During the growing season, soybean may develop bacterial, fungal, and viral diseases, or the plants may be damaged by nematode infestations. The development of these diseases depends on several factors and the interactions between those factors, including the environmental and agronomic conditions. Disease development is greatly influenced by the amount of rainfall (or lack thereof), seasonal temperatures, and timing of infection. Soybean products’ genetic tolerance, or susceptibility, can also influence the development of diseases. Knowing the tolerance levels of soybean products can help determine if a disease could become economically damaging. Finally, for some diseases, while infection may occur early in the growing season the identifiable effects will not appear until just prior to or during the reproductive growth stages.
During the growing season, soybean may develop bacterial, fungal, and viral diseases, or the plants may be damaged by nematode infestations. The development of these diseases depends on several factors and the interactions between those factors, including the environmental and agronomic conditions. Disease development is greatly influenced by the amount of rainfall (or lack thereof), seasonal temperatures, and timing of infection. Soybean products’ genetic tolerance, or susceptibility, can also influence the development of diseases. Knowing the tolerance levels of soybean products can help determine if a disease could become economically damaging. Finally, for some diseases, while infection may occur early in the growing season the identifiable effects will not appear until just prior to or during the reproductive growth stages.
Development of a fungal disease depends on the presence of fungal spores, which can come from infected residue from a previous crop, infected seed, or from spore carrying winds. With the right temperature and sufficient moisture, spores can be produced on residue and be transmitted to the growing crop through splashing rain or wind. However, some fungal diseases are unable to overwinter on previous residue in the local climatic conditions. For these diseases, the infecting spores will need to be carried by wind from areas where the disease can overwinter. Foliar fungicides may help protect yield potential against certain fungal diseases, depending on the fungal disease and application timing.
Viral diseases are generally vectored by insects such as aphids or bean leaf beetles, but they can also be seedborne. If the insect vectors are present, their associated viral diseases may be present as well. Scouting for insects that have the potential to vector viral diseases and applying a timely insecticide may help protect plants from becoming infected, but many viral diseases cannot be adequately managed by insecticides.
The potential for microscopic nematodes to infest fields depends on geography, soil type, previous crop, tillage, wildlife and livestock movement, and other factors. Soil and plant sampling can help identify the species and level of infestation.
Bacterial Diseases
Bacterial Blight
Identification, Characteristics, and Diagnosis:
• Caused by the bacterium Pseudomonas syringae pv. glycinea.
• More prevalent during the early part of the growing season but can appear late.
• Late-season symptoms include the development of angular lesions that form from small, yellow to brown leaf spots (Figure 1).
• Spots appear first in the mid to upper canopy.
• Spot centers turn dark reddish-brown to black and dry out.
• Tissue around the spots appears water soaked and develops a yellowish-green halo.
• Dried out lesions can drop from the leaf, giving the leaf a shot-holed appearance.
• Plant seeds may be shriveled and discolored.
• The bacterium overwinters in crop residue and seed.
• Spread by wind-driven rain or splashing water, cultivation when foliage is wet, and moving wildlife.
• Bacterium enters the plant through natural openings and wounds when leaf surfaces are wet.
• Favored by temperatures in the range of 70 to 80
Management:
• Consider planting soybean products with higher tolerance ratings.
• Rotate to non-susceptible crops such as corn or small grains.
• Incorporate residue.
• Avoid cultivation when foliage is wet.
• Fungicides containing copper may provide control if applied early in the disease cycle.
Figure 1. Bacterial blight on soybean leaf.
Bacterial Pustule
Identification, Characteristics, and Diagnosis:
• Caused by the bacterium Xanthomonas axonopdis (syn. Campestris) pv. glycines.
• Causes premature defoliation, reduced seed size, and reduced seed set.
• Initial symptoms include tiny, pale-green leaf spots with raised centers. Spots can be on either side of the leaf but primarily occur on the lower surface and are near the main leaf veins (Figure 2).
• Later symptoms include the development of light-colored pustules in the center of the spots.
• Pustules may have linear cracks across the top, distinct from the round openings that appear in soybean rust pustules. Additionally, bacterial pustules do not produce spores.
• In contrast to bacterial blight (Pseudomonas syringae pv. glycinea), water soaking is not present with bacterial pustule.
• Spread by wind-driven rain or splashing water, cultivation when foliage is wet, and moving wildlife.
• Bacterium enters the plant through natural openings and wounds.
• Favored by temperatures in the range of 86 to 92 °F.
Management:
• Consider planting soybean products with higher tolerance ratings.
• Rotate to non-susceptible crops such as corn or small grains.
