Downy mildew of tobacco

Bulletin 405 Library. December, 1937 ^^^C^

Downy Mildew of Tobacco

P.J. ANDERSON

Agricultural Jxperim^ut ^tattou

CONNECTICUT AGRICULTURAL EXPERIMENT STATION

BOARD OF CONTROL

His Excellency, Governor Wilbur L. Cross, ex-officio, President

Elijah Rogers, Vice-President Southington

Edward C. Schneider, Secretary Middletown

William L. Slate, Treasurer New Haven

Joseph W. Alsop Avon

Cheirles G. Morris Newtown

Albert B. Plant Branford

Olcott F. King South Windsor

AdministratioD

STAFF

WiixiAM L. Slate, B.Sc, Director

Louise M. Braxttlecht, Chief Clerk and Librarian

Katherine M. Palmer, B.Litt., Editor

G. E. Graham, In Charge of Buildings and Grounds

Analytical
Chemistry

E. M. Bailey, Ph.D., Chemist in Charge

C. E. Shepard 1

Owen L. Nolan

Harry J. Fisher, Ph.D. \ Assistant Chemists

W. T. Mathis

David C. Walden, B.S. J

Rebecca B. Htjbbell, Ph.D., Assistant Biochemist

Janetha Shepard, 1 General Assistants

V. P. Ryan, J

Chas. W. Soderberg, Laboratory Assistant

V. L. Churchill, Sampling Agent

Mabel B. Vosburgh, Secretary

Biochemistry

H. B. Vickery, Ph.D., Biochemist in Charge

George W. Pucher, Ph.D., Assistant Biochemist

L. S. Nolan, \ General Assistants

T. P. Stickney /

J. Datillo, Laboratory Assistant

Botany

E. M. Stoddard, B.S., Pomologist, {Acting Botanist in Charge)
Florence A. McCormick, Ph.D., Pathologist
A. A. Dunlap, Ph.D., Assistant Mycologist
A. D. McDonnell, General Assistant

Entomology

W. E. Britton, Ph.D., D.Sc, Entomologist in Charge, State Entomologist

B. H. Walden, B.Agr. 1

M. P. Zappe, B.S. I

Philip Garman, Ph.D. > Assistant Entomologists

Roger B. Friend, Ph.D.

Neely Turner, M.A. J

John T. Ashworth, Deputy in Charge of Gypsy Moth Control

R. C. Botsford, Deputy in Charge of Mosquito Elimination

J. P. Johnson, B.S., Deputy in Charge of Japanese Beetle Control

Helen A. HuLSE ) Secretaries

Betty Scoville I

Forestry

Walter O. Filley, Forester in Charge

H. W. HicocK, M.F., Assistant Forester

J. E. Riley, Jr., M.F., In Charge of Blister Rust Control*

Pauline A. Merchant, Secretary

Plant Breeding

Donald F. Jones, Sc.D., Geneticist in Charge

W. Ralph Singleton, Sc.D. 1 Assistant Geneticists

Lawrence Curtis, B.S., J

Elizabeth Williams, B.S , Research Assistant

Mildred H. Preston, Secretary

Soils

M. F. Morgan, Ph.D., Agronomist in Charge

H. G. M. Jacobson M^., 1 Assistant Agronomists

Herbert A. Lunt, Ph.D., J

DwiGHT B. Downs, General Assistant

Geraldine Everett, Secretary

Tobacco Substation
at Windsor

Paul J. Anderson, Ph.D., Pathologist in Charge

T. R. SwANBACK, M.S., Agronomist

O. E. Street, Ph.D., Plant Physiologist

C. E. SwANSON, Laboratory Technician

Dorothy Lenard, Secretary

* In cooperation with the U. S. D. A

Printing by The Peiper Press, Inc., WaUingford, Conn.

CONTENTS

History 65

Name of the Disease 67

Symptoms 67

In the seedbed 67

In the field 70

Causal, Parasite 71

Oospores 73

Name of the Fungus 73

Other Host Plants 74

Source of Infection in the Spring 74

In the seed 74

In perennial hosts 74

Mycelium or summer spores in the soil 75

Winter spores 75

Summer spores blown from states to the south 75

How THE Weather Affects Mildew 75

Temperature 75

Moisture 76

Sunshine 76

Wind , 76

Prevention and Control 76

Cultural practices 77

Spraying the seedbeds with fungicides 78

Vapor treatment 79

Figure 1. A healthy bed of plants contrasted with a diseased bed.
from opposite ends of the same bed.

Photographs taken

D<

DOWNY MILDEW OF TOBACCO

P. J. Anderson

'OWNY MILDEW, a serious disease of tobacco plants, made its first
appearance in Connecticut during the late seedbed season of 1937.
After causing considerable damage in the beds, it spread to the fields dur-
ing June, but disappeared with the coming of hot weather in July. How-
ever, it is primarily a disease of the seedbeds, and the greatest losses may
be expected there. The erratic behavior of downy mildew in other tobacco
sections makes it difficult to predict how serious it is destined to become in
the Connecticut Valley. Since it is now thoroughly established and distri-
buted here, we must assume that it may recur each year. Therefore, if
growers are to insure against heavy losses they must aplply the best known
preventive or remedial measures to protect their seedbeds.

The purpose of this bulletin is to acquaint the growers with the essential
facts about mildew so that they may be able to recognize the symptoms of
the disease in every stage, to understand how it spreads and develops, and,
most important, to have before them explicit directions for applying the
best known methods of control.

Since downy mildew appeared so late in the seedbed season here, the
time for conducting experiments on methods of controlling or preventing
the disease has been too short to warrant conclusions. Therefore we are
obliged to rely on results obtained in other states where the mildew has
been prevalent for a longer time and, for the present at least, to use the
methods found best in those states assuming that they will be practicable
under our conditions in Connecticut ^

HISTORY

The first published reference to a mildew disease of cultivated tobacco
came from Queensland, Australia, in 1890 (59) ^ and a few months later
from New South Wales (18). In succeeding years it was reported progres-
sively from all the tobacco growing states of Australia, where it is con-
sidered the most serious and destructive of all tobacco diseases (7). Some
tobacco growers in Australia said that it was present in the seedbeds of that
continent as early as 1860, but since such opinions are usually expressed
by some growers whenever any new disease occurs, this early date is ques-
tionable.

