is carried over
winter relatively free of disease, scab may be held in check the
following season by using lime-sulfur-wettable sulfur sprays.
It may be observed from Table 3, however, that the sulfur sprays
applied for scab control have little effect in reducing melanose
blemish on the fruits. In trees old enough for melanose to be
important, therefore, it seems inadvisable to recommend the
use of lime-sulfur sprays for the control of scab, even in case
the infection is expected to be mild.
MERCURY SPRAYS FOR THE CONTROL OF SCAB
An oil emulsion containing ethyl mercury oleate as fungicide
was introduced to the trade in 1932 as a substitute for bordeaux-
oil emulsion. Samples of the spray were furnished by the
manufacturer for experimental tests.
Spraying experiments on potted seedlings 1of rough lemon in
the greenhouse indicated that the mercury-oil emulsion was








TABLE 3.-RESULTS OBTAINED WITH A LIME-SULFUR SCHEDULE IN 1934 FOLLOWING A BORDEAUX SCHEDULE IN 1933 FOR
THE CONTROL OF SCAB AND THE RELATIVE EFFICACY OF SULFUR AND BORDEAUX SCHEDULES FOR THE CONTROL OF MELANOSE.


Condition of
trees in 1933 Spr


Unsprayed- bordeaux-c
scab severe lime-sulfur


Unsprayed- bordeaux-c
scab severe bordeaux


Unsprayed- unsprayed
scab severe


Sprayed with lime-sulfu
copper sprays- bentonit
scab slight lime-sulfu


ay Schedule I


il 6-8-100-1%
1-40


il 6-8-100-1%
3-4-100


r 4-100,
e sulfur 6-100
r 2.5-100,


bentonite su]

*0-10 lesions per sq. inch.
tll-50 lesions per sq. inch.
tMore than 50 lesions per sq. inch.


lfur 3-100


1934


-Jan. 25
-Apr. 13


-Jan. 25
-Apr. 13






-Jan. 25

-Apr. 13


Degree of Scab Infection of Fruits
by Classes
None I Slight I Moderate Severe
percentt Percent Percent Percent

91.4 5.4 2.2 1.0


95.3 3.1 1.2 0.4


3.1 6.4 8.9 81.6



93.6 3.2 1.5 1.7

1I


Degree of Melanose Blemish
of Fruits by Classes
Slight* iModeratetf Severet
Percent Percent Percent

70.3 30.7 1.0


82.9 16.1 1.0



30.5 60.2 9.3



46.9 50.4 2.7







Florida Agricultural Experiment Station


highly toxic to the spores of the causal fungus shortly after
it was sprayed on leaves. This formula was compared with
bordeaux mixture in 1932 for the control of scab in a com-
mercial grapefruit grove. The disease was as severe in the
trees sprayed with this emulsion as in the controls.
In 1934 two variations of the formula were again tested com-
paratively with copper sprays on grapefruit and tangelo trees.
The formulas and the results obtained from these tests are shown
in Table 4.
The results show that the organic mercury-oil emulsions are
relatively ineffective for the control of scab. The failure to
secure better control was probably due to the lack of good ad-
hesive properties of the emulsion. Since the oil in the spray
was not superior to ordinary cheaper oil emulsions for insect
control, further tests with this spray were discontinued.

COPPER SPRAYS FOR THE CONTROL OF SCAB
When the spraying experiments were started in 1932 home-
made bordeaux mixture was the only copper spray used exten-
sively in Florida citrus groves. A proprietary copper-oil spray
and commercially prepared bordeaux powders were used in a
limited way but other copper compounds were practically un-
known. An effective bordeaux mixture requires careful mixing
and involves considerable labor and loss of time. A number
of instances of poor control of scab with bordeaux mixture due
to improper mixing of the spray have been brought to the
writers' attention. There was a need for citrus spray materials
that would leave little residue on the fruit and foliage, would
be easier to employ than bordeaux mixture, would possess good
fungicidal properties, and would be less objectionable than bor-
deaux in promoting insect increase.
A number of insoluble copper spray materials have appeared
on the market since 1932, and growers quickly became interested
in the possibilities of using them as substitutes for bordeaux
mixture. In general these newer copper materials are easier
to employ than bordeaux, since they do not require the careful
mixing with lime for neutralization. Since the use of lime is
not required less residue is deposited on the foliage. Thomp-
son (12) has shown that hydrated lime sprayed on citrus foliage
distinctly favors the increase of scale-insects, and there was a
possibility that the insoluble copper materials would prove less
objectionable than bordeaux in this respect.






TABLE 4.-CoMPARISON OF COPPER AND MERCURY SPRAYS FOR THE CONTROL OF CITRUS SCAB IN COMMERCIAL GROVES.


Locality
and
Year



Bradenton
1932


Scab Infection of Fruits by Cl
Stage of Growthl None I Slight ]Moderate| S
Percent Percent Percent P
bloom 69.6 22.2 7.6

bloom 27.6 42.0 21.2


Variety



Seeded
grapefruit







Marsh
grapefruit







Samson
tangelo


Spraying Schedule
Fungicide_


bordeaux mixture 6-6-100

mercury-oil emulsion CS-1-M, 1-48

check


bordeaux mixture 6-8-100
bordeaux mixture 3-4-100

mercury-oil emulsion CS-2, 1-60
mercury-oil emulsion CS-2, 1-60

mercury-oil emulsion CS-9, 1-60
mercury-oil emulsion CS-9, 1-60

check


basic copper sulfate 3-100
basic copper sulfate 3-100

mercury-oil emulsion CS-9, 1-60
mercury-oil emulsion CS-9, 1-60

check


25.8

3.1


14.9

0.8


dormant
bloom

dormant
bloom

dormant
bloom


Vero Beach
1934







Lake Alfred
1934


dormant
bloom

dormant
bloom


asses
evere
percent Q

0.6
-t
9.2 2.

9.6 C


0.4 -

19.4

49.6
Ce

81.6

0-
62

63.2


99.2


.---






Florida Agricultural Experiment Station


The chief objection to most of these newer sprays is the rela-
tively high cost when compared with the cost of home-made
bordeaux mixture. Several of the insoluble copper materials
were included in the spraying experiments in comparison with
bordeaux mixture and the proprietary copper-oil spray men-
tioned above. More tests were made with basic copper sulfate
than other insoluble copper sprays mainly because other copper
sprays were not available or failed to prove desirable after trial.
The comparative results in control of scab are recorded in
Table 5.
The results show that most of the insoluble copper sprays
included in these tests will give about as effective control of
scab as bordeaux mixture, when used at approximately equal
concentrations of metallic copper. They did not all prove to
be desirable for use on citrus, however.
The copper cyanamide tested in 1932 left unsightly black
deposits of spray residue on the fruit and foliage and increased
scale-insects fully as much as bordeaux mixture. Since the
control of scab was not as effective as with bordeaux, further
tests were not made with this form of copper.
The proprietary copper-oil spray included in the experiments
in 1934 and 1935 was used, in accordance with the recomfimenda-
tions of the manufacturer, at concentrations relatively low in
copper content. Under the conditions of mild infection in 1935
it gave as effective control of scab as the other copper sprays,
but was less effective in 1934, under conditions of severe in-
fection. A severe burn of the fruits resulted from a post-bloom
application of this material to the test plots at Vero Beach in
1935. Approximately 60 percent of the fruits were affected
and other growers using this spray in the same locality suffered
a similar experience that year. Because of this possibility of
damage to the fruits, further tests were not made with this
spray.
Basic copper sulfate, containing 53 percent metallic copper,
gave as effective control of scab as bordeaux mixture in experi-
ments carried on over a period of three years and under condi-
tions of infection ranging from mild to severe. Since it is
also reasonable in price and may be combined with insecticides
that are compatible with bordeaux mixture, it is recommended
for the control of scab.
It may be observed from Table 5 that a delayed dormant
application was made in two of the test groves in 1935. This





TABLE 5.-COMPARISON OF BORDEAUX MIXTURE WITH OTHER COPPER SPRAYS FOR THE CONTROL OF CITRUS SCAB IN
COMMERCIAL GROVES.


