• TABLE OF CONTENTS
HIDE
 Front Cover
 Front Matter
 Introduction
 Paradichlorobenzene treatment
 Preserving the cloth
 Summary














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 342
Title: Controlling tobacco downy mildew (blue mold) with paradichlorobenzene
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00015120/00001
 Material Information
Title: Controlling tobacco downy mildew (blue mold) with paradichlorobenzene
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 16 p. : ill. ; 23 cm.
Language: English
Creator: Tisdale, W. B ( William Burleigh ), 1890-
Kincaid, Randall R ( Randall Rich ), 1903-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1939
 Subjects
Subject: Tobacco -- Diseases and pests -- Control -- Florida   ( lcsh )
Tobacco blue mold   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by W.B. Tisdale and Randall R. Kincaid.
General Note: Cover title.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00015120
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000924572
oclc - 18214811
notis - AEN5199

Table of Contents
    Front Cover
        Page 1
    Front Matter
        Page 2
    Introduction
        Page 3
    Paradichlorobenzene treatment
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
    Preserving the cloth
        Page 14
    Summary
        Page 15
        Page 16
Full Text



December, 1939


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








CONTROLLING

TOBACCO DOWNY MILDEW (BLUE MOLD)

WITH PARADICHLOROBENZENE


By W. B. TISDALE and RANDALL R. KINCAID


Fig. 1.-Diagram showing construction of tobacco plant bed. using boards for vaporizing
PDB, and gathering the sheeting cover to one side of the bed.





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


Bulletin 342










EXECUTIVE STAFF
John J. Tigert, M.A., LL.D., President of
the University3
Wilmon Newell, D.Sc., Director3
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
Clyde Beale, A.B.J., Assistant Editor3
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager3
K. H. Graham, Business Managers
Rachel McQuarrie, Accountant3

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 Husbandmani 3
R. B. Becker, Ph.D., Dairy Husbandman3
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. WEmmel, D.V.M., Veterinarian
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, B.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., Chemist'1
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 Economist1
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. B. Watson, A.M., Entomologist'
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.B.A., Assistant
HORTICULTURE
G. H. Blackmon, M.S.A., Horticulturist'
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' 3
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 O. 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, Meteorologists
B. H. Moore, A.B., Asst. Meteorologist2
'Head of Department.
2In cooperation with U.S.D.A.
3Cooperative, other divisions, U. of F.
'On leave.










CONTROLLING
TOBACCO DOWNY MILDEW (BLUE MOLD)
WITH PARADICHLOROBENZENE

By W. B. TISDALE and RANDALL R. KINCAID

CONTENTS
Page Page
Paradichlorobenzene Treatment .................. 3 Amount of PDB Required .................... 9
Location of Beds ..................................... 4 How to Use PDB .................................... 10
Size of Beds ......................................... 4 The 3-night treatment ................ 10
Construction of Frames ...................... 5 The 48-hour treatment .................. 11
Boards for Vaporizing PDB .............. 6 Removing Covers from Beds ............... 13
Cloth for Vaporizing PDB .................... 7 Cost of Treatment .............................. .... 13
Sheeting Covers ........................................ 8 Preserving the Cloth .................................... 14
W hen to Use PDB ................................ 8 Summ ary .......................... ............... ....... 15

INTRODUCTION

Paradichlorobenzene for controlling tobacco downy mildew
was reported to have been used successfully in 1938 in South
Carolina, but further information was needed concerning the
best methods of applying the treatment under Florida condi-
tions. Therefore, in 1939, experiments were set up in early
February in plant beds where downy mildew appeared at that
time. Other experiments were conducted in March when the
general outbreak of downy mildew occurred. From these experi-
ments considerable information was obtained concerning the
amount of paradichlorobenzene necessary to use in different
weather conditions, and on the methods for applying it. Farm-
ers who saw the experiments became very enthusiastic over
the effectiveness and simplicity of the treatment, and as a re-
sult more than 6,000 pounds of paradichlorobenzene were used
in two counties where sales records were obtained.
The purpose of this bulletin is to report the most effective
methods developed for applying the paradichlorobenzene treat-
ment and to give some of the results obtained.

