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January, 1939
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA
WILMON NEWELL, Director
DOWNY MILDEW (BLUE MOLD)
OF TOBACCO
RANDALL R. KINCAID and W. B. TISDALE
Fig. 1.-Tobacco leaf showing downy mildew on lower surface.
Bulletins will be sent free to Florida residents upon request to
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA
Bulletin 330
EXECUTIVE STAFF
John J. Tigert. M.A., LL.D., President of
the University
Wilmon Newell, D.Sc., Director
Harold Mowry, M.S.A., Asat. Dir., Research
V. V. Bowman, M.S.A., Asst. to the Director
J. Francis Cooper, M.S.A., Editor
Jefferson Thomas, Assistant Editor
Clyde Beale, A.B.J., Assistant Editor
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager
K. H. Graham, Business Manager
Rachel McQuarrie, Accountant
MAIN STATION, GAINESVILLE
AGRONOMY
W. E. Stokes, M.S.., Agronomist'
W. A. Leukel, Ph.D., Agronomist
G. E. Ritchey, M.S., Associate'
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'
R. B. Becker, Ph.D., Dairy Husbandman
L. M. Thurston, Ph.D., Dairy Technologist
W. M. Neal, Ph.D., Asso. in Dairy Nutrition
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M., Veterinarian
N. R. Mehrhof, M.Agr., Poultry Husbandman
O. W. Anderson, M.S., Asst. Poultry Husb.
W. G. Kirk, Ph.D., Asst. An. Husbandman
R. M. Crown, B.S.A., Asst. An. Husbandman
P. T. Dix Arnold, M.S.A., Assistant Dairy
Husbandman
L. L. Rusoff, M.S., Asst. in An. Nutritions
CHEMISTRY AND SOILS
R. V. Allison, Ph.D., Chemist'
F. B. Smith, Ph.D., Soil Microbiologist
C. E. Bell, Ph.D., Associate Chemist
R. B. French, Ph.D., Associate
H. W. Winsor, B.S.A., Assistant
J. Russell Henderson, M.S.A., Assistant
L. W. Gaddum, Ph.D., Biochemist
L. H. Rogers, M.A., Spectroscopic Analyst'
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
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., Horticulturist'
A. L. Stahl, Ph.D., Associate
F. S. Jamison, Ph.D., Truck Horticulturist
R. J. Wilmot, M.S.A., Spec. Fumigation Res.
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'
George F. Weber, Ph. D., Plant Pathologist
R. K. Voorhees, M.S., Assistants
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
L. O. Gratz, Ph.D., Plant Path. in Charge
R. R. Kincaid, Ph.D., Asso. Plant Pathologist
J. D. Warner, M.S., Agronomist
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., Asst. Entomologist
W. W. Lawless, B.S., Asst. Horticulturist
EVERGLADES STATION, BELLE GLADE
J. R. Neller. Ph.D., Biochemist in Charge
J. W. Wilson, Se.D., Entomologist
F. D. Stevens, B.S., Sugarcane Agronomist
Thomas Bregger, Ph.D., Sugarcane
Physiologist
Jos. R. Beckenbach, Ph.D., Asso. Horticul.
Frederick Boyd, Ph.D., Asst. Agronomist
G. R. Townsend, Ph.D., Asso. Plant Path.
R. W. Kidder, B.S., Animal Husbandman
W. T. Forsee, Ph.D., Asst. Chemist
B. S. Clayton, B.S.C.E., Drainage Engineer'
SUB-TROPICAL STATION, HOMESTEAD
W. M. Fifield, M.S., Asst. Horticulturist
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., Plant Pathologist in
Charge
K. W. Loucks, M.S., Asst. Plant Pathologist
C. C. Goff, M.S., Assistant Entomologist
Plant City
A. N. Brooks, Ph.D., Plant Pathologist
R. N. Lobdell, M.S., Asst. Entomologist
Cocoa
A. S. Rhoads, Ph.D., Plant Pathologist
Hastings
A. H. Eddins, Ph.D., Plant Pathologist
Monticello
Samuel O. Hill, B.S., Asst. Entomologist'
Bradenton
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. Meteorologist'
'Head of Department.
'In cooperation with U.S.D.A.
'On leave.
DOWNY MILDEW (BLUE MOLD)
OF TOBACCO
RANDALL R. KINCAID and W. B. TISDALE
CONTENTS
PAGE
DESCRIPTION OF THE DISEASE ... ..... --... ............ .... ...... 4
RELATION OF THE WEATHER TO DOWNY MILDEW ............................... 7
NAMES AND HOSTS OF DOWNY MILDEW ................. ... ....... ......... 11
DESCRIPTION AND LIFE HISTORY OF THE FUNGUS ............................... 11
SURVIVAL OF DOWNY MILDEW ........... ......... ................................. 12
CONTROL MEASURES .....................-....... ....... ............ 14
Control in plant beds .............. ....... ......... ..................... 14
Control on cigar-wrapper tobacco in the field ............................ 27
Breeding for resistance ........ .. .... .... .-............... 27
LITERATURE CITED .................. .......... .. ......... ....................................... 28
INTRODUCTION
Downy mildew (blue mold) of tobacco in Florida is primarily
a disease of seedlings in the plant bed. In Australia it has
been considered the most serious of all tobacco diseases for
almost a half century. It first appeared in Florida in 1921, but
a disease similar to it, if not the same, was observed previously
in the western part of the United States. After 1921 it was
not seen again in Florida until 1931, when there was a general
outbreak of the disease that spread up the East Coast to North
Carolina and Virginia. Since that time it has been more or
less prevalent every year. One cannot predict very long in ad-
vance how serious it may become, and therefore its appearance
each year causes much apprehension for the outcome of the
current crop. In 1932 and 1937 serious losses of plants and
considerable delay in transplanting were incurred. In other
years losses have been due to the use of inferior plants and the
extra cost of preparing the additional plant bed space necessary
to insure an ample supply of plants.
It is the purpose of this bulletin to present a description of
downy mildew, the fungus which causes it, some factors known
to influence its severity, and the methods that have been fonmd
to date for its control, especially those that may be practical
for use under Florida conditions.