Locality
and
Year


Bradenton
1932


Vero Beach
1934


Variety



Seeded
grapefruit


Marsh
grapefruit


Vero Beach Seeded
1935 grapefruit


Spraying Schedi
Fungicide


bordeaux mixture 6-6-100
copper cyanamide 8-100
check


bordeaux mixture 6-8-100
bordeaux mixture 6-8-100

basic copper sulfate 3-100
basic copper sulfate 3-100

proprietary copper-oil 2%-100
proprietary copper-oil 2-100

check


bordeaux mixture 6-6-100
bordeaux mixture 3-3-100

basic copper sulfate 4-100
basic copper sulfate 3-100

proprietary copper-oil 2Y-100
proprietary copper-oil 2-100

check


ule
Stage of Growth


bloom
bloom


dormant
bloom

dormant
bloom

dormant
bloom



dormant
post-bloom*

dormant
post-bloom*

dormant
post-bloom*


Num-
beur o Scab Infection of Fruits by Classes
IPlotP I None I Slight IModeratel Severe


1

1

1

1


1

1

1

1


Percent

69.6
58.0
26.4


3.1


98.6

98.6


98.3

69.6


Percent Percent Pe

22.2 7.6
32.8 7.6
42.0 22.0


3.1 1.2


'~ ~u~Y


*Two to three weeks after petals have shed.


i


recent a

0.6 g
1.6
9.6 -


0.4

0.4 o

5.6

81.6 e


0.1 o

0.1

0.1 p







TABLE 5.-COMPARISON OF BORDEAUX MIXTURE WITH OTHER COPPER SPRAYS FOR THE CONTROL OF CITRUS SCAB IN
COMMERCIAL GROVES-Concluded.


Locality
and
Year


Bradenton
1935


Haines City
1935






Vero Beach
1936








Homestead
1937


Variety



Seeded
grapefruit


King
orange


Scab Infection of Fruits by Classes
None Slight IModeratel Severe


bordeaux
basic copp
check

bordeaux
basic copp
check

bordeaux
bordeaux

basic copi
basic copp

ammonium
ammonium

bordeaux
bordeaux

check

bordeaux
bordeaux

copper hy
copper hy

check


*Two to three weeks after


Percent Percent Percent


Num-
Spraying Schedule hber ofi
Fungicide Stage of Growth Plots


mixture 6-6-100 delayed dormant 2
)er sulfate 4-100 delayed dormant 2


mixture 6-6-100 delayed dormant 1
per sulfate 4-100 delayed dormant 1
--_ 1

mixture 6-6-100 dormant 4
mixture 3-3-100 post-bloom*

)er sulfate 4-100 dormant 4
)er sulfate 3-100 post-bloom*

a copper silicate 3-100 dormant 4
a copper silicate 3-100 post-bloom*

mixture 3-3-100 dormant 2
mixture 3-3-100 bloom

2

mixture 6-6-100 dormant 1
mixture 3-3-100 bloom

droxide 6-100 dormant 1
droxide 3-100 bloom

1


Percent

84.4
81.1
60.6

70.3
73.5
36.7

98.3

96.4

96.7

96.7

56.1

99.0

98.7

25.0


0.2

22.2


2.0
2.0
5.3

5.8
3.4
19.4

0.0

0.0

0.2

0.2

13.7

0.0

0.0

37.1


Marsh
grapefruit








Marsh
grapefruit


petals have shed.


petals have shed.






Commercial Control of Citrus Scab in Florida


was following the freeze which occurred during the previous
winter, and it was very difficult to find groves that were not
damaged more or less by low temperatures. Some new growth
appeared very early in these trees, and in some instances they
continued to flush irregularly in a lush manner during the spring
months. Since the bloom appeared irregularly the usual bloom
spray was omitted. Some of the earliest growth became in-
fected before the spray was applied, and although scab infection
was relatively mild in 1935 the single delayed dormant spray
gave poor control of scab under these conditions.
It has been observed rather frequently in commercial groves
that delayed dormant sprays give poorer control of scab than
similar sprays applied before any new growth appears, although
this depends greatly upon the presence or absence of suitable
moisture conditions for infection prior to the application of the
delayed spray. Young expanding leaves of citrus are very dif-
ficult to cover thoroughly with spray material and may be in-
fected at the time the copper is applied, although such infection
may not be of sufficiently long standing to be visible to the
unaided eye.
The ammonium copper silicate included in the tests at Vero
Beach in 1936 contained a copper content equivalent to 20 per-
cent metallic copper and gave the same control of scab as the
3-3-100 bordeaux mixture under the conditions of the experi-
ment. Scab infection was relatively mild in the control plots,
and it is doubtful whether as good results would be obtainable
with either spray material at these concentrations, under con-
ditions of severe infection.
Copper hydroxide used at concentrations of copper equal to
those in the bordeaux mixture schedule gave as effective control
of scab as bordeaux in a single test made at Homestead in 1937.
This material is recommended because it also has been observed
to give good control of scab in other commercial groves in Dade
County.

EFFECT ON SCAB CONTROL OF WETTABLE SULFUR OR OIL
EMULSION ADDED TO COPPER SPRAYS
In connection with the phase of the problem dealing with
insect control, sprays were applied in which copper compounds
were mixed with wettable sulfur or with oil emulsions at con-
centrations of /2 and 1 percent actual oil.






Florida Agricultural Experiment Station


It has been known for many years that copper sprays are
compatible with oil emulsions and the combination of bordeaux
mixture with oil emulsion containing 1 percent actual oil was
widely used for the combined control of scab, melanose and
scale-insects. Since the use of this combination did not obviate
the necessity of spraying for scale control later in the season
and since it is expensive to use, the question arose as to whether
it would not be more practical to reduce the concentration of oil.
In test plots at Vero Beach in 1935 and 1936 the combination
of bordeaux mixture with 1 percent oil added was compared
to bordeaux mixture with 1/2 percent oil added. Reducing the
concentration of oil to as low as 1/4 percent gives efficient
wetting and spreading qualities to the combination spray. The
oil was used at 1/2 percent concentration in these tests because
experiments by Thompson (13) have shown that purple mites
and rust mites are reduced in population by this amount of oil
in the dormant spray.
The results in scab control are recorded in Table 6. They
show that varying the concentration of oil from 1/2 percent to
1 percent in the combination spray does not materially change
its efficacy for scab control.
The use of wettable sulfur with bordeaux mixture or other
copper sprays was a relatively new combination prior to 1935.
In that year the writers used the combination experimentally
for the control of melanose and rust mites with good results.
In 1936 the combination was compared with the bordeaux-oil
emulsion spray for the control of scab in test plots at Vero
Beach. In addition, wettable sulfur was added to basic copper
sulfate and ammonium copper silicate in post-bloom applica-
tions, comparing these combinations with application of these
sprays with a non-casein colloidal spreader added. The formulas
and results in scab control are recorded in Table 6. The results
show that wettable sulfur is compatible with these copper sprays
and apparently does not reduce their effectiveness for the con-
trol of scab.
COMPARATIVE EFFECT ON SCAB CONTROL OF FOLLOWING DOR-
MANT BORDEAUX SPRAYS WITH SULFUR, MERCURY OR
BORDEAUX SPRAYS APPLIED IN OR AFTER THE BLOOM
A comparison of bordeaux mixture, organic mercury-oil emul-
sion, and lime-sulfur as bloom or post-bloom sprays following
a dormant application of bordeaux mixture was made in 1934,
1935, and 1936 in test plots at Vero Beach. In the same groves




