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INTE STATES DEPARTMENT OF AGRICULTURE
In Cooperation with the
College of Agriculture, University of Wisconsin
DEPARTMENT BULLETIN No. 1256
ny Washington, D. C. October 13, 1924
TOBACCO DISEASES AND THEIR CONTROL
By
JAMES JOHNSON, Agent, Office of Tobacco Investigations
Bureau of Plant Industry, and Professor of Horticulture
College of Agriculture, University of Wisconsin
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Unimportant or Rare Diseases Not Otherwise Classified ............-. 41
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WASHINGTON
GOVERNMENT PRINTING OFFICE
1924
UNITED STATES DEPARTMENT OF AGRICULTURE
In Cooperation with the
College of Agriculture, University of Wisconsin
DEPARTMENT BULLETIN No. 1256
Washington, D. C. October 13, 1924
TOBACCO DISEASES AND THEIR CONTROL.
By JAMES JOHNSON, Agent, Office of Tobacco Investigations, Bureau of Plant
Industry, and Professor of Horticulture, College of Agriculture, University
of Wisconsin.
CONTENTS.
Page. Page.
Introduction= ===. eee ie | ROOtaCISCAS CS FE ee eee 18
The nature of plant diseases_______ Sp PUed ieGIScases = een. Sa eee 25
WOSSCSHELOIM GISCASC mt es eee ee 3 | Injuries due to physical agencies___ 41
The plant bed as a source of infec- Unimportant or rare diseases not
LO Tae es oes SF OT ee eee as ae 4 otherwise classified] =") ss 41
BRlantibedssanitationy === 4 | Damage in curing and fermentation_ 43
The tenia uon of soil for plant Diseases of tobacco in fcreign coun-
Th a Ee ee LOS eae Rese Te Oi Me etic aay 48
Stem OE Stalk discascs== sae (ol oibiorraph ya ees ee eae eee 50
INTRODUCTION.
The tobacco plant has been cultivated in the United States for
more than 300 years. Nearly 1,750,000 acres of land are devoted to the
production of the crop, which is now approximately one and one-third
bilhon pounds per year. While new areas have been gradually
opened to the culture of the crop, the growing districts have re-
mained remarkably constant in location, so that most tobacco soils
are subject to continuous or repeated intensive culture, naturally re-
sulting in an accumulation of diseases and insects which affect this
crop in particular. The production of a good yield and high quality
of tobacco up to the manufacturing stage has become increasingly
difficult, in spite of the greatly increased knowledge and experience
on the part of the growers concerning the various aspects of the cul-
ture of the crop. Some of the most “important and obscure difficul-
ties confronting the growers are those which belong to the class of
plant diseases. It is the purpose of this bulletin to describe these
diseases, giving the causes where known, and to suggest the best
means of control available.
The tobacco-growing districts are relatively small areas, located
largely east of the Mississippi River, but ranging from the warmest
to almost the coolest climate in the United States (fig. 1). These dis-
tricts, furthermore, produce distinct types of tobacco, used for va-
rious purposes, as a result of employing different varieties of seed,
91061°—
1
2 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
rowing the crop on widely different soils, together with variations
in climatic conditions and methods of handling. The main prob-
lems confronting the growers in the different sections, therefore,
are not always the same. Any one disease may be much more serious
Eee
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ff i,
Vite IAT erate = TOBACCO
ie TASS SO ACREAGE
Gas - yaaa EACH DOT REPRESENTS
TI & 500 ACRES
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STATUTE MILES
00 S00 100 200 300 400 500
Fic. 1.—Map of the eastern half of the United States showing the tobacco-growing dis-
tricts. Intensive culture in relatively small districts is favorable to the accumulation
of plant diseases.
in one section than in another or may not occur at all in some dis-
tricts, either because of conditions of environment or varietal dif-
ferences in resistance or due to the accident of introduction of the
parasite concerned. The control measures recommended therefore
may not apply equally well to all districts, but these details can not
be given full consideration in a brief treatise on the subject.
TOBACCO DISEASES AND THEIR CONTROL. 3
THE NATURE OF PLANT DISEASES.
Before giving separate consideration to each disease it is well to
recognize the general facts in regard to plant diseases and their
control. :
The most fundamental consideration to be borne in. mind is that
most of our plant diseases are caused by fungi or bacteria (germs).
These organisms (Pl. I, figs. 1 and 2), usually so extremely small in
size that. they can be seen only with high-power microscopes, are
capable of attacking and feeding upon plant tissues, thereby injur-
ing the plant. The particular organism causing the injury is called
the parasite, and the plant attacked is conveniently referred to as
the host. Each parasite usually produces a distinct type of injury,
so that it is ordinarily possible to recognize the disease readily by
the general symptoms. In the case of most diseases caused by para-
sites it is possible to isolate the fungus or bacterium concerned and
to grow it in pure culture, that is, free from all other living organ-
isms, and thereby study many aspects of its character and behavior.
By inoculating plants with such cultures it is possible to prove posi-
tively that the organism is the cause of the disease in question and to
study many problems bearing on the control of the disease. |
Diseases of the above-described type are infectious, that is, unde
favorable conditions they can spread from one plant to another.
They may be harbored in the soil, or by certain agencies may be
carried over unfavorable periods for infection and later spread to
susceptible plants which become diseased, environmental conditions
permitting. The part played by weather conditions in plant diseases
is an important one, but from the standpoint of disease control is
of little importance, since weather can not be controlled. About
the only practical method for the control of plant diseases is there-
fore that of preventing infection either by eliminating the parasite
or else by safeguarding the host plants from its attack by spraying
or by the use of resistant varieties.
A second class of plant diseases are those generally referred to
as the nonparasitic group. Such diseases are either known to be
or believed to be not due to parasites but result rather from unfavor-
able soil or climatic conditions which disturb the normal progress of
plant development. Several other types of injury may be included
here, however. The principles of control are therefore radically dif-
ferent from those used against parasitic diseases.
Another class of troubles which must be considered in the case of
tobacco and which strictly does not fall into either of the above
classes is damage occurring during the curing, fermenting, or storage
processes, when the plant tissue is dead. While these troubles usually
are of the same nature as the parasitic diseases, in that they are due
to fungi and bacteria, the organisms concerned are termed sap-
rophytes because they live and grow normally on dead tissue.
Insect damage is not considered in this bulletin except in so far
as it plays a part in spreading plant diseases.
LOSSES FROM DISEASE.
The annual losses caused by diseases of tobacco in the United
States are difficult to estimate with any degree of accuracy. Fairly
4 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
careful study of some diseases in certain sections at one time or
another furnishes some basis, however, for the conclusion that the
average annual loss amounts to many millions of dollars. The
losses from root rots in certain years certainly exceed $10,000,000.
Some of the recent wild-fire and black-fire epidemics have alone
caused losses to the extent of millions of dollars in certain districts.
Other leaf-spot diseases and mosaic are always present to a consid-
erable extent in one section or another and materially reduce both
yield and quality. The damages during curing and fermentation
alone run into hundreds of thousands of dollars annually. If we
estimate the average annual loss from tobacco diseases in the United
States at 5 per cent of the er op value, which is believed to be a very
conservative figure, the loss based on the present value of the crop
would be close to $25,000,000 per year.
THE PLANT BED AS A SOURCE OF INFECTION.
A very considerable number of diseases which cause great damage
in the field have their origin in the plant bed. Preventive measures
should, therefore, be used in the preparation of the seed beds to
assure success in controlling the diseases in question. The most
common seed-bed disease is known as damping-off or bed-rot and
may be caused by several different fungi. The parasites of this
class are, however, ordinarily important only in the seed_beds,
although they may occasionally carry over into the field as black-
leg, canker, or sore-shin.
In the case of black root-rot the most serious injury follows trans-
planting to infested soils, although frequently much injury results
from the occurrence of this disease in plant beds and the subsequent
transplanting of infected plants to disease-free soils.
The mosaic disease of tobacco, which is very rarely noted or re-
garded as of any importance in "the plant beds, usually originates,
however, in the seed beds, and its presence there is largely respon-
sible for the general field infections frequently observed ‘in many
sections.
Recently it has been shown that several bacterial diseases, i. e.,
wild-fire, black-fire, and Wisconsin leaf-spot, some of which are very
important in the districts where they occur, are in practically all
cases traceable to plant-bed infection. Many other diseases of tobacco
may also make their first appearance in the seed bed.
The production of plants for transplanting entirely free from
such diseases as mosaic, wild-fire, and black-fire is therefore highly
important, since at no other point ‘can the prevention of these
diseases be accomplished more efficiently or economically.
The principles of control are essentially similar for all the diseases
concerned, that is, eliminating the chances of the introduction to the
seed bed of the parasites concerned in these diseases or destroying
them if already present. The measures which can be applied can be
included under the general term “ sanitation.”
PLANT-BED SANITATION.
The following summary of sanitary measures which should be ap-
plied in seed-bed preparation, particularly on farms where mosaic
TOBACCO DISEASES AND THEIR CONTROL. 5
or the bacterial leaf spots have recently caused concern, is presented.
In the case of damping-off and root-rot the use of new soil or
sterilized soil will ordinarily suffice.
(1) Plant beds should preferably be placed a considerable dis-
tance from the location of the seed beds or fields of the previous year
and away from tobacco-curing barns and weedy areas.
(2) No tobacco refuse (trash) or stems should be used as fertilizer
on seed beds. Refuse should preferably be cleaned up and destroyed
before seed beds are started.
(3) Plant-bed frames should be made of new material, or the old
material should be disinfected by painting or sprinkling with some
disinfecting solution such as formaldehyde or corrosive sublimate.
(4) If new seed-bed covers are not used, the old cloth covers should
be sterilized by boiling one hour, and sash should be disinfected with
the frames.
(5) If new land is not used for seed beds—1. e., woods lands, new
breaking, or sod—the soil for the seed beds should be sterilized, pref-
erably by steaming for at least 80 minutes.
(6) In case the tobacco seed to be used is likely to be infested with
wild-fire or black-fire it should be disinfected with corrosive subli-
mate (1 part to 1,000 parts of water). Place the seed in a cheese-
cloth bag, dip it in the solution, and stir it about for 10 to 15 min-
utes. Rinse thoroughly with pure water and dry as rapidly as
possible. This method of sterilization can be used only with seed
sown in the soil without previous sprouting, since sprouting is other-
wise interfered with. Silver-nitrate (1 to 1,000) treatment for 15
minutes gives as good disinfection as corrosive sublimate and permits
sprouting of the seed before sowing.
(7) Do not sow the seed too thickly (1 ounce to each 800 to 1,000
square feet is usually sufficient). Do not overwater the plants, and
ventilate frequently by raising or removing the covers, especially
when glass covers are employed.
(8) Inspect the beds at intervals for diseased plants, and if any
are noted destroy them at once and soak the infected area with form-
aldehyde solution (1 to 25). Carefully avoid these infected areas
when weeding or pulling plants, since the parasites are readily spread
at such times. If infection is general in the seed bed it is safest to
procure healthy plants from other sources for transplanting.
THE STERILIZATION OF SOIL FOR PLANT BEDS.
The sterilization of seed beds by steam, using the inverted-pan
method, gives the most satisfactory results (fig. 2). Surface firing,
roasting, and the formaldehyde-drench methods are useful under
certain conditions or in the absence of facilities for steam steriliza-
tion. The principal benefits of soil sterilization are the killing of
parasites which may be harbored in the soil, such as those which
cause damping-off and root-rot, and the killing of weed seeds. By
bringing about other changes in the soil, however, a considerably
more uniform and rapid growth of the seedlings ordinarily results
(fig. 3).
To steam beds by the inverted-pan method, a boiler, pan, and con-
necting hose or pipe are required. The boiler should have sufficient
capacity to discharge a continuous flow of steam at a pressure of at
6 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
least 80 pounds into the inverted pan. While the pressure in the
pan itself may be considerably less than a pound, the advantage in
high pressure lies in the fact that dry steam is preferable to wet
steam and in that through a small connecting hose more steam can be
supplied to the pan per minute. The ordinary steam traction en-
gine, when obtainable, is especially suitable for this work.
The pan is preferably made of No. 16 galvanized iron reinforced
with angle iron, although other materials may be used. Its size,
shape, and construction may be made to fit the conditions at hand.
The pan may ordinarily vary from 50 to 100 square feet in area, 6
by 12 feet by 8 inches in height being common dimensions, although
pans as long as 14 and 16 feet are used. Handles, usually of iron
pipe, are ordinarily attached. The lower edge of the pan should be
sufficiently sharp so that it may be pressed readily into loose soil.
The inlet for the.steam may be made through the top or side of the
Fic. 2.—Steaming tobacco seed beds. This method is now widely used to prevent
plant-bed diseases, and it is also beneficial in other ways.
pan, but it should be so placed that the steam is not thrown directly
into the soil.
Soil steaming is usually done in the spring, some time before sow-
ing the seed, although fall steaming is sometimes practiced with
good results, especially where some protection can be given to the
beds to prevent reinfestation with parasites and weed seeds.
The seed beds should be made practically ready for sowing before
sterilization. ‘The soil should be fairly loose, not too dry nor too
wet, when steamed. The length of time required to sterilize the soil
will depend largely upon its condition, especially with reference to
moisture content and compactness, but ordinarily 80 minutes of
steaming suffices. The cost of steaming is usually between 1 and 14
cents per square foot.
Surface firing consists simply in burning brush or other material
on the surface of the seed bed for a period of 30 to 60 minutes. The
TOBACCO DISEASES AND THEIR CONTROL. %
roasting process consists in principle of shoveling about 4 inches
of the seed-bed surface soil upon an iron pan under which a fire is
maintained and heating it sufficiently to convert most of the mois-
ture to steam before replacing the soil.
For the formaldehyde-drench method use 1 gallon of formaldehyde
to 50 gallons of water, sprinkling this over the beds at the rate of
about 50 gallons to every 100 to 150 square feet of seed-bed surface
and regulating the flow so as to allow the solution to soak into the soil
to a depth of at least 4 inches. The soil must then be given several
days to dry out and all fumes of formaldehyde allowed to disappear
before sowing the seed. For this reason the formaldehyde method
is quite unsatisfactory unless its application can be made a consid-
erable time before the time of sowing, since weather unfavorable for
Fic. 3.—Growth of plants on soil steamed and not steamed. The area to the left of
center was steamed; the area to the right was not steamed. Steamed soil produces
healthy plants.
drying may delay sowing. Fall applications of the formaldehyde
drench, however, may sometimes be used to advantage.
