xt7dr785k59q https://exploreuk.uky.edu/dips/xt7dr785k59q/data/mets.xml   Kentucky Agricultural Experiment Station. 1939 journals kaes_circulars_003_329 English Lexington : The Service, 1913-1958. Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Circular (Kentucky Agricultural Experiment Station) n. 329 text Circular (Kentucky Agricultural Experiment Station) n. 329 1939 2014 true xt7dr785k59q section xt7dr785k59q I
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COLLEGE OF AGRICULTURE
IH -
in Extension Division l
gy THOMAS P. COOPER, Dean and Director
B2 r
30 _t__ I
ss I
06 CIRCULAR NO. 329  
1
12  
63 1
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1
BLUE MOLD OF TOBACCO AND I
ITS CONTROL i
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Lexington, Ky.  
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January, 1939 I
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Published in connection with the agricultural extension work carried on by co-operation I
of the College of Agriculture, University of Kentucky, with the U. S. Department of Agriculture ,·
and distributed in furtherance of the work provided for in the Act of Congress of May S, l9l4. {
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   QI _‘ This report was prepared cooperatively by a committee composed
    of representatives from Duke University, the Agricultural Experi-
 ii   ment Stations and Extension Services of Virginia, North Carolina, B-
.~;f   and South Carolina, and the North Carolina Department of Agricul- on SE.
JQ _'`.’   . ture. The information contained in it seems to be as applicable to ` 0th€l‘
it     [ Kentucky as to the states for which it was prepared. It is reprinted dews
g_ it [ I.- with permission of the committee. gram
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Blue Mold (Downy Mildew) of Tobacco and
Its Control
The purpose of this report is to give the grower a better understanding of the
blue mold disease of tobacco and to describe effective methods to combat it. i
NAME OF THE DISEASE _  
Blue mold derives its name from the color of the downy growth found on the  
lower side of diseased leaves. Pathologists call similar diseases on other plants  
downy mildews. The names downy mildew and blue mold, when applied to Q
tobacco, refer to one and the same disease.  
PLANTS ATTACKED  
Blue mold of tobacco may be found on all cultivated varieties of tobacco, and  
on seedlings of tomato, pepper, and egg—plant, but it is not known to involve any E
other kinds of plants. The disease should not be confused with the downy mil- ;
dews of any other crops or with the white powdery mildews common on trees, i
grasses, clovers, roses, etc., all of which are caused by other species of fungi. Q
HISTORY OF THE DISEASE  
Blue mold of tobacco has probably been present in Australia since about 1850,
and it may be native to Australia since the disease occurs there on certain native A
tobaccos. Apparently the first outbreak of this disease on cultivated tobacco i
within the United States occurred in 1921, when it appeared in Florida and Georgia. 4
For some unknown reason it did not again attract attention until 10 years later
when it reappeared in Florida and Georgia, and spread rapidly into Louisiana, ,
South Carolina, North Carolina, Virginia, and Maryland. Since then it has `
invaded all of the tobacco-producing states east of the Mississippi River, except i
Wisconsin. It has also been reported from Canada, where it made its first ap- j
pearance in 1938. The disease is now firmly established in the eastern area of  
the United States, and it is reasonable to expect that growers will be confronted ,
with the problem of its control every year. ’
O
1
ECONODIIC IDIPORTANCE E
From the standpoint of individual growers, the economic importance of blue  
mold cannot be over-estimated because plants in entire seed beds have been  
destroyed within a few days. In certain communities, losses up to 95 percent l
have been observed. In 1932 and 1937, the disease was so severe that hundreds I4
of growers were forced to secure plants from adjoining communities. In these i
epidemic years the disease was responsible for a reduction of from 20 to 50 percent  
in the tobacco acreage in some communities.  
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~ __   G SYMPTOMS OF THE DISEASE ., h b
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  E E The symptoms of the disease on young and old plants frequently differ. ‘ large y
Q Q   I n Seed Beds. The first appearance of the disease generally coincides with A coating
  § l the blooming of the flowering dog-wood. As a rule the disease is first found in _V attack
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 `
BLUE Mom on ToBAcco AND Irs CoN*r1zoL 5
A has been dry and warm and the plants are hardened, dark brown specks within
large yellow spots may appear. During such weather, there IS little if any downy
i coating of mold on the leaves (Fig. 2), and the seedlings may recover from an
I attack. If the weather again becomes cool and moist they may again be attacked.
