A Guide to Common Soybean Diseases in The Midwest

The potential always exists for a fungal, viral, or bacterial disease to develop in a soybean crop. Yield loss related to a disease can be minimal or dramatic depending on the disease, the environment, time of infection, and the soybean product. Knowing the disease history of a field and keeping abreast of reported disease developments during the growing season can help in making important management decisions to help protect yield potential. 

Many of the diseases caused by fungi overwinter on infected crop residue. Because of climatic conditions within an area, some fungi-caused diseases are unable to overwinter; however, the spores can arrive via wind from areas where the disease can overwinter. Plants become infected when splashing rain, water movement through the soil onto roots, or wind brings the pathogen to the plant. Some fungal pathogens can also be seedborne. The fungi then enter plant tissue by 1) growing a germ tube that pierces into tissue, 2) through stomata, or 3) through wounds caused by hail, wind, insects, or mechanical means. 

Fungicidal seed treatments can help protect seed and seedlings from most fungi-caused seed and seedling diseases. Foliar fungicides can help protect yield potential, depending on the fungal disease and application timing for the disease. Fungicide resistance has developed with some fungi; therefore, it is important to properly identify the causal fungus for proper fungicide selection. To help deter the development of fungicidal resistance, it is highly suggested that fungicides be rotated based on mode of action. Depending on the pathogen, improved soil drainage and soil compaction management can help reduce the potential for fungal infections. Residue destruction through tillage can help speed up the deterioration of a pathogen source.

Bacterial diseases can also overwinter on infected residue. The bacterium can contact plants through water movement in the soil, splashing rain, wind-driven rain, and mechanical means such as cultivating when plants are wet. Some bacterial pathogens also can be seedborne. Infection occurs when bacteria enter the plant through wounds or natural tissue openings such as stomates.  

Most viral diseases rely on a vector such as aphids or thrips to infect a plant. As the vector feeds, infected juices are injected into plant tissue. Scouting for insects that have the potential to vector viral diseases and applying a timely insecticide may help protect plants from becoming infected via insect feeding for some viral diseases However, insecticide applications to manage vectors do not provide adequate control of many viral diseases. Caution should be taken not to spray insecticides without proper identification as many beneficial insects can also be killed.

 

Bacterial Diseases

Bacterial Blight

Figure 1. Bacterial blight lesions on a soybean leaf. 

Identification, Characteristics, and Diagnosis:

  • Caused by the bacterium Pseudomonas savastanoi pv. glycinea.

  • Pathogen overwinters on residue and can be seed-transmitted.

  • Pathogen spreads by wind, rain, and by cultivation when plants are wet. 

  • Lesions can occur on all aboveground plant tissue; leaves in the mid to upper canopy are favored.

  • Initial infection appears as small, angular, water-soaked spots that turn yellow, become brown with a yellow halo around the spot, then die (Figure 1). 

  • Dead tissue may drop from leaves; however, leaves usually remain attached to the plant.

  • Favored by cool, wet weather. Hot, dry weather stops development. 

  • Infected seedlings may be stunted or die.

Management:

  • Plant resistant soybean products and disease-free seed.

  • Rotate with non-host crops. 

  • Avoid cultivating when plants are wet.

  • Manage residue through tillage.

 

Bacterial Pustule

Figure 2. Bacterial pustule lesions on a soybean leaf. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org

Identification, Characteristics, and Diagnosis:

  • Caused by the bacterium Xanthomonas axonopodis pv. glycines.

  • Pathogen overwinters on infested soybean residue and seed.

  • Pathogen is spread by splashing or windblown water, and cultivation when plants are wet.

  • Initial symptoms include the development of small, yellow-green lesions with higher reddish-brown centers on upper leaf surfaces (Figure 2).

  • More mature lesions produce a small, slightly raised, pale-colored pustule at the center of the lesion on the underside of the leaf.

  • Lesions can be small to large, irregular, and necrotic. 

  • Leaves can become ragged in appearance as dead tissue drops from the leaf.

  • Premature defoliation may occur, which can reduce seed count and size.

  • Favored by warm (86 to 92°F) and wet weather.

Management:

  • Plant resistant soybean products.

  • Utilize tillage to help decompose residue.

  • Avoid cultivation when plants are wet.