In the following pages the parasitic diseases are roughly grouped
according to the parts of the plant primarily affected. The non-
parasitic diseases and those due to saprophytes are in separate
groups.
STEM OR STALK DISEASES.
DAMPING-OFF OR BED-ROT.
Description.—A rot of young plants starting in the stem of the
plant near the soil surface in plant. beds is usually referred to as
damping-off. This disease may appear at almost any stage of growth
of the seedlings in the bed, and, as will be pointed out later, a very
similar condition may be met with on isolated plants in the field.
Usually, however, the disease is most likely to occur when the plants
become much crowded and the ventilation at the surface of the soil
is very poor during periods of wet weather. Under such conditions
the disease spreads rather rapidly, ordinarily in circular areas from
isolated centers of infection. The plants generally topple over, the
whole stem and leaves becoming decayed, although in other cases
8 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
only brownish or black lesions appear on the stem of the plant (fig.
4). In case of an attack of damping-off when the plants are very
young the symptoms above described are not very evident, and the
first indication of disease may be a poor and uneven stand of yel-
lowish plants.
Cause.—Damping-off may be caused by one or more of several
plant parasites. The most common organisms causing this disease
are, however, two fungi, Pythiwm debaryanum and Rhizoctonia
solani (Cor ticium vag gum). The symptoms produced by these or-
ganisms are essentially identical, but the organisms themselves are
readily distinguishable under the microscope. : Both of these fungi
Fic. 4.—Tobacco plants affected with damping-off, or bed-rot. This disease is char-
acterized by a decay of the stems of young plants in the plant beds.
are very common soil-inhabitating organisms and attack a large
variety of other plants. They are most “likely to be found in arden
soils or other soils where their common host. plants have previously
been grown. They are apparently least likely to be present in soils
where only grasses or grains have been recently grown. Both or-
ganisms Ov erwinter in the soil. They do not spread as readily as
many other parasites, since spores are not ordinarily produced as
abundantly, nor are they so readily spread through the air.
Conditions favoring the. disease. ants remaining wet
for relatively long periods furnish conditions especially favorable for
the spread of the disease. High humidity around the plants as a
Bul. 1256. U. S. Dept. of Agriculture PLATE |
Fic. |.—A PARASITIC FUNGUS GROWING FROM PIECES OF TOBACCO STALKS
Most plant diseases are caused by organisms of this type
FIG. 2.—BACTERIA FROM DISEASED TOBACCO
Many of the diseases of tobacco are caused by these extremely small organisms, visible only when
greatly enlarged under the microscope
PLATE ||
1256, U. S. Dept. of Agriculture
Bul.
[IOs oy} UL YseIOd Jo AouoTOYyop & OPLoIpUT ‘SOOT YSTMOT[OA YITA “Jvor oy} JO Buptoyond puv sup)
NOILVAUYVLS HSVLOd—’S ‘SIS
(JOAN JO ASoJINOD) *Us/jo4
unyolpog snduny 94} Aq poonpoid st Aanfur yyeys Jo edAqd sTUL
WIVLS OOOVEOL AO LOY-WALS—'| ‘SIS
TOBACCO DISEASES AND THEIR CONTROL. 9
consequence of atmospheric conditions or poor ventilation in covered
beds is also a predisposing factor. High temperatures are also
favorable for the development of Pythium, although Rhizoctonia is
apparently favored by relatively lower temperatures. Generally
speaking, the thicker the stand of plants the more favorable the con-
ditions for the spread of damping-off.
Control.Seed beds should not be located where danger of damp-
ing-off exists unless the soil is well sterilized. Ordinarily, damping-
off can be avoided by locating the seed beds on grassland or where
only grains or corn have been grown the preceding year, unless the
parasites are introduced with the animal manures used. Garden
soils, old plant beds, weedy patches, or areas where refuse has been
thrown are likely to be infested. Well-drained sandy soils are best,
since they permit more rapid drying out after wet weather. Avoid
the common error of seeding too thickly. Water the beds thoroughly
rather than frequently, and if damping-off appears permit them to
become well dried off if possible. Ventilate the beds by removing
the covers during favorable weather. If the disease occurs only in
isolated small areas remove the infected plants and those immediately
surrounding, and if the plants are still young apply a formaldehyde
solution (1 part to 50 parts of water) to the infested area, leaving
off sash covers of such areas, however, to permit the escape of the
gas.
The occasional occurrence of damping-off in steam-sterilized beds
may result either from insufficient heating of the soil or from rein-
festation due to unsterilized soil being accidentally transferred to
the seed bed.
Plants showing lesions of damping-off on the stem should not be
transplanted to the field, since such plants may subsequently rot off.
SORE-SHIN, STEM-ROT.
Description.—The sore-shin type of disease has been reported from
various parts of the country under different names. In addition to
those given, sore-shank, canker, black-leg, collar-rot, bed-rot, or
damping-off may be considered synonymous as far as symptoms are
concerned. As a matter of fact, however, it has not been found that
these diseases are all one and the same, and it is not improbable that
at least three or more different causal organisms are concerned in
their production.
As the names imply, this disease is characteristically a rot, nor-
mally located on the stem or stalk of the plant and usually starting
in the vicinity of the soil surface. This decay may appear only as a
blackening of the surface of the stalk, but more commonly it pene-
trates to the woody portion in the case of large plants or completely
through the stem in young plants. Under favorable conditions the
blackening may extend up the stalk of the plant for a considerable
distance and enter the midrib and veins of the leaves, causing them
to drop (fig. 5). Affected plants are usually dwarfed and yellowish
in color, owing to interference with the transfer of water and plant
food. The weakening of the plant at the soil surface also renders it
quite likely to be broken off at that point by the wind. If the dis-
ease does not encircle the entire stem the plant may partially or
10 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
wholly recover under conditions which are unfavorable for the
disease.
Cause.—The similarity of this disease to bed-rot, or damping-off,
previously described, is evident, excepting that the former rot is
characteristically located in the plant beds, whereas the disease now
under consideration occurs under field conditions. It seems to be
: fairly well estab-
lished that the dis-
eases are identical
and that often the
field disease .arises
from the _ trans-
planting of infected
seedlings, although
the causal organisms
concerned are com-
mon soil organisms
and infection from
field soil is not im-
probable. The dis-
ease has been re-
ported from differ-
ent sections as due to
Lhizoctonia solani
and Pythium debary-
anum, both common
damping-off fungi,
and to undescribed
bacteria. Stem-rot,
eaused by WSclero-
tium rolfsiz (Sclero-
tinia sp.), falls into
this group of dis-
eases as far as gen-
eral symptoms, ori-
gin, and control are
concerned (Pl. II,
fig. 1). chase
ease is frequently
characterized by the
production of small
sclerotial bodies on
the surface of the
diseased area. A
number of other
Fic. 5.—Sore-shin, or black-leg. The rotting of the stalk at cultivated plants are
the surface of the ground is characteristic. Sometimes affected in a similar
sort Agra i Seam a considerable distance up the stalk and way by Rhizoctonia
in particular.
Conditions favoring the disease.—It is believed that most cases
of sore-shin are a result of transplanting plants more or less affected
with the damping-off disease in the plant beds. This primary lesion
may make little or no development for weeks after transplanting,
but finally, under favorable weather conditions and especially when
TOBACCO DISEASES AND THEIR CONTROL. thik
the soil remains wet for a considerable time, the parasite may again
obtain a foothold and make a fairly rapid development, with accom-
panying decay of the plant, before being again checked by unfavor-
able conditions.
Control—The control of this disease naturally centers around the
use of disease-free seedlings for transplanting. When pulling plants
all those infected should be discarded. Soil sterilization and other
measures should be used to prevent damping-off in the seed beds
where sore-shin is of common occurrence. The disease is not usually
evident sufficiently early to permit replanting. Since wet, poorly
drained soils favor the disease, they should be avoided if possible,
and fields frequently giving a high percentage of the disease may
be advantageously rotated.
BLACK-SHANK.
Description.—A serious disease of tobacco which has long been
known in Sumatra and Java as “ lanasziekte” or “ bibitziekte” is
now said to be found in the shade-growing sections of Florida and
Georgia. This disease may attack the plants in the seed beds, caus-
ing a rot similar to damping-off. It is in the field, however, that the
greatest damage is done. The symptoms may be a general wilting
of the plant, following marked signs of decay at the base of the
stalk and extending upwards as far as 24 inches. In this respect it
resembles to some extent severe cases of sore-shin previously de-
scribed, but it more often resembles Granville wilt. The rot may
also extend into the pith and roots of the plant. This disease also
attacks the leaves, producing large brown blotches, especially after
rains. The loss may frequently be very severe, involving most or all
the plants in the field (Pl. III, fig. 1). The introduction and
spread of this disease into tobacco districts of this country should
be guarded against.
Cause.—In Java and Sumatra considerable work has been done
upon this disease. It has been shown to be due to a fungus which
has been named Phytophthora nicotianae. Experiments in this
country indicate that the same fungus is concerned. This parasite
is closely related to the fungus causing our common potato late-
blight, which produces a serious disease of potatoes under favorable
conditions for its development. The mycelium and spores of the
fungus may be spread in various ways, and apparently in the case of
tobacco they may persist in the soil for at least two years.
Conditions favoring the disease—Aside from wet or humid
weather little is known concerning the conditions favoring the
disease. It is not unlikely that high temperatures are also favorable.
Varieties and strains differ markedly in susceptibility.
Control.Seed-bed sanitary measures heretofore described (p. 4)
are important. Plants showing signs of this disease, of course,
should not be transplanted, and infected seed-bed areas should be
sterilized or avoided. When fields have once become infested it. is
unsafe to plant them to tobacco again. Especial care should be taken
to prevent the transfer of infested soil from the diseased areas to
new fields, by men, animals, tools, or other equipment. If the disease
should become arnually serious in any section the development of
disease-resistant strains offers a promising method of control.
12 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
HOLLOW-STALK.
Description.—Hollow-stalk is a fairly common disease in most
tobacco districts, but it is rarely important, usually occurring only on
a few isolated plants. This disease i is referred to in some of the old-
est hterature on tobacco culture, in which it is usually attributed to
water-logging of the soil. The disease is characterized by the decay
of the pith of the stalk in particular. It most commonly occurs on
full-grown plants, where the entire pith may be rotted out, leaving
the stalk hollow (figs. 6 and 7). Quite frequently the symptoms
appear in the leaves, as mndicated by wilting only, but occasionally
the actual rot extends into the
veins of the leaf. The rot may
start either from the base or from
the top of the stalk, but usually
through wounds produced either
by insects, by accidental breaking
of the leat, or by the topping or
suckering operations.
Cause.—Hollow-stalk has been
shown to be due to the invasion of
bacteria of the common soft-rot
type which produce decays in
many vegetables, especially in car-
rots and ‘cabbages, during storage.
While the organism causing hol-
low-stalk has not been carefully
studied, it is similar to the soft-rot
organism in the common charac-
ters, and the soft-rot organism
from carrots will produce the de-
cay of tobacco pith. Hollow-stalk
is therefore attributed to Bacillus
carotovorus.
Conditions favoring the dis-
ease—Wet or poorly drained
soils no doubt favor infection, and
continued wet weather favors both
infection and rapid growth, which
See ee ary in turn would produce a succulent
plant (at are the complete de- pith. Relatively high tempera-
guruction of ine pith Dy hollow stalk tures accompanying such moist
stalk shown at the right. weather also favor the disease.
When plants are topped or suck-
ered during such periods of weather, exceptionally favorable condi-
tions for infection thr ough the wounds produced occur before they
can heal sufficiently. While only a very few isolated plants affected
with hollow-stalk occur before topping, the disease is commonly
spread to many neighboring plants in topping and suckering, the
infective material being carried on the hands of workers from one
plant to another.
Control—The only measure of control which is Snitnarse worth
while practicing in the case of hollow-stalk is to take precautions
against spreading the disease from plant to plant during topping
and suckering. This is best done by leaving the topping and sucker-
te
e-
}
ay
% ~
3
2
i
E
TOBACCO DISEASES AND THEIR CONTROL. 13
ing of diseased plants until all healthy plants have been handled.
decayed pith is touched by the worker’s hands they should be cleaned
before continuing work on healthy plants.
GRANVILLE WILT.
Description.—This disease was first found in Granville County,
N. C., and is therefore commonly
although it is often referred to
simply as tobacco wilt. It is also
known to occur in tobacco districts
of Virginia, Florida, and Georgia
as well as in foreign countries. me
though the disease has been a se-
rious one for a long time, particu-
larly in North Carolina, it has not
and probably never will become so
in the more northern tobacco dis-
tricts, on account of the fact that it
thrives well only under relatively
high temperature conditions ‘The
disease has made a considerable
though not a rapid spread in the
southern districts. Fields ordinar-
ily become gradually infested, so
that under repeated cropping to
tobacco practically complete crop
failure results.
The plants ordinarily are first in-
fected through the roots from in-
fested soil. Subsequently the dis-
ease affects the stems and leaves.
Usually the symptoms first appear
two to four weeks after transplant-
ing, the number of wilting plants
eradually increasing. The _ first
characteristic sign of the disease
on a plant is the drooping of one or
more leaves, followed by distortion
or wrinkling, yellowing, and finally
browning and drying up (fig. 8).
If the tobacco stalk is cut across in
an early stage of the disease, the
woody portion will show yellowed
streaks, which in later stages turn
to brown or black as contrasted with
the normal white tissue. In late
stages of the disease parts or all of
the surface of the stalk turn black
and the pith also rots out, accom-
panied by a decaying root system.
The cut ends of the stalks or mid-
ribs when pressed yield a dirty
referred to as Granville wilt,
Fic. 7.—Hollow-stalk of tobacco pro-
duced by inoculation. If the bacteria
causing this disease are inserted into
a stalk they cause a rot, as shown in
this split stalk.
white ooze, which is characteristic and aids in distinguishing it from
certain oe wilt diseases.
If
14 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
Cause.—Granville wilt is a bacterial disease, and the organism
concerned has been named Bacterium solanacearum. ‘his parasite
causes similar destructive rots of tomatoes and potatoes, being cap-
able also of attacking many other plants. The bacteria pass through
and multiply particularly in the woody vessels of the plants, thereby
clogging or otherwise injuring these vessels, so that the water and
food supphes are cut off and the plant naturally wilts and gradually
dies.