• Incorporate residue.
• Avoid cultivation when foliage is wet.
Figure 2. Bacterial pustule on soybean leaf. Photo courtesy of Daren Mueller, Iowa State University. Bugwood.org.
Fungal Diseases
Aerial Blight (Web Blight)
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Rhizoctonia solani AG1-1A.
• Overwinters as sclerotia in soil or on plant residue.
• Foliar symptoms usually occur during late-vegetative growth stages on lower leaves.
• Initial leaf lesions appear water soaked and grayish green.
• Mature lesions are tan to brown (Figure 3A).
• Reddish-brown lesions can develop on petioles, stems, pods, and petiole scars.
• Long strands of web-like hyphae can spread along affected tissue and small, dark brown sclerotia can form on diseased tissue (Figure 3B).
• Favored by temperatures 77 to 90 °F, high relative humidity, and wet weather.
Management:
• Plant soybean products with higher tolerance ratings.
• Avoid planting soybean in previous rice fields with a history of sheath blight of rice, which is the same pathogen with a different disease name.
• Rotate with poor or non-host crops such as corn or sorghum for two years.
• Wider row widths and reduced plant populations are recommended.
Figure 3A. Aerial blight on soybean leaf. Picture courtesy of T. Allen, Mississippi State University Extension.
Figure 3B. Aerial blight webbing on soybean stems. Picture courtesy of M. Emerson, University of Arkansas- Division of Agriculture, Cooperative Extension Service, Lonoke Extension Center.
Alternaria Leaf Spot
Identification, Characteristics, and Diagnosis:
• Caused by fungal species of Alternaria, a seedborne pathogen.
• Leaves may become reddish or yellowish.
• Dark-brown lesions, usually with concentric rings, ¼ to 1 inch in diameter, usually appear on leaves and pods near soybean maturity throughout the canopy (Figure 4).
• Leaf lesions enlarge and merge together to produce large, dead areas.
• Leaves eventually die and fall from the plant.
• Infected seeds are smaller, shriveled, and dark brown to black.
• Favored by warm, moist conditions late in the growing season.
• Usually, a secondary disease after mechanical injury, insect injury, or another disease.
Management:
• Infection usually occurs very late, so management is generally not necessary.
• Many soybean products have tolerance to Alternaria leaf spot.
• Seed treatments may offer protection.
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about these fungicides please visit https://www.cropscience. bayer.us/products/fungicides/delaro or contact your retailer
Figure 4. Alternaria leaf spot lesions on soybean leaf. Photo courtesy of Robert Mulrooney, University of Delaware.
Anthracnose
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Colletotrichum truncatum.
• Brown to black, irregularly shaped lesions on stem, pods, and petioles (Figure 5).
• Premature defoliation can occur from petiole girdling.
• Infected pods may be filled with mycelium instead of seeds, or seeds may be fewer and/or smaller, and can also be brown, moldy, shriveled, or normal in appearance.
• Dark spines or setae stick out from the acervuli (fruiting bodies) within the lesions.
• Leaves roll and exhibit necrosis of veins between the major veins.
• Favored by warm, wet, humid conditions.
• Infected seeds may fail to germinate.
• Infected seedlings develop dark, sunken cankers on the cotyledons, epicotyl, and radicle resulting in seedling damping-off.
Management:
• Crop rotation to a non-legume (non-host) crop.
• Incorporation of infested residue.
• Plant disease-free seed and/or treat seed with a recommended fungicide.
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about these fungicides please visit https://www.cropscience. bayer.us/products/fungicides/delaro or contact your retailer.
Figure 5. Anthracnose lesions on a soybean stem. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.
Brown Stem Rot (BSR)
Identification, Characteristics, and Diagnosis:
• Caused by the soilborne fungus Cadophora gregata.
• Foliar symptoms occur when pods begin to fill, about R3 to R4 growth stages, but infection occurs early in the season through the roots.
• Depending on the environment and pathogen genotype, leaf necrosis may occur (genotype A) or may not occur (genotype B), along with vascular browning.
• A pathogen-produced toxin is believed to cause interveinal chlorosis and necrosis.
• Infected leaves remain attached to the plant.
• The pith of longitudinally split stems is a light to dark, chocolate-brown color (Figure 6).
• Favored by cool weather during pod fill and with soil pH less than 6.5.
• BSR foliar symptoms resemble those of sudden death syndrome (SDS), northern stem canker
(NSC), and southern stem canker (SSC). Distinguishing symptoms include a brown pith and no root rot in BSR, a white pith and have root rot in SDS, and reddish-brown cankers near the nodes and no root rot in NSC and SSC.