TABLE 6.-EFFECT ON SCAB CONTROL IN MARSH GRAPEFRUIT OF WETTABLE SULFUR OR OIL EMULSIONS ADDED TO
COPPER SPRAYS.
Locality
and Spray Schedule Scab Infection of Fruits by Classes
Year I Materials and Concentrations Stage of Growth| None I Slight Moderatel Severe
Percent Percent Percent Percent o
Vero Beach bordeaux mixture 6-6-100 + '/2% oil dormant 96.6 2.0 0.7 0.7
1935 bordeaux mixture 6-6-100 + 1% oil dormant 94.7 2.8 1.1 1.4 re
I check 69.6 11.8 7.2 11.4

basic copper sulfate 4-100 + %% oil dormant 96.5 2.5 1.0 0
basic copper sulfate 3-100 + wettable sulfur 5-100 post-bloom
basic copper sulfate 4-100 + %% oil dormant 96.4 2.8 0.8 0
basic copper sulfate 3-100 with non-casein colloidal post-bloom
spreader
ammonium copper silicate 3-100 + V% oil dormant 96.6 2.4 0.7 0.3 C<
ammonium copper silicate 3-100 + wettable sulfur 5-100 post-bloom
Vero Beach I
1936 ammonium copper silicate 3-100 + %% oil dormant 96.8 2.3 0.7 0.2 %
ammonium copper silicate 3-100 with non-casein post-bloom
colloidal spreader
bordeaux mixture 6-6-100 + %2% oil dormant 98.3 1.6 0.1 0 .
bordeaux mixture 3-3-100 + non-casein colloidal post-bloom
spreader I
bordeaux mixture 6-6-100 + wettable sulfur 10-100 dormant 98.3 1.6 0.1 0 .
bordeaux mixture 3-3-100 + wettable sulfur 5-100 post-bloom I
bordeaux mixture 6-6-100 + V % oil dormant 96.1 3.2 0.4 0.3
bordeaux mixture 6-6-100 + 1% oil dormant 96.7 2.5 0.7 0.1
check -- 56.1 18.1 12.1 13.7 ,






Florida Agricultural Experiment Station


a comparison was obtained of single and double applications of
bordeaux mixture for the control of scab.
The spray formulas and results are recorded in Table 7. The
results show that while the second bordeaux application gave
added protection from scab infection and was superior to the
sulfur and mercury sprays, the increased percentage of clean
fruit was in each year insufficient to justify the expense of the
second application. From the standpoint of scab control the
benefits derived from applying lime-sulfur or mercury-oil emul-
sion in and after the bloom were negligible.
It should be pointed out that these groves were in a thrifty
condition, with the spring flush of leaves appearing generally
over the trees and maturing in the space of a few weeks. Scab
control is usually less complete from a single dormant spray
on trees thrown out of balance by drouths, low temperatures
or other unfavorable conditions which may cause intermittent
flushing during the spring months. Under these conditions a
second copper spray applied in the bloom is necessary for good
protection from scab infection on the fruit.
CONTROL OF MELANOSE WITH THE SCAB SPRAYS
In sections of the citrus belt where citrus scab has occurred
annually and copper sprays have been regularly applied for
its control, melanose has not been a very serious problem. This
was not because the trees in these sections were not subject
to melanose infection but rather because the sprays applied for
scab control were giving fairly effective control of melanose
as well. It was observed repeatedly that melanose blemish was
frequently very severe on the fruit and foliage produced in un-
sprayed test plots, while the fruits and foliage of trees sprayed
with effective schedules for the control of scab were generally
free of serious melanose infection.
To secure comparative records of control of melanose by the
scab schedules, samples of fruits from the various plots in
some of the experiments were examined at maturity for severity
of melanose blemish. Severity of infection was determined by
counting the number of melanose spots in one square inch field
of surface selected at random on the upper half of each fruit.
The counts were made with the aid of a metal ring which cir-
cumscribed an area of one square inch and 1,000 fruits were
examined from each plot. The fruits were grouped into three
classes of infection, as follows: (1) slight, 0 to 10 spots per










Commercial Control of Citrus Scab in Florida


4.m



0 40









0 0 ..0 '


0 00





00.


cc jo


'0..0QP '*0.Q P.


'0


ow 00








00, 0? 0?








00 0 0
.0 Q 00Q "


0 0



-4 -






r4~

9S 00
.0.


ce


_~~


I








Florida Agricultural Experiment Station


0) C)
m y 5


0 01



03 -P V 6 0
0 0
rz4 ______




Z




Cd C)




g 0
0^
| C ))


00 00
14
T-4


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Il


(0
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.++

4-
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04 C4k





.4z 0 .7CZ
is ti n ~




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CO t?
0 0 0




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is^
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ce
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03o3(


~/i-






Commercial Control of Citrus Scab in Florida 29

square infh; (2) moderate, 11 to 50 spots per square inch; and
(3) sever more than 50 spots per square inch.
In gen ral it was found that spray schedules which gave poor
control cf scab were also relatively ineffective for the control
of melanose. Thus, the sulfur and organic mercury sprays gave
poor control of melanose, while the copper sprays were quite
effective. The results obtained with some of the copper sched-
ules with formulas and dates of application of the sprays are
shown in Table 8.
The results obtained in 1935 show that a 6-8-100 bordeaux
mixture applied before growth starts in the spring reduced
considerably the amount of melanose blemish on the fruit. From
Table 8 it may be noted that trees so sprayed yielded 74 percent
slightly infected fruit as compared to 31 percent from the con-
trol plot. By following this application with a half-strength
bordeaux mixture in the last of the bloom a still further reduc-
tion of serious melanose blemish was obtained with 83 percent
slightly infected fruits. The results in 1935 were slightly better
with basic copper sulfate than with bordeaux mixture.
In 1936, with melanose infection less severe in the control
plots, the dormant and bloom applications of bordeaux mixture
applied as above gave 91 percent of slightly infected fruits as
compared to 44 percent in the controls. Delaying the second
spray application until three weeks after the petals were shed
increased the percentage of the better grade to 95 percent. In
1936 the bordeaux mixture and basic copper sulfate schedules
gave practically the same reduction of melanose infection. In
the same series of tests a single treatment with 6-6-100 bordeaux
mixture applied before the spring flush of growth appeared gave
68 percent of slightly infected fruits as compared with 88 per-
cent in this grade resulting from a single application of the
same spray in the bloom.
From these results it was concluded that two sprayings with
copper fungicides are necessary for effective control of both
scab and melanose. The timing of the second spray should
depend upon the relative importance of the two diseases in
the grove to be protected. In older trees, where severe melanose
infection may be expected and scab was well controlled the previ-
ous season, the second spray should be applied two to four weeks
after the petals have shed. In younger trees and in trees with
an abundance of scab on the foliage of the previous season, the
second spray should preferably be applied in the last of the
bloom.