The germs live over winter in the soil or in the decaying roots
and refuse of preceding crops for at least four or five years. In the
yearly presence of tobacco or any other plant on which the bacteria
can live the disease, of course, continues to infest the soil indefinitely
instead of gradually dying out. The organism may be spread from
field to field in various ways like any other soil-infesting parasite,
but its spread is more frequent through direct transfer of soil from in-
fested fields, as carried by men, animals, water, tools, or other equip-
Fic. 8.—A field of tobacco plants affected with Granville wilt. The symptoms shown
here, accompanied by blackening in the woody layer of the stalk, which yields an
ooze on pressure of the cut end, are typical of this disease.
ment, and these facts should be borne in mind when new land is being
used for tobacco. )
Conditions favoring the disease.—Granville wilt seems to be es-
pecially favored by hot weather. Dry weather usually exaggerates
the disease symptoms, in that it hastens the drying up of the plants;
but as a matter of fact wet weather and wet soils are more favorable
for actual disease development. Sandy soils are apparently more
likely to become heavily infested than clay soils. Wounding of the
roots markedly favors infection, and a certain amount of this is un-
avoidable in transplanting and cultivating. Nematodes, which cause
the root-knot disease, are common in the wilt-infested districts, and
it seems that wounding of roots by these organisms greatly favors
infection by the wilt organism. 3
Control—rThe control of this important disease in the badly in-
fested area lies in crop rotation. A number of other methods of
control have been tried, among them resistant varieties and various
TOBACCO DISEASES AND THEIR CONTROL. 15
soil treatments, but none applicable to practical conditions has
proved satisfactory. .
Badly infested fields should be cropped for at least five years with
crops not attacked by wilt, before replanting to tobacco. Four-year
and three-year periods of cropping may suffice on land not too
heavily infested. A second crop of tobacco in succession on in-
fested soil is almost certain to result in failure. The wilt disease
attacks tomatoes, potatoes, peanuts, peppers, eggplants, velvet beans,
and garden beans. These crops therefore should not be grown in
rotation with tobacco on infested ground. Ragweed, jimson weed,
and certain other weeds are also attacked and should be carefully
controlled on infested land intended for tobacco. Such crops as
corn, wheat, rye (as a cover crop), sweet potatoes, cowpeas, grasses,
red and crimson clover, and cotton may be safely used in rotation
on infested land. General sanitary measures should also be used,
aiming especially to prevent plant-bed infection by giving due re-
gard to the location of beds with reference to infested fields and the
Fic. 9.—Fusarium wilt of tobacco. This disease resembles Granville wilt in many
respects. Pressure on the cut ends of stalks does not yield an ooze.
sterilization of seed-bed soil. The use of tobacco stalks or stems
as fertilizer should be avoided.
FUSARIUM WILT.
Description.—This recently described disease of tobacco was first
definitely noted in Maryland, but it has also been found in Ohio,
Kentucky, and Tennessee. The disease does not seem to be of com-
mon occurrence even in these districts, although in a few isolated
cases as much as 20 per cent of the plants in a field was destroyed.
The disease is a typical wilt and has been called Fusarium wilt
(naming it from the causal organism) to distinguish it from the
ordinary tobacco or Granville wilt previously described.
The general symptoms of this disease are much like those of Gran-
ville wilt, except that on cutting and pressing the infected stalk no
slimy ooze is extruded from the vessels, as is common in the case of
Granville wilt. Fusarium wilt may first become evident upon very
young or upon mature plants. Under field conditions it is probable
that infection occurs soon after transplanting, but that marked
symptoms may be delayed for several weeks. In large plants the
16 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
earliest symptom is a wilting of one or more leaves of the plant,
often characteristically localized on one side of the leaf or plant
(figs. 9 and 10). Yellowing, browning, and drying up of the in-
fected leaves follow, and in severe cases the entire plant is killed.
Roots, stalk, midrib, and veins, as well as suckers, may become in-
fected, as indicated by cutting across these parts even in early stages
of the disease.
Cause.—This wilt disease is due to a fungus which belongs to the
large group of Fusariums, rather common fungi, members of the
same group produc-
ing similar but not
identical diseases in
various. other
plants. The par-
ticular species con-
cerned with tobacco
wilt has been named
Fusarium oxyspo-
rum var. nicotianae.
The strands of this
fungus exist in
abundance in the
vessels of infected
plants and are re-
sponsible for the
wilting. The or-
ganism lives over
winter in the soil
and may possibly
exist there in the
absence of tobacco
for three years. In-
fection, therefore,
takes place from ~
the soil, and as far
as known only
through wounds on
the roots or basal
portion of the
plant.
Conditions favor-
ing the disease.—
Fusarium wilt ap-
pears to be most
common in _ rela-
Fie. 10.—Fusarium wilt of tobacco. Quite commonly this tively warm sea-
gleeate jo loenlggd ape in fhe eanly stages ates nly 132.) ore, ma
temperatures re-
main as high as 80° to 90° F. for some time. Infection can occur,
however, only on certain especially susceptible strains or varieties,
among which White Burley and Maryland Broadleaf are most
susceptible, cigar types of tobacco being generally very resistant to
infection. The extent of infestation in the soil, as well as the
amount of wounding, together with rainfall, naturally also plays an
important part in the severity of the disease.
Bul. 1256, U. S. Dept. of Agriculture PLATE III
Fic. I|.—BLACK-SHANK OF TOBACCO
A disease of tobacco new in this country, at present limited to the Florida-Georgia district.
(Courtesy of W. B. Tisdale)
Fig. 2—LIGHTNING INJURY TO TOBACCO
Plants on the border of spots in the field damaged by lightning usually show peculiar distortions
and markings
PLATE IV
Bul. 1255, U. S. Dept. of Agriculture
Aysrodoad Mois 04 [rey WOO Pp oy} 04 poyueydsuvyy jr nq Sspoq Oy} UT ATMOS MOI3 ATUO 1OU sjuRId posBostc{
SONIIGSSAS OOOVESOL NO LOH-LOOY MOVIG
Bul. 1256, U. S. Dept. of Agriculture PLATE V
Fic. |.—ROOTS OF TOBACCO PLANTS GROWN AT VARIOUS TEMPERATURES
ON SOILS FREE FROM AND SOILS INFESTED WITH ROOT-ROT
A comparison of the growth in healthy (H) and diseased (D) soil shows that cold soils favor
root-rot
;
FIG. 2.—LESIONS OF BROWN RooT-RoT
Roots of tobacco and tomato plants, showing characteristic decayed areas
a te 2 pe 5 a
a
TH0}SAS JOOI [BULIOU B SOSSossod JYSL OY) 4B
uMOYS JULI OY, “WoT Oy} 7B QuLyd OddVqGo} oYY AG UMOYS SB ‘ULOJSAS O01 OY VOT OY} IB UMOYS SB ‘4NO 4UAS 9.10.M S}OOI MO
jo squvd odavp SAOLSep A[oJo[AULOD OSBoSTP STY} S}OOI OYJ BUIUMOIG OF UOT}Ippe UT pues ‘[IOS ULIBM OUT Jos SBA FYB OY} OF BUO OY} OHI] JULTd PosvosIp A[IAveY y
LOY-LOOY NMOY Ad GaONdGAY WALSAGS LOOYU—'S “YI LOY-LOOY WOYS GSYSAO0SY LNVId O00VEO|—'| ‘DI
PLATE VI
1256, U. S. Dept. of Agriculture
Bul.
TOBACCO DISEASES AND THEIR CONTROL. ; 17
Control—Where Fusarium wilt threatens to become serious,
growers are advised not to grow tobacco on the infested soil for at
least three years. So far as definitely known, this particular para-
site does not attack any other plants. It is so similar, however, to
the potato Fusarium wilt that, as a matter of general precaution,
it is not advisable to grow potatoes in rotation with tobacco on
«
Ite. 11.—A tobacco plant stunted by black root-rot.. A healthy root system is shown
at the right.
infested soils. Precautions should also be taken to prevent the
spread of the disease to uninfested fields through the use of infected
plants for transplanting or the transfer of soil. A hopeful means
of control, should conditions warrant it, lies in the development of
disease-resistant strains within the various susceptible varieties.
91061°—24——_2
18 BULLETIN 1256, U. 8S. DEPARTMENT OF AGRICULTURE.
ROOT DISEASES.
BLACK ROOT-ROT.
Description.—Black root-rot is one of the most widespread dis-
eases of tobacco and undoubtedly has caused more losses to tobacco
growers as a whole than any other disease with which they have to
contend. This disease frequently appears in the seed bed to a serious
extent (Pl. IV), but the most important damage occurs in the fields
through reduced yield and poor quality of tobacco.
_'The disease proper is limited to the root system and the base of
the stem below the surface of the soil. The roots are distinctly
rotted and usually distinctly black in parts, but in severe cases the
marked absence of roots as a result of the disease is especially
noticeable (fig. 11). This depletion of the root system, together
with the diseased condition of the remaining roots, naturally results
in the starvation of the plant from lack of food and water. The
symptoms on the aboveground parts of the plant are, therefore, only
marked stunting of growth, usually with a yellowish or chlorotic
appearance, commonly with premature budding out. Such symptoms
Fic. 12.—A field of tobacco varieties, showing differences in susceptibility to black
root rot. Many varieties of tobacco are very susceptible to the disease, as illus-
trated by these alternate rows of ordinary White Burley, which have made little
growth in comparison with the resistant types in neighboring rows.
are not ordinarily distinguishable from those produced by low soil
fertility, lack of soil moisture, and certain other unfavorable con-
ditions for plant. growth, or in some instances symptoms produced
by other parasites. Examination of the root system must therefore
be made to determine with certainty the presence of the disease. The
importance of this similarity of symptoms lis also in the fact that,
more often than not, low soil fertility or dry weather has been held
responsible for stunted crops where black root-rot or other plant
parasites have been the actual cause of the conditions existing.
It frequently happens, however, in the case of crops infested with
black root-rot that a very uneven growth develops. This condition
may result either from different degrees of infection on the seedlings
when transplanted or inherent differences in the resistance of individ-
ual plants to the disease where impure seed is used. Less striking
unevenness may result. also as a consequence of the uneven distribu-
tion of the parasite concerned or from irregular environmental con-
ditions favorable for infection existing in the soil. Very uneven
TOBACCO DISEASES AND THEIR CONTROL. 19
growth of tobacco does not ordinarily result from unfavorable soil
or weather conditions alone, and consequently such fields commonly
indicate the presence of root-rot or in some cases other parasites.
The rapid recovery of stunted tobacco following a change in
weather conditions is also characteristic of root-rot. Infested soils
do not ordinarily favor a gradual and continuous growth throughout
the season, but are affected by short periods of drought and cool as
compared with warm weather followed by rains, so that the develop-
ment of the crop occurs in short intervals rather than as uninter-
rupted growth. ?
The extent of the damage from black root-rot ranges from complete
crop failures to imperceptible losses even in the presence of consider-
able evidence of the parasite on the roots, the latter condition re-
- sulting from the power of the tobacco plant to recover from injury
to the root system. More commonly the reduction in yield varies
between 25 and 50 per cent.
Fic. 13.—Field showing the growth of susceptible and resistant strains of White
Burley tobacco with respect to black root-rot. The ordinary strains of White Bur-
Bee (left side) failed to grow where the resistant strain (right side) made a nor
mal crop.
Cause.—Black root-rot is caused by the fungus 7hielavia basicola.
This parasite is also capable of attacking a considerable number of
other plants, usually, however, to a lesser degree than tobacco. Varie-
ties of tobacco differ markedly in their susceptibility to the disease.
This fungus produces in great abundance three kinds of spores upon
the roots. The presence of one of these forms, the chlamydospores,
is responsible for the jet-black appearance of some of the infected
roots. .
Thielavia is capable of living over from year to year in the soil
and may, in fact, persist for several years even in the absence of the
culture of its common host plants. This parasite does not readily
spread from field to field except through the transfer of infested soil
or the transplanting of infected plants. With continuous culture of
tobacco most soils become heavily infested with Thielavia. The ex-
tent of the damage from this fungus depends upon the extent of soil
infestation, the susceptibility of the variety grown, and the environ-
mental conditions to which it is exposed in the soil.
Conditions favoring the disease—Low soil temperatures during the
growing season are especially favorable to serious damage from black
root-rot (Pl. V, fig. 1). Since such temperatures are most likely to
occur in the early part of the growing season it is at this time that
=
20 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
the effects of the disease are most evident. Periods of warm weather
following, which are usually accompanied by dry conditions, exag-
gerate the stunting but permit new roots to grow out into warm, dry
soil without decay, hence permitting a rapid recuperation of growth
as soon as sufficient soil moisture is supphed (Pl. VI, fig. 1). Occa-
sionally, however, a season as a whole is relatively cool, and crops
remain stunted throughout the season on infested soils. On the other
hand, during exceptionally warm seasons excellent crops may be
produced. on heavily infested soils.
The injury from black root-rot will, of course, also vary with a
number of other factors of an environmental nature in the soil, some
of which are not fully understood. It is certain that highly acid
soils tend to reduce the disease and that soils alkaline or moderately
acid in reaction or soils made less acid by the application of lime are
more likely to favor the disease. Again, there is some evidence to
show that more damage from the disease occurs on soils which are
compact than in the'‘same soils kept loose. Within ordinary limits
the amount of water in the soil does not seem to affect materially the
extent of the disease, though poorly drained wet soils may favor
disease largely because of their lower temperature.
Control——Black root-rot can be adequately prevented in the seed
beds by selecting land for beds which has not been in tobacco or
tobacco beds for several years. If such a change of seed-bed soil
can not be made, soil sterilization, preferably steam sterilization,
should be resorted to. Do not transplant infected plants to the field,
since such plants will not only fail to start growth properly but 1 in
case of planting on new tobacco land serve as a source of introduc-
tion of the parasite, which will affect later tobacco crops grown on
the land.
The most evident way of avoiding black root-rot in the field is by
repeatedly changing the land for the tobacco crop, since continuous
culture favors root. rot, whereas soils not planted to tobacco for 5 to
10 years are generally relatively free from the parasite. The fre-
quency with which this change of land should be made, however, will
depend largely upon the susceptibility to the disease of the variety
grown, the time of introduction, and the rate of the increase of the
parasite in the soil. Land may become too heavily infested for Bur-
ley tobacco in one or two years, whereas certain fields in the Con-
necticut Valley, where shade-grown Cuban or Breadleaf are grown,
frequently permit continuous culture, although in certain seasons
high percentages.of damage from Thielavia occur.