Management:
• Residue management through tillage can help reduce pathogen survivability.
• Crop rotation to non-host crops such as corn or small grains for a minimum of three years can help reduce the fungal population in the soil.
• Plant tolerant soybean products.
Figure 6. Brown discoloration of soybean stem pith due to brown stem rot.
Cercospora Leaf Blight
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Cercospora kikuchii.
• Usually noticed during reproductive growth stages.
• Light- to dark-purple areas develop on sun-exposed leaves and eventually cover the entire leaf.
• Leaves become leathery, dark, reddish purple, bronzed, and/or blighted (Figure 7).
• Infected pods may have a purplish discoloration.
• The fungus also causes purple seed stain (Figure 8).
• Overwinters in infested debris and in infected seed.
• Host plants include many weed species such as lambsquarters, pigweed, mallow, and bindweed. • Favored by extended periods of dew and high relative humidity.
Management:
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about applying these fungicides please visit https://www.cropscience.bayer.us/products/fungicides/delaro or contact your retailer.
• Earlier-maturing soybean products may not be infected.
• Some soybean products have higher tolerance levels.
• Rotate away from soybean crop as soybean is the only main row crop that is a host.
• Residue management through tillage can help reduce pathogen survivability.
Figure 7. Cercospora leaf blight on soybean leaf.
Figure 8. Purple seed stain on soybean seed. Picture courtesy of Adam Sisson, Iowa State University. Bugwood.org.
Charcoal Rot
Identification, Characteristics, and Diagnosis:
• Caused by the soilborne fungus Macrophomina phaseolina.
• Infection generally occurs within two to three weeks after planting when soils are wet. However, the disease becomes dormant unless hot, dry conditions occur during the growing season.
• During reproductive growth stages, developing leaves may be small, rolled, lose vigor, turn yellow, wilt, die, and remain attached to petioles. • Infected plants may mature early and develop tiny, black sclerotia that resemble charcoal powder beneath the epidermis on the lower stem, taproot, and pith (Figure 9).
• Black streaks may develop in the woody portion of the crown.
• Lower stems may appear silvery or light gray.
• Favored by high temperatures and light-colored soils under drought conditions. Infected plants may be noted first on field edges and ridges where soil is more prone to drought.
Management:
• Plant soybean products that have higher levels of tolerance.
• Plant early maturing soybean products early to reduce the potential of plants achieving reproductive growth stages during typical high-heat months.
• Plant a non-host crop such as cotton or cereal grains for one to two years to help reduce pathogen populations.
• Use conservation tillage and reduce tillage planting methods to conserve soil moisture.
• Maintain fertility.
• Avoid high seeding rates and irrigate to help reduce drought stress, if possible.
• Maintain good weed control to reduce stress.
Figure 9. Soybean Charcoal Rot in soybean crown.
Downy Mildew
Identification, Characteristics, and Diagnosis:
• Caused by a fungus-like organism, Peronospora manshurica.
• Overwinters in the soil and in infested crop debris.
• Infection occurs in the spring when oospores germinate and infect seedlings.
• Upper surfaces of young leaves develop pale-green to light-yellow spots which enlarge into pale to bright yellow lesions (Figure 10).
• White to gray fungal tufts develop on the underside of the lesion.
• Oldest lesions become grayish brown to dark brown with yellowish-green margins.
• Favored by high humidity levels.
Management:
• Plant tolerant soybean products.
• Rotate soybean with a non-bean crop for at least one year.
• Rarely affects yield, so foliar fungicides are not recommended.
• Bury infested residue where feasible.
• Seed treatments can help protect seedlings from initial infection.
Figure 10. Downy mildew on soybean leaves.
Frogeye Leaf Spot
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Cercospora sojina.
• Symptoms initially appear during reproductive growth stages as dark, water-soaked lesions on younger leaves with centers that become ash gray to light brown.
• Later, the lesions become circular to angular with a purple to dark-brown margin around the tan to gray center (Figure 11).
• On leaf undersides, the center of the lesions may have a dark black area where spores are being produced.
• Favored by warm 77 to 86 °F temperatures and prolonged periods of dew or light rain.
Management:
• Plant tolerant soybean products.
• Crop rotation to a non-host crop like corn, small grains, or grain sorghum. However, long rotations may be necessary if the disease has been severe.
• Tillage encourages residue decomposition and can help reduce pathogen levels.
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about these fungicides please visit https://www.cropscience. bayer.us/products/fungicides/delaro or contact your retailer.