DOWNY MILDEW (BLUE MOLD)
OF TOBACCO
RANDALL R. KINCAID and W. B. TISDALE
CONTENTS
PAGE
DESCRIPTION OF THE DISEASE ... ..... --... ............ .... ...... 4
RELATION OF THE WEATHER TO DOWNY MILDEW ............................... 7
NAMES AND HOSTS OF DOWNY MILDEW ................. ... ....... ......... 11
DESCRIPTION AND LIFE HISTORY OF THE FUNGUS ............................... 11
SURVIVAL OF DOWNY MILDEW ........... ......... ................................. 12
CONTROL MEASURES .....................-....... ....... ............ 14
Control in plant beds .............. ....... ......... ..................... 14
Control on cigar-wrapper tobacco in the field ............................ 27
Breeding for resistance ........ .. .... .... .-............... 27
LITERATURE CITED .................. .......... .. ......... ....................................... 28
INTRODUCTION
Downy mildew (blue mold) of tobacco in Florida is primarily
a disease of seedlings in the plant bed. In Australia it has
been considered the most serious of all tobacco diseases for
almost a half century. It first appeared in Florida in 1921, but
a disease similar to it, if not the same, was observed previously
in the western part of the United States. After 1921 it was
not seen again in Florida until 1931, when there was a general
outbreak of the disease that spread up the East Coast to North
Carolina and Virginia. Since that time it has been more or
less prevalent every year. One cannot predict very long in ad-
vance how serious it may become, and therefore its appearance
each year causes much apprehension for the outcome of the
current crop. In 1932 and 1937 serious losses of plants and
considerable delay in transplanting were incurred. In other
years losses have been due to the use of inferior plants and the
extra cost of preparing the additional plant bed space necessary
to insure an ample supply of plants.
It is the purpose of this bulletin to present a description of
downy mildew, the fungus which causes it, some factors known
to influence its severity, and the methods that have been fonmd
to date for its control, especially those that may be practical
for use under Florida conditions.
Florida Agricultural Experiment Station
DESCRIPTION OF THE DISEASE
IN THE PLANT BED
Since the characteristics of downy mildew vary greatly from
day to day and are affected by the age of the tobacco plants and
by weather conditions, it is difficult to give a complete descrip-
tion of the disease. Consequently, only the most conspicuous
characters that may be seen under certain conditions will be
described. With very young plants first indications of downy
mildew are small areas of yellow plants which soon die and
disappear. At this stage downy mildew cannot be distinguished
without the aid of a microscope from nutritional troubles, cold
injury, or damping-off. On larger plants the disease usually
shows as yellow spots on the leaves and can be distinguished
from the somewhat similar spots due to cold injury by the
presence of a downy growth of fungus on the lower surface of
the leaves (Fig. 1).
.04.
Fig. 2.-Portion of a tobacco plant bed showing downy mildew on plants in a small area
in the center.
Downy Mildew (Blue Mold) of Tobacco
Under usual weather conditions these spots on the leaves and
areas of diseased plants on the beds enlarge and undergo other
characteristic changes each day. The fungus at first exhibits
the familiar bluish or purplish color; on older spots the color
is light brown. Aside from the slight change in color the upper
surfaces of diseased leaves remain practically normal in appear-
ance for a day or two and then the diseased spots die and dry
out to a light brown color. The fungus continues to develop
in the living tissues around the dead spot and produces the
downy growth on the surface. Diseased leaves become distorted
and twisted until some lower surfaces are turned upward. In
such cases the bluish color of diseased plants becomes conspicu-
ous at some distance, especially when they are wet with dew.
With favorable weather conditions the disease spreads rapidly
from the small areas of early infection (Fig. 2) over entire beds
and appears in many other beds almost simultaneously. The
leaves disintegrate, dry out and shrivel to mere strings within
a few days, making the bed look as if scalding water or some
toxic chemical had been poured over it (Fig. 3). Under such
Fig. 3.-Portion of a tobacco plant bed after a severe attack of downy mildew.
conditions there is a rank odor, suggesting decaying vegetable
matter. However, development and spread of the disease is
often erratic, and for no apparent reason some beds or portions
of beds may escape infection even during a severe general out-
break. With weather conditions unfavorable to the disease, such
as warm, clear days, spots on the leaves and affected areas on
the beds may remain small and localized for days or even weeks.
The diseased spots may not turn brown but remain yellow, and
as the leaves grow they become puckered. Then with a few days
Florida Agricultural Experiment Station
of cloudy, wet, cool weather the disease may develop with aston-
ishing rapidity.
If plants are not over a month old when attacked they are
usually killed, while plants about ready for transplanting may
suffer only a delay in growth. Large plants may lose all of
their leaves and their roots may turn yellow. Within a week
or two surviving plants produce new leaves and fresh, normal
roots. If the buds are killed suckers may develop from lower
parts of the stems. Plants which have had the disease and
recovered are seldom seriously affected again in the plant bed
and grow normally when set in the field.
IN THE FIELD
Downy mildew may continue to develop on tobacco plants
after they have been set in the fields, provided weather condi-
tions are favorable for
its continued activity.
SIn some years this has
resulted in a poor stand
or complete failure and
the additional expense
of replanting. On the
other hand if the weath-
er is clear and warm
there may be no fur-
ther development of the
disease after infected
plants are transplanted
and a good stand is ob-
tained. In certain years
downy mildew has not
appeared on some beds
until after plants have
been drawn and set in
the fields, and under
such conditions the
plants may be attacked
by the disease in the
field, causing spots or
blotches on the leaves.
Such spots rarely cause
Fig. 4. Leaf of cigar-wrapper tobacco showing
downy mildew spots of several types. anyreduction in market
Downy Mildew (Blue Mold) of Tobacco
value of flue-cured, cigar-filler and binder varieties grown in
the open field. However, the disease sometimes attacks cigar-
wrapper tobacco in shaded fields during cool weather in April
and May and has caused some loss in value during several
seasons.
The disease causes spots of various shapes and sizes on cigar-
wrapper leaves (Fig. 4). They are sometimes about an inch
in diameter and light brown in color. Sometimes they are small
and clustered, indicating that the development of the disease
was arrested before the entire invaded area was killed. Small
spots are sometimes surrounded by a yellow halo resembling
that caused by wildfire. Except during damp weather and on
leaves close to the ground the majority of the spots show no
spores or fungous growth on the surface.
RELATION OF WEATHER TO DOWNY MILDEW
Tobacco growers know that downy mildew behaves very dif-
ferently from season to season; that in some years it is very
destructive while in other years it causes little or no permanent
damage. Research work in Australia and the United States
during recent years has shown that weather conditions are very
important in the development and distribution of the fungus.
Principal weather factors are temperature, moisture, sunshine
and wind. Each of these factors exerts its own effect, but the
greatest damage is done when all of them combine in a manner
favorable to the disease.
