Plant Pathology Fact Sheet
Some Common Soybean Leaf and Stem
Tom Kucharek, Professor, Plant Pathology Department, IFAS, University of
Florida, Gainesville. 1981; Revised February 2001
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean
Soybeans are susceptible to dozens of
parasitic organisms. In Florida, fungal parasites
predominate, but bacterial and viral diseases
occur to some extent. If total control of fungal
diseases could be achieved in Florida, soybean
yields would increase by more than 50 percent.
This publication will discuss some common
fungal diseases that occur on leaves and stems
of soybean plants in Florida. These diseases
have never been observed by this author to kill
a plant; rather they cause yield loss by continual
debilitation of the plant. All eleven disease-
causing organisms discussed in this publica-
tion can remain in soybean residue in the soil
from one year to the next. Except for the web
blight organism, all can be carried in or on the
Fungi are organisms without chloro-
phyll, with microscopic spores and with micro-
scopic threads called hyphae. Some types of
spores are survival structures while other types
are disseminated by wind or rain, land upon a
leaf, and germinate with the germ tube penetrat-
ing (infecting) the tissue. A downy mildew
spore conidiumm) is about 1/1000 of an inch
Bacteria are single-celled organisms that
multiply rapidly by fission. Those that cause
plant diseases lack chlorophyll and generally
are about 1/12,000 of an inch long.
Viruses are noncellular, being com-
posed of protein and nucleic acids only. They
reproduce in living plant cells by redirecting
cellular functions of the plant. A typical soy-
bean mosaic particle is about 1/35,000 of an
Brown spot is caused by the fungus,
Septoria glycines. This disease may occur in
plants any time during the season. It is prima-
rily a leaf disease, but seeds, stems, and pods
can be infected. Primary inocula comes from
infected seed or old plant debris in the field.
Brown spot is more apt to be severe in low, wet
fields or after periods of heavy rains.
Brown spot (Figs. 1 & 2), old downy mildew
lesions (brown spot in upper center of Fig. 3)
and bacterial pustule (Fig. 10) produce simi-
lar appearing leaf lesions. Brown spot lesions
vary from pinpoint in size up to 5 mm. Leaf
spots are angular or somewhat circular with
irregular edges. They are brown in color and
leaf spots are usually aggregated as opposed
to being randomly dispersed.
A yellow halo may surround individual
spots with leaves becoming prematurely yel-
low and falling off, usually from the lower part
of the plant first. Field identification can be done
with the aid of a hand lens. Embedded in the
dead tissue within the lesion are tiny (1/10
mm), black, pimple-like structures (pycnidia).
If the tissue is dry the leaf may have to be re-
moistened to swell these structures. Micro-
scopic diagnosis may be necessary in some situ-
Downy mildew is caused by the fungus,
Peronospora manshurica. In Florida, downy mil-
dew usually appears in abundance at flower-
ing time or somewhat later. Distinct light yel-
low spots appear on the upper leaflet surfaces.
Later these spots become bright yellow and
eventually turn brown with a yellow margin
(Fig. 3). At this stage of symptom development,
it can be confused with brown spot (Figs. 1 & 2)
and bacterial pustule (Fig. 10). On the lower leaf
surface tufts of a grey-purple, downy growth
may be seen associated with the lesions. These
tufts are spores and spore stalks of the fungus.
Spores are dispersed primarily by wind. Spores
are produced from 50 to 860F. Older leaves, or
younger leaves exposed to high temperatures
prior to infection become resistant to the fun-
gus. This fungus can exist in seeds or as thick-
walled resting spores in soybean crop debris.
Numerous races of this fungus exist, and some
varieties may be more susceptible than others.
At this time, downy mildew does not appear
consequential in relation to yield. However,
seed from plants infected severely might be less
vigorous in germination.
Frogeye Leaf spot
Frogeye leaf spot is caused by the fun-
gus, Cercospora sojina In Florida, frogeye leaf
spot occurs from the time soybeans are flower-
ing until the leaves mature. Frogeye leaf spot
occurs primarily on the leaves and petioles, but
stems, pods, and seeds are infected also. Leaf
spots are 1 to 5 mm in size, usually circular,
but occasionally angular. The spot begins as a
pinpoint reddish-brown area that expands with
a reddish-brown-purple ring on the outside,
with the center of the spot being tan to grey to
white. No yellow halo surrounds these spots
(Fig. 4). The fungus survives between seasons
in seeds and old soybean debris. Young leaves
are more susceptible than older leaves. Frog-
eye leaf spot reduces yield directly by reduc-
ing active photosynthetic tissue and by the se-
cretion of toxins by the fungus.