Figure 11. Frogeye leaf spot lesions on soybean leaf.
Northern Stem Canker (NSC)
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Diaporthe phaseolorum var. caulivora.
• Initial infection can occur around the V3 growth stage, at which point seedlings can die quickly or survive and develop stem symptoms during pod set.
• Symptoms during reproductive growth stages appear as small, reddish-brown spots on stems near a node within the canopy (Figure 12).
• Spots develop into one- to three-inch-long cankers running up the stem from the point of infection.
• Cankers can girdle the plant causing plant death from interrupted nutrient and water flow.
• On dead plants, the cankers are hard to distinguish from non-infected stem tissue. Plant death can occur from a fungal toxin.
• Foliar symptoms appear during reproductive growth stages as yellowing between the veins, usually on one side of the leaf.
• Leaves turn brown, die, and remain attached to the stem.
• The pith of dead plants is light brown and stems can easily snap because of brittleness.
• Favored by cooler temperatures and extended periods of rain occurring early in the growing season.
• Foliar symptoms of NSC resemble those of brown stem rot (BSR), sudden death syndrome (SDS), and southern stem canker (SSC). Distinguishing symptoms include reddish-brown cankers near the nodes and no root rot in SSC, brown pith and no root rot in BSR, and white pith and root rot in SDS.
Management:
• Plant tolerant soybean products.
• Rotate to a non-host crop such as wheat and corn for two years to reduce pathogen populations.
• Use tillage to help destroy infested residue.
• Applying a labeled fungicide at or before V3 infection may help protect plants.
Figure 12. Northern stem canker lesion on a soybean stem.
Phyllosticta Leaf Spot
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Pleosphaerulina sojicola.
• Overwinters in infected soybean residue.
• Infection appears as circular, oval, irregular, and V-shaped lesions on leaves (Figure 13).
• Lesions are gray or tan with a narrow, dark margin. Black specks (pycnidia) may be visible in older lesions.
• Favored by cool, moist conditions.
Management:
• Rotate to non-host crops.
• Utilize tillage to help destroy residue.
Figure 13. Phyllosticta lesions on a soybean leaf. Picture courtesy of Daren Mueller, Iowa State University Extension and Outreach.
Phytophthora Root Rot
Identification, Characteristics, and Diagnosis:
• Caused by the soilborne fungal-like pathogen Phytophthora sojae.
• Can infect seedlings and plants at reproductive growth stages.
• Seedlings and plants infected at early vegetative stages have stems that appear bruised and soft, secondary roots are rotted, leaves are yellow and brown, and plants can wilt and die (Figure 14).
• Plants infected later in the season have brown lesions on the roots, rotted roots, and dark, chocolate-brown colored stem lesions extending upward several inches from below the soil line (Figure 15).
• Leaves turn yellow, wilt, and remain attached to the plant after dying.
• Favored by wet, poorly drained soils; clay soils; and compacted soils.
• There are 25 different races of this pathogen.
Management:
• Plant tolerant soybean products relative to the identified race within the field.
• Utilize seed treatments such as Acceleron® Seed Applied Solutions Basic or Acceleron® Seed Applied Solutions Standard.
• Improve field drainage.
• Crop rotation is not an effective method to reduce disease because the oospores are very long lived in soil.
• Consider tillage to help destroy residue.
• Cultivation may promote new root growth when soil is thrown against the stem base.
Figure 14. Soybean seedlings killed by Phytophthora.
Figure 15. Phytophthora lesion on a soybean stem.
Phomopsis Seed Decay
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Diaporthe longicolla.
• Infected seed can be shriveled, undersized, and have a white or chalky appearance (Figure 16).
• The interior of pods can contain a white, cottony mold.
• Favored by warm, wet weather during pod fill.
• Early maturing soybean products may be more prone to infection.
Management:
• Do not plant infected seed.
• Seed treatments may help improve emergence.
• Plant tolerant soybean products.
• Select fuller season (for the area to be grown) soybean products.
• Utilize tillage to help promote residue deterioration.
• Control weedy hosts such as velvetleaf.
• Harvest in a timely manner to reduce the risk of extended exposure of the mature crop to wet weather.
Figure 16. Phomopsis seed decay on soybean seeds.
Pod and Stem Blight
Identification, Characteristics, and Diagnosis:
• Caused by various species of the fungi Diaporthe and Phomopsis. Diaporthe sojae is the preferred scientific name.
• Pathogens overwinter on infected seed and soybean residue.
• Linear rows of dark specks (fungal fruiting bodies) develop on stem nodes, pods, and petioles (Figure 17).
• The upper plant canopy turns yellow and dies.