     

Fungal Diseases

Alternaria Leaf Spot

Figure 3. Alternaria leaf spot lesions. Picture courtesy of Robert Mulrooney, University of Delaware.

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 3).

  • 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 or insect injury, or another disease.

Management:

  • Infection usually occurs very late; therefore, management is generally not necessary.

  • Many soybean products have resistance.

 

Anthracnose

Figure 4. Anthracnose lesions on a soybean stem. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Colletotrichum truncatum.

  • Brown, irregularly-shaped lesions on stem, pods, and petioles (Figure 4).

  • 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 minor 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 non-host crops.

  • Incorporation of infected residue.

 

Brown Stem Rot

Figure 5. Brown discoloration of soybean stem pith due to brown stem rot.

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; however, infection occurs early in the season through the roots.

  • Depending on the environment and pathogen genotype, leaf necrosis may (genotype A) or may not (genotype B) occur 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 5).

  • Favored by cool (60 to 80°F) weather during pod fill (R4 to R6 growth stages), soil pH less than 6.5, adequate access to soil moisture at R1 to R2 growth stages, and when soils are low in phosphorus and potassium.

  • May be more severe when soybean cyst nematode (Heterodera glycines) (SCN) infests the field.

Management:

  • Residue management through tillage can help reduce pathogen survivability. 

  • Crop rotation to non-host crops such as corn or small grains can help reduce the fungal population in the soil and the presence of SCN.

  • Use resistant soybean products.

  • Seed treatments should be used in early plantings and no-till systems.

 

Cercospora Leaf Blight

Figure 6. Cercospora leaf blight.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Cercospora kikuchii.

  • Pathogen overwinters on soybean residue and infected seeds.

  • Pathogen is dispersed by splashing water and wind.

  • 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 6).

  • Infected pods may have a purplish discoloration.

  • The fungus also causes purple seed stain.

  • Favored by warm (at or above 82°F) and humid (heavy dew) conditions.

Management:

  • Apply Delaro® 325 SC Fungicide.  To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

  • Earlier-maturing soybean products may not be infected.

  • Individual soybean products may have higher tolerance levels.

  • Utilize tillage to help decompose infected residue.

 

Charcoal Rot 

Figure 7. Charcoal rot.

Identification, Characteristics, and Diagnosis:

  • Caused by the soilborne fungus Macrophomina phaseolina.

  • Infection generally occurs within 2 to 3 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 7).

  • Black streaks may develop in the woody portion of the crown.

  • Lower stems may appear silvery or light-gray.

  • Favored by high temperatures (82 to 93°F) 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 or resistance.

  • 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 for 1 to 2 years to help reduce pathogen populations.

  • Use conservation tillage and planting methods to conserve soil moisture.

  • Maintain fertility.

  • Avoid high seeding rates and irrigate (if possible) to help reduce stress. 

 

Pod and Stem Blight

Figure 8. Linear lesions produced by pod and stem blight. Picture courtesy of Daren Mueller. Iowa State University, Bugwood.org

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 8). 

  • Upper plant canopy turns yellow and dies.

  • Seed quality can be reduced.

  • Pod infection can occur at flowering; however, most are infected around the R7 growth stage (beginning pod maturity).

  • Injury to pods by insects favors pod infection.

  • Favored by wet weather during maturation growth stages and delayed harvest.

Management:

  • Crop rotation.

  • 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.  To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

     

Phomopsis Seed Decay

Figure 9. 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 9).

  • 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 resistant 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.

 

Northern Stem Canker

Figure 10. Northern stem canker lesions on a soybean stem.

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 10).

  • Spots develop into long (1 to 3 inches) 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 stem tissue. Plant death can occur from a fungus-generated toxin.

  • Foliar symptoms appear during reproductive 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 cooler temperatures and extended periods of rain occurring early in the growing season.

Management:

  • Plant resistant soybean products.

  • Plant a non-host crop for 2 years to reduce pathogen populations.

  • Use tillage to help destroy infected residue.

  • Applying a labeled fungicide at or before V3 infection may help protect plants.

 

Downy Mildew

Figure 11. Downy mildew on soybean leaves

Identification, Characteristics, and Diagnosis:

  • Caused by a fungus-like organism, Peronospora manshurica.

  • Infection occurs in the spring when the 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 11).

  • White to gray fungal tufts develop on the underside of the lesion.