TABLE 8.-REDUCTION OF MELANOSE BLEMISH ON GRAPEFRUIT RESULTING FROM SPRAYS TIMED FOR SCAB CONTROL.


Spraying Program
5 Dates
Materials and Concentrations Applied


Locality
and
Year




Vero Beach
1935


bordeaux 6-6-100 + %% oil
bordeaux 3-3-100

bordeaux 6-6-100 + %% oil
bordeaux 3-3-100


basic copper sulfate
basic copper sulfate

bordeaux 6-6-100 +

bordeaux 6-6-100

check


4-100 + 2'% oil
3-100

S%% oil


*0-10 lesions per square inch.
Mll-50 lesions per square inch.
tMore than 50 lesions per square inch.


Jan. 25
Apr. 13

Jan. 25

Jan. 25
Apr. 13



Feb. 1
Apr. 5

Feb. 1
Apr. 18

Feb. 1
Apr. 18

Feb. 1

Apr. 5


Stage of
Growth


dormant
bloom

dormant

dormant
bloom



dormant
bloom

dormant
post-bloom

dormant
post-bloom

dormant

bloom


Degree of Melanose Blemish of
Fruits by Classes


Slight*
Percent

82.9

74.0

87.7

30.5


90.7

94.5

93.9

68.2

88.0

44.2


Moderate
Percent

16.1

25.2

11.9

60.2


9.1

5.3

5.3

30.3

11.7

50.1


Severe$
Percent

1.0

0.8

0.4

9.3


0.2

0.2

0.6 .

1.5

0.3


bordeaux 6-8-100 + 1% oil
bordeaux 3-4-100

bordeaux 6-8-100 + 1% oil

basic copper sulfate 3-100 + 1% oil
basic copper sulfate 3-100

check


Vero Beach
1936







Commercial Control of Citrus Scab in Florida


INSECT CONTROL IN CONNECTION WITH SPRAYING
FOR CONTROL OF SCAB

In a spray schedule for control of scab, adequate provision
must be made for the control of scale-insects and rust mites,
since usually they increase more rapidly following copper sprays
than when such fungicides are not applied. During the course
of this investigation an effort has been made to develop a satis-
factory and economical control of both insects and scab. More
emphasis was placed on the development of control measures
for the scale-insects, particularly the Florida red scale (Chry-
somphalus aonidum Linn.) and the purple scale (Lepidosaphes
beckii Newm.), because of the danger of severe damage to the
trees which may follow infestations of these insects. Some of
the results on scale increases with observations on rust mite
increases have been previously reported by Thompson (12, 13),
who is continuing investigations of this phase of the problem.
The method of procedure was to secure data on seasonal in-
creases in populations of the scale-insects following the various
spray schedules. This was accomplished by making counts of
scales on samples of leaves collected in a uniform manner from
the trees in plots to be compared. From these counts the num-
ber of living scales per leaf was calculated. From counts made
before the dormant sprays were applied and again in the fall,
the seasonal increases or decreases in living scales per leaf were
determined.
In collecting the leaves care was taken to secure a representa-
tive sample from each plot. In any given test a uniform number
of leaves per tree (usually 30) was collected at random from
the same number of trees in each plot. They were taken from
approximately the same distance from the ground and half the
number were gathered from the outer exposed twigs and the
remainder from the shaded twigs inside the tree spread. The
leaves were placed in paper bags and kept in a refrigerator until
examined. Before the scale were counted the leaves from each
sample were mixed carefully, replaced in the original bag and
withdrawn at random for examination. Usually leaves were
withdrawn and examined from each sample until 500 living scale
insects were counted, or if 500 were not present, the total from
all the leaves in the sample was taken.







Florida Agricultural Experiment Station


COMPARISON OF SCALE INCREASE FOLLOWING BORDEAUX
MIXTURE AND SOME INSOLUBLE COPPER SPRAYS
Since lime residues on citrus foliage favor an increase in popu-
lation of scale-insects, the insoluble copper sprays which do not
require the use of lime in the spray mixtures may be expected
to be less conducive to scale increase than bordeaux mixture.
This point was checked by means of scale counts, and the re-
sults are shown in Table 9.
The results show that scale infestations are favored more by
the use of 6-6-100 bordeaux mixture than by the insoluble copper
sprays. On the average there were slightly more scale follow-
ing 3-3-100 bordeaux than the insoluble coppers but the differ-
ence was very small. During 1934 and 1935 in the experiments
near Vero Beach there was a greater increase of red scale
following two applications of bordeaux mixture than there was
following the same number of applications of basic copper sul-
fate, although less metallic copper was present in the bordeaux
mixture. The bluestone used in making the bordeaux mixture
contained approximately 26 percent copper as compared to 53
percent copper in the basic copper sulfate. At Bradenton in
1935 purple scale decreased less following bordeaux mixture
than following basic copper sulfate. In 1936 in the experiment
at Vero Beach the increase of red scale was much greater fol-
lowing bordeaux mixture than following either basic copper
sulfate or copper ammonium silicate. Since the latter contained
only 20 percent metallic copper, less copper was applied in this
case than with the other two sprays.
These results are in accordance with results of similar counts
following these and other copper sprays in experiments for the
control of melanose carried on by the writers in other groves
(to be reported elsewhere).
Although scale infestation may be more pronounced following
bordeaux mixture than the insoluble copper sprays, it should be
clearly understood that it is never advisable to disregard scale
control after any form of copper has been previously used as
fungicide. Scale increases may be very marked following any
copper sprays unless the proper scalecides are applied.
CONTROL OF SCALE INSECTS WITH OIL EMULSIONS AND SULFUR
SPRAYS IN COMBINATION WITH OR FOLLOWING
BORDEAUX MIXTURES
The comparative effect on seasonal scale increase of the
dormant bordeaux mixture combined with oil emulsions at 1





TABLE 9.-COMPARISON OF BORDEAUX MIXTURE WITH NEUTRAL COPPER FUNGICIDES ON INCREASE OR DECREASE OF SCALE
INSECTS IN COMMERCIAL GRAPEFRUIT GROVES.