A similar disease on a wild species of the tobacco genus ( Nicotiana
glauca) was reported from southern California in 1885 (26), and one on an-
other species ( A^. longiflora) from the Argentine in 1891 (56). StUl a third
was collected on Nicotiana biglovii in Nevada (60). There is no positive
proof, however, that the disease on any of these wild species of the tobacco
genus is the same as the disease of cultivated tobacco. The first record of
a similar or identical disease on cultivated tobacco in the United States is
furnished by a dried specimen in the fungous collection of the United

1 The writer gratefully acknowledges the invaluable assistance rendered in correspondence and
personal conference by Dr. E. E. Clayton of the Division of Tobacco and Plant Nutrition of the United
States Department of Agriculture; by Dr. F. A. Wolf of the Department of Botany of Duke University, and
by Mr. E. G. Moss and Mr. Thomas Smith of the Oxford Tobacco Station of the North Carolina Depart-
ment of Agriculture.

2 Numbers in parentheses refer to list of publications on p. 81

66 Connecticut Experiment Station Bulletin 405

States Department of Agriculture, collected in Texas in 1906 (51). There
is no indication that this caused any damage to the tobacco crop in that
state as it is not mentioned in the literature of plant diseases.

The first outbreak of mildew in the United States was in the spring of
1921 when it became widespread in the seedbeds of northern Florida and
southern Georgia. At that time there was great alarm lest the shade
tobacco industry in those states should be wiped out (51, 52, 53). For
some unknown reason, however, the disease did not appear the next year
in the destructive proportions anticipated. In fact, it caused no trouble
for the next 10 years and did not spread to neighboring states.

In the early spring of 1931, however, it was destructive and widespread
in Florida and Georgia, and was reported from Louisiana. Before the end
of the transplanting season it had also spread to South Carolina, North
Carolina and Virginia. In 1932 it was prevalent in all the tobacco-growing
Atlantic seaboard states as far north as Maryland. In 1933 it extended its
range to Tennessee and Pennsylvania, and became more widespread in
all the states where it was previously found. Since 1933 it has occurred
every year in all these states with variable degrees of severity and has
spread to Kentucky. The most extensive and destructive epidemic was
in 1937.

It was anticipated that sooner or later mildew would reach Connecti-
cut and therefore every suspicious disorder in the beds during recent years
has been closely scrutinized but no mildew was found. It is practically cer-
tain that there was no tobacco mildew in this state before May, 1937.

The first case observed was on May 25 in one seedbed of a series cover-
ing an acre or more in Bloomfield. Since all the leaves on one whole end
of this bed were dead and dry, the disease must have started at least a week
before. Later it spread to all the beds of this plantation. Within less than
a week after the first case was seen, similar spectacular and destructive
infections were found in seedbeds on seven farms in Windsor, South Wind-
sor, East Windsor, Manchester, Suffield and Glastonbury. It was widely
distributed and appeared on all three types of tobacco. The simultaneous
occurrence of the disease in such widely separated spots precludes the
probability that it spread from one of these as a center to the others. It
seems more likely that all the infections were primary and probably started
from spores blown into this region from Pennsylvania or more southerly
states where the disease was prevalent this year. Letters of warning with
a full description of the mildew were sent to all Connecticut growers. A
great deal of publicity was given to it in the newspapers and all suspicious
cases were investigated. If there had been other infections besides the
seven mentioned, this publicity would have brought them to light, but no
others were found during that first week.

Beginning June 6, reports of additional cases came in rapidly and from
various quarters indicating that the mildew was now spreading from the
seven primary infections. By the twentieth of June it had been reported
from about aU the tobacco growing towns of Connecticut and the southern
towns of Massachusetts, and additional cases were found every day until
the seedbed period was over.

The first field infections were reported about the middle of June. Ex-
amination of a large number of such fields usually showed that the worst
infections were in sections nearest to the seedbeds and there was unmistak-
able evidence that the spores were blowing from infected seedbeds into the
fields. June was a rainy month and frequently the leaves had no oppor-

Symptoms 67

tunity to dry off for several days at a time, thus furnishing ideal conditions
for the disease to spread. This weather continued until the first week in
July when it became hot and dry stopping all further advance.

Mildew now appears to be established in the entire tobacco growing
area of Connecticut and the southern part of Massachusetts.

NAME OF THE DISEASE

This same disease has been called by at least two common names to the
confusion of growers. In Australia and in the southern states it is more
often referred to as blue mold than as downy mildew. The use of this term
seems unfortunate because it is confusing. In the first place, the blue
color of the fungus is the most difficult of all symptoms to find and usually
requires considerable imagination. In the second place, the same term is
popularly used to refer to a truly blue colored mold of fruits and vege-
tables produced by fungi of the genus Penicillium, a genus far removed
from the fungus causing tobacco mildew. The growers are cdso familiar
with a blue mold (Penicillium) which sometimes runs over the soil of the
seedbeds just after sowing, but which has no connection with mildew.

"Downy mildew" is much more descriptive of the appearance of the
fungous growth on the back of the leaves, and, moreover, is the term in
common use for diseases caused by this class of fungi (Peronosporales) :
for ex£unple, downy mildews of grape, onion, etc.

To avoid confusion it would seem best to use only the terms "downy
mildew", or just "mildew", since there is no other kind of mildew on tobacco
here.

SYMPTOMS '

The appearance of the infected plants or seedbeds shows great varia-
tion depending on the kind of weather prevailing during the development
of the disease, on the age of the plants, the stage of the disease, and possi-
bly other environmental factors. Downy mildew is essentially a disease of
the seedlings in the seedbeds but this year has shown that it may also occur
here sometimes in the field. Since the field symptoms are not just the
same as those in the seedbed, it will be necessary to describe the two sepa-
rately.

In the seedbed

A badly diseased bed, such as those first seen this year, looks as if it had
been thoroughly burned by pouring scalding water or a toxic chemical,
like formaldehyde, over it. All the leaves are dead, dry, and shrivelled to
mere strings flattened out on the surface of the ground (Figure 1, page 64).
Usually the plants are not affected equally in all parts of the bed. At one
end they may be completely withered while they are progressively less
affected as one approaches the other end. This gives the impression that
the disease enters at one end and spreads toward the opposite.

Another symptom that is unmistakable after a little experience is the
rank odor — especially if the beds have been closed — suggesting rapidly dry-
ing, decaying or steaming vegetable matter. It is not unlike the odor of
potato mildew.

1 This description was made by the writer with the diseased plants before him and is based only on
observations of the season of 1937. Some symptoms described by persons in other sections were not ob-
served here.