  • Oldest lesions become grayish-brown to dark-brown with yellowish-green margins.

Management:

  • Plant resistant soybean products.

  • Rotate with a non-host crop for one year or more.

  • Rarely affects yield; therefore, foliar fungicides are not recommended.

  • Residue management.

  • Seed treatments can help protect seedlings from initial infection.

 

Frogeye Leaf Spot 

Figure 12. 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 12).

  • 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 resistant soybean products.

  • Crop rotation and tillage to encourage residue decomposition can help reduce pathogen levels.

  • Apply Delaro® 325 SC Fungicide.  To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

 

Fusarium Root Rot

Figure 13. Root damage caused by Fusarium root rot. Picture courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the several species of Fusarium a soilborne fungus.

  • Infected seedling roots become reddish-brown or dark- to light-brown and decay (damping-off) (Figure 13).

  • Older plants have reddish to black lesions on lateral roots and the tap root. As disease progresses, roots become black, the cortex decays, and fissures develop on the surface of the tap root.

  • Plants may be stunted, and leaves turn yellow from the edges inward with the veins remaining green for a short time; leaves eventually die and drop from the petioles.

  • Associated with stress (drought, herbicide damage, soybean cyst nematode pressure, and in combination with iron chlorosis).

Management:

  • Rotate with non-host crops.

  • Utilize seed treatments.

  • Plant into warm, well-drained soils.

  • Manage compacted soils through tillage.

  • Maintain good fertility.

  • Cultivation may promote new root growth when soil is thrown against the stem base.

 

Fusarium Wilt

Figure 14. Wilting associated with Fusarium wilt. Picture courtesy of Daren Mueller, Iowa State University. Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the soilborne fungus Fusarium oxysporum.

  • Plant wilting typically occurs when plants are in late reproductive growth stages. Hot, dry conditions increase wilting potential.

  • The vascular system in the stem and roots becomes brown, leaves become yellow, and defoliation can occur (Figure 14). 

  • Favored by cool temperatures and wet soils during early vegetative growth stages.

Management:

  • Manage soil compaction.

  • Delay planting until soil temperatures favor rapid germination.

  • Rotate to non-host crops.

  • Utilize seed treatments.

 

Phyllosticta Leaf Spot

Figure 15. Phyllosticta lesions on a soybean leaf. Picture courtesy of Daren Mueller, Iowa State University Extension and Outreach.

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 15).

  • 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.

 

Phytophthora Root Rot

Figure 16. Soybean seedlings killed by Phytophthora. 
 Figure 17. Phytophthora lesion on a soybean stem.

Identification, Characteristics, and Diagnosis:

  • Caused by the soilborne fungal-like pathogen Phytophthora sojae. 

  • Can infect seedlings and plants in reproductive growth stages.

  • Infected seedlings and plants 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 16). 

  • Plants infected later in the season have brown lesions on the roots, rotted roots, and a dark, chocolate-brown stem lesion extending upward several inches from below the soil line (Figure 17). 

  • 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 resistant soybean products relative to the identified race within the field. 

  • Utilize seed treatments.

  • Improve field drainage. 

  • Rotate to non-host crops.

  • Consider tillage to help destroy residue. 

  • Cultivation may promote new root growth when soil is thrown against the stem base.

 
Pythium Damping-Off

Figure 18. Damping-off resulting from Pythium infection.

Identification, Characteristics, and Diagnosis:

  • Caused by a group of soilborne fungal-like organisms (Oomycetes) related to Phytophthora. Prominent Pythium species include Pythium ultimum, P. heterothallicum, P. irregulare, P. attrantheridium, P. sylvaticum, and P. perplexum.

  • Pre-emergence infection results in seeds failing to germinate and deteriorating.

  • After emergence, Pythium-infected roots develop lesions, become discolored, deteriorate, and rot, resulting in seedling death (seedling blight or damping-off). Because of rotted roots, seedings can be easily pulled from the soil (Figure 18).

  • Seedlings can be killed within 24 hours if environmental conditions are favorable for disease development.

  • Pythium species can survive up to 10 years in soil and be reintroduced when plants become infected.

  • Secondary infections can occur because of the pathogen’s ability to travel with water and be introduced into previously non-infested fields through flooding.

  • Favored by saturated soils.