No recommendations for crop rotation with tobacco which will
safely meet all conditions can be given. Not only do the above-de-
scribed factors vary the case, but the actual rest period from tobacco
which is required and the crops to be grown in rotation in ordinary
farm practice vary in the different growing districts. As a general
principle, however, heavily infested soils should be rested four to
eight years and should not be cropped in the meantime to other good
host plants of Thielavia, such as beans, cowpeas, or soybeans. Corn,
small grains, and most truck crops are immune to the disease.
Another means of control, which can not yet be used to its fullest
advantage, lies in the use of disease-resistant varieties or strains
(fig. 12). The representative types of tobacco now grown in the
al Patt
TOBACCO DISEASES AND THEIR CONTROL. oF
United States show very marked differences in resistance to the dis-
ease. White Burley, Maryland Broadleaf, Orinoco, Pryor, and Big
Cuban are susceptible. Havana Seed types and Pennsylvania Broad-
leaf are of intermediate resistance, and Connecticut Broadleaf, Con-
necticut shade-grown Cuban, Little Dutch, and other types are quite
resistant.
A large number of subvarieties or strains may be similarly classi-
fied, and certain strains of types now growing in some districts are
sufficiently resistant to warrant their general use provided their
quality is otherwise equally desirable. The testing of the numerous
strains on this basis and the development of new resistant strains
for the various districts will undoubtedly greatly reduce the damage
from this disease, although much experimental work remains to be
done before the general usefulness of this measure will be recognized.
Up to this time the development of special resistant strains with suf-
Fic. 14.—Tobacco field affected with brown root-rot. A stunted and uneven growth
over the field results from this disease.
ficient quality to be grown commercially has only been accomplished
with Wisconsin Cigar Binder and White Burley types (fig. 13), al-
though much still remains to be done even with these.
BROWN ROOT-ROT.
Description.—Brown root-rot is new to tobacco growers, in that
it has not been previously described, but it has undoubtedly existed
for many years, particularly in the Connecticut Valley. Brown root-
rot is also suspected of occurring in Maryland, Tennessee, North
Carolina, and Wisconsin. This disease should not be confused with
black root-rot. The general aboveground symptoms are the same
(fig. 14), and the roots are rotted as in black root-rot (Pl. V, fig. 2),
but the two diseases arise from different causes, and therefore the
control measures differ in some respects. The signs of the two dis-
eases on the root are quite different, black root-rot usually showing
many distinctly black roots, whereas in brown root-rot no blackening
occurs. Microscopical examination of black root-rot shows the pres-
22 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
ence of spores of Vhielavia basicola, whereas these are absent in
brown root-rot cases unless the two diseases occur together. The
stunting of crops as a result of brown root-rot is of course frequently
laid to unfavorable conditions of the soil or to weather conditions,
but repeated experiments have shown that some parasite is probably
concerned.
Cause —The cause of brown root-rot is not yet definitely estab-
lished. If soils producing this disease are sterilized with heat or
formaldehyde, the disease is completely controlled. This, together
with other facts, practically demonstrates that some organism is the
cause of this disease. Species of fungi, especially Fusarium,
Rhizoctonia, and Actinomyces, as well as other fungi and bacteria,
are commonly present in the diseased roots, but none of these has yet
been definitely shown to be primarily concerned with the produc-
tion of the disease. This same disease attacks a large number of
other plants, such as tomatoes, potatoes, and other closely related
plants, as well as many leguminous plants. The relation of the
disease to grasses and grains is not yet fully understood. The dis-
ease is common on sod land the first year to tobacco, however, indi-
cating that the cause may have some connection with such crops.
The causal agent persists in the soil from year to year, although it
seems that under unfavorable conditions, such as excessive drying or
absence of host plants, it may die out with relative rapidity.
Conditions favoring the disease—Conditions favoring this dis-
ease are much the same as for black root-rot; that is, primarily,
relatively cool weather (Pl. VI, fig. 2). Dry hot weather tends to
exaggerate the symptoms of the disease in the aboveground parts
of the plant, but rains following such a period permit considerable
recovery.
Control.—The losses from this disease are as far as known greatest
in the Connecticut Valley. The high cost of production of shade-
grown and other primed tobacco especially tends to make even low
percentages of reduction in yield an important consideration. On the
other hand, the tendency of crops to recover or land to “ come back ”
from this disease naturally induces growers to take risks more often
in planting infested fields. The experimental evidence on the rela-
tion of the crop-rotation system to: this disease is as yet incomplete.
Drying or aeration of the soil is favorable to its recovery from this
trouble.
There seems to be no difference in varietal resistance to brown
root-rot.
ROOT-KNOT.
Description.—The root-knot disease is sometimes known as root-
gall or fe root. This malady is quite common in the Southern
States on various plants, including tobacco, but it is practically
unknown in the Northern States. The disease proper is confined
to the root system. The aboveground symptoms of the disease are
stunting or dwarfing of the plant, accompanied by wilting, espe-
cially in dry hot weather. These signs of the disease are conse-
quently very similar to injury produced by other root diseases or
by excessively dry weather. If the plants are pulled out of the
ground, however, the roots will be found to be more or less enlarged
TOBACCO DISEASES AND THEIR CONTROL. 23
or swollen in sections, presenting a very irregular appearance in size
and shape (fig. 15). This deformity is produced by the activity of
the parasite concerned and results both in lessened feeding area of
the root system as well as interference with the proper transfer of
water and food to the plant. The number, size, and extent of the
development of the enlargements on the roots will usually be propor-
tional to the extent of infestation in the soil and other conditions
favorable to- the disease.
Cause.—Root-knot is caused by nematodes, or eelworms, which are
very small animals, often barely visible to the naked eye, fairly
abundant in nature. Fortunately, only a few species are commonly
parasitic on plants. The common root-knot disease is due to the
nematode Heterodera radicicola. 'The tissue of the roots attacked
by this parasite becomes stimulated to excessive growth in diameter,
Vie. 15.—Root-knot of tobacco. The enlarged roots produced-by nematodes are typical
of this disease.
thus causing the enlargements. This nematode also attacks a large
number of other plants, but corn, grains, and grasses as a class are
not susceptible; neither are peanuts, sorghum, velvetbeans, or re-
sistant cowpeas (Iron or Brabham).
The organism lives over in the soil from year to year and can
apparently survive for at least three years, even in the absence of its
host plants. It can, of course, be spread from field to field in the
same manner as any other soil-infesting parasite.
Conditions favoring the disease-—Root-knot is usually most serious
in hght sandy soils. Heavy, clayey, or marshy soils are not favor-
able to its existence. High soil temperatures are favorable to its
development, and overwintering in soil seems to be dependent on
mild weather, such as exists in the more southern States. Moist
soils are more favorable for the development and parasitism of the
nematode than excessively dry or wet soils. Soils low in fertility
appear to be more seriously affected than soils properly fertilized.
24 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
Control—tThe only satisfactory way of controlling root-knot is
by crop rotation, using crops not attacked by the nematodes between -
the tobacco crops. Cropping with corn, wheat, oats, rye, peanuts,
sorghum, velvetbeans, or Iron or Brabham cowpeas for three years
usually suffices to rid the land of nematodes if weeds acting as host
plants are kept down to a reasonable extent. Precautions should
be taken, of course, to prevent seed-bed infection through the soil
as a harboring agent.
BROOM RAPE.
Description.—Broom rape is a fairly large flowering plant, devoid
of leaves or green coloring matter (chlorophyll), which is capable
of parasitic growth upon tobacco and other plants. Lacking
chlorophyll, it is wholly incapable of manufacturing plant tissues
Fic. 16.—Earlier stages of development of broom rape on tobacco. This para-
sitic plant attaches itself to the roots of tobacco, feeding upon them and con- -
sequently producing damage in case of heavy infestation.
from the elements of the soil, water, and air and can exist only by
attachment to certain other green plants from which it draws its
nourishment, much like fungi and bacteria. It therefore resembles
in its behavior in many respects the well-known dodders. The
broom-rape seeds germinate in the soil, and the young plants attach
themselves to the roots of the tobacco plants and grow to a size as
large as 18 inches in length, often in very thick clumps of white,
yellow, brown, or purplish stems arising from the ground to some
height at the base of infected plants (figs. 16 and 17). These stems
produce flowers and seeds, the latter being very small, but once in
the soil they are capable of remaining dormant for long periods,
subsequently germinating and attacking susceptible crop plants.
Just how much damage broom rape may do to tobacco in lowered
yield and production is not fully understood. In many cases heavy
infection with broom rape apparently has not done much harm. In
other cases, however, stunting of infected plants is apparently
TOBACCO DISEASES AND THEIR CONTROL. 95
marked. As far as known, broom rape is largely limited to Kentucky
tobacco fields, especially in the White Burley district, although it has
been known to occur on tobacco in other States. The infested area
seems to be limited to the hemp-growing district, the same broom
rape being also parasitic on hemp. The seeds are commonly mixed
with hemp seed and are sown with it; hence they infest the soil upon
which tobacco is subsequently planted.
A large number of species of broom rape are known, especially
in foreign countries. ‘The common species in the Kentucky fields is
Orobanche ramosa.
Aside from tobac-
co, this species also
attacks hemp, to-
mato, potato, rape,
cabbage, and pars-
nip; but corn, cere-
als, grasses, and
legumes as a class
are immune to this
species. The con-
trol of the trouble,
therefore, is largely
based on crop rota-
tion, using plants
not susceptible to
attack on infested
land for several
years before re-
planting to tobacco
or other _ crops,
where such precau-
tions are deemed
necessary.
LEAF DISEASES.
WILD-FIRE.
Description.—A
serious outbreak of
an apparently new
disease which be-
1 sta f devel t of b e on cane known =
= room rap .
Fie. 17.—Later ges 0 evelopment o I wild-fire occurred
tobacco.
in North Carolina
in 1917. This disease has since become introduced into practically all
tobacco districts. The occurrence of wild-fire is more or less sporadic,
being markedly influenced by weather conditions, but the seriousness
of the disease, under conditions favorable for its spread, is well estab-
lished by the large losses which it has caused since its recognition.
Wild-fire may attack plants at almost any stage of growth, and
so far as is definitely known all field infection originates from in-
fected seed beds. The most typical symptoms in the plant bed as
well as in the field are on the leaves in the form of spots which are
26 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
usually circular, of a bleached or yellowish color (chlorotic area or
halo), and often as large as a dime (PI. VII, fig. 1). In the center
of each spot is a dried area often only the size of a pin point, but
varying in size up to as much as half an inch in diameter. The
chlorotic area may consequently be only a narrow border or entirely
lacking, depending upon the age of the spot and other conditions.
These spots when numerous may run together, forming irregular
areas and sometimes causing the whole leaf to collapse and dry up.
In other cases the bleached or chlorotic area may spread into and
throughout an entire leaf, apparently from a center of infection in
or near the bud or stem of the plant. Such plants usually fail to
make satisfactory growth. |
Fic. 18.—Tobacco leaves showing wild-fire. Affected leaves finally become more or
less ragged and may often be made practically worthless.
The bottom leaves are most likely to become heavily infected, not
only because they are exposed to infection for a longer period of
time but also because they are in a, more favorable environmental
position for infection (fig. 18). Under favorable conditions for
infection, however, the disease may spread to the uppermost leaves
on the plant and even to suckers and seed pods. Cigar wrapper and
binder tobaccos especially may be rendered worthless for the pur-
poses for which they are used by a comparatively few infections per
leaf, while other types may not be seriously damaged unless the spots
are sufficiently numerous to markedly affect the color of the leaf
or cause such extensive drying up in the field as to render harvesting
impossible without much breaking, thereby lowering the yield and
producing a ragged crop.
TOBACCO DISEASES AND THEIR CONTROL. uy
In many districts mild cases of wild-fire are likely to be mistaken
for rust or firing of a less serious kind. Many other forms of leaf
spots or rust occur, as will be described later, but with close observa-
tion the grower, in most cases, can learn to distinguish wild-fire from
other diseases. When found it behooves the grower to use all possible
precautions to prevent its further spread.
Cause.—Wild-fire is caused by an organism belonging to the bac-
terial group and known as Bacterium tabacum. This organism is
eapable of entering tobacco leaves, especially through. wounds, and
when once started proceeds to multiply to enormous numbers, so that
hundreds of thousands of bacteria eventually may be present in a
single spot and serve as a source of infection on the same or other
leaves as soon as conditions become favorable.
Under unfavorable conditions for the growth of the bacteria, how-
ever, they are usually short lived. While the great majority of
them die during the season without producing any infection, a few
manage in some way to live over winter and become the source of
seed-bed infection the next season. Under just what conditions
and on what materials used in or about seed beds the bacteria over-
winter is not yet definitely known in all cases. While strong sus-
picion rests on dried leaves, seed, soil, seed-bed covers, and so forth,
yet all observed cases can not be explained on these grounds. Never-
theless, our present basis of control must rest on the supposition
that some of the bacteria do in some manner live over winter on one
or more of the materials or equipment used in growing plants. It
now seems, however, that tobacco trash from infected crops, trans-
ferred in some manner to seed beds in the spring, is the most Hey
source of infection.
Once infection has occurred in the seed ped or field, sienna
spread is readily explained. In addition to being spre ead through
the air by rain and wind, it may be carried by men, animals, tools,
or any other equipment in various ways from leaf to leaf, plant to
plant, and farm to farm unless proper precautions are observed.
Conditions favoring the disease—Beating rainstorms are well
known to be especially favorable to the spread of wild-fire in the
fields or in unprotected seed beds. This follows from the fact that
such storms not only spread broadcast the organisms causing the dis-
ease, but also wound the leaves sufficiently to markedly aid infec-
tion, at the same time furnishing the necessary moisture. Few new
infections occur during dry weather, although heavy dews remaining
on the leaves for some time permit some infection. To wound the
leaf tissue 1s not. necessary for infection in all cases, but it enhances
infection markedly. Rainy periods without. accompanying winds
may result therefore in a very considerable spread of the disease.
As far as temperature is concerned, wild-fire seems to work over a
wide range, although some difference occurs in the incubation period
and the type of injury produced. Some infection will take place
with the temperature as low as 60°, but the best infection results when
it ranges between 80° and 90° F.