• Seed quality can be reduced.
• Pod infection can occur at flowering, but most are infected around the R7 growth stage (beginning pod maturity).
• Injury to pods by insects promotes pod infection. • Favored by wet weather during maturation growth stages and delayed harvest.
Management:
• Rotate crops to non-host crops such as wheat or corn to help reduce the amount of infected residue.
• Utilize tillage to promote decay of infected residue.
• Harvest in a timely manner to reduce the risk of extended exposure of the mature crop to wet weather.
• Utilize seed treatments to protect seed.
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about these fungicides please visit https://www.cropscience. bayer.us/products/fungicides/delaro or contact your retailer.
Figure 17. Linear lesions produced by pod and stem blight. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.
Red Crown Rot
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Calonectria ilicicola.
• The pathogen overwinters on infected residue in and on the soil.
• Root infection can occur soon after planting, but initial symptoms may not appear until mid to late reproductive growth stages.
• Brick-red reproductive structures appear (usually during high soil moisture) on the base of the stem at the soil line (Figure 18), roots become black with areas of rot, and leaves have interveinal yellow or brown blotches (Figure 19).
• Favored by moderate soil temperatures ranging from 77 to 86 °F and wet soil.
Management:
• Rotate to non-legume gain crops for two or more years; avoid planting peanuts.
• Delay planting until soil conditions are favorable for rapid emergence.
• Manage nematode populations.
• Utilize tillage to help destroy residue.
Figure 18. Red lesions on soybean stems resulting from red crown rot.
Figure 19. Foliar symptoms of red crown rot. Picture courtesy of Dr. Guy B. Padgett, LSU AgCenter.
Septoria Brown Spot
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Septoria glycines.
• Irregular, dark brown lesions or spots that often have a surrounding yellow halo develop on lower plant leaves (Figure 20).
• Lesions can be small specks to 1/5 inch in diameter and coalesce to form larger spots.
• Defoliation can occur.
• Favored by wet weather and temperatures ranging from 79 to 83 °F.
Management:
• Foliar fungicide application are rarely justified; however, it may be economically justified if conditions are extremely favorable, and the disease develops in the upper canopy. Delaro® 325 SC fungicide or Delaro® Complete Fungicide are options. To learn more about applying these fungicides please visit https://www.cropscience.bayer.us/products/fungicides/delaro or contact your retailer.
• Rotation with non-legume crops and tillage may be beneficial (avoid continuous soybean crops).
• If possible, improve field drainage.
• Planting later may reduce the potential for a saturated environment.
Figure 20. Septoria brown spot lesions on soybean leaf.
Sclerotium Blight, Southern Blight
Identification, Characteristics, and Diagnosis:
• Caused by the soilborne fungus Sclerotium rolfsii.
• Sclerotia overwinter in soil and can remain viable for three to four years.
• The fungus infects plants when conditions are wet and hot ranging from 77 to 95 °F.
• Seedlings are subject to damping-off.
• Brown spots develop and expand on leaves. Leaves finally turn brown and remain attached.
• Lesions can develop at the soil line and extend up the stem several inches (centimeters) with a white fungal mass on or above the lesion.
• Residue near infected plants may have fungal growth.
• Small, yellow/red/brown fruiting structures (sclerotia) can be observed on the stem (Figure 21).
• Soybean plants are susceptible from emergence through pod fill. The greatest concern is when infection occurs during vegetative growth stages.
Management:
• Rotate to corn, small grains, or grain sorghum for at least one year.
• Do not plant tomatoes because of susceptibility.
• Reduce the potential for transfer of soil or residue to non-infected fields.
• Deep tillage to bury sclerotia may reduce their longevity.
Figure 21. Sclerotium fruiting bodies on a soybean stem caused by Sclerotium blight. Picture courtesy of Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.
Southern Stem Canker (SSC)
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Diaporthe phaseolorum var. meridionalis.
• Initial infection can occur around the V3 growth stage, at which point seedlings can die quickly or survive and develop stem symptoms during pod set.
• Symptoms during reproductive growth stages appear as small, reddish-brown spots on stems near a lower node.
• Spots develop into one- to three-inch-long cankers running up the stem from the point of infection (Figure 22).
• Cankers can girdle the plant causing plant death from interrupted nutrient and water flow.
• On dead plants, the cankers are hard to distinguish from stem tissue. Plant death can occur from a fungal toxin.
• Foliar symptoms appear during reproductive growth stages as yellowing between the veins, usually on one side of the leaf. Leaves turn brown, die, and remain attached to the stem.