Management:

  • Seed treatments should be used, particularly for fields with a history of Pythium infection.

  • Improve field drainage through tiles, ditching, and compaction reduction.

  • Delay planting until soil temperatures favor rapid germination.

 

Red Crown Rot

Figure 19. Red lesions on soybean stems resulting from red crown rot.
Figure 20. Foliar symptoms of red crown rot. Picture courtesy of Dr. Guy B. Padgett, LSU AgCenter.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Calonectria ilicicola .

  • Pathogen overwinters on infected residue in and on the soil.

  • Root infection can occur soon after planting; however, 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 19), roots become black with areas of rot, and leaves have interveinal yellow or brown blotches (Figure 20).

  • Favored by moderate soil temperatures (77 to 86°F) and wet soil.

Management:

  • Rotate to non-host 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.

 

Rhizoctonia Damping-Off and Root Rot

Figure 21. Rhizoctonia lesions on soybean stems. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the soilborne fungus Rhizoctonia solani.

  • The pathogen can survive for years by overwintering in infected soybean residue and in the soil. 

  • The disease can cause early-season stand loss because of seed rot, root rot, and lesions that form on the hypocotyl (Figure 21). The hypocotyl lesions are reddish-brown and sunken.

  • Dead seedlings can be scattered within a field or in concentrated areas. 

  • Mid- to late-season symptoms appear as premature yellowing, stunted growth, and have the reddish-brown sunken lesion. 

  • Favored by warm, wet conditions; however, infection has occurred across a wide range of temperatures and soil moisture conditions.

Management:

  • Improve soil drainage and reduce compaction.

  • Utilize fungicide seed treatments.

  • Rotate with wheat and corn.

  • Fertility and soil pH should be managed per soil test results. 

 

Septoria Brown Spot (Brown Spot)

Figure 22. Septoria brown spot.

Identification, Characteristics, and Diagnosis:

  • Caused by the fungus Septoria glycines. 

  • Irregular, dark-brown lesions or spots that are often surrounded by a yellow halo develop on lower leaves (Figure 22).

  • 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:

  • Rotate to a non-host crop for at least one year (avoid continuous soybean). 

  • Improve field drainage if possible.

  • Planting later may reduce the potential for a saturated environment. 

 

Sclerotium Blight, Southern Blight

Figure 23. Sclerotium fruiting bodies on a soybean stem caused by Sclerotium blight. Picture courtesy of Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Caused by the soilborne fungus Sclerotium rolfsii.

  • Sclerotia overwinter in soil and can remain viable for 3 to 4 years.

  • The fungus infects plants when conditions are wet and hot (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 with a white fungal mass on or above the lesion.

  • Residue nearby infected plants may have fungal growth. 

  • Small, yellow/red/brown fruiting structures (sclerotia) can be observed on the stem (Figure 23).

  • Soybean plants are susceptible from emergence through pod fill. The greatest concern is when infection occurs during vegetative growth stages.

Management:

  • Rotate to corn or other non-host crops for at least 1 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.

 

Soybean Rust

Figure 24. Soybean rust

Identification, Characteristics, and Diagnosis:

  • Caused by the windborne 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 surface of mature leaves of the lower canopy (Figure 24).

  • 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 6 hours of leaf wetness (10 to 12 hours considered very favorable) and temperatures ranging from 70 to 80°F (infection can occur as low as 59°F). 

Management:

  • If local sentinel plots indicate the presence of Asian soybean rust, scouting should be diligent and thorough, particularly in early-planted fields, early-maturing soybean products, low-lying or fields with prolonged wetness, and fields with early canopy closure.

  • Apply Delaro® 325 SC Fungicide.  To learn more about Delaro® 325 SC Fungicide, please visit https://www.cropscience.bayer.us/products/fungicides/delaro and contact your retailer.

 

Sudden Death Syndrome

Figure 25. Sudden death syndrome of soybean.

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 (infection usually occurs on seedlings).

  • The spots progress to interveinal chlorosis (yellowing) and eventually the leaf tissue dies (Figure 25). 

  • Leaves may fall prematurely, leaving petioles attached.

  • Foliar symptoms can resemble stem canker, charcoal rot, and chemical injury.

  • Roots are rotted, pith tissue remains white (Figure 25), and xylem (cortical tissue) is gray to brown.