It is also fairly evident that rapidly growing tobacco, as a rule,
is more hkely te become heavily infected than stunted tobacco. It
also seems that some relation exists between the type of soil or the
fertilizer applied and the extent of infection, but this relation is not
yet fully understood.
28 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
Control—The control measures recommended are largely con-
cerned with the production of disease-free plants in the seed beds.
Failing in this, healthy plants should be sought elsewhere in pref-
erence to taking the chance of introducing even a seemingly small
amount of infection into the field.
The principles of plant-bed sanitation previously stated (p. 4)
apply particularly to wild-fire prevention. In addition to this, it
may be necessary to resort to other methods. In districts where wild-
fire is almost-certain to be of general occurrence and the probability
of obtaining completely disease-free plants seems small, it may be ad-
visable to spray or dust the seed beds with Bordeaux mixture once or
twice a week to hold down the amount of infection.
Where wild-fire infection is found in isolated spots in the seed
beds, destroy the plants in such areas, together with surrounding
plants, by burying or applying formaldehyde (1 part to 25 parts of
water) and cover the the treated area to hold the fumes. The covers
should not be replaced on the treated beds until most of the formalde-
hyde fumes have escaped into the air; otherwise the healthy plants
may be damaged. The disease may be easily spread. throughout the
bed from such infected areas during the process of weeding beds or
pulling plants for transplanting.
If wild-fire develops in the field on only a few isolated plants
early in the season it is well to destroy them and replant with
healthy plants. In case of a high percentage of infection, it is often
advisable to plow up and replant the entire field if the season is
not too far advanced. If such a procedure can not be profitably at-
tempted owing to lateness of the season, serious effort should be
made to remove all the infected leaves as thoroughly as possible
early in the season with the hope that such reduction of infective
material, together with the chances of an unfavorable season for
the disease, may hold its spread down to a minimum.
With persistent effort on the part of the grower to hold down the
disease and to destroy or otherwise dispose of all infected material
on the farm, including the plowing down of sucker growth in the
fall before it becomes infected, much headway will be made toward
reducing the chances of wild-fire in the crop the following year.
BLACK-FIRE.
Description.— tobacco leaf-spot which has probably been existent
for many decades, especially in Virginia and other southern dis-
tricts, but which has not been carefully studied until recently, is
the disease now properly referred to as black-fire (angular leaf-
spot). The disease is in many respects similar to wild- fire, occurring
in the same districts, and doubtless the two are often confused by
growers. Since these diseases are caused by different parasites,
though similar, and as the symptoms of the two are fairly dis-
tinct, they must be considered separately, although the control
measures for both diseases are much the same. It may likely de-
velop that from a practical standpoint the chief reason for the
erower recognizing the difference between these and other leaf spots
lies in the fact that one or the other of such diseases: will prove
much more dangerous in some sections than others, which will in-
fluence the control measures undertaken. So far as known, black-
.
TOBACCO DISEASES AND THEIR CONTROL. 29
fire is more or less confined to the Southern States, whereas wild-
fire seems likely to be a serious disease in the Northern States as well
as in the South. In 1920 black-fire is said to have been especially
serious in Virginia and was found in 85 per cent of the fields in-
spected, causing an estimated loss of more than $5,000,000.
In the seed bed the black-fire leaf spots are quite small, angular
in shape, and black or dark brown in color, with a narrow, ‘clear
border especially distinct when held up to the light Gel V-LLE, fig. A).
The spots on seedling leaves always remain small and do not ap-
proach the size attained in the field. When a considerable number
of infections occur the leaf often becomes deformed and ragged.
In the field young black-fire spots in the early stages are much
like those in the seed bed, but later they become as lare ge as half an
inch in diameter. On thin leaves the spots are usually quite angular
in outline, whereas on thicker leaves they are more rounded, some-
times zonated, from tan to dark brown, sometimes almost black in
color (Pl. VII, fig. 2). The conspicuous bleached border or halo
so characteristic of wild-fire is absent, although some yellowing
occurs around the spot.
Cause.—Black-fire is caused by a bacterium which has been named
Bacterium angulatum. This organism closely resembles the wild-
fire bacterium in appearance and behavior. So far as known the
overwintering and spread of this organism is practically the same as
that of wild-fire.
Conditions favoring the disease.—As in the case of wild-fire, fre-
quent rainfall and especially rain with strong wind is favorable to
infection and spread of the disease. Good and well-balanced fertili-
zation appears to favor the occurrence of infection rather than poor
fertilization, which results in less rapid and succulent growth. Low
topping, as ‘compared with high topping, has been found to favor
the disease. Low topping is, however, usually essential to the pro-
duction of proper size and body of leaf, especially in the dark export
types of tobacco.
Control.—Seed-bed sanitation measures, as described earlier (p. 4),
and general precautions as recommended for wild-fire control (p. 28)
apply equally well to black-fire. There seems to be definite evidence
that im the case of black-fire the germs may live over from year to
year on seed and on cloth covers, and special attention should be paid
therefore to seed disinfection and the use of new cloth covers or to
the sterilization of old covers every season. As an aid to the pro-
duction of disease-free seed, it is advised that seed pods showing no
sign of infection should be selected. To cover flower stalks with
paper bags will aid in protecting pods against infection. Seed 2
or more years old is quite unlikely to carry infection, but in all
doubtful cases these precautions should be supplemented by seed
disinfection. The plowing under of stubble as soon as the tobacco
is harvested is recommended for infected fields, especially if tobacco
is to follow on the same land.
THE WISCONSIN LEAF-SPOT.
Description.—Two bacterial leaf-spot diseases have already been
described, namely, wild-fire and black-fire. The Wisconsin leaf-spot
is a third form of bacterial disease, causing similar but not identical
.
30 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
\
symptoms. As far as known, it is rarely as serious as the two above-
mentioned diseases. This leaf-spot is undoubtedly the so-called
ordinary “rust ” of Wisconsin, but on account of the fact that prac-
tically all leaf-spots, regardless of actual cause, are commonly re-
ferred to as “rust” by growers, the term “ Wisconsin leaf-spot,” for
want of a better name, is used at present to distinguish it from other
rust spots. So far as known, this disease is limited to Wisconsin,
where it is fairly common in some years, but occurs only rarely
to a damaging extent.
The disease is in many respects similar to wild-fire, as far as
symptoms are concerned, except that these are less pronounced (PI.
VII, fig. 2). ‘The bleached area or halo surrounding the center of
infection is usually much narrower than that characteristic of wild-
fire spots. However, wild-fire spots also often appear without
marked halos surrounding them, and these are almost identical with
the Wisconsin leaf-spot. In such cases it is necessary usually to
resort to laboratory methods to distinguish the two. This can be
done readily, since wild-fire is caused by a white bacterium, whereas
a yellow bacterium causes Wisconsin leaf-spot.
‘Wisconsin leaf-spot also makes its first appearance in the seed
beds, where the spots resemble those of black-fire in that they are
usually quite small and more or less angular. In the field the disease
is usually confined to the bottom leaves, only under exceptionally
favorable conditions spreading as high upon the plant as wild-fire or
black-fire.
Cause.—Wisconsin leaf-spot is caused by an organism which has
been named Bactertwm melleum. Aside from its yellow or honeylike
appearance on some culture media, it resembles the wild-fire and
black-fire organisms closely in shape and size. Wounding is es-
pecially favorable for’ infection. The spread and overwintering of
the organism are most probably the same as for wild-fire.
Conditions favoring the disease-—The occurrence of the disease in
the seed bed has usually followed protracted periods of wet weather.
Its occurrence in the field is also dependent upon rainy weather, or
at least moisture and high humidity on and surrounding the leaves.
Storms no doubt favor the disease by spreading the germs and
wounding the leaves.
Control.—Very little experimental work has been done aiming at
the control of this disease in particular. Where control measures
need to be practiced the same measures that were described for wild-
fire control will apply.
FROG-EYE.
Description.—This leaf-spot disease of tobacco is fairly common,
though not often serous, in the southern tobacco- growing districts,
but as far as known it is rare or does not occur in the northern dis-
tricts. The term “ frog-eye” is unquestionably applied to various
leaf-spots similar in appearance but arising from different causes.
The description here applied, however, refers to one particular dis-
ease ordinarily recognized in liter ature as properly called frog-eye
because of the presence of the particular causal organism concerned.
This leaf-spot is characteristically located on the “bottom leaves of
the plant, especially toward their maturity. More or less circular
brownish spots, with a grayish white area in the center, upon which
TOBACCO DISEASES AND THEIR CONTROL. 31
commonly very small dark specks (spores) can be seen, are. typical
of the disease. Considerable variation in the shape, size, color, and
relative size of the colored areas occurs, however, and correct diag-
nosis of the disease is dependent upon the occurrence of the spores
of the causal organism on the spot (PI. IX, fig. 1). The diseased
spots ordinarily “do not break and fall out of the leaf in dry or
cured tobacco. While the disease is not usually regarded as serious
and in fact may be regarded as desirable in mild ‘form, yet in iso-
lated cases the damage “from it is reported by individual growers to
be relatively very high.
Cause.—The frog-eye disease of tobacco has not been carefully
studied, but sufficient work has been done to show that it is caused
by the fungus Cercospora nicotianae, to which it has long been
attributed. The long, narrow, septate spores appear in small
bunches on the surface of the central area of the spot and can readily
be seen with a small hand magnifying lens. Practically nothing is
known about the overwintering and spread of this particular dis-
ease, but from closely related diseases we know there is nothing un-
usual in overwintering in the spore form and subsequent dissemina-
tion from primary infections by wind and rain.
Conditions favoring the disease —Moisture is essential for the ger-
mination of the spores and subsequent leaf infection. Frog-eye is
especially likely to develop or spread following long periods of rainy
or damp weather, or even following heavy dews, which remain on the
basal leaves for long periods in the case of large tobacco. Mature
leaves appear to be more susceptible than unripe leaves, but no par-
ticular difference in varietal susceptibility seems to have been observed
in this country.
Control.—Practically nothing is known about satisfactory means
of controlling this disease, and since it occurs infrequently to a
serious extent no effort, so far as known, has been attempted in that
direction. While it seems hkely that the soil may be the common or
only source of infection in the spring, this is not established with
certainty. Until this fact is established recommendations for con-
trol practices can not be satisfactorily made. Where the disease oc-
curs on primed tobacco, something can be done in the case of heavy
outbreaks by attempting to keep ahead of the disease by harvesting,
but this is practicable, of course, to only a limited extent.
BLUE-MOLD.
Description—In March, 1921, there appeared in the seed beds of
the shade-growing district of Gadsden County, Fla., and Decatur
County, Ga., a disease not previously found on tobacco in this coun-
try. This disease proved to be one commonly called “ blue-mold ”
in Austraha and Malaysia, where it had often appeared as a more
or less serious trouble of tobacco. The disease spread very rapidly
in the seed beds in the Florida-Georgia section and for a time
seemed to threaten the industry, but fortunately it disappeared
rapidly, doing little or no damage in the field, owing, possibly, to a
change in weather conditions. The disease did not re appear in 1922,
although judging from the experience of other countries, it is not
unlikely that it may reappear at any time to a more harmful extent.
32 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
The signs of the disease are especially distinctive in one respect,
namely, in the appearance of the white to hght-purple or bluish
mildew, which is usually found on the lower “surface of the leaf.
The leaf tissue is broken down by the mildew and subsequently turns
brown and dries up in patches of irregular size and shape, frequently
limited by the veins of the leaf (PI. ie fig. 2). In severe cases the
seed bed as a whole has a patchy and scalded appearance. Only the
leaves appear to be attacked.
Cause-—Blue-mold is caused by a fungus belonging to the group
of downy mildews, some of which are well-known ‘parasites on other
crops. The tobacco mildew is known as Peronospora hyoscyami,
having been first found on black nightshade, a plant related to
tobacco. The fungus produces spores in great ‘numbers. These are
powdery and light and easily carried by the wind thr ough the beds,
which accounts for its rapid spread through beds when once infected
if favorable weather conditions prevail. These ordinary spores are,
however, very short lived, although other longer lived spores may be
produced under certain conditions and carry the fungus over un-
favorable periods, thus accounting for its overwintering when this
occurs.
Conditions favoring the disease —Parasites of this class are usually
very sensitive to environmental conditions and consequently are quite
sporadic in their occurrence. Moist warm weather and sometimes
alternating cool nights and warm days are most favorable for in-
fection. In the Florida- Georgia section the outbreak was seemingly
associated with several days “of heavy dew and fog in the morn-
ings. With the cessation of these dews and fogs the disease became
less conspicuous and finally disappeared entirely.
Control.—As soon as signs of this disease appear in a seed bed the
plants in the infected area, together with the surrounding bordering
plants, should be destroyed at once, preferably by applying form-
aldehyde solution (1 to 25). This should be followed up by burn-
ing over or other means of eradication. All the bed should then be
sprayed daily with Bordeaux mixture (2-2-50) in an attempt to hold
the disease in check, destroying with for maldehyde solution any new
infection which may appear.
In the event of failure to control the disease in the seed bed it is
inadvisable to transplant diseased plants to the fields if it is at all
possible to obtain healthy plants elsewhere. Although the disease
appears to be primarily a seed-bed trouble, it is not unlikely that
it may prove equally serious in the field, weather conditions being
favorable. In case of repeated occurrences of the disease, general
seed-bed and field sanitary measures, such as have been previously
described, should be undertaken, but until more experience with the
disease is had in this country, further recommendations can not be
given with any certainty as to their value.
OTHER PARASITIC LEAF-SPOT DISEASES.
Although a considerable number of other parasitic leaf-spot dis-
eases are reported from foreign countries, they do not seem to occur
in this country.
In the early nineties three different fungi were reported as causing
different leaf spots in North Carolina. A “brown rust” concentri-
ee
Bul. 1256, U. S. Dept. of Agriculture PLATE VII
Fic. |.—WILD-FIRE FROM PLANT BEDS
Transplanting should not be done from infested beds if it is at all possibie to get plants from
healthy beds
Fic. 2.—A BACTERIAL LEAF-SPOT
This disease is rarely serious and so far as known is practically limited to Wisconsin. It is
easily confused with wild-fire in some cases
PLATE VIII
Bul. 1256, U. S. Dept. of Agriculture
(OUITIOAT "CF “yy JO ASOVANO*D)) = *O.A-PTLA WTA
poyemosse Ajpensn oyey snondtdsuod oy} OAvY YoU Op A0T4
pue oly-p[IM JO osoy) UvYyy Ie~nsue o10ur ore sjyods oyL,
aisi4 wous ay¥l4-xov1g—'s ‘dI4
Te ee
(OM ‘Vy ‘1 JO Asoynog) *syods ory
-PpTIM se snonordsuod se ApiweuIpso you o1e sods 9,
sqaq daag Wous ayl4-xovIg—"| “SIs
ee ee ee a eae ay ae. tee ee ee eee a Ne ee eee
- - - ——
Sere OE
PLATE IX
Iture
Icu
256, U. S. Dept. of Agri
4
!