Spray Schedule



Material and Concentration


unsprayed

6-8-100 bordeaux mixture + 1% oil
3-4-100 bordeaux mixture
oil emulsion (83% oil) 1%4-100

3-100 basic copper sulfate + 1% oil
3-100 basic copper sulfate
oil emulsion (83% oil) 1/4-100


unsprayed

6-6-100 bordeaux mixture + 12%% oil
dry lime-sulfur 5-100, bentonite sulfur 2-100
dry lime-sulfur 5-100, bentonite sulfur 2-100
dry lime-sulfur 5-100, bentonite sulfur 2-100


3-100 basic copper sulfate + %% oil
dry lime-sulfur 5-100, bentonite sulfur
dry lime-sulfur 5-100, bentonite sulfur
dry lime-sulfur 5-100, bentonite sulfur


2-100
2-100
2-100


Locality
and
Year


Dates of
Application




Jan. 25
April 13
June 7

Jan. 25
April 13
June 7




Jan. 29
April 1
April 29 I
July 2

Jan. 30
April 1
April 29
July 2


5.7



negligible

negligible



negligible


+5.0


Living Florida Red
Scale Per Leaf
Average
increase
After (+) or
Treat- decrease
ment (-)
for Season

1.8 +1.2


7.7 +6.7


negligible


negligible



3.05


1.3


Living Purple
Per Lea
Av
inc
After (+4
Treat- de
ment
for

negligible


1.27 -4.67


Scale
f -
verage 0
crease
-) or
crease -
(-) g.
Season g


--







-0.4

-2.7
Q




0.


Vero Beach
1934


Bradenton
1935


---:--


1.27 --4.67







TABLE 9.-COMPARISON OF BORDEAUX MIXTURE WITH NEUTRAL COPPER FUNGICIDES ON INCREASE OR DECREASE OF SCALE
INSECTS IN COMMERCIAL GRAPEFRUIT GROVES-Concluded.


Spray Schedule


Locality
and
Year






Vero Beach
1935










Vero Beach
1936


Material and Concentration


unsprayed

6-6-100 bordeaux mixture + %% oil
3-3-100 bordeaux mixture
oil emulsion (83% oil) 2-100
4-100 basic copper sulfate + %% oil
3-100 basic copper sulfate
oil emulsion (83% oil) 2-100

unsprayed

6-6-100 bordeaux mixture + %% oil
3-3-100 bordeaux mixture
lime-sulfur 11%-100, wettable sulfur 5-100
oil emulsion (83% oil) 2-100
4-100 basic copper sulfate + %% oil
3-100 basic copper sulfate
lime-sulfur 1%-100, wettable sulfur 5-100
oil emulsion (83% oil) 2-100
3-100 copper ammonium silicate + %% oil
3-100 copper ammonium silicate
lime-sulfur 1%4-100, wettable sulfur 5-100
oil emulsion (83% oil) 2-100


Living Florida Red
Scale Per Leaf


Dates of
Application




Jan. 31
April 18
June 26
Jan. 31
April 18
June 26


Jan. 21
April 14
May 12
June 16

Jan. 21
April 14
May 12
June 16
Jan. 21
April 14
May 12
June 16


After
Treat-
ment


0.62

3.78


3.98


2.09

6.06



1.78



1.63


Living Purple Scale
Per Leaf


Average
increase
(+) or
decrease
(-)
for Season

-0.26

+3.52


+3.38


+0.82

+4.11



+1.64



+1.52


Average
increase
(+) or
decrease a
(-)
for Season p

. -1.10

-0.33


-0.06


+0.18

-0.50


-0.19 -
-0.19


After
Treat-
ment


0.85

0.5


0.53


0.61

0.09



0.0



0.09


-0.60







Commercial Control of Citrus Scab in Florida


percent and 1/2 percent concentrations of oil was determined
during 1935 and 1936. In 1936 the efficacy of a combination of
bordeaux mixture and wettable sulfur was compared with the
combination of bordeaux mixture and oil emulsion. In the same
series of tests the efficacy for scale control of oil emulsions
and sulfur sprays following bordeaux mixture was determined
by scale counts. The various spray schedules used and the
results in seasonal increase of scale insects are recorded in
Table 10.
In the experiments at Vero Beach the results of both seasons
show definitely the desirability of adding oil emulsion at 1 per-
cent concentration of oil rather than reducing the oil content
of the spray to 1/2 percent. This was particularly desirable for
the suppression of red scale. Purple scale did not increase at
the same rate as red scale and they were readily controlled in
all the schedules. In Table 10 it may be noted that in 1936 red
scale increased by 4.11 living scale per leaf in plots sprayed
with 6-6-100 bordeaux mixture before the spring flush, followed
after the bloom with a 3-3-100 bordeaux mixture, a sulfur spray
and one oil spray at 1.66 percent concentration. On the other
hand red scale increased by only 2.17 living scale per leaf in
adjacent plots receiving the same concentrations of bordeaux
mixture but with wettable sulfur added to the dormant spray
at 10 pounds to 100 gallons and to the post-bloom application
at 5 pounds to 100 gallons and followed by the same concentra-
tion of oil emulsion. The sulfur spray applied in May for rust
mite control in the first schedule proved unnecessary where the
wettable sulfur was combined with the bordeaux mixture. Simi-
lar results in rust mite control were obtained with the addition
of wettable sulfur to copper sprays in melanose experiments.
In was concluded that the addition of wettable sulfur to the post-
bloom copper spray may be expected to eliminate one sulfur
spray or dust application in the spray schedule.
In both seasons a single application of oil emulsion made
in June was equally as efficacious or more so than three post-
bloom applications of sulfur sprays for the control of red scales.
In Table 10 it may be noted that in the experiment near Vero
Beach in 1936 red scale increased by 59 living scales per leaf
after a dormant spraying of 6-6-100 bordeaux mixture was fol-
lowed by three post-bloom applications of a lime-sulfur-bentonite
sulfur spray. It was concluded, therefore, that oil emulsions
should be relied upon for the control of red scale where this
insect is expected to be a serious pest.










TABLE 10.-CONTROL OF SCALE-INSECTS IN GRAPEFRUIT GROVES WITH OIL EMULSIONS AND SULFUR SPRAYS FOLLOWING
COPPER FUNGICIDES.


Spray Schedule


Locality
and
Year












Vero Beach
1935


Living Florida Red
I Scale Per Leaf


Dates of After
Application Treat-
ment


Feb. 1
June 26

Jan. 31
June 26

Jan. 31
April 18
May 22
June 26

Jan. 31
April 18
May 22
June 26

Feb. 1
April 18
May 22


Average
increase
(+) or
decrease
(-)for Season
for Season


-0.16

+0.03

+1.76


+6.30



+1.73



+0.94


I Living Purple Scale
S Pper LeT.paf


After
Treat-
ment


i

d

fo


Average
increase
+) or
ecrease
(-)

r Season

-1.10

-0.36

+0.10


+0.09



-0.31
.


Material and Concentration



unsprayed control

6-6-100 bordeaux mixture + 1% oil
oil emulsion (83% oil) 2-100

6-6-100 bordeaux mixture + %% oil
oil emulsion (83% oil) 2-100

6-6-100 bordeaux mixture + %% oil
dry lime-sulfur 5-100 + bentonite sulfur 3-100
dry lime-sulfur 5-100 + bentonite sulfur 2-100
dry lime-sulfur 5-100 + bentonite sulfur 2-100

6-6-100 bordeaux mixture + %% oil
lime-sulfur 2%-100 + bentonite sulfur 3-100
lime-sulfur 2-100 + bentonite sulfur 2-100
lime-sulfur 2-100 + bentonite sulfur 2-100

6-6-100 bordeaux mixture + Y% oil
mercury-oil emulsion 1-60
mercury-oil emulsion 1-60


+0.16


I i


'










TABLE 10.-CONTROL OF SCALE-INSECTS IN GRAPEFRUIT GROVES WITH OIL EMULSIONS AND SULFUR
COPPER FUNGICIDES-Continued.