68

Connecticut Experiment Station

Bulletin 405

The smaller plants in these badly diseased areas are dead but the strong-
er ones still have green bud leaves although all the larger outer leaves are
withered.

But if one wishes to see the beginnings of infection and observe the
stages by which such destruction has come about, he must examine the
opposite end of the bed or find beds where the infection is still new. In
such places he will find the first indication of disease in small areas where
the tips of the leaves, or indefinite spots on the leaves, are faded or rusty
yellow. Such leaves are not flat, as they should normally be, but are irregu-
larly puckered, humpy, contoured or cupped, or sometimes twisted until
the lower sm-face faces upward.

Figure 2. A healthy plant, right, contrasted with a badly- mildewed plant, left.

If it is early in the morning or the weather is cloudy, the lower surface of
some of these leaves will be covered with a downy felt of fungus (Figure 3),
the symptom which gives this disease its name. The color of the down
varies. Commonly it is white or gray, or, if older, rusty brown. Some-
times, however, especially if viewed obliquely, it has a distinct violet tint
which accounts for the name, "blue mold". The presence of this felt-like
growth on the lower surface of the leaves is the only infallible symptom of
the disease visible to the naked eye. Later in the day and during dry
weather, as well as during the later stages of the disease, this disappears and
diagnosis becomes more difficult. The subsequent changes in appearance
of the affected spots vary with the weather. When it is wet, the diseased
tips take on a dark green to black, dead, water-soaked appearance as they
wilt and wither progressively downward. In dry weather the affected spots
become brown, dry and brittle. The colors which the dead tissue takes on

Symptoms

69

are so varied that they furnish no criterion for diagnosis. Neither is the
shape of the spots regular or characteristic and it furnishes no proof of
identity of the disease. ^

*

W^ '-ffc^-

Figure 3. Leaves from the seedbed showing the fungus covering a part
of the lower surface. Somewhat enlarged.

A remarkable characteristic of downy mildew is the capacity of badly
diseased beds to recover. Except on the smallest plants, the bud and ' 'chit' '

70

Connecticut Experiment Station

Bulletin 405

leaves are not killed. » After the initial attack, the plant appears to acquire
a certain degree of immunity and develops normally. Beds which seemed
to be completely ruined when first observed this year were examined
after 10 days and appeared perfectly normal, with no mildew on them.
Plants from such beds showed no injurious effect when set in the field.

In the field

Entire leaves do not die in the field but the disease appears as spots of
a half-inch to more than an inch in diameter, one to a dozen on a leaf
(Figure 4) . In the first stages one sees only a faint, indefinite yellow blotch

Figure 4. Mildew spots on shade leaf in the field.
About one-third natural size.

on the upper side of the leaf. This blotch rapidly becomes more definite
and more yellow, and as the leaf tissue dies, it turns to a light brown. The
majority of the spots show no fungus on the lower surface at this time, but
if the weather is damp one may find it, especially on leaves close to the

■ Pathologists in other tobacco states report that often 80 or 90 percent of the plants are killed when
infection occurs while the seedlings are quite small. This may well happen here if the paildew starts earlier
in the season. ' - • '. . .

Causal Parasite 71

ground. On examining the young spots closely one notices numerous little
brownish or blanched or sunken specks visible on both surfaces. Some
persons have mistaken these for flea beetle injuries but examination under
the glass shows that there has been no chewing of the tissue.

In wet weather the spots on the leaves enlarge to a certain extent but
when dry they quickly cease to show any further development. When the
tobacco is cured, the spots appear as blanched, dry areas which greatly re-
duce the value of the leaves as wrappers or binders.

Field infections were more common in shade tobacco than in the other
types this year. The higher humidity under cloth may explain this, or
possibly the fact that shade tcbacco was set out earlier than the other
types. There is no indication that any one type of our tobaccos is any
more or less susceptible to mildew than the others.

CAUSAL PARASITE

The dead spots are due to the attack of a parasitic fungus which lives
inside the tissues between the upper and lower epidermis of the leaves.
This parasite forages its food from the leaf cells, causing them to die from
starvation and poisoning, and thus producing a dead spot on the leaf.

The part of the fungus which lives in the interior of the leaf, the myce-
lium, consists of numerous, microscopically fine, branching threads, hyphae,
running in every direction between the host cells. Specialized branches of
these hyphae, haustoria, bore through the walls of the cells to reach the in-
terior from which they absorb the food. A poisonous substance secreted
by the mycelium also causes the death of cells not actually invaded.
After fattening a few days on the food they have robbed from the leaf, the
hyphae grow out to the lower surface, or occasionally the upper, making
their exit through the numerous stomata, "breathing pores".

After passing through the stomata, each hyphal tip develops into a
branched, tree-like structure (Figure 5A) called a sporophore. One or
several sporophores may arise from each stoma. On the tips of the branches
are borne egg-shaped, or lemon-shaped, colorless spores, variously called
conidia, summer spores or sporangia (Figure 5 A and B) . It is these sporo-
phores and spores emerging in enormous numbers from the lower surface
of the leaf that form the cottony or felt-like patches and furnish the most
characteristic sym ptom of the disease. The development of these structures
occurs early in the morning or on cloudy days, which accounts for the fact
that the downy covering can be seen best at such times. The dust-like
spores which are produced in enormous numbers are so light that they can
be wafted about like the finest dust particles with the slightest air currents
and can easily travel many miles through the air. The rapid spread of the
disease and its wide distribution are thus accounted for by the quick de-
velopment, enormous numbers and especially the easy aerial transportation
of these summer spores. The spores may also be carried on the hands or
clothes of workmen, by the splashing of water, and possibly by some insects.
Later in the day, if it is clear, they blow away and the sporophores shrivel
so that nothing can be seen on the surface of the leaf.

When the air is full of spores floating about, some of them are sure to
fall on other tobacco plants in the same or other beds. Whether or not
they infect the leaf on which they fall depends entirely on moisture condi-
tions. If the leaf is dry and remains so, the spores die because they are
short lived and most of them lose their power to germinate after a few

72

Connecticut Experiment Station

Bulletin 405

FiGUBE 5. The causa] fungus, Peronospora tabacina. A. A single spoorphore
showing young summer spores in several stages of development. Most of the
spores have already fallen off the tips of the branches. B. Mature summer
spores (sporangia or conidia). C. Germination of the summer spores in var-
ious stages from one to fom- hours in a drop of water. D. An oospore (winter
spore) from the interior of a dead leaf. (Magnified, 400 times natural size.)