Bul.
u10}durAs poounouoid 4sour oy) ST
SOABO Pd}O0YR JO OpIsIopuN oy) UO MopTIUL Jo vouesead
oq} nq
‘
poonpoid 018 onsst} peep JO seyoyolq sepnBoerI]
OO0OVdOL AO AIOW-ANIG—'S ‘SIF
OSLOSTP OY} JO UOT}BOTpPUT
d1Ns B ST VLOdSOd.10,) JO Sa10ds dy} JO 9
ouosoid
oy} ynq ‘[wordAy oie syods SuNOA osoy}y JO [Le 40
OO0OVEOL AO 3AQ-S0"4—'|
‘SI
N
PLATE X
1256, U. S. Dept. of Agriculture
Bul.
[108 Jo sodA} ouLos UO 0d9RqO4 JO SOToLIwA UTe4.100
JO OLJstOJoVALYO Ssoy IO odouL sy Bury,ods Auonbo.ayur JON
LOdS-sAVAqT] OILISVHVdNON YW—'S “DIS
yeoy 944 JO UIsIeU pus sdiy 944 wor yorq ZurAp [er9ues @ Aq pozt1epRILYO St STILT,
OO0OVEO!, AO ,,ONIYI4,,—| ‘SIF
Bul. 1256,-U.-S. Dept. of Agriculture
NONPARASITIC LEAF-SPOTS
A great variety of spots occur on tobacco leaves which are believed to be
of nonparasitie origin
Bul. 1256, U. S. Dept. of Agriculture PLATE XII
NONPARASITIC LEAF INJURIES
Many forms of injuries of comparatively rare occurrence show in their distribution a more or
less definite relation to the position of the leaf veins
TOBACCO DISEASES AND THEIR CONTROL. 33
cally zonated was said to be due to Macrosporium longipes n. sp.: a
“ white-speck ” disease was attributed to Wacrosporium tabacinum n.
sp.; and an irregularly shaped spot, pale dirty brown, lighter in the
center, with a reddish zonate margin was attributed to Phyllosticta
nicotiana. No mention is made of inoculation experiments, and the
pathogenicity of these fungi is therefore questionable. So far as
known, they have not since been described on tobacco in America,
except for one recently reported occurrence of P. nicotiana in North
Carolina and a fungus, apparently Phyllosticta, on tobacco seedlings
in Virginia.
A recently reported disease which is most commonly found produc-
ing a leaf-spot in seed beds, where it may also cause damping-off,
but which may also apparently appear in the field, has been found
to be due to a species of Fusarium, closely resembling Fusariwm
afine. Exceptionally high moisture accompanied by some pre-
hminary weakening of the plant due to unfavorable conditions ap-
pears to favor infection. Only isolated cases are known, and the dis-
ease 1s not to be regarded as an important one.
NONPARASITIC LEAF-SPOTS.
Under this general term may be included a considerable number of
leaf-spots or injuries which differ considerably in appearance and
are evidently due to various causes arising from the effect of soil or
weather conditions on plant growth rather than following the at-
tack of any parasitic organism. No distinct line can be drawn be-
tween this class and other abnormalities of plants resulting from
malnutrition or other causes, but arbitrarily we may include those
diseases in which the early symptoms are mor or less characterized
by spotting of the leaves. This class of tobacco diseases has con-
siderable historical interest, since such diseases were apparently
among the earliest observed maladies of tobacco, or at least all early
observed spotting was attributed to nonparasitic influences.
Such terms as “ firing,” “specking,” “rusting,” and “spotting,”
with various modifications usually descriptive of the color of the
diseased areas, have been used from the earhest days of tobacco cul-
ture. Their origin has been laid to all sorts of causes, none of them,
however, with any satisfactory proof for the theories presented.
There can be no doubt that some of these diseases, such as frog-
eye, black-fire, and Wisconsin leaf-spot, were early included in the
list of diseases attributed to nonparasitic agents which are now
known to be due to specific organisms. The belief held from the
earliest time was that spotting of tobacco leaves was due to an excess
or deficiency of fertilizers or water in the soil or to a succession of
certain weather or atmospheric conditions. These theories are, of
- course, sufficiently broad so that some of them are doubtless respon-
sible for some forms of spotting, but they are not of much help
in explaining the frequently distinctive types of nonparasitic spot-
ting which are now known to exist and that usually can not be
| produced at will by subjecting the plants to variations of the above-
| named conditions. In view of this fact, control measures for many
of these forms of spotting are not at hand.
91061 °—24——3
34 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
From recent observational and experimental evidence it appears
that at least four main types of nonparasitic spotting exist on the
basis of causal agents, namely, those due to (1) a hereditary pre-
disposition toward spotting, (2) unbalanced nutrition, (3) absorp-
tion of toxic agents, (4) toxic agents applied externally. As illus-
trations of these groups the following may be cited: Under certain
conditions the Sumatra and Connecticut Broadleaf varieties com-
monly show spots when other varieties do not. Phosphorus deficiency
in the soil may result in ae Soils rendered toxic by steriliza-
tion or by the addition of certain salts may produce spotting of leaves.
Spraying with certain insecticides or fungicides produces spotting.
In all these cases, however, the weather conditions apparently affect
the occurrence of these spots to a greater or lesser extent.
Attempts to classify the nonparasitic tobacco leaf-spots occurring
naturally on the basis of symptoms with some consideration of their
origin are not usually satisfactory. A few more or less distinctive
types have been named, and the predisposing weather conditions are
at least partially understood.
Firing, as most properly used _now, refers to a drying up of the
basal leaves, usually starting at the leaf margins and proceeding i in-
ward between the veins, in blotches rather than spots (Pl. X, fig. 1).
This is conceded in most cases to result from dry weather, resulting
in the starvation of the plant as a whole for food and water and the
withdrawal of these from the basal leaves to supply the growing
point. A more or less sudden deficiency of food alone in the soil
in relatively wet weather may result similarly, especially on sandy
soils artificially fertilized and subject to leaching or otherwise losing
the reserve food supply. On the other hand, an excess of phosphoric
acid on certain soils is said to result in firing.
The true forms of nonparasitic spotting are usually sharply de-
marcated, round to angular in shape, varying from a considerable
size to mere specks (PI. X, fig. 2) and from almost black to white
in color (P]. XI). Others exist more or less as continuous lines or
circles showing marked relation to the veins (Pl. XIT).
Dead blotches in tobacco leaves sometimes follow after distinct
symptoms of specific malnutrition have existed for some time, as in
the case of potash and magnesium starvation, but these will be given
separate consideration, as ‘they are not regarded as typical leaf. -spot
troubles.
Leaf spotting sometimes results from physical causes, such as
bruising by rain, hail, or wind.
The control of the main forms of nonparasitic spotting is for the
present limited to avoiding varieties or strains, where possible, which
are predisposed to this injury, maintaining a proper balance of fer-
tilizers in the soil, or avoiding the use of soils which are not ame-
able to fertilizer treatment.
THE MOSAIC DISEASE.
Description.—Mosaic is one of the most common and widespread
diseases of tobacco. ‘The disease is often referred to by different
names in various districts, such terms as “ calico,” “mottling,” § * oray-
top,” and “mongrel” being often used. “ Frenching” is another
term commonly applied to mosaic, but this term properly used ap-
plies to a distinct though somewhat similar disease. Mosaic is not
ee)
TOBACCO DISEASES AND THEIR CONTROL. 35
ordinarily feared by growers, since it often occurs without doing ap-
preciable damage, although in some years, in certain districts par-
| ticularly, the losses from the
disease are very large both in
yield and quality.
The symptoms of mosaic in
tobacco are numerous, varying
from only faint signs to those
that are most distinct and pro-
nounced. The most common
and characteristic symptom is
mottling of the leaves—that is,
alternate dark green and yel-
lowish (chlorotic) areas irregu-
larly scattered over the leaf
surface, usually being most dis-
tinctive in the top or youngest
leaves, particularly in the early
stages of the disease (fig. 19).
The more pronounced symp-
toms are blistering, curling, and
distortion of the leaves, fol-
lowed by dwarfing of the entire
plant 1CPlgo WE Ps fiex «lye: in
older plants the flowers may
also become dwarfed, distorted,
and bleached. Under certain
weather conditions it is not un-
common for mosaic plants, even -
though showing only faint
symptoms in the leaves, to de-
velop more or less spotting, or-
dinarily referred to as “rust”
or “ firing,” but such cases are
by no means always a conse-
uence of mosaic (Pl. XIV).
On plants where the suckers
show characteristic mosaic
symptoms and the spotting is
not due to any other evident
cause, mosaic is usually respon-
sible. The losses from mosaic
follow as a consequence of the
reduced yield due to dwarfing
of the plants and the mottling
or abnormality of the leaves,
Fic. 19.—Tobacco mosaic. The mottled ap- which render them unsuitable
Pace emiste Sign of | for use for certain purposes of
manufacture.
Cause.—From the standpoint of science mosaic is one of the most
puzzling diseases of plants. Repeated efforts to demonstrate the
actual cause of this disease have so far failed. It is well known,
_ however, that the disease is infectious, being readily transmitted from
Ct
ee al
diseased to healthy plants in various ways. It practically follows
36 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
that some parasite must be concerned, which has so far escaped de-
tection either because of its extremely small size or because of other
peculiarities. Not only tobacco but a considerable number of other
plants are affected with mosaic identical with or closely similar to
that of tobacco. Tobacco mosaic can be transmitted to certain other
plants, particularly to some belonging to the same botanical family,
such as tomato and pepper. This fact is especially important, since
the ability of this disease to live on other plants may account
for overwintering in some cases. Experimental and observational
evidence in tobacco mosaic thus far indicates that the causal agent
does not lve over winter in the soil or in the seed. In the case
of mosaic of other plants it has been shown that the disease per-
sists over winter in the living underground parts of perennial
plants. Such a general carrying-over agent for tobacco mosaic has
not yet been demonstrated. On this principle, however, weeds in and
around tobacco beds should not be tolerated on farms where mosaic
is to be reckoned with as a serious disease. _
In this connection it should be stated here that probably all mosaic
of tobacco occurring in the field originates from seed-bed infection.
The signs of the disease in the bed usually are only very slightly vis-
ible, if at all, and usually occur only on isolated plants. From these
plants, however, it is readily transmitted to many other plants in
the operations of pulling and transplanting or by other means. The
importance of preventing seed beds from becoming infected is evi-
dent. Aside from the overwintering of the parasite in perennial
host plants and its subsequent transfer to plant beds, it should also
be stated that under some conditions of storage of infected material
it may survive for a year or more, so that overwintering on tobacco
-refuse, plant-bed covers, frames, or other equipments is not beyond
reason.
When infection has once occurred in the spring, its spread from
plant to plant may usually be accounted for. Aside from transmis-
sion during the numerous times the individual tobacco plants are
handled, it is likely that insects, especially plant lice, may spread the
disease under field conditions, as can be readily demonstrated ex-
perimentally. oe
Conditions favoring the disease—Tobacco mosaic is dependent
upon specific infection from other mosaic plants. Once inoculated,
plants are almost certain to show the disease at one stage or an-
other in their growth. Aside from the fact that weather conditions
influence the length of the incubation period—that is, the length
of time between inoculation and the first signs of the disease—their
influence does not seem to be important as affecting the occurrence of
mosaic. It is quite evident that tobacco mosaic is favored in develop-
ment by relatively high temperatures (80° to 85° F.), although tem-
peratures higher than this tend to retard its development until at
about 100° F. it is no longer active, and diseased leaves tend to re-
cover. Closely associated is the fact that rapidly growing plants
tend to show the symptoms earlier and more markedly, though sub-
sequent growth may be retarded. Rainfall and humidity do not seem
to have any such direct relation to infection and progress of mosaic
as to certain other leaf diseases. Where insects are important car-
riers of the disease, conditions affecting their occurrence and numbers
appear to have a direct bearing on the occurrence of mosaic.
TOBACCO DISEASES AND THEIR CONTROL. 37
Control—More facts in regard to the cause, overwintering, and
dissemination of mosaic must be ascertained before control measures
especially applicable to this disease can be assured. In general, how-
ever, the measures which are applicable fall under the head of seed-
bed sanitation. The location of seed beds a considerable distance
away from weedy patches of ground where the disease may be hiber-
nating in perennial plants, soil sterilization to kill weed seeds or any
other possiblé infective material in the soil, and persistent destruc-
tion of solanaceous weeds around seed beds and in other locations are
recommended. Plant-bed frames or covers previously used with beds
in which mosaic plants appeared should not. be again used without
sterilization. Special precautions should be taken to prevent the
introduction into the seed bed of any trash from the preceding
year’s crop. Before planting, the beds should be carefully examined
for mosaic plants, and even if only a very slight infection is notice-
able it is advisable not to use any plants from such beds if good
plants can be obtained elsewhere. Where infected plants appear to
be numerous in areas of the bed, it 1s practically certain that. a poor
crop will result from transplanting from such beds. Infection may
occur in the seed bed, however, without being noticeable. If it is
found that the symptoms show up in the field early in the season on
a large percentage of the plants, it is usually advisable to disk up
and replant. No possible means of controlling the disease in the
field are known. On the other hand, the disease does not ordinarily
spread in the field early enough to cause any considerable damage.
FRENCHING.
Description—tThe disease of tobacco commonly known as french-
ing occurs in practically all tobacco districts of the United States
and in many foreign countries. It is often referred to under other
names, such as shoestring, sword-leaves, strap-leaves, rosette, and
polyphyllie. The disease is often mistaken for the mosaic disease and
confused with it on account of the leaf deformities and chlorosis pro-
duced which are also characteristic of mosaic. There is no relation
between the two diseases, however, from the standpoint of cause, as
sometimes seems possible when the two diseases occur simultaneously
on the same plant.