Spray Schedule


Material and Concentration



unsprayed control

6-4-100 bordeaux mixture + 12%% oil
lime-sulfur 2-100 + wettable sulfur 2-100
oil emulsion (83% oil) 1.8-100

6-6-100 bordeaux mixture + 1 %% oil
lime-sulfur 2-100 + wettable sulfur 2-100
oil emulsion (83% oil) 1.8-100

6-6-100 bordeaux mixture + 1 %% oil
lime-sulfur 2Y2-100 + wettable sulfur 2-100
lime-sulfur 2-100 + wettable sulfur 3-100
lime-sulfur 1%-100 + wettable sulfur 4-100


Dates of
Application




Jan. 29
April 1
July 2

Jan. 29
April 1
July 2

Jan. 30
April 1
April 29
July 2


Living Florida Red
Scale Per Leaf


After
Treat-
ment


negligible

negligible


negligible


Locality
and
Year









Bradenton
1935


Average
increase
(+) or
decrease
(-)
for Season

I


SPRAYs FOLLOWING


Living I
Pe


After
Treat-
ment


3.05


0.24


0.28


1.30


I I


negligible


~ilmru


----------


Purple Scale W
r Leaf n
Average .
increase
(+) or
decrease 0
(-)
for Season

+0.40


-3.16


-2.83


-3.02
Q









TABLE 10.-CONTROL OF SCALE-INSECTS IN GRAPEFRUIT GROVES WITH OIL EMULSIONS AND SULFUR SPRAYS FOLLOWING
COPPER FUNGICIDES-Concluded.


Locality
and
Year












Vero Beach
1936


Dates of
Application


Spray Schedule



Material and Concentration


unsprayed control
6-6-100 bordeaux mixture + %% oil
lime-sulfur 14-100 + wettable sulfur 5-100
oil emulsion (83% oil) 2-100
6-6-100 bordeaux mixture + 1% oil
lime-sulfur 11/-100 + wettable sulfur 5-100
oil emulsion (83% oil) 2-100
6-6-100 bordeaux mixture + 1% oil
lime-sulfur 2Y2-100 + bentonite sulfur 2-100
lime-sulfur 1%-100 + bentonite sulfur 10-100
lime-sulfur 1%-100 + bentonite sulfur 10-100
6-6-100 bordeaux mixture + %% oil
3-3-100 bordeaux mixture
lime-sulfur 114-100 + wettable sulfur 5-100
oil emulsion (83% oil) 2-100
6-6-100 bordeaux mixture + wettable sulfur 10-100
3-3-100 bordeaux mixture + wettable sulfur 5-100
oil emulsion (83% oil) 2-100


Living Florida Red
Scale Per Leaf
Average
increase
After (+) or
Treat- decrease
ment (-)
for Season

2.09 +0.82

3.40 +2.79


2.91 -0.78


60.89


+58.96



+4.11



+2.17


Living Purple Scale
Per Leaf


After
Treat-
ment


0.61

0.06


0.04


0.13



0.09



0.06'


Average i
increase 5"
|(+) or .
decrease ?
I (-)
I for Season

+0.18

-0.36


-0.99


-0.48



-0.50 'Z



-0.36


Jan. 21
April 14
June 16
Jan. 21
April 14
June 16
Jan. 21
April 14
May 12
June 16
Jan. 21
April 14
May 12
June 16
Jan. 21
April 14
June 16


i







Commercial Control of Citrus Scab in Florida


Purple scales were quite effectively controlled with the post-
bloom sulfur applications following a single copper spray. At-
tempts to control these insects after two earlier applications of
bordeaux mixture had been made were not always satisfactory.
Since wettable sulfur may be added to the second copper spray
for rust mite control, there appears to be little justification for
substituting lime-sulfur for oil emulsion in the schedule.

CONCENTRATIONS AND TIMING OF OIL SPRAYS
Infestations of the Florida red and the purple scales vary
considerably in different localities and in different seasons. The
results of experiments to date indicate that red scale are more
abundant and more difficult to control in groves along the East
Coast of Florida than along the West Coast, and in the central
part of the state they are seldom a serious problem. Purple
scale have been less common and more readily controlled along
the East Coast than along the West Coast and in central Florida.
Therefore, a spray schedule and concentration of spray best
adapted to a particular locality should be used.
Oil emulsions at concentrations of 11 to 12/3 percent of oil
have proven more satisfactory for the control of red scale than
weaker concentrations, but emulsions containing 11/4 to 11 per-
cent oil have given good control of purple scale in trees where
the scale population was moderate. In experimental test plots
receiving the same copper sprays but different concentrations
of oil in June, there was, three months after the oil application,
an average increase of 0.43 living red scale per leaf following
a 12/3 percent oil and an increase of 4.85 per leaf following a
11/4 percent oil. Purple scale were held in check by the weaker
concentrations of oil in these tests.
There is still some question as to the proper time for apply-
ing the oil sprays for maximum seasonal control with minimum
expense to the growers, and experiments are being continued
to secure more information on this phase of the problem. For
the control of red scale, results to date indicate that the most
opportune time for applying the oil is between the latter part
of May and the first of July, especially if the fruit is to be kept
free from serious scale infestation. Early infestation of fruits
results commonly in more or less pitting of the skin which may
lower the grade. In seasons particularly favorable for the in-
crease of red scale, a second oil application may be necessary in
the fall to prevent tree damage. In such seasons, if the first oil







Florida Agricultural Experiment Station


application is delayed until late in the summer, a much higher
percentage of the fruit will be infested and there is danger of
damage to the trees before the spray is applied. If the second
application of oil is made within six weeks of harvest, it may
prevent uniform coloring of the fruit by artificial means.
Regardless of the .date of spraying one of the most important
factors in scale control is thoroughness of application. Good
coverage is absolutely essential for good control.

DISCUSSION AND CONCLUSIONS
These experiments have demonstrated that citrus scab can
be effectively and economically controlled in grapefruit, tangelo,
and King orange groves by spraying with copper fungicides,
provided the spraying be done with thoroughness and at the
proper time.
Citrus scab is more serious in some localities than in others,
and in a given locality its importance may vary greatly from
year to year and even from tree to tree. Since the disease is
carried primarily from one year to the next on diseased foliage,
in localities where scab is regularly serious there is a marked
tendency for it to build up in the trees to the point where
practically all of the fruits and foliage will become severely
blemished during average years, provided fungicides are with-
held. In these same localities the yearly application of copper
fungicides for scab control reduces the number of hold-over scab
stromas on foliage and fruits, and the prevention of fruit blemish
in subsequent years becomes progressively easier of accomplish-
ment. Thus two, and in wet seasons three, sprayings are usually
necessary to protect susceptible parts from infection in trees
with the foliage of the previous year severely infected. The
following year, if the foliage of these trees goes through the
winter months relatively free of scabs, a single spraying usually
will give adequate protection from infection, provided the fungi-
cide is thoroughly applied at the proper time.
Just before the spring flush of growth appears is considered
to be the best time to spray for the control of scab. An effective
fungicide thoroughly applied to both the upper and lower surface
of the leaves at this time will cover the old scabs and prevent
infection on the new foliage, thus eliminating the chief source
of spores which may infect the fruits later in the season. New
scab infections on young expanding leaves may, under favor-