PARADICHLOROBENZENE TREATMENT

The paradichlorobenzene treatment is a new method for com-
bating plant diseases. Paradichlorobenzene may be purchased
under various trade names, such as "Paracide" and "Para-Baco",
and is commonly called PDB. For brevity, it will be called PDB
in this bulletin. PDB may be obtained in the form of white
crystals of various sizes, ranging from that of coarse ice cream










CONTROLLING
TOBACCO DOWNY MILDEW (BLUE MOLD)
WITH PARADICHLOROBENZENE

By W. B. TISDALE and RANDALL R. KINCAID

CONTENTS
Page Page
Paradichlorobenzene Treatment .................. 3 Amount of PDB Required .................... 9
Location of Beds ..................................... 4 How to Use PDB .................................... 10
Size of Beds ......................................... 4 The 3-night treatment ................ 10
Construction of Frames ...................... 5 The 48-hour treatment .................. 11
Boards for Vaporizing PDB .............. 6 Removing Covers from Beds ............... 13
Cloth for Vaporizing PDB .................... 7 Cost of Treatment .............................. .... 13
Sheeting Covers ........................................ 8 Preserving the Cloth .................................... 14
W hen to Use PDB ................................ 8 Summ ary .......................... ............... ....... 15

INTRODUCTION

Paradichlorobenzene for controlling tobacco downy mildew
was reported to have been used successfully in 1938 in South
Carolina, but further information was needed concerning the
best methods of applying the treatment under Florida condi-
tions. Therefore, in 1939, experiments were set up in early
February in plant beds where downy mildew appeared at that
time. Other experiments were conducted in March when the
general outbreak of downy mildew occurred. From these experi-
ments considerable information was obtained concerning the
amount of paradichlorobenzene necessary to use in different
weather conditions, and on the methods for applying it. Farm-
ers who saw the experiments became very enthusiastic over
the effectiveness and simplicity of the treatment, and as a re-
sult more than 6,000 pounds of paradichlorobenzene were used
in two counties where sales records were obtained.
The purpose of this bulletin is to report the most effective
methods developed for applying the paradichlorobenzene treat-
ment and to give some of the results obtained.

PARADICHLOROBENZENE TREATMENT

The paradichlorobenzene treatment is a new method for com-
bating plant diseases. Paradichlorobenzene may be purchased
under various trade names, such as "Paracide" and "Para-Baco",
and is commonly called PDB. For brevity, it will be called PDB
in this bulletin. PDB may be obtained in the form of white
crystals of various sizes, ranging from that of coarse ice cream







Florida Agricultural Experiment Station


salt to table salt. Intermediate sizes, which average about the
size of rice grains or slightly smaller, are the most desirable
to use for controlling tobacco downy mildew. The largest sizes
vaporize too slowly and the smallest sizes are difficult to dis-
tribute evenly. About 75 percent of the "rice" crystals will be
retained on a 10-mesh screen, whereas about 95 percent of the
smaller size will pass through a 10-mesh screen and about 78
percent of them will be retained on a 20-mesh screen. When
the crystals are exposed to the air they form a vapor or gas
that is heavier than air. Neither the solid nor the gas is in-
flammable.
LOCATION OF BEDS
Tobacco plant beds should be located on new land as far as
practicable from the previous year's beds to avoid infection
while the plants are small. They should be prepared for the
treatment before sowing. The land should be cleared of all
stumps, which would interfere with the treatment and would
occupy valuable space. The land should be well drained, but
not too steep, preferably with a slope of not more than 5 feet
in 100. Beds should be protected from sweeping winds, if pos-
sible, but should have a maximum of sunlight, preferably with
an eastern exposure. Beds running north and south are slightly
better than those running east and west, because fewer plants
are shaded by the frames. A space of 1 or 2 feet should be
left between the beds to serve as a walkway.
SIZE OF BEDS
If the PDB treatment is used it is not necessary to plant the
excess yardage of bed that has been commonly used during
the last 6 years. The necessary plant bed area is 100 square
yards for 3 or 4 acres of flue-cured or for about 1 or 2 acres
of shade-grown cigar-wrapper tobacco.
Beds to be treated with PDB should be approximately 6 or
9 feet wide (not over 12 feet), and preferably not over 75 feet
long. The heavy cloth covers required for this treatment are
usually made by sewing together strips of cloth 1 yard wide.
The actual width of the beds should be at least 6 inches less
than the width of the cover so that the cloth can be drawn
down over the top edges and fastened on the sides rather than
on top of the frames. If the cloth is fastened to the top edges
of the frames it sags between the nails and leaves openings
through which the vapor escapes. Narrow beds are easier to