TEMPERATURE
Spores of the downy mildew fungus develop during early
morning hours at temperatures between 400 and 63 F. with
greatest production at about 56 (7)1. Thus it is apparent that
when the night temperatures are above 63 or below 400, few
or no spores are produced and there is little chance for the dis-
ease to spread. Normal winter weather in the tobacco-producing
section of Florida is too cold for the development of the fungus,
but often there are warm periods in January and February of
sufficient duration to initiate its development. Beginning in
March and continuing through most of the remainder of the
season temperatures are usually favorable, although occasionally
it is too warm and dry. However, in some years many beds
show first signs of the disease after transplanting is completed.
'Italic figures in parentheses refer to "Literature Cited" in the back
of this bulletin.
TABLE 1.-MONTHLY MEAN (AVERAGE) TEMPERATURE (M) AND LOWEST TEMPERATURE (L) RECORDED AT FOUR U. S. WEATHER
BUREAU STATIONS IN NORTHERN FLORIDA FOR THE WINTER SEASONS 1930-31 TO 1937-38, INCLUSIVE.
(Readings in degrees Fahrenheit)
Quincy (Gadsden County) M adison (Madison County)
Winter Mar Dec. Jaan. Feb. Mar. Feb.
M L ML ML ML ML ML ML ML
1930-31 ....... 48 30 51 25 54 27 57 30 49 31 52 25 55 30 58 34
1931-32 ....... 64 40 62 33 64 32 56 23 67 43 62 33 66 36 58 24
1932-33 ....... 60 29 57 30 55 17 60 32 62 36 59 33 58 20 62 34
1933-34 ....... 59 32 53 20 50 25 58 28 62 35 56 20 52 27 61 28
1934-35 ...... 52 16 53 21 54 22 65 23 54 19 55 22 56 24 67 31
1935-36..... 46 18 52 21 51 18 62 33 47 20 54 21 51 20 64 38
1936-37 ....... 56 30 64 39 54 29 57 26 56 32 66 42 56 32 58 30
1937-38 ...... 51 15 52 21 59 33 66 31 52 18 54 23 61 34 68 35
Normal* .... 54 54 56 61 55 55 57 62 -
Lake City (Columbia County) Gainesville (Alachua County)
Winter Dec. Jan. Feb. Mar. Dec. Jan. I Feb. Mar.
ML ML IML ML ML M L ML ML
1930-31 ........ 50 27 53 25 56 31 59 32 51 26 54 27 57 33 58 32
1931-32 ........ 67 42 63 33 67 38 59 25 69 46 65 32 67 40 59 28
1932-33 ........ 63 37 59 33 60 23 63 36 64 33 61 34 63 28 64 37
1933-34........ 59 34 55 24 52 28 63 29 62 33 60 27 57 32 63 30
1934-35....... 55 20 57 28 56 25 68 28 57 16 58 27 58 26 69 30
1935-36........ 54 53 64 40 50 23 57 26 57 26 64 39
1936-37 ...... 54 31 63 42 34 58 33 60 30 69 44 59 35 61 33
1937-38 ....... 50 17 52 23 35 67 38 56 20 57 24 63 31 70 42
Normal* ...... 56 56 58 63 57 57 58 64-
*Based on records of not less than 20 years, up to and including 1935.
Downy Mildew (Blue Mold) of Tobacco
If cool weather prevails downy mildew may develop on plants
set in the fields. However, no serious field damage has been
reported except in a few instances on cigar-wrapper tobacco
grown under cloth shades. Within a few weeks after trans-
planting the disease usually disappears, probably because the
high temperatures and sunshine during the day inactivate or
kill the fungus.
An analysis was made of the monthly mean (average) tem-
perature and the lowest temperature recorded at four U. S.
Weather Bureau stations in northern Florida for the winter
seasons of 1930-31 to 1937-38, inclusive, (Table 1). These
stations, located at Quincy (Gadsden County), Madison (Madi-
son County), Lake City (Columbia County), and Gainesville
(Alachua County), are well distributed throughout the tobacco-
growing area of Florida, as shown in Fig. 5. During the winter
55 30 32
7 37* 33* 40*
61 1* 2*
62
1 51
ALPHABETICAL LIST OF COUBIES 15 21 54
inMBERED TO AGREE WITS THE MAP 42
1. Alachua 30. Holmes 51. Pasco
2. Baker 32. Jackson 53. Polr 35
4. Bradford 33. Jefferson 54. Putnam
12. Colu obta 35. Lake 58. St. .Tohns
15. Mxie 37. Leon 60, Smnter 51
20. e adsden 38. Lety 61. Suwannee 55
21. Gilchrist 40. Madison 6?. Taylor
24. Hamilton 14. Marion 63. Union
27. Hernando 45. Wassan 67. Washington
Fig. 5.-Map of the tobacco-growing area of Florida. Counties in which tobacco was
grown in 1938 are numbered. An asterisk (*) indicates that downy mildew occurred in
the county in 1938.
of 1931-32 the months of December, January, and February had
average temperatures from 7 to 12 degrees above normal, with
no freeze until March; downy mildew was severe in Florida and
also in other states. During the next four winters, 1932-33 to
1935-36 inclusive, January temperatures declined rather steadily
and the severity of the disease likewise declined. January 1937
was again far above normal, in fact the warmest during the
eight-year period, and the disease during that season was the
most widely distributed, as well as the most severe. January
1938 was cool, with a freeze and average temperatures normal
Florida Agricultural Experiment Station
or slightly below, and the attack of downy mildew was again
light; some counties in which tobacco was grown were not
affected (Fig. 5). These observations indicate that it may be
possible to predict, in a general way, the severity of downy
mildew during the plant bed season from the temperatures
prevailing during the month of January (12).
MOISTURE
Like the seeds of higher plants, summer spores of the downy
mildew fungus germinate only in the presence of sufficient
moisture. After they germinate water must remain on the
tobacco leaf an hour longer for the fungus to grow into the
leaf tissues and cause infection. Even though night tempera-
tures are favorable for spore production, water must be present
on the leaves for three or more hours after the spores are pro-
duced for them to germinate and start the disease. Therefore,
conditions which prolong the period during which the leaves
are wet tend to make the disease more severe. Since dews are
rather common and heavy during the spring months in Florida,
there is usually an adequate supply of moisture for several hours
each morning, especially after the plants grow large enough
to become crowded in the beds. Drying of dew is further
delayed by trees that shade the beds. Irrigation of the beds
and light rains also provide favorable conditions.
SUNSHINE
Sunshine naturally hastens the evaporation of dew and rain
and in this way may prevent germination of the spores, thereby
reducing the severity of downy mildew. Direct sunlight also
kills summer spores within an hour (14). Thus downy mildew
develops less rapidly during periods of bright, clear days. Indeed,
growers recognize "blue mold weather" as cool, damp nights,
or cool mornings with heavy dew, followed by cloudy, warm days.