Name of the Fungus 73

hours or a day or two. They are also killed by exposure for an hour to
direct sunlight or a temperature of 84° F. or more (61). If, however, there
is moisture on the leaf, the spore germinates by pushing out a slender tube
which elongates very rapidly (Figure 5C) and passes into the interior of the
leaf through the stoma. Here it develops into a mycelium, as described
above, and the life cycle is started again. In laboratory tests, the writer
found it was only necessary for the spore to be in a drop of water two or
three hours before it started to germinate, and the germ tube grows with
unbelievable rapidity. According to Wolf, et al. (61) the life cycle, from
inoculation to the production of a new crop of spores, requires four to seven
days.

Oospores

A second type of spore, the "winter spore", is produced, not on the
surface like the short-lived summer spores, but buried in the interior of the
affected leaf. These occur in the collapsed dead leaves which are in contact
with the soil. They have hard, thick, resistant shells (Figure 5D) and do
not germinate at once when mature, but, after the leaf has decayed, re-
m£iin in the soil until the following spring and then germinate at the right
time to start new infections in the young beds. These oospores have been
found rather frequently in the southern states where mildew is common,
but rarely in Australia. The writer has found them in Connecticut and
no doubt they occur here commonly, although not produced in such
abundance as the summer spores.

NAME OF THE FUNGUS

The causal fungus belongs to the- lowest or most simple of the three great classes of
fungi, the Phycomycetes, in the genus Peronospora. The numerous species of this genus,
Peronospora, are all parasitic on plants and produce diseases to which the name "downy
mildews" has been given because of the plainly visible downy covering produced on the
surface of the leaves during sporulation.

When Farlow (26) first found a Peronospora produciag a mildew on a wild tobacco,
Nicotiana glauca, he beUeved it was the same species as DeBary had described in Europe
as Peronospora Hyoscyami on the black nightshade, Hyoscyamus niger, another plant of
the same family as tobacco. Then when a similar disease was found on cultivated to-
bacco in Australia and later in Florida (51), the fungus was considered to be the same
species and in the hterature up to 1933 was referred to as Peronospora Hyoscyami de B.
Inoculation experiments, however, by Angell and Hill (7) in AustreJia, and Wolf et al.
(61) in North Carolina, in which spores from tobacco failed to produce any disease on
the black nightshade, showed that the latter plant is immune to tobacco mildew although
the two fungi are morphologically very similar. Because of these host differeiices it is
now generally accepted that the tobacco mildew fungus should not be called Peronospora
Hyoscyami.

In 1891 Spegazzini (56) described a mildew fungus on another wild tobacco species,
N. longiflora, in the Argentine as Peronospora nicotianae. Since this fungus was mor-
phologically very much like the tobacco mildew fungus, and since the latter was able to
produce the disease on N. longiflora when inoculated, Wolf et al. (61) suggested that
the pathogen on cultivated tobacco should be regarded as P. nicotianae Speg. Investiga-
tions by Adam (3) in Australia and later by Clayton and Stevenson (17) in America
showed, however, that there were distinct morphological differences between the two
species, particularly in the oospores. Also the latter point out that although N. nico-
tianae was shown by Spegazzini to infect various species of Nicotiana, still the tobacco
growers in the Argentine are not troubled with any disease similar to our tobacco mildew.

Adam (3) in 1933 described the morphological differences between the three species
and decided that the pathogen of cultivated tobacco is distinct from the other fungi and

Eroposed a new name, Peronospora tabacina. This is the name now generally accepted
y Austreilian and American pathologists working on tobacco mildew.

74 Connecticut Experiment Station Bulletin 405

OTHER HOST PLANTS

In Australia, Angell and Hill (7) found the same mildew on about 20
other species of Nicotiana, the genus to which the cultivated tobacco be-
longs. They express the opinion that probably all species of this genus are
susceptible. The genus Nicotiana comprises some 40 species, only two of
which are of economic importance: N. tabacum, to which all of our culti-
vated types here belong, and A^. rustica, used in some regions for smoking
and grown in various places for the extraction of nicotine. All the others
are wild weeds, none of which occur in New England. However, one spe-
cies, TV. alata, is sometimes cultivated in flower gardens here and is known
popularly as "flowering tobacco".

Outside the genus Nicotiana, this fungus has been found to afiect seed-
lings of tomato, eggplant and pepper (61, 10, 11). Albert and Sumner in
South Carolina (11) report one case in which it did serious damage to
pepper seedlings. Outside of this instance, there is no record of real damage
to other plants and it seems unlikely that downy mildew will ever become
a menace to other crops. Other host plants are of importance only be-
cause of their possible connection with the spread and overwintering of the
disease.

Among the numerous varieties of cultivated tobacco, none has yet been
found to be immune or highly resistant to mildew.

SOURCE OF INFECTION IN THE SPRING

Since the summer spores under ordinary conditions live only a few days
at the most and the mycelium inside the cured leaf is no longer alive, how
does the fungus live over the winter to start infection in the beds the next
spring? The following possibilities suggest themselves: (1) Mycelium may
remain alive over winter in the seed; (2) mycelium may remain alive on
some perennial weed host and produce spores in the spring ; (3) mycelium or
summer spores may overwinter in the soil; (4) winter spores (oospores)
may winter in the soil of the seedbeds ; (5) summer spores may blow into
New England from warmer southern states.

In the seed

In Australia, Angell (4) and Angell and Hill (7) found that the fungus
sometimes occurred on the seed pods. By microtechnique they demon-
strated the presence of mycelimn in the seed of such infected pods but
were not able to show that this mycelium lived until the next year, nor that
such infected seed, when sowed, would produce diseased plants. American
investigators have not reported the occurrence of mildew on pods or seeds.
This possibility should be investigated further but at present there is no
indication that infected seed is responsible for primary infection in the
spring.

In perennial hosts

In Australia and in some of our southern states, the tobacco plants
sometimes remain alive over winter and produce a ne"w crop of suckers in
the spring. Early spring infections sometimes found on these, and the
fact that in Australia it has been shown that the mycelium is not always
local in leaves but may become systemic and live in the tissues of the stalk,
has led to the suggestion that such overwintering plants may furnish the

Source of Infection in the Spring 75

medium for starting spring outbreaks. Under Connecticut winter condi-
tions, however, no part of the tobacco plants survive and therefore this
possibility may be dismissed. Neither are there any other species of plants,
kncwn to be susceptible to this mildew, which survive the winter here.