Controlling Tobacco Downy Mildew


treat than wide beds, and they can be treated and weeded with-
out walking over them, thus saving many plants. Also, heavy
cloth covers wider than 12 feet are very difficult to handle,
especially when wet, without injuring plants and brushing off
the PDB from the boards.
All beds in a given location should be of the same size so
that the heavy cloth covers may be used interchangeably on
them as needed. Thus, if only about 100 square yards of plant
area are required, this may be divided into two beds of equal
size and enough heavy cloth bought for only one to effect a
saving in the cost of cloth. In 1939 many farmers bought only
one-half as many yards of cloth as the plant bed area and suc-
cessfully treated the entire area by first treating one portion
of the bed and then shifting the treatment to another. This
can be done if treatment is started as soon as first signs of
the disease appear.
CONSTRUCTION OF FRAMES
Plant beds should be tightly framed before the seeds are
sowed. The frames are constructed preferably of boards from
10 to 12 inches wide and free of holes and cracks. Logs may
be used if they are fitted to make the top edge of the frame
smooth and straight. The frames should be banked around
the outside with soil to fill any low places.








-- -- -- -- -- --.- - -- -- -

Fig. 2.-Diagram showing construction of tobacco plant bed, using ordinary plant bed cloth
for vaporizing PDB, and rolling the sheeting cover to one end of the bed.

If boards are used for vaporizing the PDB a wire should be
stretched lengthwise over the center of the bed and supported
about 3 inches higher than the frame by small posts placed
every few feet. Poles may be used instead of this wire. Other
wires stretched crosswise over the bed should be used to sup-
port the cloth at least 6 or 8 inches above the plants. These
crosswise wires should be supported in such way as to keep







Florida Agricultural Experiment Station


the cloth from sagging and brushing the crystals off the boards
while it is being placed over the bed, and also to keep water
from running through the cloth directly over the boards and
washing off the crystals. The construction of this type of bed
is shown in Figure 1.
If the ordinary plant bed cloth is used for vaporizing the
PDB the center wire may be omitted or placed on a level with
the side walls so that the cloth will be in a horizontal position
(Fig. 2).
BOARDS FOR VAPORIZING PDB
To promote adequate vaporization of PDB and to obtain an
even distribution of the vapor in the beds it is necessary to
distribute the crystals on a suitable support above the plants.
Boards have been found satisfactory for this purpose. Rough
boards, 5 or 6 inches wide, running the length or width of the
bed, are placed above the plants. The PDB crystals slide off
of dressed boards very easily. Six-inch boards are better than
narrower ones because this width gives a greater surface for
spreading the PDB; wider ones shade too many plants and may
interfere with the treatment, as shown by the fact that in one
instance downy mildew was not killed under 12-inch boards.
The boards should be at least 3 or 4 inches above the tops of
the plants and so placed in the beds that all plants are within
3 feet of the crystals.


Fig. 3.-Experimental plant bed showing method of applying PDB in a 6-foot bed.
(Photo by Agronomy Department.)


I ___ ___ I ___I


~6~~







Controlling Tobacco Downy Mildew


In a bed 6 feet wide one board placed lengthwise in the center
gives adequate vaporizing surface (Fig. 3). In a bed 9 or 12
feet wide two boards may be placed lengthwise of the bed and
about 2 or 21/3 feet from the side walls, or crosswise every 6
feet, beginning 3 feet from the ends (Fig. 1).
Bricks, wooden blocks, or shingles driven in the ground may
be used as supports for the boards, but probably it would be
more convenient to nail legs of the proper length and width
on the boards, as shown in Figure 1. The PDB can be dis-
tributed on the board outside the beds and then be placed in
position. These boards should be removed from the beds when
the treatment is completed to prevent shading the plants.