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    6 Krcxruoicr Ex·r1;Ns1oN C1ucULA1n N0. 329  
  F 7   l I n the Field. Plants may be affected in the field, if protracted cool wet weather l   1 q
  _   follows transplanting. During such weather groups of small yellow spots appear     arise
  "   forming circular blotches similar to those on seed bed leaves (Fig. 3). Some-   {_ plow
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  -4 {   circular decayed areas are formed and the leaves become torn and ragged. The Z
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  Fig. 3.-Advanced symptoms of downy mildew on the lower leaves of field grown plums. Q
  Both large and small dead spots are commonly formed. for 3
  -:3* A 2 .
.,;=~§;;‘  . . . . . ap 9
;_g;i;·=;·L  mfection of plants IH the field has been reported on several occasions but it has Wh;]
{Z  sl` never been so abundant as to cause alarm except in 1938. As hot weather ap- late
l   I proaches the disease disappears from the field. I"

 ` BLUE Mom) or Toencco AND Irs CoNTnoL 7 1
if Since downy mildew (blue mold) infection of field-grown tobacco plants may {
“ arise from air-borne sporangia from nearby seed beds, it appears advisable to
E` plow under or burn over tobacco seed beds as soon as possible after adequate l
59 field stands are obtained. _
1e I
CAUSES OF THE DISEASE AND LIFE CYCLE OF THE CASUAL `
; FUNGUS `
. The cause of this tobacco disease is a mold or fungus that reproduces itself by I
' two kinds of seed-like bodies or spores. One of these kinds occurs in the downy
coating on the lower surface of diseased leaves and is the chief means by which §
i the mold is spread in the spring. These seed-like bodies are called "sporangia." I Q
The other kind forms in the decaying leaves, remains dormant throughout the l
A summer, fall, and winter, and starts the disease the following spring. These g
" seed-like bodies are known as "oospores."  
l Under the microscope sporangia appear as minute egg-shaped bodies about l ¤
i one-thousandth of an inch in length (Fig. 4, C). They are formed at the ends of ` l
L the branches of small tree-like structures on the lower side of diseased leaves 4
A (Fig. 4, A). The sporangia usually begin their development in early morning .  
T with the coming of dawn and are mature by sunrise. On dark cloudy days  
sporangia may be produced continuously so long as the leaf is able to support l
the growth of the mold. l
Sporangia must germinate on healthy leaves in order to cause them to become l
l diseased. Upon germination slender tubes (Fig. 4, B), are formed if there is a 1
film or drop of water on the leaf. Following germination, the slender tubes enter `  
the leaf (Fig. 4, D), where they develop by branching into a web that may extend y
‘ throughout the entire leaf and into the stem. From this web within the leaf, T
` tree-like structures emerge and bear sporangia at the ends of the branches. i
The sporangia are produced in countless numbers. Since they are small and ’
. dust-like they may be readily distributed by air currents, thus making it possible ‘°
for the fungus to be spread widely. Observations show that these small bodies  
readily stick to the hands and clothing, so that, while weeding, one may spread 1
the sporangia from infected areas within the bed to non-infected ones, even to ‘ 1
near-by healthy beds. g
After sporangia have lodged on the leaves of healthy plants a period of 3 to 7  
days only is required before such plants are visibly diseased as shown by the ,,
presence of sporangia. These sporangia may then be spread by air currents to  
other healthy tobacco seedlings, and the cycles of infection and production of Q
sporangia may be repeated several times in a season.  
The oospores are formed within a few days after the tissues of the leaves have _ 1
I been killed. They are ball—shaped, and slightly less than one-five-hundredth of 1
. an inch in diameter (Fig. 4, F, G, H, I). A thick, resistant wall aids them in  
surviving from one spring until the next, even though the leaves within which  
they formed are completely decayed. Oospores, remaining alive in the soil of  
mm the bed, may cause the seedlings to become infected in case the same site is used  
for a tobacco seed bed the next year. This explains why downy mildew may j
_ has 8DI?€9·1‘ Gafliest in beds sown on old bed sites and why these beds are s0uI‘C€S fl`0m ` `T
Q ap_ {7V};1€h the fungus spreads to cause 9, general outbreak of the disease about 3 Weeks  
a er.

