A number of fungi produce spots on soybean leaves. During a particular
growing season, crop damage in any given area varies greatly, depending
on weather conditions and the disease-causing fungi present. Foliage diseases
are worse in wet years than dry ones, but they rarely cause serious economic
losses. In most years, one or more of these diseases can be found in 90
percent or more of the soybean fields in Illinois.
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1. Upper and lower leaves with symptoms of Septoria glycines.
Brown spot, or Septoria brown spot, is caused by the fungus Septoria
glycines (teleomorph Mycosphaerella uspenskajae). The disease has
become more important in Illinois during recent years. It appears
early in the growing season as small spots that are somewhat angular
to irregular and light to dark brown. The spots develop on both
surfaces of the lower leaves (Figure 1). Some heavily infected leaves
quickly turn yellow and drop off. During warm wet periods the fungus
moves progressively up the plant; and, if severe, may cause extensive
defoliation and major yield reduction. Late in the growing season,
infected leaves become a rusty-brown before dropping off prematurely.
Brown lesions of an irregular size and shape with indefinite margins
develop on the main stem, branches, petioles, and pods of the maturing
plants. Symptoms on these organs are not sufficiently distinct from
those of other diseases to be diagnostic.
Brown spot is most severe when soybeans are grown continuously
in the same field. Areas with poor drainage favor the spread of
the Septoria fungus. Yield reductions of 8 to 15 percent or more
The Septoria fungus overwinters in diseased crop debris as minute
brown fruiting bodies (pycnidia) and mycelium in infected leaf and
stem debris and in diseased seeds. During wet weather, the microscopic
spores (conidia) produced within the pycnidia on living or dead
plant parts are discharged. The needle-like spores are splashed
by rain or blown by the wind onto healthy leaves where infection
takes place. Lesions that develop on infected cotyledons and unifoliate
leaves provide spores for later infections. Infection and disease
development are favored by warm, moist weather. The spread of the
disease is checked by hot, dry weather. The fungus enters the soybean
plant through stomata and grows between the cells, killing those
next to the fungal hyphae. Pods are penetrated through stomata or
by growing through placental and funicular tissue, later invading
the seed coat.
Resistant cultivars are not available. Some differences in susceptibility
between cultivars has been observed. Determinant cultivars tend
to suffer more from this disease than do indeterminant cultivars.
This disease is caused by the fungus Peronospora manshurica (synonym:
P. sojae). Downy mildew frequently is prevalent over most of Illinois.
The first symptoms are the appearance of indefinite, yellowish green
areas on the upper leaf surface. These areas later enlarge and turn
a pale to bright yellow to a grayish brown or dark brown and are
surrounded by yellowish green margins (Figure 2).
Downy mildew can be readily distinguished from other soybean diseases
by tufts of a grayish to pale purplish mold on the lower leaf surface
in humid weather (Figure 3A). Severely infected leaves turn yellow,
then brown, curl at the edges, and drop early. The causal fungus
also grows within the pods and may produce a whitish, crusty growth
of mycelium and oospores on the seed. Oospore-encrusted seeds often
appear dull white and have cracks in the seed coat (Figure 3B).
The disease causes some defoliation, lowers seed quality, reduces
seed size, and may cause yield reductions of up to 8 percent.
2. Downy mildew: Left, early stage; Center and right, advanced
3. Downy mildew. A. Tufts of mold on lower leaf surface (courtesy
C.T. Schiller); B. Oospore-encrusted seed.
The Peronospora fungus overwinters as thick-walled oospores in
infected leaves and on seed. Seedling hypocotyls are also infected
by soilborne oospores. Planting infected seeds can produce a few
systemically infected seedlings under cool conditions. Such plants
remain smallwith mottled, gray-green leaves that curl downward.
Microscopic conidia (or sporangiospores) produced in the downy growth
on newly infected leaves during moist weather are disseminated by
air currents. Spores that land on leaves may germinate within 12
hours and penetrate the leaf through stomata or directly by forming
appressoria and penetration pegs.
Older leaves are resistant to infection. The number of lesions
increases and the size decreases as the leaves age. Downy mildew
is favored by high humidity and a temperature of 68 to 72 F (20
to 22 C). The production of spores occurs between 50 and 77 F (10
to 25 C) with no sporulation above 86 F (30 C) or below 50 F (10
C). Over 30 physiological races of the causal fungus have been identified
in the United States on the basis of disease reactions, using a
set of differential cultivars. This greatly complicates the development
of resistant varieties.
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FROGEYE LEAF SPOT
4. Frogeye leaf spot caused by Cercospora sojina. A. Lesions
on upper and lower leaf surfaces; B. Lesions on pods (courtesy J.T.
|This disease, caused by the fungus Cercospora sojina
(synonym: C. daizu), was formerly common in the southern half of Illinois.