CLOTH FOR VAPORIZING PDB
Instead of using boards to vaporize the PDB, this material
may be distributed on the regular plant bed cloth left stretched
over the beds. The heavy sheeting necessary to retain the vapor
is then placed over this and fastened to the frames. When
this method is used the cloth should be supported in a horizon-
tal position over the beds as shown in Figure 2. This method
of distribution was used in several instances in 1939 and gave
complete control of downy mildew wherever used. Figure 4
shows the first bed in which the PDB was distributed on the
regular plant bed cover. The regular plant bed covers used in
these instances were new and of a heavy grade with 24 to 28
threads per inch. To be satisfactory for this purpose the cloth
should be free of holes and of mesh fine enough to keep the
PDB crystals from falling through on the tobacco plants. This
appears to be the most economical and effective method for
distributing and vaporizing PDB.

Fig. 4.-Plant bed in which downy mildew was controlled by distributing the PDB on the regular
plant bed cover. The bed cover and sheeting are removed from half of the bed to show condition of
plants at end of treatment.
If) I.am I -







Florida Agricultural Experiment Station


SHEETING COVERS
A cover tight enough to retain PDB vapor effectively must
be kept over the bed during the time when the treatment is
being applied. Such a cover is made of unbleached sheeting,
woven with about 52 to 60 threads per inch each way, and weigh-
ing about 4 ounces per square yard. This grade of cloth was
purchased in 1-yard widths in the North Florida counties in
1939, and sewed together on the farms to make the desired
width of cover. Wider cloth costs more per square yard, cloth
of coarser weave does not retain the vapor effectively, and heav-
ier sheeting is difficult to handle. The cloth generally used for
flue-cured tobacco bed covers or tobacco shade cloth is not ef-
fective, even when doubled several times. Tarpaulins are too
heavy for handling conveniently and may cause injury to the
plants in warm weather.
The cloth may be fastened to the frame with small nails driven
through the edges, but this often tears the cloth. The life of
the cloth can be increased by inserting small metal eyelets or
grommets about every 18 inches along the edge of the cloth.
These eyelets are slipped over nails driven in the outside walls
of the frames at the proper distances to insure a close fit of
the cloth on the top edge of the frame, making the operation
much more convenient than when nails are driven through the
cloth.
A wet cloth retains the vapor much better than a dry one,
and for this reason the cloth should be thoroughly wetted after
it is placed over the bed. It may be wetted with a hose, sprinkled
with a sprinkling can or a broom, or sprayed with a spray pump.
Water should not be dashed on with a bucket because of the
danger of washing off the crystals from the boards. Since new
cloth is not readily wetted uniformly, it is advisable to wash
new cloth before using it.
WHEN TO USE PDB
The PDB treatment can be applied during rainy weather
when it is likely to be needed most.
The treatment should be started when first signs of downy
mildew are found in the plant bed. However, the beds should
be properly arranged and framed, and the inside boards, sheet-
ing covers and PDB should be ready in advance. Plants are
grown under the regular plant bed covers until time to start
the treatment. When the disease is reported in the community,







Controlling Tobacco Downy Mildew


a careful daily inspection of the beds should be made to discover
its presence as soon as possible.
There seems to be little advantage in applying PDB before
the disease appears in the plant beds. Smaller quantities or
less frequent applications of PDB than those required to check
the disease will not keep it out of the beds, especially when used
under the regular plant bed cover. Furthermore, the plants are
susceptible to infection again as soon as the treatment is dis-
continued, provided weather conditions are favorable for the
disease. Therefore, if the treatment be used as a preventive
it would probably be necessary to keep PDB on the bed con-
tinuously during the period when downy mildew threatens. For
these reasons it seems more economical to delay treatment until
downy mildew is found in the bed and then use enough PDB
to kill it.
The disease may reappear within a week or 10 days after
the treatment is completed, and if it does the treatment should
be repeated. In 1939 the disease reappeared in all except one
of the beds treated in February. On the other hand, it re-
appeared in only a few of the numerous beds treated in March
after which weather conditions were unfavorable for the dis-
ease. Thus, when the disease does not appear until late in the
season it is probable that only one treatment will be required
in most cases.
If the treatment is started in time half of the plant bed area
can be treated and then the covers and also the inside boards,
if these are used instead of the regular plant bed cloth for
vaporizing the PDB, can be moved to the other half of the plant
bed area and used as before. This effects a considerable saving
in the cost of sheeting and of lumber. Many farmers used
this system in 1939 with satisfactory results.
AMOUNT OF PDB REQUIRED
The amount of PDB which must be actually vaporized to con-
trol downy mildew is about 1 to 11/2 pounds per 100 square
yards of plant bed per night. The amount that must be applied
to give this required concentration of vapor depends on the tem-
perature, moisture and size of crystals. It also depends upon
the area and nature of the surface over which the crystals are
scattered, as discussed above.
Temperature has a large effect on the rate of vaporization
of PDB. In average weather during the plant bed season an