 x     `  H ; T 
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  .>·-“ * —`    
Y l   8 KENTUGI{Y EXTENSION C11tcULArt N0. 329 _Q
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· ' Fig. 4.--The appearance of the downy mildew parasite under the microscope. A. A tree- SE
.   z like fruiting branch of the dowuy mildew fungus. The stalk is folded and most of the esti- TBD
  °"_` , shaped sporangla have fallen 0l’l’. The downy coating is caused by the large number of tl1pS*¢ for
    structures. B. Germinating sporangia, showing thread-like tubes. C. Stages in the f0l`HlilU0ll
      of sporangia at the tips of the branches. D. The tube from a germinating sporangium sh0Wl¤E Ob
=. · "- . *3. its entrance into a breathing pore. E, F, G, H, and I. Stages in the fornmtion of 00sp0r€S» f
  l *>_ J. Branclhexg thread of the downy fungus by means of which it obtains food from the cells of th? , OY
j   g Q tobacco ea . Ven
    tl0I

 BLUE Mom or Tomxcoo AND Irs Coxruot 9 `  
FACTORS AFFECTING DISEASE DEVELOPMENT
i Growers are well aware of the variable nature of the blue mold disease since
it may appear in seed beds either early or late, and be severe or mild depending
upon such factors as seed bed site, time of infection, size and vigor of seedlings, ‘
and weather conditions.
) Seed Bed Sites. Observations show that ordinarily the first outbreaks of
blue mold occur in beds located on sites of old beds that were diseased the previous .
year. However, the disease may not always appear first in all beds sowed on
old sites in a given community. These first outbreaks arise from oospores that
have survived in the soil of old beds. For this reason it is best that old bed sites .
be avoided and that beds be located in a different place each year. As an addi- ~ »
tional precaution to be taken in locating beds it is advisable that good air drainage I  
with proper exposure to the sun be provided, since swampy, shaded places, closely ` ,
surrounded by woods, are favorable for the growth of the mold. {
Time of Infection. Tobacco seedlings of any age may become infected. If    
V the seedlings are growing on sites where diseased beds were located the previous ` l
year and if oospores survived, an outbreak may occur by the time the seedlings `  
have reached the 4-leaf stage. Observations show that blue mold may occur A Q
·· shortly after the first spell of warm weather. At this time there may be so few T
i affected plants that they escape notice, unless a most careful inspection is made. ‘  
About a week later, however, the disease is easily noticed because small patches of , i
~ plants will have become affected. The sporangia formed on these few diseased =  
plants are carried by the wind throughout that bed and to other beds, frequently - 4
for several miles. A general outbreak of blue mold in all seed beds then follows ` i
within 3 to 4 weeks after the disease makes its first appearance in the community. `  
Size and Vigor of Seedlings. Plants are less likely to survive if attacked V  
while small than they are if attacked when they have reached the stage for trans-
planting. Larger plants may recover even though all but the bud leaves are ‘
. destroyed. A greater number of seedlings are killed if they are growing densely ‘
crowded than if the stands are thinner. Afected seedlings should not be trans- f
planted in the field until after new roots have formed. ;
Weather Conditions. Blue mold may be very destructive in some seasons and
_ cause little damage in others. It may spread throughout the bed very rapidly .
or it may progress slowly. The peculiar behavior of the disease can be explained,  
in part, by weather conditions. Continued cloudy, rainy weather, especially if I
accompanied by winds, is favorable for the development and spread of the disease,  
, k and bright warm days are known to retard it. If the temperature at night ranges if
fl V around 56°F. conditions are very favorable for the abundant production of spo- ,
rangia; but, if the temperature is 68°F. or above, few sporangia will be formed.  
Low temperatures at night retard the growth of blue mold, but frosts do not kill {
the sporangia. High temperature during the day also hinders the growth of ,
the mold.  
Moisture is equally important as temperature in governing the course of this  
rw diSG&1S€. Rain or dew on the leaves is necessary for the germination of the SDO- 2
2::- Tangiil and for the entrance of the mold into the leaves. Favorable conditions i
  for infection are provided if leaves remain wet from 2 to 3 hours after sunrise. ,
  Obviously the longer the leaves remain wet the more favorable the conditions V]
me for infection and the more severe the disease. It is for these reasons that proper A
V€Y1i3ii?ltiOI‘1 and air drainage be provided, and that swampy, poorly dfainéd loca-  
tions be avoided.  