WIND
Windy days, especially following nights when spore produc-
tion is abundant, are conducive to the spread of downy mildew
spores from one bed to another. Even a light breeze can effec-
tively distribute the spores over all parts of a bed. On the other
hand, winds at night prevent the formation of dew and also
hasten the drying of leaves after they are wet, thereby helping
to check further development of the disease.
Downy Mildew (Blue Mold) of Tobacco
NAMES AND HOSTS OF DOWNY MILDEW
Downy mildew of tobacco is caused by a fungus technically
known as Peronospora tabacina Adam. All members of the
group Peronospora and certain closely related groups produce
diseases to which the name "downy mildew" has been given,
because of the visible downy growth of the fungus on the sur-
face of the diseased parts. The name "blue mold" is rather
confusing because it has long been applied to fungi that grow
on fruits and vegetables, and sometimes on the soil of plant
beds soon after they are sown, especially if cottonseed meal has
been used as a fertilizer.
Downy mildews of cabbage, onions, grapes and other crops
are similar in appearance but are entirely distinct diseases. For
example, downy mildew of tobacco will not attack cabbage, and
in turn downy mildew of cabbage will not attack tobacco.
Wild tobaccos in Australia have been reported as susceptible
to the downy mildew of cultivated tobacco (1). Tomato, egg-
plant, and especially pepper, which belong to the same plant
family as tobacco, may be affected (3).
DESCRIPTION AND LIFE HISTORY OF THE FUNGUS
A knowledge of the different stages and activities of the
downy mildew fungus should give the grower a better under-
standing of the reasons why certain measures are effective in
controlling the disease, while others are less successful.
After a spore germinates on a tobacco leaf, thread-like fila-
ments myceliumm) of the fungus grow into the leaf and obtain
food from the tobacco leaf cells, causing them gradually to dry
out and die. These thread-like structures of the fungus corre-
spond roughly to the roots of a tree. After four to seven days,
depending upon the weather, minute tree-like structures (sporo-
phores) grow out of the leaf and produce numerous branches
(Fig. 6). On the tip of each branch is borne an oval or lemon-
shaped spore (summer spore or conidium), which serves as a
seed for the fungus. These structures are very small, but a
dense mass of the spore-bearing branches is visible to the naked
eye as the familiar downy coating on the leaf (Fig. 1). Enor-
mous numbers of these spores are produced. They are easily
removed and may be carried by wind, man and other agencies
to other leaves of plants in the same and other plant beds. With
many spores in the air some of them will fall in drops of water
Florida Agricultural Experiment Station
on tobacco leaves, and
after a few hours they
germinate, grow into the
leaves and start the pro-
cess again. Spores of this
p kind are short lived, rarely
surviving for more than a
Sfew hours. They serve to
Perpetuate the disease on
S living plants as long as
"" .... weather conditions are
i *favorable.
SAnother kind of spore,
called resting spore
oosporee), is produced
'. Within diseased leaves
fk that come in contact with
the soil. These spores are
not so abundant as the
*" '- summer spores, and hav-
ing thick, hard coverings,
Fig. 6.-Sporophores of downy mildew, as seen
with a microscope. Sporophore at left shows spores can remain alive in the
attached. (Magnified 150 times.)
soil for an indefinite time.
Their probable significance is discussed below.
SURVIVAL OF DOWNY MILDEW
All of the means by which downy mildew survives from one
plant bed season to another are not definitely known, and be-
cause of the strictly parasitic habit of the fungus it is difficult
or impossible to obtain experimental proof.
Extensive observations indicate that summer spores cannot
survive from one plant bed season to another, therefore it is
not considered likely that summer spores could cause the first
infections in spring. Although resting spores are known to
survive for a long time, and a few have been observed to ger-
minate in the laboratory (15), infection from these spores has
not actually been observed. However, since the earliest infec-
tions each spring are found on old beds where downy mildew
occurred during the preceding year, it is possible that resting
spores are responsible, as has been shown for downy mildew
diseases on certain other crops.
Downy Mildew (Blue Mold) of Tobacco
Also, it is possible that downy mildew survives between plant
bed seasons as mycelium in living volunteer plants. It is known
to attack plants in the fields, and at various times throughout
the year it has been found in active form on seedlings grown
for experimental purposes. In south Georgia it has been found
on volunteer seedlings in winter before the new seedlings were
up in the plant beds (9). Therefore, it is reasonable to believe
that the disease occurs, at least in an inconspicuous form or
as a dormant mycelium, on plants in abandoned plant beds and
fields in the South throughout the season. Such holdover plants,
volunteer plants and suckers provide successive crops of tobacco
plants on which the fungus may possibly live from one plant
bed season to the next.
It has been reported from Australia (1) that downy mildew
attacks the seed pods and that the fungous mycelium has been
found within the seeds. The disease has never been found on
seed pods in the United States, and it is not considered neces-
sary to treat the seed for the control of downy mildew in
Florida.
So far, downy mildew is not known to attack any wild plants
in or around tobacco fields and plant beds, and there appears
to be no opportunity for the fungus to survive on wild hosts
between tobacco plant bed seasons.
In 1938 the first seven beds in which early infections were
found in Florida were located on lands used for plant beds in
1937, when practically all beds were infected with downy mildew.
Most of these beds were found when only one or more small
areas showed the disease, and it was a month or more before
the disease began to appear in beds made on new land. Unused
plants were allowed to remain on most of these beds in 1937
and were raked off or burned when the beds were being prepared
for 1938 sowing. Only one farmer reported finding living plants
at that time. Some growers reported that they had burned the
land before sowing, but the early appearance of downy mildew
indicates that burning cannot be relied upon to destroy all stages
of the fungus that may remain in the soil. No cases of early
infection have been found in beds in the cigar-wrapper area
where the soil is steamed before the seed are sown, or in new
land beds. Although these observations do not prove in what
stage the fungus survives, they do show that it remains in or
near the old bed site to start early infection.
Florida Agricultural Experiment Station
CONTROL MEASURES
Although downy mildew has been a serious tobacco disease
in Australia for many years and has occurred in the United
States for almost two decades, it is only recently that much
progress has been made in developing methods of control. This
slow progress has been due to several factors, chief among which
are: (1) Cultural practices in connection with the growing of
tobacco seedlings, (2) growth habits of the seedlings, (3) their
susceptibility to injury by fungicides, and (4) the strictly para-
sitic habit of the fungus and the limiting effect of weather con-
ditions on its activities. In spite of these difficulties, a number
of control methods have been developed, but the cost of some
of these has been found too great to permit of general use.
Obviously, a method of plant disease control, no matter how
effective, is not practical unless it is less costly than the losses
caused by the disease.