  • 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.

 Management:

  • Plant soybean products with higher tolerance ratings. Earlier-maturing products may have a lower potential for infection. 

  • Utilize soybean seed treatments.  

  • Delay planting until soil conditions are drier and warmer.

  • Improve field drainage, reduce soil compaction, and manage SCN.

 

Target Leaf Spot

Figure 26. Target spot lesions on a soybean leaf.

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 (Figure 26).

  • Mature spots may be 3/8 to 5/8 inches or more in diameter. Some may have a zonate appearance.

  • Areas of infection on stems and petioles are dark-brown and range from specks to elongated lesions.

  • Lesions developing on pods are circular, usually small (1/32 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.

  • Fungicides are not recommended because the disease has a low potential for yield reduction.

 

White Mold

 Figure 27. Wilting in the top of the soybean canopy due to white mold.
Figure 28. White mold growth on soybean stems.

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 27).

  • Stems become soft, watery, and covered with white mold (Figure 28).

  • 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 high population fields and narrow rows, high fertility, and possibly earlier planting.

Management:

  • Plant disease-free seed.

  • Select soybean products that may have some level of resistance.

  • 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; therefore, 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.

     

Soybean Nematodes

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.

  • Taproot and secondary roots are pruned. 

  • Nitrogen uptake and nodulation can be 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:

  • Several species of Pratylenchus .

  • 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.

  • Delay planting.

  • Rotate with corn.

     

Soybean Cyst Nematode (SCN)

Identification, Characteristics, and Diagnosis:

  • Heterodera glycines.

  • More common in sandy soils; however, SCN is well distributed throughout most soil types.

  • Penetrate 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

Management:

  • Plant soybean products that are resistant to field-identified races.

  • Utilize an in-furrow or seed treatment nematicide.

  • Rotate to non-host crops.

  • Manage weedy hosts.

  • Maintain adequate fertility.

  • Soil sample in the fall to determine SCN populations and HG types.

 

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, but no fibrous roots.

Management:

  • Utilize an in-furrow or seed treatment nematicide.

  • Rotate to non-host 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 resistant soybean products.

  • Utilize an in-furrow or seed treatment nematicide.

  • Rotate to non-host crops.

     

Viral Diseases

Bean Pod Mottle Virus (BPMV)

Figure 29. Bean pod mottle virus. Picture courtesy of Edward Sikora, Auburn University, Bugwood.org.

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 in lower leaves (Figure 29).

  • Delayed maturity or green stems are often observed near harvest.

  • Seed coat mottling may be present.

  • The virus overwinters in bean leaf beetles, which infect seedlings as they feed.

  • The virus can also overwinter in perennial weeds and infected seed. 

  • Plant infection by BPMV and soybean mosaic virus (SMV), 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 (cowpea (Vigna unguiculate), other bean species, and Demodiumspecies) can help reduce the inoculum source.

  • Delayed planting may increase early-season death of bean leaf beetles, reducing the vectoring population.

     

Soybean Mosaic Virus (SMV)

Figure 30. Soybean mosaic virus symptoms. Picture courtesy of Daren Mueller, Iowa State University, Bugwood.org.

Identification, Characteristics, and Diagnosis:

  • Vectored by the soybean aphid, Aphis gylcines.

  • A green/yellow mosaic pattern is the most common initial symptom on leaves.

  • More mature leaves may exhibit a yellow/brown mosaic pattern.

  • Premature defoliation is common.

  • Infected seeds exhibit a brown or black mottling (Figure 30).

  • The virus is spread 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 resistant 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.

 

Soybean Vein Necrosis Virus (SVNV).

Figure 31. Soybean vein necrosis virus.

Identification, Characteristics, and Diagnosis:

  • Vectored by soybean thrips, Neohydatothrips variablilis Beach.

  • Virus infection can occur throughout the growing season; however, symptoms are most visible after flowering, around mid-June.

  • 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 31). 

  • 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 thrips populations.

Management:

  • Control soybean thrips with timely and labeled insecticides.

  • Control alternate virus hosts, ivyleaf morning glory (Ipomoea hederacea Jacq), cowpea (Vigna unguiculate), and mung bean (Vigna radiata), can help reduce the inoculum source. 