 L     10 Kicwruoxr EXTENSION CIRCULAR No. 329
I _A:   Sunlight affects the development of the disease in several ways. Exposure " fi to]
  `   of sporangia to sunlight for an hour or two destroys them. Seedlings in shaded T   Sho
Q     beds remain wet longer and thus are more subject to infection than those in beds -° is
  § { exposed to sunlight. Removal of covers from seed beds makes the plants grow ’   the
i .     more slowly and as a consequence they are hardened and less likely to be killed 1 `Q
Q _ .. _ E by blue mold. YQ
S . ·   Fertilization. The use of commercial fertilizers and fertilizer materials to con-  
i   A i § trol blue mold has been uniformly unsuccessful in spite of claims to the contrary, ` {
1 .   l The judicious and timely use of nitrate of soda, however, has proved useful in ~ S " mb
é i promoting the recovery of diseased seedlings. Egg
i . -
    PLANT BED PREPARATION , 01**5
_ °     As a result of extensive investigations on the control of blue mold of tobacco, ·  
‘   ‘ two methods of treatment have been developed. These are (1) fumigation, and · Suit
I     (2) spraying. Regardless of the treatment used, however, a standard type of ;
        seed bed should be constructed. The use of narrow seed beds is strongly recom- l `
_ j ; mended because experience has shown that narrow seed beds have the following L
        advantages: . V can
A 7 »   { (1) More plants are produced per square yard in narrow beds.   bac
    (2) The use of narrow beds avoids the necessity of trampling on plants ‘ the
. ·‘¥f ._   while weeding the bed or pulling plants. .
_ ` _—·- »_ § (3) Narrow seed beds are easier to weed and to water. l 1`3-ti
’   i , (4) Treatments of any kind are more easily applied in narrow beds. ¤€€·
—   I (5) The control of tobacco mosaic or walloon is easier to accomplish in ' for
A I ,’ narrow beds.
_. 7 ·~ · }   (6) Cloth to make covers of the proper width can be purchased, avoiding Sm,
  l l the necessity of sewing covers together. fun;
.     (7) Tight fitting of covers can be accomplished more easily on narrow I
_   I   beds, which results in less damage from tobacco flea beetle. _ eaci
Q·` _yf~.[ L, The seed bed may be of any desired length but should be only 2 yards wide. Squ:
J ,·   iT * A pathway 18 inches or 2 feet wide should be left between the beds. The
_: `_ ,‘·, is Obviously the seed bed should be located near the owner’s residence and near of t
_ .;,4:     a source of water. A warm location is desirable but if fumigation is to be used, of H
~` elk ,:§ the seed bed site is of little concern since blue mold can be held in check. If tra;
    V spray treatments are to be used, the seed bed should have every possible advantage blu
_· `‘’o a*_,=;;=_;_ . of sunlight, warmth, and air drainage. P
`§_f=*i    ~ Prior to the appearance of the blue mold disease as an economic problem in ro
        the United States, 100 square yards of seed bed was considered sufficient to pro- one
_ `;    - ·; duce seedlings to plant 3 to 4 acres. Within the past few years, however, growers a W
      have learned to plant 2 to 3 times this amount of seed bed area in the hope of Olfel
. if   IZ obtaining a sufficient number of plants. If fumigation is practiced, no additional lvm
      ‘ seed bed area need be provided, but, if the spray treatment is applied, it is advisable · IS Sl
  i` `_*: ¤~  to plant additional seed bed area since there may be loss of plants even though ‘ _ gpm
  Q; they are sprayed. ‘
li    The beds should be framed tightly with boards 8 to 10 inches wide. Soil be l
  j  should be drawn up against the boards along the outside of the bed. All cracks . wise
  ¥?_'   and places where vapors might escape should be carefully covered. foot
   l Seed bed covers of the usual kind should be used up to the time the blue mold Shen
· -Z._ff  }’ { fungus appears. Thereafter, if the beds are fumigated, a heavier cover will have ` be I

 BLUE Mono or Tormcoo Arm Irs Coivrrzor, 11  
V9 to be substituted as described later. If the beds are to be sprayed the usual cover '
’d should be used throughout the season.
is It is very important that the seed beds be inspected daily for the first signs of l `
W the disease in order to avoid unnecessary loss of plants.
ed `
CONTROL TREATMENTS ` l
H- Many difficulties have been met while developing methods for the control of ·
y' tobacco blue mold because the treatments used for diseases of other crops were
m not entirely satisfactory when applied to tobacco, and therefore, new methods of `
procedure had to be devised. The grower should bear in mind that the methods ‘
outlined below are the best available at the present time and that if improved ‘  
methods appear they will be made available. Three methods of treatment are =
Y0, given and it is hoped that each grower will select the one which seems to best , s
ld suit his needs. , l
if BENZOL (BENZENE) FUMIGATION    
F12 Preparation for Fumigation and Materials Used. Fumigation was tried be-  
cause vapor or gases could be expected to reach all above ground portions of to- .l
bacco seedlings. Benzol was selected because its vapors are very poisonous to Q
the mold but are tolerated in much greater strengths by tobacco seedlings.  
If narrow seed beds are used fumigation is materially simplified. Pans (evapo-  
rators), benzol, and cotton sheeting must be obtained to complete the equipment J I
needed. The evaporators, sheeting, and seed bed