Commercial Control of Citrus Scab in Florida


able moisture conditions, give rise to myriads of spores capable
of spreading infection very rapidly.
During dry springs relatively little of the new foliage may
become infected. If this occurs an effective fungicide applied
in the bloom may give good protection from scab infection to
the young expanding fruits and will markedly reduce fruit
blemish from melanose as well. In a wet spring, however, if
the fungicide is withheld until the bloom, there is serious danger
of considerable infection taking place on many of the small
fruits that set from the first of the bloom before the spraying
can be completed, particularly in large groves or if rains inter-
fere with spraying at this time. There is also the danger of
strong fungicides, such as bordeaux mixture and especially if
combined with oil emulsion, injuring blossoms and small fruits.
There has been strong evidence that reduction of yield or of
grade occurred in some instances in some of the experimental
test plots where copper fungicides were applied in the bloom.
Homemade bordeaux mixture has consistently given good
control of scab when properly made and thoroughly applied.
Until rather recently crystals of copper sulfate and quick lime
have been used in stock solutions to prepare bordeaux mixture.
While a very satisfactory bordeaux can be prepared by this
method, it has several disadvantages which have frequently
resulted in the production of an inferior fungicide. The main
difficulties arise from the lack of uniform quality of the quick
lime and the care needed to slake it properly. Bordeaux made
from improperly slaked lime contains gritty materials which
clog the strainers and spray nozzles, cause excessive wear of
spray machinery and sometimes injury to foliage and fruit of
citrus.
From experimental tests it was found that the so-called
"instant" method of preparing bordeaux mixture (10) from
pulverized copper sulfate and superfine hydrated lime yielded a
bordeaux equal to that produced by the old stock solution method
for the control of scab. The "instant" method entirely elimi-
nates the necessity for using stock solutions, effects a saving
of time, and makes a more uniform spray. The fineness of the
hydrated lime is very important in determining the texture and
quality of the resulting bordeaux mixture. The fineness should
be such that at least 99.5 percent will pass a 300-mesh sieve.
There is no apparent necessity for increasing the lime content
of the formula for use on citrus. Under certain conditions bor-







Florida Agricultural Experiment Station


deaux mixture will cause burning of tender citrus foliage even
with an excess of lime in the formula and regardless of whether
hydrated or quick lime is used. In experimental test plots there
was little difference in scale increase following a 6-6-100 or a
6-4-100 formula, and there appears to be little advantage to be
gained from reducing the lime content below that of the copper
sulfate in the formula.
In groves where the over-wintering foliage is severely infected
by scab, a bordeaux mixture containing 6 pounds of copper sul-
fate and 6 pounds of superfine hydrated lime to 100 gallons of
water is recommended for the dormant or pregrowth spray.
Excellent control of infection on the new leaves has resulted
from using concentrations as low as 3-3-100 in groves where
the infection on the over-wintering foliage was mild. For the
bloom and post-bloom sprays the 3-3-100 formula has given
practically as good control of fruit infection as the higher
concentrations, provided the dormant spray had been previously
applied.
Several commercially prepared insoluble copper spray mate-
rials have given effective control of scab in the experimental
tests and in commercial groves. None of these materials re-
quires the use of lime for neutralization. The advantages of
such products in the saving of time and labor in preparation,
and the gain in uniformity and stability of the spray mixture
is evident. Basic copper sulfate, containing 53 percent metallic
copper, was as effective as bordeaux mixture under conditions
of infection ranging from mild to severe. When used at the
rate of 3 to 4 pounds to 100 gallons of water as a pre-growth
spray and at 3 pounds to 100 gallons of water as a bloom spray
this material was as effective as 6-6-100 bordeaux mixture ap-
plied at the same stages of growth. Copper hydroxide (40 per-
cent) and ammonium copper silicate gave as good control of
scab as bordeaux mixture in experimental tests of one season,
when used in formulas containing approximately equal amounts
of metallic copper. Insecticides compatible with bordeaux mix-
ture may also be combined safely with these insoluble copper
sprays.
Lime-sulfur solution with wettable sulfur added, when used
at concentrations close to the limits of safety from foliage in-
jury, gives only partial control of scab under conditions of
moderate to severe infection. In trees protected from infections
by copper sprays during one season to the extent that the foliage







Commercial Control of Citrus Scab in Florida


is carried over winter relatively free of disease, scab may be
held in check the following season by using strong lime-sulfur-
wettable sulfur sprays. Sulfur sprays have little effect in re-
ducing melanose blemish on the fruits. In trees old enough for
melanose to be important, therefore, it seems inadvisable to use
sulfur sprays for the control of scab, even in case the infection
is expected to be mild.
Organic mercury-oil emulsions gave results very similar to
those of the lime-sulfur sprays and proved unreliable for the
control of scab where the infection was moderate to severe.
From the standpoint of scab or melanose control the benefits
derived from applying lime-sulfur or mercury-oil sprays in or
after the bloom are negligible. The use at this time of a 3-3-100
bordeaux mixture or its fungicidal equivalent in one of the in-
soluble copper sprays will prove much more effective.
Scale infestations are more pronounced and more difficult to
control following bordeaux mixture than following the insoluble
copper sprays, but it is never advisable to disregard scale con-
trol, whatever the form of copper used as a fungicide. Scale
increases may be very marked following any copper sprays un-
less scalecides are applied.
Oil emulsion at 1 percent concentration of oil should be com-
bined with the pre-growth copper fungicide when the scale
infestation is moderate to heavy, especially if red scale are
present. In case the infestation is light, the concentration of
the oil may be reduced to 1 percent, or if rust mites are
abundant, wettable sulfur at 5 to 10 pounds to 100 gallons
may be substituted for the oil but never combined with it. The
toxicity of the sprays to the scab fungus is reduced little if
any by the addition of oil emulsion or wettable sulfur at the
recommended concentrations. In some instances growers have
reported difficulty in coloring mature seedless grapefruit from
young trees after it has been sprayed with a pre-growth copper-
oil combination spray.
During the period between the latter part of May and July 1,
oil emulsion should again be applied for scale control. The oil
for this application should preferably be one of the so-called
"summer emulsions" as these are considered to be less liable
to cause defoliation or other injury, and the concentration may
vary with the degree of scale infestation. If red scale are
present in dangerous numbers the concentration of oil should







Florida Agricultural Experiment Station


be from 11 to 12/3 percent, preferably the latter. If red scale
are absent or nearly so, the concentration may be reduced to
11/4 to 1 percent for the control of purple scale. Oil sprays
applied at this time are more effective for the prevention of
fruit pitting by red scale and reduce mealybug infestation more
effectively than later applications.
Rust mites usually increase more rapidly following copper
sprays than when such sprays are not used. The use of either
oil or wettable sulfur in combination with the pre-growth fungi-
cide is of benefit in checking these pests during the early spring.
Early infestations of rust mites on the young fruits are readily
controlled by the addition of wettable sulfur at 5 to 6 pounds
to 100 gallons to the bloom or post-bloom sprays. If the wettable
sulfur is added to these applications, it is usually unnecessary
to apply further control measures for rust mite until the latter
part of August or later in some seasons. Then sulfur spray or
dust applications may be made as needed.
In older trees infected with both scab and melanose two spray-
ings with copper fungicides are necessary for effective control
of both diseases. The pre-growth spray should be followed by
a second application in or after the bloom, the time depending
upon the relative importance of the two diseases in the grove
to be protected. When severe melanose may be expected and
scab was well controlled the previous season, the second spray
should be applied from two to four weeks after the petals have
shed. In trees with an abundance of scab on the over-wintering
foliage the second spray should be applied in the last of the
bloom. In very wet springs or in case the bloom appears inter-
mittently, it may be desirable to repeat the second application
four weeks later for good control of melanose.
/The success of the spray schedules for the control of scab
and scale-insects will be proportional to the thoroughness with
which the sprays are applied. Good coverage is absolutely
essential for good control.
Of some benefit for the control of scab is the prompt removal
of sour orange and rough lemon sprouts from the rootstocks
of the grove trees. Nursery trees of these highly susceptible
species should be grown in locations removed some distance from
susceptible grove trees or should be kept fairly free of scab
infection by spraying with effective fungicides.