Mycelium or summer spores in the soil

Although the summer spores and mycelium ordinarily are very short-
lived, Angell and Hill (7) were able to cause spores to germinate after 117
days when kept in dry soil at a very low temperature, 3° to 5° C. Little is
known about the longevity of the mycelium itself under various conditions.
This possibility needs further investigation before it is completely dis-
missed.

Winter spores

By analogy with many other downy mildews, one would expect the
oospores to be the most important if not the only source of spring infection.
As stated on page 73, these spores occur in the decaying leaves and are
known to be able to survive the winter in the soil and to germinate the
following spring (62). Wolf et al. (61, 62) present evidence to show that
this is the principal source of primary infection in the southern states.

Summer spores blown from states to the south

Reasons for believing that this was the source of primary infection in
1937 have been presented on a previous page. Repetition of this per-
formance may be anticipated in the future if weather conditions are right.
Investigations in the southern states have fully demonstrated that the
spores blown for many miles in great numbers, remained capable of produc-
ing infection.

HOW THE WEATHER AFFECTS MILDEW

Mildew comes on suddenly and disappears as suddenly. Some years
it is very destructive ; other seasons it causes little or no damage. Some-
tunes it spreads with almost unbelievable rapidity, and again it remains
stationary. Its erratic and puzzling behavior makes it impossible to pre-
dict what it will do at any one time. For the most part, these peculiari-
ties of the disease can be explained by the effects of the weather on the
development and distribution of the causal fungus.

Continued cool, moist weather is most favorable to the disease. Since,
in general, our early growing season is cooler than that of the southern
states, it may be anticipated that the disease will at least be as destructive
here as it has been in the South.

Temperature

Dixon et al. (25) , after exhaustive investigations on the effect of tempera-
ture, found that the spores in the beds are developed during the early hours
of the morning at temperatures of 42° to 63° F. with the most abundant
production at about 56°. Little, if any, production of spores occurs above
68° F., or below 36° F. Naturally the disease spreads little when no spores
are produced. In the beginning of our seedbed period the nights are too
cold for spore production; at the end of the season they may, at times, be-
come too warm ; but for most of the period, the night temperature range in
seedbeds is quite favorable to sporulation.

76 Connecticut Experiment Station Bulletin 405

High temperatures during the day inactivate or kill the mycelium.
Thus in July of 1937, when the weather suddenly became very warm,
spots on the leaves in the field made no further progress during the entire
season and no more spores were found. That mildew has never caused
damage in the field in the South is probably due mostly to the high tempera-
tiu-es that prevail during the summer. The critical temperature aJjove
which the mycelium does not develop is around 84° F. Except during un-
usual seasons, it is unlikely that this will be a serious field disease here.

No attempts have been made to determine the effect of winter tempera-
tures on the fungus. Judging from analogy to other fungi of this group,
we may anticipate that the severity of our winters will not kill the oospores
and will give us no protection. The only advantage of cold weather is that
it prevents the survival of suckers or any living part that might harbor
the fungus until the following spring.

Moisture

Like most fungi, the mildew pathogen is favored by moisture. Since
our beds are constantly watered and the sash prevent too much evapora-
tion, the humidity is close to saturation most of the time. Even though the
uppermost leaves may be dry, when the plants are crowded, there can be
very little ventilation around the basal leaves and a high humidity near the
ground is inevitable. Such conditions are ideal for the luxuriant develop-
ment of the sporophores and spores that form the felt-like covering on the
lower surface of the leaf.

Water plays a more important role, however, in the germination of the
spores and infection of leaves. When a spore alights on a leaf it can germin-
ate only when it is in water, i.e., it will not push out an infection tube on
the dry sm-face of the leaf. It is naturally not possible to keep the leaf
surfaces dry all the time. In a drop of water, as previously stated, the
spore germinates within two hours or less and in another hour the germ
tube could have entered the stomate of the leaf. Thus any condition under
which drops of water remain on the leaf as long as three or four hours will
permit infection. Naturally, the longer the leaves remain wet, the greater
the chances for infection and the more severe the disease. In the field also,
long periods of rain may easily result in spreading the disease.

Sunshine

Direct sunlight is lethal to the summer spores, killing them within
a few hours. It also dries the leaf off more quickly and thus delays infec-
tion.

Wind

Since the spores are disseminated mostly by air currents, a windy day
is most favorable for spreading the disease. On the other hand, however,
wind may have a beneficial influence in evaporating the drops of water from
the leaves.

PREVENTION AND CONTROL

Only recently have satisfactory methods of control been developed
and even these have not been tested long enough. Certain methods de-
veloped in Australia and in our southern states appear promising but need
further trial before we can be sure that they are effective under Connecticut

Prevention and Control 77

conditions. For convenience in discussion we may group the suggested
methods of prevention and control under three heads: (1) Modification of
cultural practices; (2) spraying the plants with fungicides, and (3) vapor
treatment of the seedbeds.

Cultural Practices

1. In the southern states it has been recommended that the beds be
located in a different place every year. This is based on the observation
that the first infections found in the spring are usually in beds sowed where
there were diseased seedbeds tht, previous year. Soil in diseased beds
would naturally contain a larger number of overwintering oospores than
new soil. Or, if there are other stages in which the fungus winters, the
chances of infection would surely be greater on old bed sites. Continual
yearly shifting would be practical on some Connecticut farms but on others,
where there has been a considerable outlay for water systems, fencing,
stationary steaming outfits, etc., such a plan would involve considerable
expense which many growers would not wish to incur.

2. Steaming the soil is a common practice among the better growers in
Connecticut and would kill the oospores or any of the other stages of the
fungus that are present in or on the soil. Whether or not formaldehyde
or acetic acid sterilization of soil would kill the oospores has not been deter-
mined. The spores probably do not remain over on the sideboards and
sash so that there would seem to be no advantage in sterilizing them. Even
when the soil is steamed there is always the chance of the fungus remaining
alive in the walks between or about the beds. Nevertheless, steaming
should not be neglected and may be an effective link in the chain of meas-
ures necessary to cope with the disease.

3. It has been recommended that beds be located on sites where good
air drainage and proper exposure to sun would dry the water off the leaves
quickly. Shaded, swampy, or poorly drained sites should be avoided. Any
environment that will permit the water to stand for long periods on the
leaves will furnish better opportunity for germination of the spores and
thus favor infection.

4. Ventilation of the beds is of even more importance. The parasite
requires high humidity of the air. When the sash are closed tightly the
humidity is close to 100 percent all the time. By keeping one end of the
sash raised, or leaving spaces between the sash, the humidity is quickly
reduced. If the weather is warm, it is better to remove the sash completely.
Plants grown with adequate ventilation are stronger and better regardless
of mildew.

