SOYBEAN MOSAIC (SMV)
A plant's reaction to SMV infection depends on the cultivar, the strains
of the virus, plant age at time of infection, and environmental conditions
(especially temperature). Mosaic is generally a minor disease appearing
to a limited extent throughout Illinois. Infected plants are somewhat
stunted and bushy, and have distorted leaves. The leaves may be dwarfed,
crinkled, or ruffled and are narrower than normal, with margins curling
downward. They may have a yellowish cast and usually show a dark-green,
blister-like puckering along the veins (Figure 1). The youngest and most
rapidly growing leaves show the most severe symptoms. Nodules on the roots
of plants infected with soybean mosaic virus are fewer, smaller, and lighter-weight
than those found on healthy plants. Reduced nodulation and leaf distortions
are thought to be the primary cause of smaller, lightweight beans in mosaic-infected
fields.
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The pods are often stunted, flattened, or curved with less pubescence;
also, they contain fewer seeds. Mosaic-infected plants may produce
seed with a black or brown mottling of the seed coat. Badly mottled
beans are subject to downgrading, especially in overseas trade,
where soybeans are used primarily in food products.
The leaf symptoms of mosaic are sometimes confused with 2,4-D injury.
Herbicide damage, however, is much more uniform in a field or area
than that caused by mosaic. The injury usually becomes progressively
less as the distance from the site of application increases. Herbicide
injury is most apparent along sprayed roadsides and near sprayed
fields of small grain and corn when the ester form of 2,4-D has
been applied on a windy day. Mosaic-infected plants are scattered
at random in soybean fields. Some strains of the virus delay the
maturity of infected plants, resulting in green plants that stand
out in the field after healthy plants have matured. Soybean mosaic
is transmitted from one season to the next through seeds. Virus-infected
plants can produce virus-infected seeds that are a mottled dark-brown
or black when the plants become infected before flowering (Figure
2). However, seed may be mottled and not carrying the soybean mosaic
virus, and virus-infected seeds may not be mottled. SMV-infected
seeds may fail to germinate or may produce diseased, spindly seedlings
with mottled, crinkled unifoliate leaves. Infection after flowering
ceases may reduce the yield somewhat (Table 1), but transmission
of the virus by seeds to the next generation does not occur. Infection
by SMV can predispose some cultivars to infection by pod and stem
blight, stem canker, and Phomopsis seed decay resulting in a loss
of seed germinability.
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Figure 2.
Dark brown mottling of a soybean seed due to infection by the soybean
mosaic virus as seen under a high-power microscope. |
The incidence of seed transmission varies from less than 1 percent to
30 percent or more depending on the soybean cultivar, the virus strain,
and the time of infection. Transmission of soybean mosaic in the field
occurs when certain species of aphids feed on an infected soybean plant
and then on a healthy plant nearby. No aphids colonize soybeans in Illinois,
but transient aphid species are plentiful (over 60 species have been identified
in soybean fields, of which at least 31 species are able to spread the
virus in a nonpersistent manner), especially late in the growing season.
Aphids become viruliferous by feeding on infected stems, stem tips, and
old or young leaves. Soybean mosaic is a minor disease in full-season
soybeans in Illinois because the principal aphid flights occur before
planting and after flowering. Soybean mosaic is more prevalent in soybeans
planted late, such as after winter wheat.
Table 1. Yield, seed coat mottling, and seed
transmission in Williams Soybeans inoculated with the Soybean Mosaic Virus
(SMV) at various times after planting
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Inoculation (days after planting)
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Yield (percent of control)
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Mottling (percent)
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SMV seed transmission (percent)
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Healthy
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100.0
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2.2
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0.1
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9
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22.7
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99.6
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8.1
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19
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44.8
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90.3
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5.2
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34
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52.3
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98.8
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3.8
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57
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61.6
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99.7
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0.3
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Control
1. Plant virus-free seed.
2. Do not plant seed produced in late-season plantings.
3. In seed-producing fields, remove (rogue) plants suspected of being
virus-infected as soon as they are found.
4. Since SMV is transmitted mechanically, avoid handling or brushing
up against infected plants whenever possible.
Although resistance to soybean mosaic virus has been identified (due
to one or more dominant genes), resistance to SMV is not yet available
in commercial soybean varieties.
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BEAN YELLOW MOSAIC (BYMV)
The bean yellow mosaic virus is widely distributed in Illinois, but has
not caused evident damage. Affected plants are not noticeably stunted.
Usually, the leaves are not distorted and puckered like those affected
by soybean mosaic but some strains of BYMV produce severe mottling and
crinkling of leaves. Younger leaves on plants infected with bean yellow
mosaic show a scattered yellow mottling or indefinite yellow bands along
the major veins (Figure 1). Rusty, dead spots develop in the yellowed
areas as the leaves mature. Mixed infections of bean yellow mosaic virus
and soybean mosaic virus are common in soybeans. Bean yellow mosaic virus
has a wide host range, is easily transmissible mechanically, and by more
than 20 species of aphids in a nonpersistent manner.