• The pith of dead plants is light-brown, and plants can easily snap because of brittleness.
• Favored by extended periods (24 to 96 hours) of moderate temperatures of 72 to 86 °F and wet weather.
• Foliar symptoms of SSC resemble those of brown stem rot (BSR), sudden death syndrome (SDS), and northern stem canker (NSC). Distinguishing symptoms of the diseases include reddish-brown cankers near nodes and no root rot in SSC, a brown pith and no root rot in BSR, and a white pith and root rot in SDS.
Management:
• Plant tolerant soybean products.
• Plant a non-host crop such as wheat and corn for two years to reduce pathogen populations.
• Delayed planting can help reduce the incidence of the disease.
• Use tillage to help destroy infected residue.
• A labeled fungicide applied at or before V3 growth stage infection may help protect plants.
Figure 22. Southern stem canker lesion on soybean stem.
Soybean Rust
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Phakopsora pachyrhizi.
• Does not overwinter in most Midwestern areas. Spores are carried by wind currents from southern locations into the Midwest.
• Initial infection may appear as small, brown or brick-red dots on the upper leaf surface (Figure 23).
• Later, raised pustules (viewable with a 30X lens) resembling small volcanoes develop in angular lesions on the underside of leaves in the center and lower canopy. The pustules release spores through a central opening.
• Optimum conditions for infection include a minimum of six hours of leaf wetness (10 to 12 hours is considered very favorable) and temperatures ranging from 70 to 80 °F, though infection can occur at temperatures as low as 59 °F.
Management:
• If local sentinel plots indicate the presence of soybean rust, scouting should be diligent and thorough, particularly in early planted fields, early maturing soybean products, low-lying fields, fields with prolonged wetness, and fields with early canopy closure.
• Apply Delaro® 325 SC fungicide or Delaro® Complete Fungicide. To learn more about applying these fungicides please visit https://www.cropscience.bayer.us/products/fungicides/delaro or contact your retailer.
Figure 23. Asian Soybean rust lesions on leaf.
Sudden Death Syndrome (SDS)
Identification, Characteristics, and Diagnosis:
• Caused by the soilborne fungus Fusarium virguliforme.
• Initial visual symptoms appear as small, yellow spots on leaves during reproductive growth stages, though infection usually occurs at the seedling stage.
• The spots progress to interveinal chlorosis (yellowing) and eventually the leaf tissue dies (Figure 24).
• Leaves may fall prematurely, leaving petioles attached.
• The foliar symptoms are almost identical to those associated with brown stem rot.
• Roots are rotted, pith tissue remains white (Figure 13), and xylem (cortical tissue) is gray to brown. Under some conditions, a light blue spore mass may form on the tap root.
• More severe in the presence of soybean cyst nematode (SCN) and in low, wet field areas.
• Favored by cool, wet conditions and may be worse following corn as the pathogen also causes stalk rot.
• SDS foliar symptoms resemble those of brown stem rot (BSR), northern stem canker (NSC), and southern stem canker (SSC). Distinguishing symptoms of the diseases include a white pith and root rot in SDS, a brown pith and no root rot in BSR, reddish brown cankers near the nodes and no root rot in NSC
and SSC.
Management:
• Plant soybean products with higher tolerance ratings. Earlier-maturing products may have a lower potential for infection.
• Utilize soybean seed treatments such as ILeVO® seed treatment.
• Delay planting until soil conditions are drier and warmer.
• Improve field drainage, reduce soil compaction, and manage SCN.
Figure 24. Leaf necrosis and white pith color resulting from Sudden Death Syndrome of soybean.
Target Leaf Spot
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Corynespora cassiicola.
• Lower leaves develop small, brown specks (spots) that are round to irregular with a possible yellow halo.
• Mature spots may be 3/8 to 5/8 inches or more in diameter.
• Some spots may have a zonate (ringed or belted) appearance (Figure 25).
• Areas of infection on stems and petioles are dark brown and range from specks to elongated lesions.
• Lesions developing on pods are circular, usually as small as 1/32 of an inch, and purple or black with brown margins.
• Favored by high humidity (greater than 80%) or free moisture and cool to moderate soil temperatures.
• Dry conditions help suppress the disease.
Management:
• Plant tolerant soybean products.
• Reduce surface residue through tillage.
• If possible, avoid planting back-to-back soybean crops.
• Fungicides are not recommended because target leaf spot has a low potential for yield reduction.
Figure 25. Target spot lesions on a soybean leaf.
White Mold
Identification, Characteristics, and Diagnosis:
• Caused by the fungus Sclerotinia sclerotiorum.
