Discolored Soybean Seed
By Laura Sweets
This may be a year when there are numerous questions concerning various
types of soybean seed discoloration. Soybean diseases are one of several factors which
can cause discoloration and deterioration of soybean seed. But soybean diseases are
only one of the factors which may be involved in this year's problem. Much of the
state was extremely dry during July and August. Drought stress may be contributing
to small seed size, wrinkles or cracks in seed coats and even some of the seed
discoloration. If the soybean plants died prematurely because of drought stress and/or
late season diseases such as charcoal rot, there may be immature, green seed in the
harvested grain. Discoloration of soybean seed this year appears to be the result of
environmental stress compounded by late season stem and pod diseases.
When the late season pod and stem diseases occur, maturing plants have a
blackish cast and black to gray spots, blotches and streaks may cover stems, branches
and pods. The late season diseases led to increased problems with discolored and
damaged soybean seed. Purple seed stain; a general blotchy brown discoloration
that might be the result of the Cercospora or Colletotrichum species which cause
anthracnose and tipblight; bleeding hilum which can be caused by virus diseases
such as soybean mosaic and bean pod mottle; a white mold growth which could be
Phomopsis seed decay or secondary fungi entering through pods damaged by insects
are all showing up in beans. The diseases which contribute to discolored soybean seed
are usually favored by wet conditions late in the season. Weather conditions from
now through harvest will have a major influence on how severe discoloration and
deterioration of soybean seed is this season.
Symptoms of the seed damage which may result from Phomopsis seed decay,
purple seed stain, frogeye leaf spot, virus diseases and Colletotrichum anthracnose
and tipblight are described below.
Phomopsis seed decay
| | Phomopsis seed decay.
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Phomopsis seed decay results when the fungi which cause pod and stem blight
move from the stems and pods onto the seed. Plants infected with pod and stem
blight may be stunted and have discolored stems. Black pycnidia or fruiting bodies
of the fungi Phomopsis sojae or Phomopsis longicolla develop on the lower portion
of the main stem, branches and pods as plants reach maturity. The pycnidia may be
limited to small patches usually near the nodes or may cover dead stems and pods.
On stems, the pycnidia are usually arranged
in linear rows while on pods they are
randomly scattered. Prolonged periods of
warm, wet weather during flowering and
pod fill favor the development of pod and
stem blight. If conditions remain warm and
wet, the fungus may grow through the pods
and infect the seed. Infected seed is oblong
or misshapen and may have a white moldy
appearance.
Purple seed stain
| | Purple seed stain.
|
Cercospora kikuchii can infect
soybean seeds, pods, stems and leaves
but is most commonly found on the
seed. However, during the last several
years leaf spot and leaf blight caused
by this fungus have been prevalent in
parts of the state. Foliage infection
occurs on the uppermost leaves and
begins as reddish purple to reddish
brown angular to somewhat circular
lesions on the soybean leaves. These
lesions may coalesce to kill larger areas
of leaf tissue. The entire uppermost
trifoliolate leaf and petiole may be blighted and brown.
One striking symptom of this disease can be the premature
yellowing and then blighting of the youngest, upper leaves over
large areas of affected fields. In most fields symptoms do not
progress down the plants more than one to two nodes. Pods
at the uppermost nodes may develop round, reddish purple
to reddish brown lesions. Infected seed show a conspicuous
discoloration varying in color from pink to pale purple to dark
purple. The discoloration may range from small specks to large
blotches which cover the entire surface of the seed coat. Warm,
humid weather favors disease development. Yields are usually
not reduced but a high percent of seed stain may be evident at
harvest.
Frogeye leaf spot
Cercospora sojina causes frogeye leaf spot on soybean.
Symptoms occur primarily on leaves although the causal
fungus may also infect stems, pods and seeds. Lesions are small,
circular to somewhat angular spots that develop on the upper
leaf surfaces. Initially the spots are dark and water soaked in
appearance. As the lesions age they develop a dark reddishbrown
border. The center of the lesion becomes light brown to
light gray in color. Lesions may merge to kill larger areas of the
leaf. Heavily spotted leaves may wither and drop prematurely.
Stem lesions usually develop later in the season. Young stem
lesions are deep red with a narrow, dark brown to black margin.
As the stem lesions age, the centers become brown to smoky
gray in color. Lesions on pods are circular to elongate, slightly
sunken and reddish brown. The fungus can grow through the
pod wall to infect maturing seed. Infected seeds may show
discoloration of the seed coat that ranges from small specks to
large blotches of light gray to dark gray or brown.
Virus diseases
There are several virus diseases which may occur on
soybean in Missouri including bean pod mottle, soybean
mosaic and tobacco ringspot or budblight. Of these, soybean
mosaic virus and bean pod mottle virus are most likely to cause
symptoms on the seed. Seed infected with soybean mosaic or
bean pod mottle virus may have a symptom called bleeding
hilum. This is a discoloration, usually black or dark in color,
that bleeds from the hilum down the sides of the seed. The
affected area may be quite small and near the hilum or may be
quite extensive and cover most of the seed. It is important to
keep in mind that bleeding hilum is also a genetic characteristic
of certain soybean varieties. The intensity of the discoloration
can be influenced by environmental conditions during the
growing season.
Colletotrichum anthracnose and tipblight
Colletotrichum truncatum and several other
Colletotrichum species cause anthracnose of soybean.
Typically, anthracnose is a late season stem and pod disease
of soybean. Symptoms occur on stems, pods and petioles as
irregularly shaped, light to dark brown spots, streaks or lesions.
Eventually black fungal structures may be evident in these
lesions. Anthracnose may also cause a tipblight. The tipblight
phase of anthracnose causes a yellowing or browning of the
uppermost leaves and pods. The blighted tips may dry up and
die prematurely. This fungus may also infect seed. Seed may be
smaller than normal and severely infected seed may be a moldy,
dark brown in color and shriveled. Anthracnose is favored by
warm, wet weather, and the tipblight phase of anthracnose is
most likely to occur after a rainy period.
The incidence and severity of the soybean diseases which
cause seed discoloration and deterioration are greatly increased
by warm, wet conditions late in the season. For grain crops
there are no potential rescue treatments. Fields should be
harvested as soon as possible to prevent further seed damage.
Many of the pathogens causing seed discoloration and
deterioration can survive on soybean seed. Heavily infected
seed, if planted, could produce diseased seedlings resulting in
stand problems. Therefore, seed from infected fields should
not be saved for planting. If infected seed must be used for
planting, it should be thoroughly cleaned, a sample submitted
for a germination test (preferably a stress test) and a fungicide
seed treatment applied.
Many of the pathogens that cause these diseases may also
survive in infested residues left on the soil surface. Thus, crop
rotation is an important means of preventing or reducing
disease outbreaks. At least one year between soybean crops in
recommended. Varieties may differ in their reaction to these
various diseases and, if possible, good quality seed of resistant
varieties should be planted.
Laura Sweets
573-884-7307
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