Now, it is rare because of the widespread use of resistant cultivars.
Frogeye can be recognized by the small, round to angular "eye
spots" on the leaves. The ash-gray to light brown central area
is surrounded by a narrow, darker, somewhat purplish to reddish brown
border. The distinct border and dark gray clusters of spore-bearing
fungus structures (conidiophores) in the center of the spotmostly
on the underleaf surfacesmake frogeye easy to identify (Figure
4A). There is no yellow zone around the spots. Several lesions may
merge to form large, irregular spots. Severely spotted leaves wither
and drop early. Stems and pods may become infected late in the growing
season. Lesions on young stems are elongated and deep red, with a
narrow, dark brown to black border. Later, the lesions turn brown,
then a pale smoky-gray, and finally almost black, due to clusters
of conidiophores and numerous conidia. Lesions on ripe pods are circular
to elongate, reddish brown to light gray and usually bordered by a
dark brown ring (Figure 4B). The fungus may grow through the pod wall
into maturing seeds. Infected seeds develop areas that are light to
dark gray or brown which vary from specks to large blotches covering
the entire seed coat. Such areas may show alternating bands of light
and dark brown. Usually, there is some cracking or flaking of the
seed coat. When susceptible cultivars are grown, yields may be reduced
as much as 15 percent. Seed quality is lowered by discoloration and
by the lower germination of infected seed.
The Cercospora fungus overwinters as mycelium in crop residue and seed.
When an infected seed germinates, it may produce a weak and stunted seedling
with lesions on the cotyledons. Large numbers of microscopic spores (conidia)
are produced on the cotyledonary lesions in warm, humid weather. The spores
begin to germinate within an hour if free water is present. The conidia
are carried short distances by air currents and splashing rain and can
result in secondary leaf, stem, and pod infections throughout the season
under favorable conditions. Young leaves are infected more rapidly than
older ones. At least five different races of the fungus are known in the
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CERCOSPORA BLIGHT AND LEAF SPOT AND PURPLE SEED STAIN
Cercospora blight and leaf spot, as well as purple seed stain,
which occur worldwide, are caused by the fungus Cercospora kikuchii.
The disease is first observed on the young upper leaves at the beginning
of and throughout seed set. The sun-exposed leaves have a light
purple discoloration which may darken and extend over the entire
upper surface of affected leaves, giving them a dark, reddish purple
to bronzed, leathery appearance. Angular, to irregular, reddish
purple lesions later appear on both leaf surfaces. The lesions,
which vary in size from pinpoint spots to irregular patches, may
merge to form large dead areas. Numerous infections result in rapid
yellowing and browning of leaf tissues followed by defoliation,
starting with the upper leaves. Green leaves usually remain below
the defoliated area.
Lesions on petioles and stems are reddish purple and slightly sunken.
The lesions may merge and girdle affected stems and petioles. The
petioles remain attached to the branches but the leaflets commonly
drop prematurely. Reddish purple lesions, which later turn purplish
black, form on the pods. A general, dark reddish purple discoloration
also occurs on the upper stems, petioles, and pods exposed to the
sun. Some foliar infections are latent and express no symptoms.
The disease is most conspicuous on the seed. A pink or lavender
to a dark purple discoloration in the seed coat easily identifies
the disease (Figure 5). The discolored areas range from specks to
large, irregular blotches, which may completely cover the surface
of the seed. Cotyledons are not usually discolored. Seeds that are
invaded by the fungus may not show symptoms.
Severely affected seed is often cracked, rough, and dull. Such
seed commonly has a reduced germination percentage. The discoloration
of the seed may lower the grade and is objectionable to the producer
of pure seed. The purple color disappears after heating and is thus
not detrimental to soybean processors. U.S. grading standards, however,
do not allow more than 5 percent purple-stained seeds in No. 1 yellow
Infected cotyledons are often shrunken, turn dark purple, and drop
early. Cotyledon infection may spread to the stem, forming dead
areas that may girdle the young stem, killing the plant. Other seedlings
are stunted but survive. During seedling emergence, if the weather
is warm and humid, a velvety, grayish white mass of conidiophores
and conidia may appear on the cotyledons and stems.