IN PLANT BEDS
Control methods which have been found promising include
heat treatments, cultural methods, chemical treatments, and
breeding for resistance. Heat treatments, which consist of
applying artificial heat to the beds to raise the temperature
.above that favorable for development of the disease, have been
tried by several investigators (4, 8, 11) and found more or less
effective, but too expensive for ordinary commercial use. Other
methods, which are more promising for use in Florida, are
described below. Effectiveness of any method depends upon
thoroughness of application, and the grower should endeavor
to follow instructions in detail if he hopes to derive benefit
from its use.
CULTURAL PRACTICES
Since large plants are known to survive an attack of downy
mildew better than small ones, the grower should prepare and
manage his plant beds in such manner as to have the plants
as large as possible when an outbreak of the disease occurs.
Much can be done to bring this about at little or no extra cost,
since in many cases only slight modifications of usual cultural
practices are necessary. Although certain of the following
recommendations are based on the nature and development of
the disease, they are beneficial for other reasons besides the
control of downy mildew.
Downy Mildew (Blue Mold) of Tobacco
1. Plow under or cut down all unused plants in the plant
bed as soon as transplanting is completed. This prevents downy
mildew from developing on the overgrown seedlings and pre-
vents the production of seeds, and of volunteer seedlings on
which the disease might appear during the fall and winter.
2. Plow under or cut down all tobacco plants in the field
after harvest, to prevent old stalks and suckers from surviving
through the winter.
3. Make the plant bed on new land, or sterilize the soil thor-
oughly with steam, if downy mildew occurred in the bed during
the previous season. Because of the uncertainty of burning
the soil deep enough to control other diseases and kill weed
seeds, a new bed is advisable even though no downy mildew
occurred on the bed during the previous season.
4. Locate beds on eastern or southern slopes, if practical, to
obtain maximum sunshine. Cut down all trees which shade
the bed during the early morning hours. A survey made in
Florida in 1938 showed that beds shaded during early morning
hours were more damaged by downy mildew than beds in the
open.
5. Bank the log or board frames with soil on the outside to
fill all cracks and depressions, and use cloth free of holes to
protect the plants from cold wind and frost.
6. Unless spray or vapor treatments as described below are
to be applied, sow a large excess of plant bed area. Two or
three beds, located as far apart as possible, are better than
one bed. This practice has been the means of obtaining a
sufficient supply of plants in most years.
7. Provide good drainage for the soil. Beds 3 or 4 feet
wide, elevated a few inches above the walks, are preferable to
one continuous, flat bed. Avoid low, swampy soil.
8. Sow the proper quantity of seed to obtain an even, medium
stand of plants.
9. Water the bed as needed to insure good germination of
the seeds, and afterwards at least until the plant leaves are
the size of a dime. Water the beds in the forenoon so the plants
will have time to dry off before night.
10. After the plants have become established remove the
cloth covers, or raise the walls of tall beds commonly used for
cigar-wrapper tobacco, on warm days to allow the sunshine and
wind to dry the dew from the plants as early as possible in the
Florida Agricultural Experiment Station
mornings. The cloth should be replaced before night if frost
threatens. This practice may not be advisable if budworm
moths or other insect pests are prevalent.
11. Nitrate of soda will not control downy mildew but when
applied after the plants have begun to recover from the dis-
ease it may stimulate new growth. Dissolve from 2 to 3 pounds
of nitrate of soda in 50 gallons of water and apply with a
sprinkling can to about 100 square yards of bed. This amount
will not burn the plants and it is not necessary to apply more
water immediately unless the soil is dry and the plants need
it. The application of nitrate of soda or other fertilizer
materials in dry form cannot be recommended because of the
danger of burning the plants. Additional nitrogenous fertilizer
should not be applied if there is already plenty of available
nitrogen present in the soil for vigorous growth, because it will
make the plants too tender.
12. Do not transplant seedlings while the disease is active
on them. They may be used after they have begun to recover,
as indicated by the growth of new leaves and fresh white
rootlets.
SPRAY TREATMENTS
Spray treatments, properly applied, have been found fairly
effective in controlling downy mildew in Florida, Georgia (6),
and certain other states, particularly in the South. Spraying
does not entirely prevent the disease, but delays its appearance,
lessens its severity, and permits most of the plants to survive.
EQUIPMENT
Size and style of sprayer used is determined by the area of
plant bed and the price to be paid. An outfit consisting of a
stout bucket pump, at least 25 feet of 3/-inch pressure hose,
a cutoff, extension rod from 4 to 8 feet long, and a nozzle,
have been found to be a fairly satisfactory outfit for growers
who have small plant beds, not more than 300 or 400 square
yards. The cost of this outfit (Fig. 7) is about $10.00 or $12.00.
The extension rod may be adjusted to the desired length by
means of standard 1/-inch water pipe and fittings, as shown
in the figure.
For larger beds, wheelbarrow (Fig. 8) or barrel sprayers
with 50 feet or more of hose and a pressure gauge are more
satisfactory. The cost of such outfits is from about $20.00 to
Downy Mildew (Blue Mold) of Tobacco
Fig. 7.-Bucket pump spray outfit, with 25 feet of hose, cutoff, two pieces of extension
rod made of I inch water pipe and fittings, and nozzle.
M W ft
Fig. 8.-Wheelbarrow sprayer with auxiliary air tank, 50 feet of hose, cutoff, bamboo
extension rod, and nozzle.
Florida Agricultural Experiment Station
$40.00, depending mainly on the type of sprayer and the amount
of hose. Wheelbarrow sprayers, with large auxiliary air tanks
(Fig. 8) in which the spray stands without agitation, are not
suitable for use with the red copper oxide-oil emulsion spray,
which is described below.
Small traction sprayers designed for spraying tobacco and
other row crops may be adapted for hand operation as stationary
sprayers. Some farmers who used these for spraying plant beds
in 1938 reported that satisfactory pressure was easily main-
tained.
Small power sprayers (Fig. 9), hauled from bed to bed on a
truck, have been used for spraying large acreages of beds. Two
or three lines of hose or spray booms with two or more nozzles
are generally used with power sprayers.
Fig. 9.-Small power sprayer mounted on a truck and used for spraying a plant bed with
line of hose and a three-nozzle boom.
In order to apply the spray as a fine mist, a pressure of at
least 100 pounds per square inch, preferably 150 or 200 pounds,
and a suitable nozzle are required. The nozzle should have re-
placeable outer discs with an aperture of 1/32 inch, or not over
3/64 inch, an efficient whirl plate to spread the spray in a wide
cone, and a strainer to retain particles of solid matter which
would clog the aperture. The nozzle body and cap and a gasket
complete the assembly, as shown in Fig. 10. A 45-degree nozzle
is more readily adjusted for spraying the plants from the de-
sired angle than is a straight nozzle. The nozzle should be taken
apart and cleaned after each day's spraying.