5. Just as soon as the setting season is over, all extra plants and debris
should be removed from the beds to prevent them from harboring the
oospores which would remain in the soil until the next spring.

6. In the South it is commonly recommended that growers increase the
size of their customary seedbed area to provide a reserve of plants.

7. In Australia, the disease has been found on seed pods and, since it
was suspected that infected seed could transmit mildew to the following
crop, growers have been warned to avoid saving seed from plants known to
be affected. In America, no one has reported the disease on pods, and it
seems unlikely that this would be a source of danger.

78 Connecticut Experiment Station Bulletin 405

8. In the southern states appHcation of nitrate of soda to diseased
plants is recommended to increase growth and recovery after infection.
Since our plants in this section are raised on a pretty high level of fertiKty,
it is questionable whether this practice would be of value.

9. The remarkable power of recovery of diseased beds has been mention-
ed on a previous page of this bulletin. It has been found that plants set
after recovery live much better than those set during the early stages of the
disease. Recovered plants appear to develop a partial immunity. If it is
necessary to set from beds that have been attacked, it is best to wait until
the plants have recovered. Naturally it is still better to set from beds
which have no disease at all.

10. Although wind is probably the principal agent in spreading the
spores, it is also certain that they may be carried from bed to bed on the
hands or clothes of workmen. As far as is practicable, workmen should
avoid handling diseased plants. Curious visitors who come to see aifected
beds may also carry the spores to healthy beds that they visit afterward.

11. Destruction of diseased suckers in the fall. In some sections, the
disease has been found late in the fall on suckers growing from old stalks.
Oospores would normally be developed as these leaves rotted and might
start infection in the spring. As yet, the mildew has not been found in the
fall here. If further investigation should show late infection, it would be
best to plow the stubs under as soon as the crop is removed, or to remove
all suckers from the field later.

Spraying the seedbeds with Fungicides

Since Bordeaux mixture has been extensively and successfully used in
the control of various other downy mildew diseases, it was naturally the
first to be tried against the tobacco downy mildew. The control obtained
in experiments with this fungicide, however, has been disappointing.
Angell and Hill (7), after one season of tests, succeeded only in delaying
the appearance of mildew to a certain extent and stated, "Our experiments
do not appear to offer much promise". Clayton and Gaines (14) in 1933,
after reviewing the investigations in the South up to that time, state:
"Bordeaux Mixture appears to be about as effective as any other spray or
dust". None of them, however, had been found very satisfactory. Man-
delson in Australia (38) conducted more extensive tests on Bordeaux in
combination with various spreading agents. Also he included in his tests
a number of other copper fungicides. Most of these gave some degree of
control. He got poor control with all of the dusts. Bordeaux mixture
alone was not as effective as when mixed with spreading agents such as soft
soap or molasses. He used a weak Bordeaux of the formula 2-1-50. Most of
the Bordeaux combinations also produced some injury to leaves.

Since Bordeaux mixtiue is in common use here on seedbeds for the
control of wildfire and other diseases, and we can use it at the full 4-4-50
strength without injury, it appears worthy of further trial under our condi-
tions. Connecticut growers who had been using Bordeaux on their seed-
beds in 1937, however, did not escape the mildew, and in view of the rather
poor results reported by pathologists in other sections, it would probably
be unwise for growers to rely on Bordeaux mixture at present.

Mandelson (38) found two other copper fungicides to be more efficient
than the Bordeaux, viz.: (1) Home-made colloidal copper with soft soap

Spraying the Seedbeds with Fungicides 79

as a spreader, and (2) copper emulsion. He recommends particularly the
former, which is prepared by adding molasses to a copper sulfate solution
and neutralizing with caustic soda. This concentrated stock solution is
stored throughout the season and diluted with water when needed for
spraying. Armstrong and Sumner (11) in South Carolina also conducted
spraying tests with colloidal copper, prepared as recommended by Mandel-
son, and obtained encouraging results. Although it did not prevent the
disease it greatly reduced the severity of attack and gave somewhat better
control than the other fungicides tried. It demands that the grower take
considerable care in neutralizing to prevent injury, a point which may
prevent the general use of this material.

Tests with a proprietary fungicide, Cal-Mo-Sul, reported by Armstrong
and Sumner (11) also gave results not quite so good as the colloidal copper
but much better than the unsprayed check.

In the same bulletin Armstrong and Sumner report favorable results
with red copper oxide. This same fungicide, mixed with cottonseed oil
and lethane spreader, has also been tested in Florida, Georgia, North
Carolina, Virginia and Maryland. It is now reconunended more than any
other spray by the tobacco pathologists of these states. The cottonseed
oil has a weak fungicidal value and when the two are combined they are
more effective than either one used separately. The lethane spreader en-
ables the copper to cover the leaf surface more thoroughly.

There are some minor variations in the quantities of materials and
technique of mixing recommended by pathologists of the different states.
The most generally accepted procedure is as follows:

1. Materials needed to make 50 gallons of spray mixture are: }/2 pound
of red copper oxide, 1 quart of lethane spreader, 2 quarts of cottonseed oil
and 50 gallons of water.

2. Moisten the copper oxide with enough lethane spreader to make a
dough. Then gradually add water, stirring all the time, to make a suspen-
sion.

3. Mix the quart of lethane spreader and 2 quarts of oil by stirring thor-
oughly.

4. Add 2 or 3 gallons of water to the above spreader-oil mixtm-e.

5. Pump this through the spray pump (nozzle attached) into another
container in order thoroughly to break up, emulsify, the oil into fine parti-
cles.

6. Add water and the copper oxide suspension (from 2 above) to bring
the total volume up to 50 gallons.

This is now ready to spray on the plants. Make up just enough to
spray all the beds each time. Do not try to store it for future applications.
Use a fine nozzle with high pressure and apply enough to cover all leaves.
Spray twice a week.

Experiments with the copper oxide oil treatment were started here late
in the seedbed season. Results were not conclusive but showed some prom-
ise. Further investigations are in progress.

In view of results obtained in other sections, however, this seems to be
the most promising treatment. It should be generally tried by the growers
during the coming season.

Vapor Treatment

This treatment is accomplished by exposing volatile chemicals in the
beds during the night, the fumes of which fill the confined air and are toxic

80 Connecticut Experiment Station Bulletin 405

to the fungus. Various chemicals have been tested in Australia and in
America (5, 8, 33, 34, 39, 42, 48). The one now commonly recommended is
benzol, a distillate of coal tar. Other coal tar distillates such as toluol and
xylol have also given control but not as complete as benzol*.