Control
Bean yellow mosaic is not seed-transmitted, thus roguing is of no value.
No control is warranted.
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BUD BLIGHT (Tobacco Ringspot Virus [TRSV], Soybean
Mosaic Virus [SMV])
Bud blight usually appears first at the border of a field, advancing
inward as the season progresses. The disease is common after clover or
alfalfa fields have been harvested and when pastures dry up. Sporadic
but serious losses have occurred in some soybean fields, especially in
southern and south-central Illinois. Yields may be reduced by 25 to 100
percent. In general, losses are greatest when young plants are infected
or when seeds with a high percentage of TRSV are sown.
The symptoms vary with the stage of development at which plants become
infected. In young plants, the terminal bud turns brown and bends sharply
downward forming a crook. Later, other buds become brown. Such buds become
dry and brittle (Figure 3) and may fall off at the slightest touch. The
leaf just below the tip bud may be bronzed and flecked in a rust color.
A reddish brown discoloration of the pith, especially at the nodes (joints),
is often evident inside the stem. Brown streaks are sometimes evident
on petioles and along large leaf veins.
Plants infected before flowering time are severely dwarfed and produce
few or no pods. The pods that do form remain small and are mostly one-seeded.
About 90 percent of the seed is infected with virus and will produce virus-infected
plants if sown. Infected plants usually remain green until harvested or
killed by frost, and they stand out conspicuously in soybean fields at
harvest. Plants infected after flowering are much less dwarfed and may
produce an almost-normal number of pods. Many pods may drop when they
are still small, or may develop dark purple blotches (Figure 4). The seeds
on plants affected after flowering do not contain the virus and will produce
healthy plants when sown. The virus also significantly reduces root and
nodule growth.
In instances of severe bud blight, both tobacco ringspot and soybean
mosaic viruses have been isolated. Apparently, an interaction between
these viruses may induce the leaf to extend its veins beyond the margin
of the leaf blade (enations).
Many common weeds (e.g., dandelion, wild carrot, purslane, jimsonweed,
nettleleaf goosefoot, lamb's-quarters, pigweed, velvetleaf, horse-nettle,
sourdock, ragweed, horseweed) and some forage legumes are symptomless
hosts of the tobacco ringspot virus. The virus is spread from these plants
to soybeans and later from soybean to soybean by unknown vectors. Thrips,
grasshoppers, and several other insects have been implicated as well as
dagger nematodes.
Control
1. Removing infected plants from fields intended for seed production
is of limited value, except in preventing the dissemination of the virus(es)
to new areas. Infected plants produce so little seed and so much of it
is lost in threshing that it is of no consequence the next year, and is
practically self-eliminating in the second year.
2. Grow 4, 6, or 8 rows of corn, forage sorghum, or other tall-growing
crop as a barrier between soybean and clover or alfalfa fields and between
soybeans and noncultivated areas, such as roadsides, ditch banks, and
pastures.
3. Apply 2,4-D or other herbicide to kill broadleaf weeds in noncrop
areas that adjoin soybean fields before the soybean plants emerge or in
the fall after the field has been harvested.
OTHER VIRUS DISEASES OF POTENTIAL IMPORTANCE
1. COWPEA SEVERE MOSAIC (CSMV). Symptoms typical of bud blight can
also be caused by infection of soybeans with the cowpea severe mosaic virus.
The virus, which causes severe mosaic and stunting, is transmitted by seeds
in cowpeas but not in soybeans. Numerous leaf-feeding beetles transmit the
virus to soybeans, but much less efficiently than to cowpeas. Cowpea severe
mosaic was first discovered in Illinois soybeans in 1978 and is not believed
to be widespread. Growers who also grow cowpeas should be sure to plant
healthy cowpea seeds and to remove virus-infected plants in order to minimize
the spread of the virus to soybeans.
2. BEAN-POD MOTTLE (BPMV). The bean-pod mottle virus is present
sporadically in Illinois soybeans. It causes a mild and generally unimportant
disease (green and yellow mottling of the younger leaves, decreased pod
formation and a reduction in seed size, weight, and numbers) in this crop
unless the plants are also infected with the soybean mosaic virus. Doubly
infected plants are severely stunted with malformed, bronzed leaves, produce
few if any pods or seeds, and have a delayed maturity. Bean-pod mottle
can spread to soybeans via the feeding of bean leaf beetles (Cerotoma
trifurcata), and other beetles including Colaspis brunnea, and C. lata,
Diabrotica balteata, and D. undecimpunctata howardi, and Epicauta vittata)
that have fed recently on infected weeds or garden (snap) beans. Maximum
losses occur when plants are infected at the seedling stage. Yield losses
from BPMV alone may reach 2 to 20 percent depending on planting date,
but in association with SMV, losses may exceed 60 percent.
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