 

Tobacco Ringspot Virus (TRSV)

Figure 32. Tobacco Ringspot Virus 

Identification, Characteristics, and Diagnosis:

  • Vectored by dagger nematode (Xiphinema americanum) at a low level; also transmitted by infected seed and mechanical distribution. The virus can be viable within the seed embryo for at least 5 years.

  • Infection causes leaves to become thicker, darker, and necrotic (Figure 32).

  • Infected stems remain green longer than healthy stems, pith and branches may become brown, and shepherd’s crooking may develop.

  • Stunting is common and fewer seeds are produced.

  • Pods are undeveloped or aborted because of insufficient pollen production, which can cause a proliferation of new buds and pods (green bean syndrome).

  • Favored by hot, dry weather. 

Management:

  • Plant virus-free seeds.

  • Manage dagger nematode populations.

  • Control weed hosts such as Palmer amaranth (Amaranthus palmeri) and lambsquarter (Chenopodium album).

  • Plant soybean fields next to corn rather than pastures.

Sources:

Fungus foliage diseases of soybeans. PPD N. 503. 1990. Reports on Plant Diseases. Integrated Pest Management. https://ipm.illinois.edu/
Thomas-Murphy, J. Alternaria leaf spot. Diseases of Soybeans. Field Crops. Cornell University. https://fieldcrops.cals.cornell.edu/
Geisler, L.J. Anthracnose. CROPWATCH. University of Nebraska – Lincoln. https://cropwatch.unl.edu/. 
Malvick, D. Anthracnose on soybean. University of Minnesota. https://extension.umn.edu/. 
Kelly, H.M. and Stewart, S. Soybean rust. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Smith, D. Asian soybean rust. Wisconsin Field Crops Pathology. University of Wisconsin. https://fyi.extension.wisc.edu/
Coker, C., Hurst, K., Kirkpatrick, T., Rupe, J., Tingle, C., and Trent, M. 2005. Asian soybean rust. FSA7531. Agricultural and Natural Resources. University of Arkansas. https://www.uaex.edu. 
Malvick, D. 2018. Bacterial blight of soybean. University of Minnesota. https://extension.umn.edu/
Faske, F., Kirkpatrick, T., Zhou, J., and Tzanetakis, I. 2014. Soybean diseases. Arkansas Soybean Production Handbook. Chapter 11. University of Arkansas. https://www.uaex.edu/
Faske, F., Kirkpatrick, T., Zhou, J., and Tzanetakis, I. 2014. Soybean diseases. Arkansas Soybean Production Handbook. Chapter 11. University of Arkansas. https://www.uaex.edu/
Giesler, L.J. Bacterial pustule. CROPWATCH. University of Nebraska. https://cropwatch.unl.edu./
Giesler, L.J. Bean pod mottle virus. CROPWATCH. University of Nebraska. https://cropwatch.unl.edu
Malvick, D. Bean pod mottle virus on soybean. University of Minnesota. https://extension.umn.edu/
Smith, D. Bean pod mottle virus. Wisconsin Field Crops Pathology. University of Wisconsin. https://fyi.extension.wisc.edu/
Westphal, A., Abney, T.S., and Shaner, G. 2006. Brown stem rot. Diseases of Soybean. BP-41-W. Purdue University. https://www.extension.purdue.edu/.  
Kelly, H.M. and Stewart, S. Cercospora leaf blight. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Hershman, D.E. 2009. Cercospora leaf blight in Kentucky. Plant Pathology Fact Sheet. PPFS-AG-S-20. University of Kentucky. https://plantpathology.ca.uky.edu
Smith, D. 2014. Cercospora leaf blight and purple seed stain of soybean. Wisconsin Field Crops Pathology. University of Wisconsin. https://fyi.extension.wisc.edu/
Melvick, D. 2018. Charcoal rot on soybean. University of Minnesota Extension. https://extension.umn.edu/. 
Hershman, D.E. 2011. Charcoal rot of soybean. Plant Pathology Fact Sheet. PPFS-AG-S-02. University of Kentucky. http://plantpathology.ca.uky.edu. 
Soybean pod and stem blight. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/
Malvick, D. 2018. Stem canker on soybean. University of Minnesota Extension. https://extension.umn.edu/. 
Kleczewski, N. 2017. Stem canker on soybeans. Field Crops Disease Management. http://extension.udel.edu/. 