The benzol is placed in shallow pans distributed throughout the beds.
The total exposure surface of benzol in our glass-covered beds should be
about .01 of the bed area to be protected. Starting when mildew first ap-
pears in a locality, the pans are set in the beds every evening about sun-
down and removed the following morning. It is also recommended that
they be left in the beds during dark days, but not during bright days.
Since benzol fumes are heavier than air, the treatment should be more ef-
fective if the evaporating pans are supported a few inches above the ground
level. It requires about one-half to two-thirds of a gallon of benzol per
night for 100 square yards of seedbed. The sash should be closed tightly
during the night. Any unused benzol should be returned to the bottle and
may be used for the next treatment. If just the required amount is used
each time, there should be none left. This should be about one fluid ounce,
(29.5 cc) per square yard of bed. (16 fluid ounces equal one pint.)

It is claimed by investigators who have worked with it in the South and
in Australia that benzol completely prevents mildew.

If the benzol splashes on the leaves it will kill them or, at least, make
dead spots on them. Water collecting on the underside of the sash may
drop into the benzol during rainy weather and cause it to splash on to adja-
cent plants. This can be prevented by using some type of covers above the
pans. There is also a possibility of an excess amount of the fumes causing
some yellowing of the plants, particularly during warm nights. This in-
jury may be prevented by mixing the benzol with lubricating oil, either
fresh or waste (42) . The mixture should consist of one part of benzol to
five parts of oil. The oil may be used again and again. Benzol is in-
flammable and naturally should be kept away from lighted matches.

The benzol vapor treatment is a new method for controlling fungous
diseases and has not been tried long enough to warrant a recomm^endation
that it be universally adopted.

Further investigations are in progress here and reconomendations must
await further results, but it is worthy of some trial by tobacco growers.

Benzol vapor has the additional advantage that it kills flea beetles and
other insects in the beds.

1 Also called benzene, but not the same as benzine which is a commercial mixture and closely related
to gasoline. Benzine should not be used.

Publications on Downy Mildew 81

LIST OF PUBLICATIONS ON DOWNY MILDEW

1. Adam, D. B. The blue mould (peronospora) disease of tobacco. Jour. Dept.

Agr. Victoria, 23: 436-440. 1925.

2. Adam, D. B. Blue mould in tobacco. Hints on its control. Jour. Dept. Agr.

Victoria, 29: 469-471,476. 1931.

3. Adam, D. B. Blue mould of tobacco. Jour. Dept. Agr. Victoria, 31: 412-416.

1933.

4. AngeU, H. R. Blue mould of tobacco ; investigations concerning seed transmission.

Aust. Jour. Council Sci. and Indus. Res., 2 (3) : 156-160. 1929-

5. AngeU, H. R., Allan, J. M. and Hill, A. V. Downy mildew (blue mould) of to-

bacco: Its control by benzol and toluol vapors in covered seedbeds. II. Aust.
Jour. Council Sci. and Indus. Res., 9: 97-106. 1936.

6. AngeU, H. R. and HiU, A. V. Blue mould of tobacco: Longevity of conidia.

Aust. Jour. Council Sci. and Indus. Res., 4: 181-184. 1931.

7. AngeU, H. R. and HiU, A. V. Downy mildew (blue mould) of tobacco in Australia.

Aust. Jour. Council Sci. and Indus. Res., 65: 9-30. 1932.

8. AngeU, H. R., HiU, A. V. and AUan, J. M. Downy mUdew (blue mould) of tobacco :

Its control by benzol and toluol vapors in covered seedbeds. Aust. Jour. CouncU
Sci. and Indus. Res., 8: 203-213. 1935.

9. AngeU, H. R., HUl, A. V. and Currie, G. A. Blue mould of tobacco; progress re-

port of studies on an insect vector. Aust. Jour. Council Sci. and Indus. Res., 3:
83-86. 1930.

10. Armstrong, G. M. and Albert, W. B. Downy mildew of tobacco on pepper, tomato

and eggplant. Phytopath., 23: 837-839. 1933.

11. Armstrong, G. M. and Sumner, C. B. Investigations of downy mildew of tobacco.

S. C. Agr. Exp. Sta., Bui. 303: 5-23. 1935.

12. Beyma thoe Kingma, F. H. van. On some moulds of the genus MonUia isolated

from tobacco. Zentbl. Bakt. Abt. II, 88: 124-131. 1933.

13. Burger, O. F. and Parham, H. C. Peronospora disease of tobacco. Quart. Bui.

State Plant Bd. Fla., 5: 163-167. 1921.

14. Clayton, E. E. and Gaines, J. G. Downy mildew of tobacco. U. S. Dept. Agr.,

Circ. 263. 1933.

15. Clayton, E. E. and Gaines, J. G. Progress in the control of tobacco downy mUdew.

Phytopath., 24: 5. 1934.

16. Clayton, E. E. and Gaines, J. G. Control of downy mildew disease of tobacco

through temperature regulation. Science. N. S., 78: 609-610. 1933.

17. Clayton, E. E. and Stevenson, J. A. Nomenclatiu:e of the tobacco downy mildew

fungus. Phytopath., 25: 516-521. 1935.

18. Cobb, N. A. Notes on the diseases of plants. Agr. Gaz. N. S. Wales, 2: 616-624.

1891.

19. Cooke, M. C. Tobacco disease. Gardeners' Chronicle, 9 (ser. 3): 173. 1891.

20. DarneU-Smith, G. P. Diseases of tobacco plants. Blue mould and a bacterial

disease. Agr. Gaz. N. S. Wales, 29: 82-88. 1918.

21. DarneU-Smith, G. P. Infection experiments with spores of blue mould disease of

tobacco. Agr. Gaz. N. S. Wales, 40: 407-408. 1929.

22. Dept. Agr. Plant Dis. Rpt. (1.9P69P). Recent information on tobacco downy mil-

dew. U. S. Dept. Agr. Plant Dis. Rpt., 17: 28-31. 1933.

23. Dickson, B. T. Downy mildew (blue mould) of tobacco. Aust. Tobacco Invest.,

Pamphlet I. 1933.

24. Dixon, L. F., McLean, Ruth A. and Wolf, F. A. The initiation of downy mildew

of tobacco in North Carolina in 1934. Phytopath., 25: 628-639. 1935.