Downy Mildew (Blue Mold) of Tobacco
O
Fig. 10.-Nozzle and parts. Top row, left to right: Angle nozzle complete, nozzle body,
strainer. Bottom row, 1. to r.: Whirl plate, gaskets, outer disk, and cap.
SPRAY FORMULAS TESTED
A large number of different spray formulas, most of them
containing compounds of copper, combined, with various wetting,
spreading and sticking agents, have been tested for the control
of downy mildew. For example, Bordeaux mixture has been
found to give some control, but it has caused severe stunting
of the plants in Florida. A few formulas have been found which
are fairly effective. One which was recommended and widely
used in Florida in 1938 is the red copper oxide-oil emulsions.
Any others which may be found more effective or otherwise
more satisfactory will be reported from time to time.
RED COPPER OXIDE-OIL EMULSION SPRAY
Materials.-The materials used in making this spray are (1)
red copper oxide, (2) cottonseed oil, and (3) a spreader. Each
of these may be obtained in two slightly different forms, as
described below.
Red copper oxide is a fine powder, generally bright red in
color. It should not be used if lumpy, nor if darkened by ex-
posure to air. "Cuprocide 54", recently placed on the market,
is red copper oxide with added spreading and sticking materials,
and is used at the rate of 11/2 ounces instead of 1 ounce of
ordinary red copper oxide. "Cuprocide 54" makes a more stable
spray, especially for use in power sprayers with violent agita-
tion, and the total cost of the spray is about the same.
Florida Agricultural Experiment Station
Cottonseed oil may be obtained from grocery stores as cooking
or salad oil, or direct from cotton oil mills. "Self-emulsifying"
cottonseed oil, another new material, has an emulsifying agent
dissolved in it, and can be easily mixed with water without the
addition of any other spreader.
Spreader keeps the oil broken up (emulsified) in water, and
also causes the spray to cover the leaf surfaces more thoroughly.
"Lethane" spreader may be used with ordinary cottonseed oil.
"Self-emulsifying" cottonseed oil is simpler to use.
Mixing the Spray.-Any one of the following formulas may
be used in making a satisfactory spray. The grower can take
his choice of formulas according to the
available.
Formula I
No.
1
2
3
4
To make
Materials 25 gallons
Red copper oxide ......... 4 ounces
Cottonseed oil .............. 1 quart
"Lethane" spreader .... 1 pint
"Cuprocide 54" ..........i 6 ounces
Cottonseed oil .............. 1 quart
"Lethane" spreader ... 1 pint
Red copper oxide ........ 4 ounces
"Self-emulsifying"
cottonseed oil ........... 1 quart
"Cuprocide 54" ............ 6 ounces
"Self-emulsifying"
cottonseed oil .......... 1 quart
materials which he has
To make
12 gallons
10 teasp.*
1 pint
Y2 pint
6 tablesp.t
1 pint
pint
10 teasp.
1 pint
6 tablesp.
1 pint
To make
61/4 gallons
5 teasp.
pint
%/ pint
3 tablesp.
pint
% pint
5 teasp.
% pint
3 tablesp.
% pint
teaspoonful (level)
t tablespoonful (level)
Other quantities can be made, using the same proportions.
There are four steps in mixing the spray, as follows:
1. In one bucket moisten red copper oxide and stir to form
a thick paste. If ordinary red copper oxide is used, mix with
either "Lethane" spreader or a little "self-emulsifying" cotton-
seed oil diluted with equal parts of water to make a paste. If
"Cuprocide 54" is used, mix with water to make a paste. Then
to the paste add slowly about a gallon of water.
2. In another bucket stir together cottonseed oil and "Le-
thane" spreader, or use "self-emulsifying" cottonseed oil without
any additional spreader, and stir in slowly about a gallon of
water. Pass this mixture through the spray pump and nozzle
to break up the oil and produce a milky white emulsion. A short
Downy Mildew (Blue Mold) of Tobacco
piece of spray hose 2 or 3 feet long is more convenient to use
in mixing than the full length of hose used in spraying.
3. Put the emulsion (mixture No. 2) in the spray tank or
mixing vessel and add, with stirring, the remainder of the water
required.
4. Finally, add the red copper oxide (mixture No. 1).
The complete spray should appear uniform throughout, with
no droplets of oil floating on the surface. Use it the same day;
do not attempt to keep it overnight. Spray which has been
allowed to stand in the hose for a few minutes should be returned
to the tank and thoroughly mixed before spraying the plants.
If the spray pump does not provide agitation, the spray must
be stirred frequently to keep red copper oxide from settling.
However, too much agitation may cause the spray to break, and
oil and red copper oxide separate and stick to the sides of the
tank. "Cuprocide 54" may be more satisfactory for use in power
sprayers because it makes a more stable spray than ordinary
red copper oxide.
For making large amounts of spray it has been found con-
venient to prepare a concentrated stock emulsion of oil and
spreader, diluting the emulsion and adding the red copper oxide
as needed. For example, in making 100 gallons of spray use
1 gallon of oil, 1/2 gallon of spreader and 21/2 gallons of water;
pass these materials through the spray pump and nozzle as
directed above, to form the emulsion; use 1 gallon of this stock
emulsion for each 25 gallons of spray.
Amount of Spray Required.-About 21/2 gallons of spray are
required for one application on 100 square yards of plant bed
when the plants are small. As the plants grow the amount of
spray should be increased gradually to about 6 or 8 gallons
when the plants are nearly large enough to be transplanted.
About 10 applications, totalling about 75 gallons of spray
per 100 square yards, may be required to give control. The
costs of materials for making this quantity will be about $2.00,
depending somewhat on the amounts purchased and on current
prices.
Application.-If the grower expects to obtain most benefit
from spraying he should begin when the leaves are about the
size of a dime, or sooner if downy mildew has been found nearby.
Spray twice a week unless the weather is very cold; then once
a week is sufficient. The spray is intended to form a protective
Florida Agricultural Experiment Station
covering on the plants and prevent infection or lessen its sever-
ity. For this reason it is very desirable to spray at least three
or four times before downy mildew normally appears. Continue
spraying until the plants are ready to transplant or until the
disease has passed completely over the bed and the plants have
begun to recover. Spray only when the leaves are dry. Apply
the spray as a fine mist, covering the leaves as thoroughly as
possible. Beds should not be more than 12 feet wide, so that
they can be conveniently sprayed from the sides without tram-
pling the plants. Plants may be set at any time after spraying.