• Germinating sclerotia (small, hard, black fruiting structures) near the soil surface produce small, tan to gray mushroom-shaped structures that produce spores which spread by wind and infect dead soybean flowers.
• Lesions develop at stem nodes during or after flowering.
• Lesions become larger (3 to 18 inches long), and the tops of plants turn grayish green, wilt, and die (Figure 26).
• Stems become soft, watery, and covered with white mold (Figure 27).
• Dry, dead stems may have a bleached, white appearance.
• Hard, black fungal fruiting bodies (sclerotia) are produced on or inside stems and pods.
• Dead plants remain upright and may be scattered or in patches throughout an infected field.
• Favored by moist soils, rainy weather, high relative humidity, cool (less than 85 °F). conditions during flowering, reduced air circulation in fields with high populations and narrow rows, high fertility, and possibly earlier planting.
Management:
• Plant disease-free seed.
• Select soybean products that may have some level of tolerance.
• Consider reducing seeding rates and utilize wider rows.
• If irrigating, reduce the frequency during flowering.
• Sclerotia can remain viable for several years in the soil, so long-term rotations to corn or other non-host crops may help reduce the potential for infection.
• Consider implementing biological controls such as Contans® WG.
Figure 27. White mold growth on soybean stems.
Viral Diseases
Bean Pod Mottle Virus (BPMV)
Identification, Characteristics, and Diagnosis:
• Vectored by the bean leaf beetle, Cerotoma trifurcate Förster.
• Foliar symptoms range from mild chlorotic mottling on upper leaves to puckering and severe mosaic on lower leaves (Figure 28).
• Delayed maturity or green stems are often observed near harvest.
• Seed coat mottling may be present.
• The virus overwinters in bean leaf beetles and can infect seedlings as the beetles feed.
• The virus can also overwinter in perennial weeds and infected seed.
• Plant infection by BPMV and soybean mosaic virus (SMV), which is vectored by soybean aphid, may cause severe dwarfing, foliar distortion, leaf necrosis, leaf mottling, and severe yield loss.
Management:
• Managing emerging and first-generation bean leaf beetles in the spring with timely and labeled insecticides can reduce populations of the virus-laden insects.
• Controlling alternative BPMV hosts such as cowpea (Vigna unguiculate), other bean species, and Demodium species can help reduce the inoculum source.
• Delayed planting may increase early-season death of bean leaf beetles, reducing the vectoring population.
Figure 28. Bean pod mottle virus symptoms on soybean leaves. Picture courtesy of Edward Sikora, Auburn University, Bugwood.org.
Soybean Mosaic Virus (SMV)
Identification, Characteristics, and Diagnosis:
• Aphids are a primary vector.
• A green/yellow mosaic pattern is the most common initial symptom on leaves (Figure 29).
• More mature leaves may exhibit a yellow/brown mosaic pattern.
• Premature defoliation is common.
• Infected seeds exhibit a brown or black mottling.
• Spreads from plant to plant by soybean aphid feeding.
• Plant infection by SMV and bean pod mottle virus (BPMV), vectored by bean leaf beetles, may cause severe dwarfing, foliar distortion, leaf necrosis, leaf mottling, and yield loss.
Management:
• Seeds should be virus-free.
• Plant tolerant soybean products.
• Early planting may minimize aphid transmission at an early crop growth stage.
• Insecticide applications are not recommended because some insecticides may increase soybean aphid movement in the field, increasing the dissemination of the virus.
Figure 29. Soybean mosaic virus symptoms. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.
Soybean Vein Necrosis Virus (SVNV)
Identification, Characteristics, and Diagnosis:
• Vectored by soybean thrips, Neohydatothrips variablilis Beach.
• Can also be transmitted by seed.
• Virus infection can occur throughout the growing season, but symptoms are most visible around mid-June after flowering.
• Initial symptoms appear as thread-shaped vein clearing along the main leaf veins.
• Severe infections may result in purple to dark-brown lesions across most of the leaf (Figure 30).
• Veins become yellow and necrotic as the growing season progresses.
• Several areas on a leaf may have lesions.
• Early lesions lack defined edges.
• Highest canopy leaves are most affected because emerging leaves are prime feeding sites for soybean thrips.
• Favored by cool temperatures and mild winters followed by a warm spring, which may help increase the thrips population.
Management:
• Control soybean thrips with timely and labeled insecticides.
• Controlling alternate virus hosts such as ivyleaf morningglory (Ipomoea hederacea Jacq), cowpea (Vigna unguiculate), and mung bean (Vigna radiata) can help reduce the inoculum source.