Florida Agricultural Experiment Station


application of from 2 to 4 pounds per night is sufficient, the
largest amount being used during cold weather. However, at
freezing temperatures and below there is practically no vaporiza-
tion of PDB and nothing is to be gained by applying it. For-
tunately, downy mildew seems to be inactive at these tempera-
tures, although it is not killed.
Coarse crystals evaporate more slowly than fine crystals. If
fine crystals are placed in piles they form a cake or crust which
slows down the rate of vaporization. This tendency to cake
or crust, however, can be eliminated by thoroughly mixing with
the PDB crystals 1 percent or less of fuller's earth, kaolin,
powdered charcoal, limestone, hydrated lime or very fine dry
soil, thereby providing a more uniform rate of vaporization.
Laboratory tests showed that wetting the crystals reduced
the rate of vaporization slightly. The disease was checked in
the plant bed, however, when it was cloudy and raining from
one to several times a day and the temperature dropped below
400 F. at night. The treatment appears to be as effective when
the plants are wet as when they are dry.
Because of these factors affecting the rate of vaporization of
PDB, the dosage which must be applied to vaporize the amount
actually required varies considerably under different conditions.
Unless the crystals left on the board are weighed, the amount
vaporized cannot be determined very accurately. However, the
grower can judge whether he is using the right amount of PDB.
If too little is used the disease will not be checked by the treat-
ment and if too much is used the plants may wilt slightly
for a day or two after the treatment is discontinued in warm
weather.
HOW TO USE PDB
Downy mildew has been successfully checked by treating with
PDB (1) for three successive nights, removing the cloth during
the day, and (2) for 48 hours, leaving the cloth over the bed
night and day.
1. The 3-Night Treatment. This treatment should be used
when the days are clear and warm. The amount of PDB applied
per night varies from 2 to 4 pounds per 100 square yards, in
order to vaporize 1 to 11/2 pounds actually required to control
the disease. During average spring weather when night tem-
peratures range between 400 and 550 F., from 2 to 3 pounds
should be used; on cool nights when the temperature drops to
400 F. or below, 4 pounds should be used. The required amount







Controlling Tobacco Downy Mildew


of PDB is applied at about 5 or 6 o'clock in the afternoon and
the cover is immediately spread over the bed, fastened to the
frame, and sprinkled or sprayed with water. The cover is
removed about 8 or 9 o'clock in the morning. If boards are
used for vaporizing, the PDB left may be brushed off and saved
for future use. This treatment, continued for three successive
nights, gave perfect control in several beds in February (Fig. 5).
Under some conditions a 2-night treatment may be sufficient,
but it has not proved to be in all instances.











.r .




Fig. 5. Experimental plant bed in which downy mildew was controlled in February by
using PDB for three successive nights. All plants in non-treated portion (right) of bed
were killed by the disease. The beJ was retreated on March 3, 4 and 5, and photographed
March 13, 1939.

2. The 48-Hour Treatment. This treatment may be used
when the days are cold, or cool and cloudy. From 3 to 4 pounds
of PDB per 100 square yards are applied at about 5 or 6 o'clock
in the afternoon, and the cover is immediately spread over the
bed, fastened to the frame, and sprinkled, provided it is not
already wet. The same amount of PDB is applied again the
next afternoon and the cloth is wetted again. It may be advis-
able to wet the cloth also during the day if the weather is dry
and windy. Dividing the total amount of PDB between two
applications reduces the danger of injuring the plants if the
weather becomes fair and warmer during the day, and also gives
the plants some fresh air while the second dose is being applied.
This treatment proved completely effective under conditions
where it was tested (Fig. 6).
The 48-hour treatment involves less labor than the 3-night
treatment. Since it is also completed in less time, the 48-hour







Florida Agricultural Experiment Station


treatment seems to be more practical whenever the weather
is favorable for its use.
Plants should be examined carefully after the completion of
either treatment. If the mildew on the leaves is bluish or light
gray in color, indicating that the disease is still active, the
treatment should be continued for another night or 24 hours.

