Effectiveness.-Many beds in Florida were sprayed in 1938.
Most of the growers who sprayed according to recommendations
reported that the treatment was beneficial, and that they were
satisfied with the results. Two growers who sprayed under
supervision, leaving portions of each bed unsprayed as checks,
obtained from two to three times as many plants per 100 square
yards from the sprayed part of the beds as from the unsprayed
parts. On the other hand, growers who used fewer or lighter
applications than were recommended obtained little or no benefit
from spraying in 1938. In some instances excessive applications
or improperly mixed spray caused injury, consisting of speaking
and sometimes curling of the leaves.
VAPOR TREATMENT
BENZOL VAPOR TREATMENT
This treatment, originally developed in Australia (2), consists
in allowing benzol to evaporate from shallow pans in beds which
are tightly framed and covered with cloth heavy enough to retain
the vapor or gas. Benzol is a coal-tar product which evaporates
rapidly and is HIGHLY INFLAMMABLE.
Methods.-Beds, preferably from 4 to 8 feet wide, are tightly
framed with boards from 8 to 12 inches wide or with logs, and
carefully banked with soil. Each bed should be partitioned with
cross boards into sections of from 10 to 20 square yards each.
Covers tight enough to retain benzol vapor effectively are
made of unbleached muslin sheeting, woven with about 56 x 60
threads per inch, weighing about 4 ounces per square yard.
Cloth of coarser weave does not retain vapor effectively, and
heavier materials shed too much of the rain which may be
needed on beds that are not equipped for irrigation.
"Tobacco sheets", made of burlap sacks sewed together and
used for wrapping cured tobacco as it is taken to market, have
Downy Mildew (Blue Mold) of Tobacco
been found to retain vapor fairly well but not nearly as well as
muslin. They are heavy and inconvenient to handle when wet.
Covers wet with rain or dew are much more effective than
dry cloth. Since new cloth is not readily wetted uniformly, it
should be washed before use. When the weather is dry and
windy the covers should be sprinkled with water after they
are drawn over the beds in the evening (10).
The covers are put on the beds at night and rolled back during
the day. To save labor in covering and uncovering the beds,
the ordinary plant bed cloth may be used until downy mildew
appears in the bed and it becomes necessary to start the treat-
ment.
Benzol is placed in shallow pans, distributed uniformly
throughout the bed, and supported 4 or 5 inches above the
soil. The area of pans used in treating 100 square yards of
plant bed should be from 1 to 112 square yards. For example,
26 pans, 8 inches in diameter, make a total of 1 quare yard.
The pans may be of any convenient material and shape, and
may be oiled to prevent rusting. They must be sheltered to
keep rain water from flooding them or splashing out the benzol,
because liquid benzol burns the plants. A stand such as those
shown in Figures 11 and 12 has a bottom shelf to support the
pan, and a top shelf to keep out rain.
Fig. 11.-Plant bed framed with logs. Photograph taken two weeks after benzol vapor
treatment had been applied under effective cover (sheeting) right, and worthless cover
(plant bed cloth doubled), left.
Florida Agricultural Experiment Station
Fig. 12.-Plant beds framed with boards and covered with sheeting. Bed in center shows
cover rolled back.
Beds of two designs, and stands for benzol pans, are shown
in Figures 11 and 12. In the beds shown in Figure 12 one edge
of the cloth is fastened to the frame, the other edge to strips
of wood 1 or 2 inches wide and 1 inch thick, which hold the cloth
tight when the beds are covered and aid in rolling the cloth back
when the beds are uncovered. One man can handle covers of
this type from 25 to 50 feet long.
The amount of benzol which should be evaporated per 100
square yards of plant bed to check the development of downy
mildew is from 2 to 4 quarts per night, depending on the wind
and the tightness of the beds, including the amount of moisture
on the covers. Benzol evaporates more slowly on cold nights
than on warm nights, and therefore more pans are required to
evaporate the necessary amount of benzol in cold weather than
in warm weather.
All pans should be filled to approximately the same depth.
The beds are then covered, left until morning, and then un-
covered. Any benzol remaining in the pans in the morning
may be returned to the supply can.
The effectiveness of this treatment depends entirely on the
concentration of benzol vapor or gas in the air around the plants.
All fresh spore production is stopped in two nights with a vapor
concentration of 0.05 percent throughout the night (10), but
the plants are not injured unless the concentration reaches about
2 percent (13).
Since the range of vapor concentration which may be used
is rather wide, the grower can judge his own results by observ-
ing the plants. If the vapor concentration is within the proper
range, the spread of downy mildew is checked after two or
three nights, and no fresh mildew can be seen on the leaves.
Downy Mildew (Blue Mold) of Tobacco
If the vapor concentration is too high, leaves nearest the pans
of benzol turn white at the edges, and the amount of benzol
used must be reduced.
Treatments may be started on the day when downy mildew
first appears in the bed, if a careful watch is kept every day
to determine this date. However, the beds must be properly
framed and sheeting or burlap covers, pans, stands and benzol
must be ready in advance. It may be sufficient under Florida
conditions to continue the treatment for three or four nights,
or until no fresh mildew is found on the plants. After treat-
ments are discontinued it may be necessary to resume them
if the disease again becomes active.
Cost.-The cost of materials, not including benzol, is about
$15.00 or $20.00 per 100 square yards for the first year. How-
ever, boards, stands, pans and muslin covers will last several
seasons with proper care, reducing the annual cost of these
materials to perhaps $5.00. The cost of benzol and of labor
for applying the treatment per 100 square yards is at least 25
cents per day during the period when the treatment is applied.
Effectiveness.-The benzol vapor treatment has been tried
in Florida and in various other states of the United States,
as well as in Australia, and found highly effective in controlling
downy mildew. In a cooperative test in a flue-cured tobacco bed
in Florida in 1938, benzol was applied for five nights under three
different types of covers, as follows: Muslin, "tobacco sheets"
made of burlap sacks, and ordinary plant bed cloth doubled. A
fourth portion of the bed was left untreated as a check. Con-
siderable control of downy mildew was obtained where muslin
was used as the plant bed cover. Burlap sacks were less effec-
tive, and plant bed cloth was of little value. The fairly good
condition of the plants treated under muslin in contrast to those
under double plant bed cloth, 16 days after the treatment was
discontinued, is shown in Figure 11.
PARADICHLOROBENZENE TREATMENT
Paradichlorobenzene, applied in a manner similar to that de-
scribed above for benzol, has recently been found as effective
as benzol in controlling downy mildew (5). This material, com-
mercially called "Paracide", is obtained in the form of large
crystals, which should be crushed to a powder just before using,
to increase the rate of evaporation.