Figure 30. Soybean vein necrosis virus.
Soybean Nematodes
Soybean Cyst Nematode (SCN)
Identification, Characteristics, and Diagnosis:
• Heterodera glycines.
• More common in sandy soils, though SCN is well distributed throughout most soil types.
• Penetrates roots to feed.
• Evidence of feeding may be unnoticed until plants are under stress.
• Common symptoms include yellowish leaves and stunting.
• Nitrogen-fixing nodule formation can be reduced.
• Feeding wounds can be entry points for other diseases.
• Female cysts (initially white) that contain up to 500 eggs develop on roots. As cysts mature, their color changes from white, to yellow, to brown. Brown cysts have died and become the overwintering stage.
• Hot weather can reduce reproduction while cool to moderate weather can increase reproduction.
• Genetic variance occurring within SCN populations creates distinct and different HG-types (Heterodera glycines-types).
Management:
• Utilizing tolerant soybean varieties is the best and most effective management tool to control SCN. There are two types of tolerance for SCN currently available in the seed industry. Peking and PI 88788-type sources of tolerance are the most widely used.
• Utilize an in-furrow or seed treatment nematicide such as ILeVO® seed treatment.
• Rotate to a non-legume crop such as alfalfa, canola, corn, cotton, sorghum, or wheat.
• Manage weedy hosts such as purple deadnettle, henbit, field pennycress, shepherd’s purse, common chickweed, small-flowered bittercress, common mallow, white clover, Canada thistle, common cocklebur, and others.
• Maintain adequate fertility.
• Soil sample in the fall to determine the SCN population and race(s).
Figure 31. Soybean cyst nematode on roots.
Columbia Lance Nematode
Identification, Characteristics, and Diagnosis:
• Hoplolaimus columbus.
• Very common in coarse-textured soils, primarily in southeastern states.
• Feeds internally and externally on soybean roots.
• Oval patches of stunted and/or wilted plants parallel to soybean rows.
• Feeding lesions can coalesce and appear like a root rot.
• The taproot and secondary roots are pruned.
• Nitrogen uptake and nodulation are reduced, resulting in yellowish plants.
• Wilting can occur regardless of ample moisture.
Management:
• Some soybean products may have tolerance.
• Utilize an in-furrow or seed treatment nematicide.
• Rotate to peanuts (corn or cotton can increase populations).
Lesion Nematode
Identification, Characteristics, and Diagnosis:
• Pratylenchus species.
• Common in coarse-textured soils, primarily in southeastern states.
• Penetrates roots to feed and lay eggs.
• Feeding lesions can coalesce and appear like a root rot.
Management:
• Some soybean products have tolerance.
• Utilize an in-furrow or seed treatment nematicide such as ILeVO® seed treatment.
• Delay planting.
• Rotate to corn.
Sting Nematode
Identification, Characteristics, and Diagnosis:
• Belonolaimus longicaudatus.
• Found in very sandy soils.
• Feeds externally on roots and lays eggs in the soil.
• Common symptoms include poor growth, stubby roots, and possibly a tap root with few lateral roots and no fibrous roots.
Management:
• Utilize an in-furrow or seed treatment nematicide.
• Do not rotate to cotton, peanuts, sorghum, soybean, or corn, as all these agronomic crops are hosts crops.
Reniform Nematode
Identification, Characteristics, and Diagnosis:
• Rotylenchulus reniformis.
• Found in any soil type.
• Survives winter as either wormlike pre-adults or eggs.
• Common symptoms include stunted plants and roots.
• Soil particles adhere to egg masses.
Management:
• Plant tolerant soybean products.
• Utilize an in-furrow or seed treatment nematicide such as ILeVO® seed treatment.
• Rotate to non-host crops such as corn or sorghum (cotton can increase populations).
Root-knot Nematode
Identification, Characteristics, and Diagnosis:
• Several species: southern root-knot (Meloidogyne incognita), guava root-knot (M. enterlobii), Javanese root-knot (M. javanica), northern root-knot (M. hapla), and peanut root-knot (M. arenaria).
• Non-uniform stunting, wilting, chlorotic patches, and possible root galls.
• Most common in sandy soils.
• Eggs can survive in soil for several years until favorable conditions for hatching exist.
• Lab analysis may be required to distinguish which species is present.
Management:
• Plant tolerant soybean products.
• Utilize an in-furrow or seed treatment nematicide.
• Rotate to corn or small grains crops (most all other major agronomic are susceptible).
• Sanitize equipment when moving from field to field.
Figure 32. Bayer soybean fungicide application timing infographic.