The symptoms of frenching vary greatly in their intensity of ex-
pression. The earliest stages of the disease, especially in very young
leaves, are characterized only by a yellowish or chlorotic appearance,
with some increased thickness and brittleness of leaves. As the
erowth of these leaves continues they will show an early tendency
toward narrowness and mottling. Later, the margins of the leaves
may curl downward and the younger leaves in the bud increase to
abnormally high numbers, though being usually closely bunched.
In later stages of the disease the leaves are frequently normal in
color but extremely narrow in proportion to their length, quite thick,
and ruffled at the margin (fig. 20). In the most severe cases the
number of these stringy leaves becomes extraordinarily high, often
reaching more than five times the normal number on the stalk, which
is usually considerably shorter than normal, hence producing a
distinctive rosette appearance. More commonly, however, the disease
is limited to chlorosis of the top leaves. The losses from this disease
,
:
r)
:
h
|
38 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
are not extremely high, although it is at times quite an important
factor in crop production (fig. 21).
Cause.—The actual cause of frenching is not yet known. Al-
though a number of predisposing factors have been observed, the
production of this disease at will, as far as known, has not been
repeatedly accomplished. Frenching is apparently not infectious;
consequently there is no reason to believe that any foreign organism
Iie, 20.—A frenched tobacco plant. The numerous shoestringy leaves are typical.
This disease is often confused with mosaic.
is associated with it. The disease is frequently found on soils ex-
posed in one way or another to excessive water. This, together with
its occurrence at times on portions of fields in poor tilth, indicates
a relation to soil aeration. On the other hand, frenching seems to
have considerable relation to soil fertility, or at least to a deficiency
of certain fertilizing elements, but in combination with these factors
certain weather conditions must apparently exist to bring about its
expression.
TOBACCO DISEASES AND THEIR CONTROL. aa
Control.—_Measures to prevent this disease are not as yet satisfac-
torily worked out. Land likely to yield the disease should not be
used for tobacco. If such soils must be used, it is probable that
proper drainage, subsoiling, and cultivation conducive to good tilth
together with well-balanced fertilization, will be very useful in
preventing it.
.
SAND-DROWN.
Description.—A. blanching or chlorosis, especially of the lower
leaves of tobacco plants grown on very sandy soils during seasons of
excessive rainfall, has long been known as sand-drown. This disease
occurs particularly in the tobacco districts of North Carolina, but
also may be found in the sandy soils of other States, as in the Con-
necticut Valley. This trouble is most likely to be confused with
another nutritional symptom fairly common in tobacco and known
Fig. 21.—A badly frenched field of tobacco. This disease is commonly characteristic
only on certain types of soil or under certain soil conditions.
as potash hunger or potash starvation. Potash hunger may occur,
however, on almost any type of soil and under quite variable condi-
tions of rainfall. The blanching in potash hunger is usually more
localized in small areas, quite yellow in color, and sometimes over-
cast with bronze or copper colors, together with crinkling or curl-
ing of the leaves. Sand-drown is characterized, on the other hand,
by more or less complete and uniform blanching until the leaf
is practically white in late stages except for the midrib and veins,
which retain their color (Pl. XV). The blanching usually starts
at the tip of basal leaves, proceeding downward along the margin
and inward between the veins toward the midribs. Crinkling and
curling are not common, but occasionally local areas die out, as in
the case of potash hunger or leaf-spot diseases.
Cause.—Sand-drown has recently been shown to be due to a short-
age of magnesium in the soil or in the fertilizer used. Most soils
40 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
naturally contain sufficient magnesium for the plant’s needs, or it
is supplied in the ordinary system of fertilization practiced. On
certain sandy soils low in natural fertility and supphed with artifi-
cial fertilizers containing little or no magnesium, the deficiency of
this element becomes evident in seasons of considerable rainfall
when the small supply of available magnesium may be leached away.
Control—The use of so-called high-grade or relatively pure
mineral commercial fertilizers, especially the high-grade potash salts,
to the exclusion of low-grade forms or organic forms (originating
from vegetable or animal matter) favors the appearance of sand-
drown under the conditions described. The addition of sulphate or
chlorid of magnesium to the fertilizer salts in relatively small quan-
tities prevents the chlorosis. Dolomitic limestone (high in magne-
sium) and fertilizers of vegetable origin, as cottonseed meal, tobacco
stalks and stems, and barnyard manure, tend to prevent the disease.
A relatively small amount of magnesium is apparently sufficient to
remedy sand-drown, even in striking cases of deficiency, less than 50
pounds per acre usually being sufficient.
POTASH STARVATION.
The lack of sufficient potash in a soil is often characteristically indi-
cated by tobacco plants grown in such soils, especially if other fer-
tilizing elements are not lacking. Aside from a general stunted ap-
pearance, the leaves become crinkled or rough and turn downward at
the margin. Yellowing usually starts at the leaf tips, becomes most
marked along the margin, but proceeds inward between the veins in
blotches, without sharp demarcation. A bronze or copper tinge over
parts of the leaf, surface is also commonly associated with the yel-
lowing. Smaller yellow spots may develop, which finally turn brown
or grayish white, and the whole leaf may finally dry up. Specking
or spotting may also be characteristically associated with these symp-
toms of potash hunger. Leaves deficient in potash are frequently
brittle both in the green and cured condition (PI. I, fig. 2).
If symptoms of potash starvation are noted sufficiently early, some
benefit may come from a liberal application of a readily available
potash salt between the rows of the growing crop. The fertilizer
formula for the following year’s crop should contain both some
readily available and some slowly available potash.
OTHER MALNUTRITION SYMPTOMS.
A deficiency of other elements in the soil is not so characteristically
indicated as in the case of potash and magnesium. The lack of nitro-
gen is indicated in most plants by a light-green to yellow color and
stunting. This condition is brought about as well, however, by a
number of other variable factors, such as deficiency or excess of wa-
ter, poor tilth, or shortage of light, so that not much reliance can be
placed upon it. Excess of nitrogen tends to produce a large dark-
green, heavy-bodied, coarse leaf usually undesirable except in dark
export types of tobacco. |
In case of phosphoric-acid deficiency the plants may remain _nor-
mally green or even turn darker green than ordinarily, particularly
in the case of young plants. In case of marked deficiency of this ele-
i
é
TOBACCO DISEASES AND THEIR CONTROL. 41
ment brown leaf spots may develop (Pl. XIII, fig. 2). Marked
stunting, with yellowing, may eventually follow.
Bertone attributing the above- described symptoms to nitrogen or
phosphoric-acid starvation the roots of the plants should be care-
fully examined for root injury by the root parasites previously de-
scribed. At the same time the weather and soil conditions in gen-
eral must be taken into account. Chemical analysis of the soil may
aid in determining which element is lacking, but trial application of
fertilizing elements is the most reliable method to determine the
fertilizer needs of any particular soil.
INJURIES DUE TO PHYSICAL AGENCIES.
The tobacco plant is subject ee a number of injuries resulting from
the action of physical agencies which may be grouped together.
These deserve brief consideration under the subject of diseases, be-
cause they may sometimes be mistaken for plant diseases.
Injuries of this nature which frequently appear on tobacco are
rain-spot, hail cuts or spots, frosted or frozen tobacco, sun-scalded
lig. 22.—Lightning injury to tobacco. This apparently diseased area was produced
by lightning striking the field.
and sunburned tobacco, wind whipping, bruises from handling,
spray injury, sand beating, and lightning damage (fig. 22; Pl. ITI,
fig. 2). To these might also be added the large group of insect in-
juries, some of which are sometimes mistaken for disease injury.
Most injuries of the above-named types soon after taking place are
ordinarily too evident to warrant description (fig. 23). Some time
after the damage has occurred, however, and especially when affected
leaves have been cured or fermented, considerable difference of opin-
ion may arise as to the source of the damage found.
UNIMPORTANT OR RARE DISEASES NOT OTHERWISE CLASSIFIED.
A considerable number of abnormalities occur in rare or isolated
cases in tobacco fields, regarding which little is known as to causal
relations, since sufficient importance is not attached to such cases to
warrant investigation. Some of these diseases have not been de-
scribed at all or not more than once in the literature, so that com-
parisons are difficult to make.
One of the more common abnormalities of this nature is sometimes
known as marbling or variegation, but in foreign countries goes un-
49 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
der a wide variety of names, panachure being especially used. This
is a condition of the leaf in which large bands or irregular areas of
the leaves of isolated plants lose their green color, becoming yellow
or distinctly white and sharply demarcated from the oreen areas.
This condition is not infectious, but is apparently hereditary. More
rarely the entire leaf or part of the plant turns yellow or white, when
it is spoken of as albinism or chlorosis.
A form of curly-dwartf is not unknown in tobacco, but is very rare
in this country. In the East Indies and Europe, similar diseases are
described as krulzieke and faltenzwerg. In this disease the leaves
become markedly stunted and curled. The cause is unknown.
A streak disease of tobacco apparently of nonparasitic origin has
been occasionally noted. A narrow brown line usually extends the
Fic. 23.—Damage to tobacco from hail and storm. Various degrees of injury result
from this cause. An extreme case of injury to shade-grown tobacco is here shown.
full length of the stalk and into the leaves, following the vascular
system and causing wilt and death of the affected leaves. It has been
seen most commonly on White Burley tobacco.
A trouble known as crookneck has been described from North Caro-
lina. The plants and leaves are twisted and deformed. The stem
and leaves on one side of the plant are usually affected and die or
fail to grow, while the growth of the other parts causes the twisting
and malformation observed, much as in the case of lightning injury.
Monstrosities or teratological phenomena are sometimes observed in
tobacco as in other plants (fie. 24). ‘These are usually characterized
by fasciations, as in the doubling of stalks, midribs, veins, or leaf
blades, or in the splitting up of one leaf into two or more apparently
distinct parts.
Crown-gall caused by Bacterium tumefaciens, a fairly common
disease on some of the fruits, produces striking tumors on the to-
bacco plant by artificial inoculation. As far as known, however, this
disease does not occur naturally upon tobacco. :
TOBACCO DISEASES AND THEIR CONTROL. 43
DAMAGE IN CURING AND FERMENTATION.
The diseases of tobacco previously described have been concerned
with maladies affecting the living plant and are either of a parasitic
or nohparasitic origin. Following harvest, the tobacco leaves must
first go through a curing process which consists essentially of a more
or less rapid drying and consequently death of the leaf tissue itself.
The conditions of curing, fermentation, and storage of the various
types of tobacco must be such as are most conducive to the proper
color, body, texture, elasticity, grain, burn, and other factors which
make up quality for the individual types. Any distinctive abnor-
malities from the desired result in these processes are naturally of
as much concern as abnormalities of the growing plant and conse-
quently must receive at-
tention as to cause and
control measures. For the
most part the damage in
curing, fermentation, and
storage is the result of the
action of undesirable mi-
croorganisms under con-
ditions favorable for
their development. These
organisms belong to the
class of saprophytes rather
than the parasites, since
they act ordinarily on
dead tissues only, in
which they tend to pro-
duce decay. In this con-
nection it is well to re-
member that the decay
and final disappearance
of all organic matter is
dependent upon the ac-
tion of such organisms
Fig. 24.—-Jack-in-the-pulpit effect of a leaf growth in and that any method
eas A Ooeeen Dione uetratng rare phe; hich eliminates | their
presence completely, such
as sterilization, or which renders conditions unfavorable for their
development can be relied upon to preserve most such materials in-
definitely.
The most common forms of damage in tobacco following harvest
are shed-burn or pole-rot, stem-rot, wet-butts (or fat-stem), white
veins, black-rot, must, and molds. These injuries will be taken up in
the approximate order of their occurrence.
SHED-BURN OR POLE-ROT.
Description.—This damage is also known under the name of pole-
burn and pole-sweat. It is characterized in mild cases by the darken-
ing and the tendency of the leaf to dry out rapidly and to become
harsh and brittle, together with loss of elastic strength. In more
severe cases the body of the leaf is actually decayed, so that affected
spots appear dark by reflected light but transmit hght more readily
ee ae
Sr eR 8 ey
44 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE,
than the normal leaf. As the decay proceeds these spots run together,
and the infected areas break up and fall out at the least disturbance
(Pl. XVI, fig. 1). This trouble is most common in tobacco which is air
cured rather than in any system of curing using more or less. heat.
Under favorable conditions for its occurrence large losses may result.
Cause.—It seems fairly well established that at least some of the
symptoms described for shed-burn are due to microorganisms.
There is not good agreement, however, as to just which organism or
organisms are primarily concerned in all cases. The most recent
experimental evidence seems to indicate that several fungi are cap-
able of damaging the leaf in a similar manner. Species of Alter-
naria, Fusarium, Botrytis, and other fungi and bacteria are prob-
ably most instrumental in actual decay.
Conditions favoring attack—Tobacco leaves ordinarily are not at-
tacked by these organisms while they are still in the green or yellow
stage of curing. Normally, in the air-curing process no danger of
damage exists until some time after harvest, or when the leaves are
turning from the yellow color to the final stage. At this stage any-
thing that is conducive to excessively high humidity may bring
about shed-burn, especially if at the same time the temperature is
quite high. The ideal conditions for air curing itself are a relatively
high temperature (80° to 100° F.) together with a relatively high
humidity (80 to 90 per cent). When the relative humidity rises
above 95 per cent, however, there is considerable danger, especially
if little or no circulation of air prevails. Under such conditions
little pockets of air form between closely hanging leaves and between
leaves and stalks in which the actual humidity reaches saturation, the
leaf surface becoming actually moist through precipitation of moist-
ure from the air or by excretions from the leaf which are not carried
off, thus producing a favorable medium for the growth of micro-
organisms which can enter the leaf tissue and consequently damage
the leaf. It follows, therefore, that the close hanging of large
tobacco or an improperly ventilated shed are conducive to shed-
burn. Foggy or damp weather at the critical stage of curing, es-
pecially if it persists for several days with a moderate temperature,
should be guarded against by control measures.
Control_—The principles of control of shed damage are primarily
concerned with proper ventilation to secure adequate circulation of
air throughout the shed or raising the temperature within the shed
by artificial heat, thereby reducing the relative humidity of the air.
The former is dependent upon the proper construction of the shed,
together with proper management of the ventilating system.