Kelly, H.M. and Stewart, S. Downy mildew. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Faske, T. and Kirkpatrick, T. Downy mildew of soybean. University of Arkansas. https://www.uaex.edu. 
Kelly, H.M. and Stewart, S. Frogeye leaf spot. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Faske, T. 2016. Soybean disease update and management: Frogeye leaf spot. University of Arkansas. http://www.arkansas-crops.com/. 
Nelson, B. Fusarium root rot. Soybean Diseases. North Dakota State University. https://www.ndsu.edu/. 
Thomas-Murphy, J. Fusarium wilt. Field Crops. Cornell College of Agriculture and Life Sciences. https://fieldcrops.cals.cornell.edu/
Soybean management guide. Clemson University. https://scsoybeans.org/
Padgett, G. Nematode management in soybean. Louisiana State University. https://www.lsuagcenter.com/
Mulrooney, R.P. 2011. Soybean cyst nematode. PP-02. University of Delaware. https://extension.udel.edu/
Sisson, A. Phyllosticta leaf spot. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/
Phyllosticta leaf spot of soybean. Crop Protection Network. https://cropprotectionnetwork.org/
Malvick, D. 2018. Phytophthora root and stem rot on soybean. University of Minnesota. https://extension.umn.edu/
Smith, D. Phytophthora root and stem rot. Wisconsin Field Crops Pathology. University of Wisconsin. https://fyi.extension.wisc.edu/
Zitnick-Anderson, K., Markell, S., and Nelson, Jr., B. 2014. Pythium damping-off of soybean. PP1737. https://www.ag.ndsu.edu. 
Allen, T. 2012. Soybean seedling disease identification: Pythium damping-off and root rot. Mississippi Crop Situation. Mississippi State University. http://www.mississippi-crops.com. 
Padgett, B. 2011. Red crown rot. Root & Lower Stem Diseases. Soybean Disease Atlas. LSU AgCenter. Louisiana State University. https://www.lsuagcenter.com/
Scott, K. and Dorrance, A. 2017. Rhizoctonia damping-off and root rot of soybeans. Ohioline. The Ohio State University. https://ohioline.osu.edu/
Kelly, H.M. and Stewart, S. Brown spot. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Faske, T. and Kirkpatrick, T. Brown spot of soybean. University of Arkansas. https://www.uaex.edu. 
Kleczewski, N. 2017. Southern blight in soybeans. Field Crops Disease Management. University of Delaware. https://extension.udel.edu/
Faske, T. and Kirkpatrick, T. Southern blight of soybean. University of Arkansas. https://www.uaex.edu/
Zhou, J. and Tzanetakis, I.E. Soybean mosaic virus. University of Arkansas. https://www.uaex.edu
Mueller, D. 2013. Soybean vein necrosis virus identified in Iowa. Integrated Pest Management. Iowa State University. https://crops.extension.iastate.edu/
Byamukama, E., Dorrance, A., Jardine, D., Malvick, D., Markell, S., Nachappa, P., Sisson, A., and Sweets, L. 2015. Soybean vein necrosis virus. Soybean Disease Management. CPN-1003. Crop Protection Network. www.soybeanresearchinfo.com/
Zhou, J. and Tzanetakis, I.E. Soybean vein necrosis virus. University of Arkansas. https://www.uaex.edu/
Kelly, H.M. and Stewart, S. Southern stem canker. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Spurlock, T. 2015. Sudden death syndrome of soybean found. Arkansas Row Crops. University of Arkansas. http://www.arkansas-crops.com. 
Kelly, H.M. and Stewart, S. Target leaf spot. Institute of Agriculture. The University of Tennessee https://guide.utcrops.com
Faske, T. and Kirkpatrick, T. Target spot of soybean. University of Arkansas. https://www.uaex.edu. 
Zhou, J. and Tzanetakis, I.E. Tobacco ringspot virus. University of Arkansas. https://www.uaex.edu. 
Malvick, D. 2018. Sclerotinia stem rot (white mold) on soybean. University of Minnesota Extension. https://extension.umn.edu. 
Smith, D. White mold of soybean (Sclerotinia stem rot). Wisconsin Field Crops Pathology. University of Wisconsin-Madison. https://fyi.extension.wisc.edu.

(Sources verified 6/2019) 1007_G1