Anthracnose is caused by the fungi,
Colletotrichum truncatum, Glomerella glycines,
G.cingulata, G. graminicola and possibly other
related fungi. Anthracnose is a major yield-re-
ducing disease on soybeans in Florida. This
disease is generally a late season (pod set to
harvest) disease but it can occur earlier. Anthra-
cnose can be associated with seedling blights
or early to mid season petiole blemishes.
Leaves, petioles, stems, pods, and pedicels
(flower and pod stalks), are susceptible. Pod
infection can occur by the fungus alone, but
stinkbug injury predisposes the plant to more
severe infections. Anthracnose will be reduced
in severity during "dry" seasons. These fungi
carry over from one season to the next in seed
and old soybean debris. The anthracnose fungi
can infect lima bean, clover, alfalfa and other
forage legume crops. Symptoms of anthracnose
are variable. The seeding blight phase of this
disease has not been observed in Florida. Leaf
petioles may become infected, resulting in elon-
gated (2 or more), reddish brown lesions.
Other diseases such as pod and stem blight or
web blight may cause similar symptoms on
petioles. Stem and pod symptoms include
blotchy areas of discoloration which are usu-
ally grey but nay be reddish-brown (Fig. 5).
Often stems may not have the blotchy discol-
oration, but rather small lack cushion-like struc-
tures (Fig. 6). With a magnifying glass you can
see black spines growing from he spore bear-
ing cushions (acervuli).
Pod and Stem Blight
Pod and stem blight is caused by the
fungus Diaporthe phaseolorum var. sojae. The im-
perfect (conidial) stage is Phomopsis phaseoli.This
fungus along with other species of Diaporthe
and Phomopsis causes significant yield loss and
is commonly associated with low quality, poor
germinating seed. Leaves, petioles, stems,
pods, seeds, and roots are susceptible. Seeds
may be infected with no visible symptoms.
However, seeds that are shriveled, cracked, or
small in size should be suspected of being in-
fected with this fungus or other fungi. The pod
and stem blight fungus can also infect other
plants such as lima bean, cowpea, snap beans,
peanuts, lespedeza, lupine, pepper, tomato,
okra, onion, and garlic.
Signs and symptoms of pod and stem blight
usually appear late in the season, but the fun-
gus can be in the plant earlier without express-
ing symptoms. Then, late in the season, black,
pimple-like, spore-bearing structures (pyc-
nidia) suddenly appear. They are arranged in
a linear fashion on the stem (Fig. 7). Shaving
the skin or bark off of a soybean stem often will
reveal black stroma (matrix of fungus myce-
lium) growing throughout the inner part of the
stem (Fig. 8). This stroma or matrix can be
present without the pycnidia mentioned ear-
lier. A related fungus, Diaporthe phaseolorum var.
caulivora, caused a major epidemic in the south-
eastern U.S. in 1983. The name of that disease is
Aerial Blight (Web Blight)
Aerial blight is caused by the fungus
Rhizoctonia solani This disease is caused by the
same fungus that causes seedling blight and
root rot (See Plant Pathology Fact Sheet No. 1).
Aerial blight may be present at low levels with-
out being noticed, but when the plants are ma-
ture with a closed canopy of leaves across the
field and when frequent rains occur, aerial
blight will spread fast. Within one week or less
an entire field will appear scorched. Small fields
bordered by trees or poorly drained fields are
more apt to have severe aerial blight. This fun-
gus has an extremely wide host range.
Symptoms are variable but often diag-
nostic in the field. Microscopic examinations
can confirm the presence of this fungus. Stems,
flowers, pods, petioles, and leaves are suscep-
tible. Root tissue is also susceptible but such
an infection may occur without causing aerial
blight. Lesions in leaflets can range in size from
a single pinpoint to coverage of the entire leaf-
let. Usually the lesion is brown to pale green in
color. If the lesion is fresh it may have a greasy
appearance. Lesions do not have a distinct
shape; lesion shape is determined by tissue
colonized by the fungus (Fig. 9). Fungal hyphae
can be seen on infected tissue; it will appear as
a brown spider-like web and is most apt to be
seen early in the day or when the canopy is still
moist. Pods and stem tissue that are infected
will be greasy, brown, and shriveled.
Bacterial pustule is caused by the bacte-
rium Xanthomonas campestris pv. glycines. The
bacterium is common in nature, but because of
the presence of a single recessive gene in most
commercially available varieties, this disease
is not a major problem. Without this resistance
it would be difficult to grow soybeans in the
Southern U.S.A. The bacterium enters leaves or
other plant parts through natural openings (sto-
mata) or via wounds. Once inside the plant, the
bacterium multiplies and lesions are produced.
Spread of the bacterium occurs primarily by
wind-blown rain and cultivation when the plant
is wet. The bacterium can multiply from 50 to
1000 F. Optimum conditions are from 86 to 91 F.
The bacterium can live from one season to the
next in seed and soybean litter in the field. The
incubation period may vary depending on nu-
merous weather variables and varieties, but a
5-7 day incubation period is standard. Young
leaves are more susceptible than older leaves.