Commercial Control of Citrus Scab in Florida


SUMMARY

The results obtained in field experiments, conducted over a
period of six years, for the commercial control of citrus scab
and of citrus scale-insects in grapefruit, tangelo, and King
orange groves are as follows:
Citrus scab is carried over from one year to the next primarily
on infected foliage. In localities where scab is regularly serious
there is a marked tendency for it to increase in severity if fungi-
cides are withheld. In these same localities the yearly applica-
tion of effective fungicides for control reduces the number of
holdover scab stromas on the foliage, and the prevention of
fruit blemish in subsequent years tends to become progressively
easier of accomplishment.
The period just before the spring flush of growth appears
is considered the best time to spray for control of scab. Appli-i
cations of effective sprays in the last of the bloom will also
give good control, but the practice is not applicable to large
groves because of the short time available to complete the opera-
tion, and there is some danger of causing injury to blossoms and /
very small fruits.
Homemade bordeaux mixture made from pulverized copper
sulfate and superfine hydrated lime will give consistently good
control of scab. In trees where the overwintered foliage is
severely infected a 6-6-100 formula is recommended for the pre-
growth spray. Excellent control of infection of the new foliage
has resulted from using concentrations as low as 3-3-100 in
groves where the infection on the overwintered foliage was mild.
For bloom and post-bloom sprays the 3-3-100 formula has given
as good control of fruit infections as the stronger concentrations,
provided the pre-growth spray had been previously applied.
If the overwintered foliage is but slightly infected, a single
thorough pre-growth application of 6-6-100 bordeaux will usually
give adequate protection from scab infection.
Some of the proprietary copper spray materials will give as
effective control of scab as bordeaux mixture, if used in formulas
containing approximately equal amounts of metallic copper.
Good results have been obtained with basic copper sulfate, copper
hydroxide (40%), and ammonium copper silicate. Insecticides
compatible with bordeaux mixture may also be combined safely
with these proprietary copper sprays.







Florida Agricultural Experiment Station


Lime-sulfur solution 4-100 with added wettable sulfur will give
only partial control of scab under conditions of moderate to
severe infection. Sulfur sprays have little effect in reducing
melanose blemish on the fruits. In trees old enough for mel-
anose to be important, therefore, it seems inadvisable to use
sulfur sprays for the control of scab, even in case the infection
is expected to be mild.
Mercury-oil emulsions are unreliable for the control of scab
where the infection is moderate to severe.
From the standpoint of scab or melanose control the benefits
derived from applying lime-sulfur or mercury-oil sprays in or
after the bloom are negligible. The use at this time of a 3-3-100
bordeaux mixture or its equivalent in some other copper spray
will be much more effective.
Scale infestations are more pronounced and more difficult to
control following bordeaux mixture than the proprietary in-
soluble copper sprays, but may be very marked following any
copper spray unless scalecides are applied.
Oil emulsion at 1 percent concentration of oil, or wettable
sulfur at 5 to 10 pounds to 100 gallons, should be combined
with the pre-growth copper spray. The oil emulsion is to be
preferred, especially if red scale are abundant.
During the period between the latter part of May and July 1, \
oil emulsion should again be applied for scale control. The con-
centration may vary from 11/4 to 12/3 percent actual oil, depend-
ing upon the severity of the infestation. Applied at this time,
oil sprays are more effective for the prevention of fruit pitting
by red scale than later applications.
Early infestation of rust mites on young fruits may be con-
trolled readily by the addition of wettable sulfur at 5 to 6
pounds to 100 gallons of the bloom or post-bloom sprays.
In older trees infected with both scab and melanose two spray-
ings with copper fungicides are necessary for the effective con-
trol of both diseases. The pre-growth spray should be followed
by a second application in or after the bloom, the time depending
upon the relative importance of the two diseases in the particular
grove to be protected.
The success of the spray schedule for the control of scab and
scale insects will be proportional to the thoroughness with which
the sprays are applied. Good coverage is absolutely essential
for good control.








Commercial Control of Citrus Scab in Florida 47

ACKNOWLEDGMENTS
The writers acknowledge their indebtedness to the several citrus growers
who cooperated in permitting the experiments to be conducted in their
groves. They are also indebted to the Niagara Sprayer and Chemical
Company, the California Spray Chemical Corporation, the Sherwin-Williams
Company, the Grasselli Chemical Company, and the Chipman Chemical
Company for furnishing some of the spray materials used in the experi-
ments.
LITERATURE CITED

1. BITANCOURT, A. A., and ANNA E. JENKINS. Elsinoe fawcetti, the per-
fect stage of the citrus scab fungus. Phytopathology 26: 393-396.
1936.
2. Perfect stage of the sweet orange fruit
scab fungus. Mycologia 28: 489-492. 1936.
3. Sweet orange fruit scab caused by Elsinoe
australis. Jour. Agr. Research 54: 1-18. 1937.

4. FAWCETT, H. S. Some relations of temperature to growth and infec-
tion in the citrus scab fungus, Cladosporium citri. Jour. Agr.
Research 21: 243-253. 1921.
5. GROSSENBACHER, J. G. Sour scab of citrus in Florida, and its preven-
tion. Phytopathology 6: 127-142. 1916.
6. JENKINS, ANNA E. The citrus scab fungus. Phytopathology 15:
99-104. 1925.
7. A Sphaceloma attacking navel orange in Brazil. Phy-
topathology 23: 538-545. 1933.
8. Sphaceloma perseae, the cause of avocado scab. Jour.
Agr. Res. 49: 859-869. 1934.
9. PELTIER, G. L., and WM. J. FREDERICK. Effects of weather on the
world distribution and prevalence of citrus canker and citrus scab.
Jour. Agr. Res. 32: 147-164. 1926.
10. SCHNEIDERHAN, F. J. Instant bordeaux. West Va. Agr. Exp. Sta.
Cir. 60. 1932.
11. SWINGLE, WALTER, and HERBERT J. WEBBER. The principal diseases
of citrous fruits in Florida. U. S. D. A. Div. of Veg. Phys. and
Path. Bul. 8: 20-24. 1896.
12. THOMPSON, W. L. Lime-sulfur sprays for the combined control of
purple scale and rust mites. Fla. Agr. Exp. Sta. Bul. 282. 1935.
13. Supplements in dormant bordeaux sprays for insect
control on citrus. The Citrus Industry 18(1): 8-9, and 21. 1937.
14. WINSTON, JOHN R. Citrus Scab: Its cause and control. U. S. D. A.
Bul. 1118. 1923.




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