Diseases Commonly Found in Soybean Fields

Soybean diseases of the leaf, root, stem and pod can cause plant damage and yield loss. Yield damage caused by diseases in soybeans depends upon several factors, including how susceptible a particular soybean variety is to a specific disease, the existing level of pathogen inoculum, and weather and other environmental conditions.

With the goal of maximum yield and excellent grain quality, growers must help their soybean crops survive multiple diseases throughout the growing season, such as frogeye leaf spot, Septoria leaf spot, soybean rust, white mold and sudden death syndrome (SDS). Many soybean disease pathogens cause blights and rots, which exhibit infection through various signs in plant tissues.

Soybean disease management is achieved through an integrated approach of best management practices, which includes disease identification, seed treatments and fungicides. This approach begins with becoming familiar with the various diseases found in soybean fields.

Septoria leaf spot, or brown spot, usually starts on the lower section of the soybean plant, moving up through the tissues under favorable conditions
Septoria leaf spot, or brown spot, usually starts on the lower section of the soybean plant, moving up through the tissues under favorable conditions. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org.

Soybean Leaf Diseases

Foliar diseases are often called “blight” because of the plant’s response to infection in the leaves, seen in the rapid chlorosis, browning and death of plant tissues.

Septoria Leaf Spot (Brown Spot)

Brown spot causes small, angular to somewhat circular brown or red spots on unifoliate and lower trifoliate leaves. Infected unifoliate leaves will yellow and drop prematurely. The disease usually appears on the lower section of the plant. Under favorable warm, wet conditions, the disease may travel up through the plant. Infection can occur at any stage of soybean development, but it often shows up after flowering. Brown spot disease survives in infested residues left on the soil surface. During wet spring weather, spores on residues are blown or splashed onto cotyledons or unifoliate leaves. Continuous soybeans are more likely to exhibit damage.

Frogeye Leaf Spot

Symptoms occur primarily on leaves. Lesions consist of small, circular to irregular dark spots, which develop on the upper leaf surfaces. Eventually the spots enlarge and inhibit photosynthesis. The centers of the lesions become gray to brown and have reddish purple margins. Heavily spotted leaves usually wither and drop off prematurely. Leaves developing during warm, wet conditions are more likely to be infected than leaves expanding during dry periods.

Prevalent across soybean growing regions, frogeye infection can occur at any stage of soybean development, most often during flowering. It is most serious in warm regions or during humid, wet weather.

Asian Soybean Rust (Soybean Rust)

The most common symptom is foliar lesions, which appear as small, yellow specks on the tissue of the upper leaf surface, concentrating near leaf veins. They are typically angular to somewhat circular in shape. These lesions then darken, with colors ranging from reddish-brown or dark brown to tan or gray-green. With heavy infections, lesions may be larger and merge together, killing large areas of leaf tissue.
When conditions are favorable for the development of this disease, foliage may yellow and defoliation and premature plant death may result.

Powdery Mildew

Powdery mildew in soybeans is a windblown pathogen, which requires cool air temperatures and low relative humidity. Symptoms on the leaves include green and yellow “islands,” interveinal necrosis, necrotic specks and crinkling of the leaf blade followed by defoliation. Necrosis causes leaves, stems and other plant structures to darken and wilt, weakening the soybean plant and making it more susceptible to other diseases and pests.

Other Foliar Diseases

Other common foliar diseases include early-season anthracnose, which strikes leaves early in the season but is more typically a stem and pod disease during later reproductive growth; and Cercospora leaf blight, which attacks both the leaves and seeds.

Common root stem and pod diseases in soybeans

Many common root, stem and pod diseases can also cause infected plants to wilt, turn brown or even die prematurely.

Pod and Stem Blight

This soybean blight disease may be more common when harvesting is delayed during wet weather. Symptoms may be confused with anthracnose because both diseases can occur together on plants late in the season. Signs of infection can appear in mid-season and on plants nearing maturity.

Key signs of soybean pod and stem blight are many small, black, raised dots arranged in rows on infected stems, pods and fallen petioles late in the season. Upper portions of infected plants may turn yellow and die. Infected seeds are cracked, shriveled and dull and may have a gray mold. Infected seeds can have low viability and decompose at harvest.

Sclerotinia Stem Rot (White Mold)

Sclerotinia stem rot, also called white mold, is particularly a problem in the upper Midwest. Early symptoms may be the wilting of leaves in the upper canopy.

Leaves may look gray-green, off-color or wilted. Cankers may appear on the stems at the nodes. White mold can cause extensive yield losses in fields that otherwise have high yield potential. It is prone to develop in moderate temperatures (less than 82 degrees F) in the canopy and under frequent rainfall, especially as the plants begin to flower and set pods.

Sudden Death Syndrome (SDS)

This soilborne fungus (Fusarium virguliforme) generally appears midsummer, after blooming. Infection actually occurs at the seedling stage, but visual symptoms above ground are late in the season. SDS is identified by small, scattered yellow blotches or spots that begin in the lower leaves and progress upward with maturity. The spots enlarge, merge and turn the tissues brown between the veins. Leaves may curl up or drop prematurely, but the petioles remain firmly attached. Fields with severe foliar symptoms will have a tan to brown cast. Next, flowers abort and pods fail to fill seed. One characteristic of SDS is that the interior, or pith, of the stem remains white. SDS can occur as a disease complex with soybean cyst nematode (SCN), often resulting in early and more severe symptoms.