5. Purple seed stain (courtesy T.M. Sjulin).
The Cercospora fungus overseasons in diseased leaves, stems, and seeds
and infects soybean plants at flowering. If heavily infected seeds are
planted, the seed coat may adhere to the cotyledons and emerge with the
seedlings. The fungus may grow into the cotyledons and then into the stem
of a small percentage of the seedlings. In warm, 73 to 81 F (or 23 to
27 C) humid weather, the fungus sporulates abundantly on the cotyledons,
stems, and leaves producing a velvety, grayish white growth of conidiophores
and conidia. The microscopic spores are wind-borne and rain-splashed to
other leaves and stems where secondary infections occur. Such infections,
in turn, produce more conidia that infect other leaves, stems and pods
during warm, moist conditions. The fungus grows through the pod well and
spreads through the hilum and into the seed coat, producing the distinctive
The fungus can establish symptomless colonization in the seed, cotyledons,
leaves and stems. Planting purple-stained seeds may introduce the fungus
into a field, but usually has little influence on the amount of purple
stain in the resulting crop. Several crops and weeds may serve as alternate
hosts of the Cercospora fungus.
No public soybean varieties currently recommended for growth in Illinois
are highly resistant.
ALTERNARIA LEAF SPOT; POD AND SEED DECAY
Alternaria leaf spot, caused by species of Alternaria, appears
as dark brown lesions, often with concentric rings, 1/4 to 1 inch
in diameter (Figure 6). The spots usually occur on leaves and pods
nearing maturity, but occasionally appear on seedlings and on young
plants. The lesions enlarge and may merge to produce larger dead
areas on the leaves. Infected leaves later dry up and fall early.
Dark brown to black pod and seed infections are usually associated
with senescence, frost injury, insect feeding, or wounding. Infected
seed are commonly smaller than normal, shriveled and discolored
dark brown to black.
The Alternaria fungi are seed-borne. Seed invasions occur most
commonly through breaks in the pod walls. Infection increases as
harvesting is delayed. The disease is favored by warm, moist weather
late in the growing season.
6. Leaf spots caused by Alternaria spp. (courtesy D.W. Chamberlain).
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ANTHRACNOSE, LEAF AND STEM SPOT
Soybean anthracnose on stems (courtesy T.M. Sjulin).
Anthracnose is usually caused by two fungiColletotrichum
truncatum (synonyms: C. dematium var. truncatum, C. dematium var.
truncata, and C. glycines) and C. destructivum (teleomorph Glomerella
glycines)that produce similar symptoms and may affect soybeans
at any stage of growth. Both fungi have wide host ranges. Symptoms
typically appear in the early reproductive stages on stems, pods,
and petioles as irregularly shaped brown areas that may resemble
pod and stem blight.
Stems, pods, and leaves may also be infected but show no symptoms.
Anthracnose fungi primarily attack older, mature plants, usually
in the late reproductive stages. Indefinite, enlarging areas that
are dark brown or reddish brown may cover the surface of infected
stems and pods. Later, these areas may become black-speckled with
fungus fruiting bodies (acervuli), which form black spines (setae)
that can be seen with the unaided eye (Figure 7). The lower, shaded
leaves and branches may die and fall prematurely. Young pods may
be attacked and killed. Infected seed within diseased pods may be
shriveled, dark brown, and moldy or fail to form. Less severely
infected seed may show no external sign of infection. Germinating
seeds may be killed before or after emergence. Dark brown, sunken
cankers develop on the cotyledons of emerging seedlings. The anthracnose
fungi may grow from infected cotyledons into young stems where small
deep-seated cankers may form that kill young plants.
Both of the principal anthracnose fungi overseason as mycelium
in diseased crop residue or in infected seeds. Planting infected
seed may cause the seedlings to die (damping-off) before or after
they emerge. Seedlings may also be infected, but may show no symptoms
until the plants begin to mature. Secondary stem and pod infections
mostly occur in the reproductive stages from bloom to pod fill during
warm, moist weather (below 95 F, 35 C) when free moisture is present
for 12 hours or longer.
PHYLLOSTICTA LEAF SPOT
Phyllosticta leaf spot or leaf blight, caused by Phyllosticta sojiecola
(synonym: P. glycines), teleomorph Pleosphaerulina sojicola, is
a minor disease in Illinois, rarely spreading beyond the first few
trifoliate leaves. Where the disease is severe, young plants may
be partially defoliated, but they recover rapidly. Areas that are
pale green, round-to-oval, or V-shaped appear at the tips or margins
of the leaves of young plants. These areas become tan to dull gray
in 1 to 3 days with a narrow, dark brown or purplish border. Numerous
small, black specks (fungus fruiting bodies, or pycnidia) form in
older lesions (Figure 8). The fungus may grow from the leaf blades
into the petioles and then to the stipules and stem tissues at the
leaf scar. Superficial, light gray, tan or brownish lesions with
a narrow, brown or purplish border may form on the petioles, stems,
and pods. The pod lesions are roundish with a reddish margin. Seeds
located beneath the pod lesions may become infected.
8. Phyllosticta leaf spots (U.S. Department of Ag photo).
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