Fig. 6.-Experimental plant bed in which downy mildew was controlled with PDB in
a 48-hour treatment. Photographed March 16, when plants in treated portion of bed were
ready to transplant.

Since the vapor of PDB is heavier than air it does not spread
very far in the bed when there are no air currents. Therefore,
it is important to scatter the crystals as evenly and uniformly
as possible on the boards (Fig. 3) or cloth in all parts of the
bed. Thoroughness with which this is done can be determined
by the amounts and location of crystals left on the boards in
the morning after it was applied the afternoon before.
No permanent injury was observed from the use of these
methods under a wide range of weather conditions. In a few
cases where tarpaulins or other heavy materials were used for
covers and the frames were low, leaving very little space be-
tween the cover and plants, considerable injury developed.
Large plants, especially when the stand was thick, wilted badly
when the cloth was left over the beds during warm, bright days,
but they revived after the cloth was removed and showed no
signs of injury after a few days. Many farmers transplanted
the plants to the field on the second or third day after com-
pleting the treatment and reported that a high percentage of
them lived.







Controlling Tobacco Downy Mildew


REMOVING COVERS FROM BEDS
The heavy cloth should be removed from the beds as soon
as the treatment is completed. To remove the cloth it is un-
fastened on the ends and one side and rolled or gathered up
along the frame on the other side (Fig. 1). Another method
which is perhaps more convenient is to unfasten the cloth on
both sides and one end and roll it lengthwise of the bed on a
pole or a narrow board (Fig. 2).
In warm weather the beds may be left uncovered between
periods of treatment to harden the plants and to make condi-
tions less favorable for downy mildew. If there is danger of
frost the regular plant bed cover should be replaced over the
beds. The sheeting covers have also been found to give ex-
cellent protection to the plants in freezing weather.

COST OF TREATMENT
Cost of the PDB treatment will vary somewhat with the
width of beds, kind of materials used for frames, difference in
price of sheeting in various localities, and the choice of boards
or the regular plant covers for distributing and vaporizing
the PDB.
A given yardage of plant bed can be framed with a smaller
amount of lumber if the bed is made 3 or 4 yards wide than
if it is made 2 yards wide. For example, 100 square yards
in a bed 4 yards wide (25 yards long) would require 174 lineal
feet of boards; 3 yards wide (331/3 yards long), 218 lineal feet;
2 yards wide (50 yards long), 312 lineal feet.
Approximately 150 lineal feet of boards will be required for
distributing and vaporizing the PDB per 100 square yards of
bed under treatment at one time, except for a bed 3 yards wide
with two boards placed lengthwise, in which case 200 lineal feet
would be required. These boards can be used interchangeably
in two areas.
The price of sheeting will vary from year to year and may
vary in different localities in any year. It was purchased in
1-yard widths in 1939 at 7 to 8 cents per yard. By treating
only half of the plant bed area at one time it is necessary to
purchase only about one-half as many yards of the sheeting
as the yardage of plant bed. With proper care, the cloth should
last for several years, although this time has not been deter-
mined by actual experience.