25. Dixon, L. F., McLean, Ruth A. and Wolf, F. A. Relationship of climatological

conditions to the tobacco downy mildew. Phytopath., 26: 735-759. 1936.

26. Farlow, W. G. Notes on some injurious fungi of California. Bot. Gaz., 10: 346-

348. 1885.

27. Gaines, J. G. Recurrence of tobacco nuldew in Georgia. U. S. Dept. Agr. Plant

Dis. Rpt., 16: 16. 1932.

28. Gaines, J. G. PreUminary notes on spraying for the control of tobacco downy mil-

dew. U. S. Dept. Agr. Plant Dis. Rpt., 16: 27. 1932.

29. Graham, W. A. Downy mildew (blue mold) of tobacco. N. C. Dept. Agr. 1934.

30. Henderson, R. G. Experiments on the control of downy mildew of tobacco. Phy-

topath., 24: 11. 1934.

31. HUl, A. V. and Allan, J. M. Downy nuldew (blue mould) of tobacco. Attempts

at control by the use of (1) sprays, and (2) heated seedbeds. Aust. Jour. CouncU
Sci. and Indus. Res., 9: 220-232. 1936.

82 Connecticut Experiment Station Bulletin 405

32. Hill, A. Y. and Angell, H. R. Downy mildew (blue mould) of tobacco. I-III.

Aust. Jour. Council Sci. and Indus. Res., 6: 260-268. 1933.

33. Hill, A. V. and Angell, H. R. Downy mildew (blue mould) of tobacco: Preven-

tion of its development in inoculated and infected seedlings by benzol. Aust.
Jour. Council Sci. and Indus. Res., 9: 249-254. 1936.

34. Kretchmar, H. H. An apparatus for the application of benzol to tobacco seedbeds.

West. Aust. Jour. Dept. Agr., (ser. 2) 13: 380-383. 1936.

35. Lamb, S. and Sutherland, G. F. Report on the tobacco-growing industry in the

Tumut District. Agr. Gaz. N. S. Wales, 4: 313-322. 1893.

36. Mandelson, L. F. Additional recommendations for the control of blue mold of to-

bacco. Queensland Agr. Jour., 40: 465-469. 1933.

37. Mandelson, L. F. Tobacco diseases. Leaflet, Queensland Dept. Agr. 1933.

38. Mandelson, L. F. Fungicidal experiments for the control of blue mold of tobacco.

Queensland Agr. Jour., 40: 470-494. 1933.

39. Mandelson, L. F. Experiments with vapotirs for the control of blue mold of to-

bacco. Queensland Agr. Jour., 45: 534-540. 1936.

40. May, R. G. Prevention of blue mould of tobacco. Agr. Gaz. N. S. Wales, 44: 745-

748. 1933.

41. Mc Alpine, D. Report by the vegetable pathologist. Ann. Rpt. Dept. Agr. Vic-

toria, 1899: 222-269. 1900.

42. McLean, Ruth A., Wolf, F. A., Darkis, F. R. and Gross, P. M. Control of downy

mildew of tobacco by vapors of benzol and of other organic substances. Phyto-
path., 27: 982-991. 1937.

43. Palm, R. T. The false mildew of tobacco introduced into the United States from

the Dutch East Indies. Phytopath., 2: 430-432. 1921.

44. Pittman, H. A. An outbreak of "downy mildew" (so-called "blue mould") of to-

bacco in Western Australia. West Aust. Jour. Dept. Agr., 7: 469-476. 1930.

45. Pittman, H. A. Downy mildew (so-caUed "blue mould") of tobacco. The indus-

try's most serious menace, and how to combat it. West. Aust. Jour. Dept. Agr.,
II. 8: 264-272. 1931.

46. Pittman, H. A. Downy mildew (so-called "blue mould") of tobacco. West. Aust.

Jour. Dept. Agr., 9: 97-103. 1932.

47. Pittman, H. A. Downy mildew of tobacco. Two recent outbreaks near Perth.

West. Aust. Jour. Dept. Agr., II. 9: 452-456. 1932.

48. Pittman, H. A. Downy mildew (blue mould) of tobacco and its control by means

of benzol and other volatile hydrocarbons. West. Aust. Jour. Dept. Agr., (ser. 2)
13: 368-380. 1936.

49. Popova, Mme. A. A. Diseases of tobacco Nicotiana rustica L. Preliminary com-

munication. Morbi Plantarum, Leningrad, 18: 1-2, 45-53. 1929.

50. Samuel, Geoffrey. Annual report of the lectm-er on plant pathology. In report of

the Minister of AgricultxKe for South Australia, for the year ended June 30, 1924.
1925.

51. Smith, E. F. and McKenney, R. E. B. A dangerous tobacco disease appears in the

United States. U. S. Dept. Agr., Circ. 174. 1921.

52. Smith, E. F. and McKenney, R. E. B. Suggestions to growers for treatment of to-

bacco blue mold disease in the Georgia-Florida district. U. S. Dept. Agr., Circ.
176. 1921.

53. Smith, E. F. and McKenney, R. E. B. The present status of the tobacco blue

mold (Peronospora) disease in the Georgia-Florida district. U, S. Dept. Agr.,
Circ. 181. 1921.

54. Smith, Temple A. J. Blue mould in tobacco. Jour. Dept. Agr. Victoria, 12: 641-

643. 1914.

55. Smith, W. G. S. Gardeners' Chronicle, 9: 211. 1891.

56. Spegazzini, Carlos. Phycomyceteae Argentinae. Rev. Argentina Hist. Nat., 1:

28-38. 1891.

57. Stevens, N. E. United States of America : hn outhreak of Peronospora hyoscyami

on tobacco. Internatl. Bui. Plant Protect., 5: 183-184. 1931.

58. Stevens, N. E. United States of America : Further distribution of tobacco downy

mildew in 1933. Internatl. Bui. Plant. Protect., 7: 268-269. 1933.

59. Tryon, Henry. Tobacco diseases. Leaflet, Queensland Dept. Agr. 1890.

60. Wilson, G. W. Studies in North American Per onospor ales. Mycologica, 6: 192-

210. 1914.

61. Wolf, F. A., Dixon, L. F., McLean, Ruth A. and Darkis, F. R. Downy mildew of

tobacco. Phytopath., 24: 337-363. 1934.

62. Wolf, F. A., McLean, Ruth A. and Dixon, L. F. Further studies on downy mildew

of tobacco. Phytopath., 26: 760-777. 1936.

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