The quantity of moisture the air can hold varies greatly with the
temperature. The ratio of the amount of moisture actually in the
atmosphere at any time to the amount which can exist in the air
at that temperature is known as relative humidity. The capacity of
the air for holding moisture is doubled for every increase of 18°
to 20° F. in temperature, and consequently its relative humidity
is reduced by half. Under normal conditions the temperature of
the curing shed rises in the daytime and the relative humidity falls.
As the temperature falls at night the relative humidity rises often
above 95 per cent but again falls the following day. During rainy
or foggy weather the relative humidity, of course, remains high,
PLATE XII1
Bul. 1256, U. S. Dept. of Agriculture
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PLATE XIV
U. S. Dept. of Agriculture
Bul. 1256,
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PLATE XV
1256, U. S. Dept. of Agriculture
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PLATE XVII
Dept. of Agriculture
Bialet256; Ul. 'S.
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TOBACCO DISEASES AND THEIR CONTROL. 45
and if this is maintained for comparatively long periods, especially
in relatively warm weather, with tobacco at the danger point in
curing, damage is likely to occur unless the relative humidity is
lowered by raising the temperature in the shed by artificial heat.
Heat is usually applied as open fires under the tobacco. A num-
ber of small fires uniformly distributed is preferable to a few large
fires. Frequently small pits are dug in the ground in which the
fire is placed. Small deflectors above the fire are also desirable.
Charcoal, very dry wood, or corncobs may be used, but coal or
coke, or material giving dense smoke should not be used with most
types of tobacco. The shed is usually tightly closed, except for roof
ventilators, which may be left partly open. The length of time fires
are kept up will depend on weather conditions, but usually 12 to 24
hours will be sufficient to prevent damage.
STEM-ROT OF CURING TOBACCO.
In the curing process the midribs or stems of the leaves and the
large veins are usually the last to dry or cure out, owing to their
thickness. Under normal conditions the moisture given off by these
tissues passes off into the air immediately on reaching the surface,
but if high relative humidity prevails they become soggy and are
therefore a favorable medium for the growth of saprophytic micro-
organisms (Pl. XVI, fig. 2, B and C). This condition does not
usually come about until the leaf web itself is quite fully cured
out and the greatest danger from shed-burn is over. The midrib
has then turned from the normal color, indicating actual death of
the tissues. At times, however, the veins decay before the leaf web
loses its green color. The saprophytic organisms concerned may be
the same as those associated with shed-burn, although this is not fully
established. Species of Fusarium, Tricothecium, Botrytis, and other
fungi are at least frequently present in dense masses of white and
pink growths, often followed in bad cases by bacterial decay.
Conditions favoring stem-rot and its control are the same as for
shed-burn.
WET-BUTTS, OR FAT-STEM.
The term “ wet-butts” refers to a wet, soggy, and discolored con-
dition of the base of the midrib and the veins, but sometimes includ-
ing the whole leaf, resulting from the failure of the leaf to cure
properly (Pl. XVI, fig. 2, 4). As far as known, no careful investi-
gation of this trouble has been conducted, but from observation it
seems possible that it is due in some cases to freezing weather before
the stem is fully cured. This is borne out by the fact that it 1s
the top leaves, generally in late green tobacco, which are usually af-
fected in this manner. In stripping and sorting, the affected leaves
are separated and allowed to dry out before being packed, when
they are placed in the poorer grades of tobacco.
When a large percentage of wet-butts is likely to occur in a crop
of tobacco it is a good plan to hasten curing with artificial heat
before freezing weather.
WHITE VEINS.
Occasionally during curing a whitened appearance of the veins
of the leaf arises, making it undesirable for cigar-wrapper putr-
\
46 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
poses. This condition has been attributed in some southern dis-
tricts to the action of an insect (thrips) feeding on the veins of
the leaf in the field, with the result that when cured these become
much lighter in color than the remainder of the leaf.
In the northern districts white veins appear occasionally, ap-
parently as a result of excessively rapid curing of the leaf during
dry weather. It is believed that the surface ‘layers of the veins,
drying out more rapidly than the deeper layers of tissue, permit a
thin layer of air to form between the two layers, thus causing the
veins to appear white. In any case it is a good plan to keep up
the humidity of the shed in dry weather by keeping the shed closed
in the daytime, opening it partly at night, and occasionally apply-
ing water to the ground in the shed to raise the humidity of the
inside air.
BLACK-ROT IN FERMENTATION.
Description.—F ollowing curing, tobacco is ordinarily graded and
bulked in various ways, depending upon the type, when it is sufli-
ciently pliable for handling without breakage. Leaf tobacco of
manufacturing types is usually “reordered” to bring it to the
proper moisture content before storage, and damage rarely occurs.
Leaf tobacco of cigar types, however, is usually allowed to come
into higher “ case’ before being removed from the curing barn, with
the result that it often contains a high percentage of moisture, and,
moreover, it is not “reordered” before packing in boxes for fer-
mentation or sweating.
Black-rot is ordinarily found, then, in boxed cigar types of to-
bacco. It usually appears as a dark-brown or black discoloration
of the packed tobacco, which varies from only small insignificant
local areas on a few leaves to almost complete ruin of the entire lot.
Commonly the affected area is more or less limited to the tip ends
of the leaf where they overlap in packing. The affected tissue is
ordinarily characterized by a dry rot, and the body, texture, and
elasticity of the leaf no longer exist, and it easily breaks up when
disturbed (Pl. XVII, fig. 1). Commonly a sooty appearance is pres-
ent, as a result of the production of large numbers of spores by the
organism causing the decay. The line of demarcation between rotted
and sound areas is frequently quite sharp, and the sound portions of
the leaf are still of value for cigar purposes if large enough to war-
rant their separation. Sometimes black-rot appears in bundled
tobacco before sorting. The loss from this trouble is frequently
quite large in the packing houses of northern cigar-tobacco districts.
Cause-—Black-rot has been found to be due to a fungus common
in nature, Sterigmatocystis nigra, more often called “Aspergillus
niger. This fungus is peculiar in that it thrives best at relatively
high temperatures (around 100° F.), such as exist in fermenting to-
bacco. It can also grow, of course, at considerably lower and slightly
higher temperatures.
The fungus produces spores abundantly, which can remain dor-
mant for a long time, are readily ¢ ‘arried about in the air, and are
quite common in tobacco warehouses. When these spores are present
on tobacco they can germinate under favorable conditions and pro-
luce decay of leaf tissue in a few days.
TOBACCO DISEASES AND THEIR CONTROL. 47
Conditions favoring attack.—The extent of damage from black-rot
will ordinarily vary with the number and distr ibution of the spores
of the causal organism present and the favorableness of the condi-
tions for their development. Moisture is naturally essential for
growth, and as a matter of practical importance the percentage of
water present in the fermenting tobacco is the most important consid-
eration in damage from black- ‘vot. The high temperatures produced
by fermentation, together with a relatively high moisture content of
the leaf, make an especially favorable condition for black-rot to
develop. It has been found that black-rot can not occur above 110°
F., since the rot fungus can no longer grow above that temperature.
Control. — The control of black-rot must center around either
reducing the moisture in tobacco to an amount necessary only for
proper fermentation or raising the temperature of fermentation as
rapidly as possible to around 110° F. or above. With the object
of keeping the moisture content down, growers should be advised
not to permit tobacco to come into too high case when taken down
in the shed. When high-case crops are received in the warehouses
they should preferably be assorted and packed as late in the sorting |
season as possible or bulk fermentation carried out before packing
in cases. Bulk fermentation usually permits temperatures as high
as 110° to 120° F. being reached in a few days, hence is unfavorable
for black-rot, and the accompanying turning of the warm tobacco in
the bulk is especially favorable for reducing its percentage of mois-
ture before packing in boxes. The number of rebulkings should
depend somewhat, therefore, on the amount of moisture in the to-
bacco, in order that it may be brought down to a desirable moisture
content before being packed in storage.
The alternative of packing directly into the boxes and storing at
once in room temperatures as high as 110° F. suggests itself. This
method has not been satisfactorily tested, as far as known. Earlier
methods of force sweating in boxes were usually at lower tempera-
tures and not satisfactory. Finally, it is not at all improbable that
cigar tobaccos may be passed through drying or “ reordering” ma-
chines, which will not only reduce danger of damage but tend to
produce a more uniform product in general.
MUSTS AND MOLDS.
True tobacco musts are characterized by a white powderlike growth
over the surface of the leaves during fermentation or storage (PI.
XVII, fig. 2). No true decay occurs, nor is the leaf ordinarily
injur ed in any way excepting in appearance and in aroma. A musty
odor accompanies this damage, which it is practically impossible to
remove satisfactorily, and consequently affected tobacco is much
reduced in value. This trouble is fairly common upon fermenting
or stored cigar tobaccos. It does not ordinarily seem to be asso-
ciated with black-rot in the same packings, and consequently the
conditions favoring the two are apparently different in one or more
respects. Practical observations indicate that when certain types
of tobacco he for several weeks in boxes before going into active
fermentation, mustiness is likely to appear. Similarly, mustiness
may develop following fermentation in storage. Mustiness is not
favored particularly by the high temperatures of fermentation nor
48 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE.
necessarily by high moisture content of the tobacco, although the
relation of moisture is not yet fully understood. The character of
the tobacco leaf itself, as regards various factors resulting from
growth and method of handling, probably plays some part in its
predisposition to musting. The presence of air in packed tobacco
also seems to favor musting, so that loosely packed leaf seems more
likely to become affected, especially if the humidity in the warehouse
is high.
True musts are caused by a group of extremely small organisms
of the fungous type. More than one species seem to be involved.
Oospora nicotianue oY ver y similar species and in some cases Actino-
myces are responsible. Some of these organisms grown on tobacco
or other media have an extremely musty odor.
Molds due to other fungi are also found on tobacco, but less
commonly and usually only under exceptionally undesirable condi-
tions of handling and storage. A condition sometimes mistaken
for must or mold on tobacco is the excretion of a white salt (usually
saltpeter) on the midrib and veins of the leaf. This is of a less
harmful nature than musts or molds and is readily brushed off
without injurious odors remaining. This condition apparently re-
sults when the leaves are subjected to more or less rapid drying
under certain conditions. The character of the soil upon whieh
the leaf is grown seems to influence the occurrence of saltpeter to
a considerable extent, and it is more or less characteristic on tobacco
of certain districts.
The prevention of musting of tobacco is not satisfactorily under-
stood. The avoidance of conditions previously mentioned as fav-
orable to it only can be advised. When tobacco has become musted
some packers resort to brushing off the musty color, redampening
the leaf with water or acetic-acid (vinegar) solution (about 4 per
cent), and repacking with the hope that a second fermentation may
be started and the undesirable odor partly removed.
DISEASES OF TOBACCO IN FOREIGN COUNTRIES.
Tobacco is grown under a wide range of conditions in various
parts of the world, although more than one-third of the total world
production is in the United States. The other principal countries
growing tobacco are India, Russia, Sumatra, Java, Hungary,
Japan, the Philippine Islands, Germany, Cuba, France, Italy, Tur-
key, Brazil, and South Africa. To this list may be added Mexico,
Porto Rico, Canada, China, Rumania, Serbia, Bulgaria, Greece,
Belgium, and several other countries growing more than 5,000 acres.
The diseases of tobacco which occur in these countries are of some
interest to the producer in the United States, since with increasingly
close commercial relations the importation of new and dangerous
parasites is not at all unlikely. It seems quite probable that at
least some of our diseases of tobacco have been introduced from
foreign countries. A few diseases described in foreign literature
are not yet known to exist in this country, and these should espe-
cially be guarded against.
Many of the foreign countries have investigated their plant dis-
eases in little or no detail, so that serious troubles may exist about
whiclt we at present know nothing. It is also true that diseases
TOBACCO DISEASES AND THEIR CONTROL. 49
which are not serious in their native country may be much more so
when introduced into other countries with .different climatic condi-
tions prevailing.
Tobacco diseases have been reported on in most detail from the
Dutch East Indies, Italy, India, Dalmatia, Russia, France, and
Germany. In general, foreign countries have most of the common
diseases of tobacco which occur in the United States, although their
occurrence as a serious problem is naturally very variable, as it is in
the tobacco districts of the United States. It does not appear from
the literature that the recently described bacterial leaf-spots of
tobacco in this country exist elsewhere, with the exception of the
possible occurrence of wild-fire in South Africa.
The diseases of tobacco are naturally described in foreign litera-
ture under a wide variety of names, and with the unsatisfactory de-
scription and diagnosis presented in many cases it is difficult to
ascertain the relationship between the diseases as described from
different districts. In the Dutch East Indies the bacterial wilt
disease due to Bacterium solanacearum and the lanasziekte caused
by Phytophthora nicotianae (our black-shank) seem to have been
given most consideration. In Italy root-rot due to Thielavia basicola
is said to be serious. Leaf-spot diseases are world-wide and have ~-
been attributed to a number of different parasites, many of which are
not known in this country. Mosaic and frenching are common, as
are a number of minor diseases attributed to nonparasitic agents or
of unknown cause. Broom rape due to various species of Orobanche
are common in southeastern Europe, as are also species of Cuscuta
(dodders).
Among the more common fungous diseases not yet known to occur
in this country is the powdery mildew due to Erysiphe to which
at least five specific names have been given, though all probably are
identical. Two species of Cercospora are reported, at least one of
which is probably our American species. Alternaria tenuis Nees
is frequently reported on tobacco in Europe, causing various types of
troubles from seed bed to curing house, as are also various species
of Sclerotinia. Parasitic species of Phyllosticta, Ascochyta, and
Septoria on tobacco are apparently more common in Europe than in
America, but none are probably of much economic importance.
Ascochyta and Septoria have not been reported on tobacco in this
country. A species of Olpidium is reported as causing a root disease
of tobacco in Dalmatia, and two new species of Pythium are re-
ported elsewhere as causing seed-bed troubles. On curing and fer-
menting tobacco, species of Botrytis, Penicillium, Aspergillus, Alter-
naria, Cladisporium, Sporidesmium, Pleospora, Sclerotinia, and
Mucor are reported, but in most cases association of these with any
of the damage occurring in these processes has not been definitely
established. Oospora nicotianae is described as causing “ must ”
in Italy.
Several diseases due to bacterial parasites have been described,
particularly in France, but the relation of the organisms described
to these diseases is still open to question. In the East Indies a
bacterial leaf-spot (black rust) due to a bacterium (4. pseudozool-
loeae Hon.) has been shown to exist. Sacillus amylobacter Van
Teigh. is said to cause a bed rot in Italy.
gidg12 94" 4
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