Leaf symptoms of bacterial pustule are similar
to brown spot and old downy mildew. Pinpoint
spots expand into brown, angular lesions with
or without a yellow halo. Small raised pustules
in the center of the lesion are conspicuous on
some lesions (Fig. 10).
Bacterial blight (Fig.11) is caused by the
bacterium Pseudomonas syringae pv. glycinea.
This disease was found for the first time in
Florida in 1982 in Madison County. It was lim-
ited to the variety "Centennial'. It occurred for
a couple more years in nearby counties but then
ceased to be a problem. The bacterium enters
tissue in leaves through stomata. Optimal tem-
peratures for growth of this bacterium are from
76-80F. With the advent of hot temperatures
in July and August, the progress of this disease
was suppressed. Control of bacterial blight is
easily accomplished with resistant varieties.
Many strains or races of this bacterium exist
around the world.
Powdery mildew is caused by the fun-
gus Microsphaera diffusa. It is an uncommon dis-
ease of soybeans in Florida. It is favored by tem-
perature from 64-75F. Powdery mildew is not
active at 860F or above. Symptoms in the field
are diagnostic. A white powdery growth is seen
on the -upper leaf surface (Fig. 12). This fungus
can infect other legumes and some solanaceous
Target spot is caused by the fungus
Corynespora cassiicola This fungus has a wide
host range including: cotton, cowpea, cucum-
ber, beans, lima beans, lupine, okra, pepper,
sicklepod, tomato, and watermelon. Leaf infec-
tion occurs only when the relative humidity is
above 80% or when free water is present on the
leaves. This is not a common disease on soy-
beans in Florida, but it is capable of reducing
yield. The fungus can overseason on old soy-
bean debris in the field or in seeds. Many vari-
eties are resistant to the disease, which accounts
for its infrequent occurrence. Leaves, petioles,
stems, pods, seeds, and roots are susceptible.
Lesions in leaves are brown, somewhat round,
and begin as pinpoint spots which can expand
up to 1/2 inch across. A thin yellow halo may
surround the spot (Fig. 13).
Leaf Bronzing and Purple Seed Stain
(Cercospora Leaf Blight)
Leaf bronzing and purple seed stain are caused
by the fungus Cercospora kikuchii. Leaves, peti-
oles, stems, pods and seeds can be infected.
This disease can cause yield reduction and
downgrading in seed quality. The fungus sur-
vives in soybean debris in the field and in seeds.
Infected seeds will have a reduced and low
vigor germination. Seeds can be infected either
through the flower or through the pod after seed
formation. Often the upper-most leaves on a full
grown plant are infected whereas lower leaves
may or may not have some degree of infection.
Symptoms on leaves include, spots 2-3 mm
across, reddish brown and angular. Spots are
often small but aggregated and appearing as a
purple to red to bronze crust (Fig. 14). Seeds
may be infected without expressing symptoms,
but a pink to purple coloration on the seed coat
is diagnostic. (Fig. 15).
Diseases of soybeans are numerous, and there-
fore, numerous controls must be used in a se-
quence to reduce damage. Specific recommen-
dations are available from your county Exten-
sion office, and these are subject to change each
year. However, briefly, the following controls
are basic and essential to control all diseases
described in this publication:
(1) Use disease-free seed. Most soybean dis-
eases are seedborne. (2) Use crop rotation with
grass crops. (3) Use a moldboard plow to bury
old stubble. (4) Control weeds because they
increase moisture retention on leaves and in-
terfere with fungicide sprays. (5) Use a seed
treatment as it can reduce inoculum of several
seedborne diseases. (6) Use resistant varieties
where available. (7) Fungicide recommenda-
tions are recommended for certain situations.
Figure 1. Brown spot in cotyledons and
(8) Harvest soybeans as soon as they mature.
Leaving soybeans in the field beyond maturity
results in a poorer quality bean or seed and can
be counterproductive to other control measures
Figure 2. Brown spot in soybean leaves.
Figure 4. Forgeye leaf spot.
Figure 3. Downy mildew.
Figure 5. Anthracnose.
Figure 7. Pod and stem blight ( Photo cour-
tesy of Clemson Extension Service).
Figure 6. Anthracnose.
rigultr o0. r uu dati sItei uig.IL IU gUn s.um1a
in inner stem tissue.
Figure 9. Aerial blight (web blight).
Figure 12. Powdery mildew.
Figure 10. Bacterial pustule.
Figure 11. Bacterial blight.
Figure 13. Target spot. Figure 14. Leaf bronzing (Cercospora blight).
Figure 15. Purple seed stain (Cercospora