Florida Agricultural Experiment Station


The price of PDB in 1939 ranged from about 15 cents per
pound in 100-pound drums to 25 cents per pound in 4-pound
cans. With general use for controlling downy mildew it may
become cheaper. At the above prices the cost of 6 to 8 pounds
of PDB necessary for a complete treatment of 100 square yards
will vary from a minimum of 90 cents (6 pounds at 15 cents)
to a maximum of $2 (8 pounds at 25 cents).
Since the heavy cloth and PDB are necessary for the control
of downy mildew, the cost of these two items must be incurred.
For the first year the maximum cost of these items per 100
square yards of bed, figuring $4 for 50 square yards of cloth
and $2 for PDB for one 3-night or 48-hour treatment, would
not exceed $6 if only one treatment is necessary, or $8 if two
treatments are necessary. For the second and succeeding years,
as long as the original cloth could be used, the maximum cost
of PDB only would be $2 for one treatment or $4 for two treat-
ments.
If suitable log frames are used and the PDB is vaporized from
the regular plant bed cover, the cost of boards for these pur-
poses can be eliminated. However, it is considered more de-
sirable to use boards for the frames.
The saving effected by reducing the yardage of plant bed to
a minimum may largely offset the cost of the PDB treatment.

PRESERVING THE CLOTH
The sheeting should last for several years if it is properly
treated, thoroughly dried, and stored where rats will not de-
stroy it. The life of the cloth can be greatly increased at
nominal cost by soaking it in a solution made up as follows:
4 pounds zinc sulfate, 4 pounds sal soda, 1 pound tartaric acid
and 150 gallons water. The solution is made by dissolving the
zinc sulfate and sal soda separately in one-half the total volume
of water. These two solutions are stirred together and then
the tartaric acid dissolved in a little water is added while stir-
ring. This quantity is sufficient to treat about 1,000 yards of
cloth. Larger or smaller volumes of the solution may be made,
using the same proportions.
The cloth is soaked in the solution for 48 hours and then
hung up without wringing and allowed to dry thoroughly be-
fore storing.







Controlling Tobacco Downy Mildew


SUMMARY
The paradichlorobenzene (PDB) vapor treatment has proved
highly effective in controlling downy mildew of tobacco. This
method is simple and comparatively inexpensive. It can be suc-
cessfully used during rainy weather when treatment is needed
most.
Narrow beds, approximately 6 or 9 feet wide, are most con-
venient and probably most economical to treat.
If the PDB treatment is used it is necessary to plant only
the regular yardage of plant bed area; that is, 100 square yards
for 3 or 4 acres of flue-cured tobacco, or for 1 to 2 acres of
shade.
It is necessary to purchase only one-half as many yards of
the sheeting as there are yards in the plant bed. When the
disease is checked on one bed or part of a bed treatment is
shifted to another.
The frames must be made tight and high enough so the crys-
tals can be placed above the tops of the plants. Ten- or 12-inch
boards give best results.
The crystals must be supported above the plants in such way
that none of the plants will be over 3 feet away. Rough 6-inch
boards placed lengthwise or crosswise of the bed have proved
satisfactory for this purpose. Better distribution may be ob-
tained by placing the PDB on the regular plant bed covers of
fine enough mesh to keep the crystals from falling through;
this is the most economical and probably the most effective
means of vaporization.
It is not necessary to keep the PDB dry after placing it in
the plant bed but if it is distributed on boards the cover should
be supported in such manner that it will not sag immediately
over the boards, permitting water to accumulate, stream through
and wash off the crystals. Wetting retards the rate of vapor-
ization slightly, but the treatment has been used effectively in
rainy weather.
Covers made of sheeting with about 52 to 60 threads per inch
each way must be fitted tightly over the beds after the PDB
has been applied to prevent escape of the vapor. The cover
should be sprinkled or sprayed with water after it is placed
over the bed to make it more effective. This cloth and the
inside boards should be removed as. soon as the treatment is
completed.







Florida Agricultural Experiment Station


Treatment is started preferably when first signs of the dis-
ease are discovered. There appears to be nothing gained by
starting the treatment before downy mildew appears in the bed.
If the disease reappears in the bed the treatment should be
repeated.
The amount of PDB which must be applied in order to vaporize
the required dosage varies considerably with temperature and
other factors. With the 3-night treatment from 2 to 4 pounds
of PDB are applied per 100 square yards of bed per night.
With the 48-hour treatment from 3 to 4 pounds of PDB are
applied per 24-hour period.
For the treatment to be effective the crystals must be thinly
and evenly distributed throughout the bed.
The PDB treatment has caused no injury to the plants when
properly applied.
The cost of the treatment over a period of years appears to
be reasonable and may be largely offset by the saving in cost
of excess plant bed area. With proper care the sheeting covers
should last several seasons.




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