Florida Agricultural Experiment Station
Paradichlorobenzene is scattered on boards 3 or 4 inches wide
which are placed as shelves around the sides and ends of the
bed. If the bed is 10 feet or more in width, other boards should
be placed across the bed. Since paradichlorobenzene is not solu-
ble in water, a little moisture on the boards does no harm, but
the material should not be washed off.
The rate of application is about 11/ pounds per 100 square
yards per night, and the beds are covered with muslin, about
56 x 60 threads per inch. There appears to be no danger of
burning the plants with the vapor from this dosage but too
much paradichlorobenzene causes the plants to wilt excessively
during the day. Methods of application are otherwise the same
as for the benzol treatment, and the precautions regarding the
tightness of frames and covers are just as important.
Some advantages of the paradichlorobenzene treatment over
the benzol treatment are apparent. Paradichlorobenzene is more
convenient to handle, and it is non-inflammable. Board shelves
are less expensive than the pans and stands required for benzol.
Otherwise, the cost of the two treatments as recommended are
about the same.
The paradichlorobenzene treatment has been tried in Florida
only in a limited way during one season. In all states where
it has been tried it appeared to be highly satisfactory, but fur-
ther experiments are needed to find the cheapest and most
convenient methods of application which will be satisfactory
under various plant bed conditions.
COMPARISON OF PLANT BED CONTROL METHODS
The selection of the most satisfactory method for the control
of downy mildew in plant beds depends on cultural practices
employed and on the expense which is justified by the returns
expected from the crop. The main advantages and disadvant-
ages of each method are given below to aid the grower in making
his selection.
Cultural Practices.-The sowing of excess plant bed area has
been largely relied upon in Florida, since downy mildew became
established to insure a sufficient supply of plants. Before the
spray and vapor treatments were developed this was the only
method available. However, the results are uncertain and the
cost is rather high, being from $10.00 to $20.00 per 100 square
yards of plant bed sowed in excess of the normal requirement.
There is also the danger of a reduction in value of the crop if
downy mildew causes transplanting to be delayed until after
Downy Mildew (Blue Mold) of Tobacco
the most favorable season. On the other hand if plants become
large enough for transplanting too early in the spring a delay
may be of some slight advantage.
Spraying with red copper oxide-oil emulsion spray is not
expensive, and is fairly effective, permitting most of the plants
to survive an attack of the disease. A disadvantage of any spray
treatment is that it cannot be applied during rainy weather.
Furthermore, for spraying to be effective it must be started
before downy mildew appears and continued at regular intervals.
The expense of this preventive treatment must be incurred
each year.
Vapor treatments with benzol or paradichlorobenzene are
highly effective in controlling downy mildew, even in checking
the disease after it has started. The treatment can be applied
in rainy weather, when it is likely to be needed most. Disad-
vantages are the high initial cost of frames and muslin covers
and the labor of tending the beds night and morning. However,
since the treatment may not be needed at all during some years,
the average cost per year may not be too high for many growers,
considering the effectiveness of the treatment. Paradichloro-
benzene will probably prove to be more satisfactory than benzol.
CONTROL ON CIGAR-WRAPPER TOBACCO IN THE FIELD
Downy mildew has occasionally caused considerable reduction
in value of cigar-tobacco in shaded fields. Preliminary experi-
ments conducted in 1938 indicated that spraying would give
control. These investigations will be continued.
In fields where the crop is very severely affected it has been
suggested that the stalks might be cut off near the ground in
order to produce a sound crop of sucker leaves. However, it
is the consensus of opinion among growers and packers that
sucker crops are likely to be chaffy and sleazy, and this prac-
tice is hardly to be recommended unless practically the entire
original crop of leaves has been ruined for wrappers.
BREEDING FOR RESISTANCE
All commercial varieties of tobacco now being grown in Florida
are highly susceptible to downy mildew. A foreign variety has
been found which is resistant. This is being crossed with com-
mercial varieties in an attempt to combine resistance with the
desirable characters of the commercial varieties.
Florida Agricultural Experiment Station
LITERATURE CITED
1. ANGELL, H. R., and A. V. HILL. Downy mildew (blue mould) of
tobacco in Australia. Jour. Counc. Sci. Ind. Res. (Austr.) 65: 9-30.
1932.
2. ANGELL, H. R., A. V. HILL, and J. M. ALLAN. Downy mildew (blue
mould) of tobacco: Its control by benzol and toluol vapours in
covered seedbeds. Jour. Counc. Sci. Ind. Res. (Austr.) 8: 203-213.
1935.
3. ARMSTRONG, G. M., and W. B. ALBERT. Downy mildew on pepper,
tomato and eggplant. Phytopathology 23: 837-839. 1933.
4. ARMSTRONG, G. M., C. B. SUMNER, and W. M. LUNN. Tobacco downy
mildew (blue mold) and its control. In 47th Ann. Rpt., S. C. Agr.
Exp. Sta., 1933-1934: 39-44, 104-105.
5. CLAYTON, E. E. Paradichlorobenzene as a control for blue mold
disease of tobacco. Science n. s., 88: 56. 1938.
6. CLAYTON, E. E., and J. G. GAINES. Blue mold (downy mildew) dis-
ease of tobacco. USDA Farmers' Bulletin 1799. 1938.
7. DIXON, L. F., RUTH A. MCLEAN, and F. A. WOLF. Relationship of
climatological conditions to the tobacco downy mildew. Phyto-
pathology 26: 735-759. 1936.
8. GAINES, J. G. Tobacco diseases. In 17th Ann. Rpt. Ga. Coastal Plain
Exp. Sta. 1936-1937 (Bul. 28): 115-123.
9. GAINES, J. G. Early appearance of tobacco downy mildew in Georgia
in 1937. Plant Disease Reporter 21: 42. 1937.
10. GUMAER, P. W. Control of blue mold of tobacco with benzene vapor.
Ind. and Eng. Chem. 30: 1076-1081. 1938.
11. HILL, A. V., and J. M. ALLAN. Downy mildew (blue mould) of tobacco.
Attempts at control by the use of (I) sprays, and (II) heated seed-
beds. Jour. Counc. Sci. Ind. Res. (Austr.) 9: 220-232. 1936.
12. MILLER, PAUL R. January temperatures in relation to the distribu-
tion and severity of downy mildew of tobacco. Plant Disease Re-
porter 21: 260-266. 1937.
13. WOLF, F. A. Control of downy mildew by fumigation. Proc. 4th Ann.
Conf. Tob. Dis. Counc. July 19-21, 1938. Page 4. (Mimeographed.)
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