Fusarium fungi over season in crop residue or soil. The disease is most severe in cool, wet growing conditions and on well-managed soybeans that have high yield potential. The disease is a serious threat from the time the seed starts to germinate and throughout the growing season A soybean field may look healthy one day and startlingly bad the next. With the above ground symptoms appearing so far into the growing season, replanting is not an option.

Rhizoctonia Root Rot

While disease caused by the fungus Rhizoctonia can cause seedling blight, it also causes root rot in more mature soybean plants. At this later growth stage, lower leaves may begin to turn yellow. Plants may appear stunted and less vigorous. Infected plants, when dug up from the soil may reveal poorly developed root systems and lateral roots that are discolored or rotted. The stem may have a brick-red discoloration at the soil line that extends in either direction. Severely infected soybean plants that are stressed by hot, dry conditions may die. If cool, wet conditions occur after a Rhizoctonia infection, a flush of secondary roots above the stem may be visible. Rhizoctonia root rot symptoms often mimic those of charcoal rot.

Charcoal rot

Also known as dry weather rot, charcoal rot is caused by a widespread soilborne fungus that attacks the roots and bottom section of the plant. Charcoal rot typically develops in hot, dry weather in combination with nutrient-deficient soils or unfavorable growing conditions. In advanced stages, leaves turn yellow, wilt and wither, but remain attached. After the flowering stage, the taproot and lower stem appear silver or light gray. In later stages of the disease, black specks appear underneath the “bark” that is peeled away from the roots and stem base – hence the name charcoal rot. Seedling infestations of charcoal rot may mimic those of Rhizoctonia, producing reddish lesions on the stem of the seedling. Where charcoal rot is present, these lesions can be scraped off with light pressure.

Phytophthora Root Rot

Plants infected early in the season with the fungus Phytophthora may appear as leaf blight, not showing symptoms until later in the season. As plants enter the flowering stage and begin to set pods, symptoms of late-season stem and root rot may develop. Infected older plants show reduced vigor or gradually die over the season. The stems show brown discoloration, which extends upward from below the soil line.

Anthracnose

Typically, this fungal disease develops as a stem and pod disease on soybean plants during later reproductive growth. Anthracnose may also cause tip blight in early pod-filling stages. Irregularly shaped brown lesions may develop on pods, petioles and stems. The fungus also produces small, black, fruiting bodies, or acervuli, that erupt in infected tissue. Anthracnose typically develops in warm, wet weather. If wet weather continues through harvest, the symptoms on stems will be more severe.

Brown Stem Rot

This disease is caused by the fungus Cadophora (Phialophora) gregats, which survives in soil and infested crop residue. Foliage symptoms usually develop as soybean plants are beginning to set pods. Leaf symptoms, which may be similar to those of sudden death syndrome, appear as light green to yellow blotches in the interveinal leaf tissue. The vascular tissues and pith of the soybean stem is also affected with a brown discoloration; this can be seen when the stems are split open. Later in the season, this brown discoloration may appear almost continuous within the stem.


Managing Soybean Diseases

Best management practices help minimize the potential for fungicide resistance and include cultural practices, planting disease-tolerant cultivars and applying fungicides with multiple modes of action.  

Disease-tolerant seed traits, specific to soybean growers, can be found in Credenz® soybean seed. Credenz can provide built-in protection against stem canker, frogeye leaf spot, Phytophthora root rot and Sudden Death Syndrome (SDS), plus multiple herbicide-tolerant traits and nematode control. When followed by timely fungicide applications, Credenz soybeans are best protected against yield-robbing diseases.

A fungicide seed treatment can provide a healthy start for seedlings. Multiple fungal pests can impact soybean seeds and seedlings immediately after planting, and EverGol® Energy SB can protect against pests causing the most damage such as Rhizoctonia solani, Fusarium spp and Pythium spp.  In cool and damp conditions, ILeVO® soybean seed treatment from Bayer protects against the fungus causing SDS in soybeans. ILeVO is an important risk-management tool for growers to protect against SDS. The disease is a serious threat from the time the seed starts to germinate and throughout the growing season. A soybean field may look healthy one day and startlingly bad the next. With the above ground symptoms appearing so far into the growing season, replanting is not an option. Additionally, since SDS can occur as a disease complex with soybean cyst nematode (SCN), planting soybean seed treated with Poncho/VOTiVO will help protect seedlings against nematodes as well as certain insects during the critical early development stage.

Also, timely fungicide applications, including foliar fungicides, help protect soybean plants from fungal diseases through the season. For farmers, a fungicide decision is a matter of considering production needs, past history in the fields, commodity prices, proper timing and risk management.

Stratego® YLD fungicide features the latest in triazole technology combined with strobilurin chemistry for soybeans. Offering two modes of action, it provides both preventive and curative benefits and systemic movement to provide broad-spectrum, long-lasting disease control and higher yield potential. Trials show that when applied at early flowering, around growth stage R3, Stratego YLD can deliver an average yield bump of 3.25 bushels per acre over untreated soybean fields. For cost efficiency, fungicides can be tankmixed with insecticides and applied in the same trip across the field.

Crop Science also offers Proline® fungicide to help farmers manage diseases in soybeans. It suppresses white mold and offers an effective tool for disease resistance management in various diseases. Proline can be used in a program with Stratego® YLD

For more information on soybean disease control options from Bayer, contact your local Crop Science representative or visit www.bayercropscience.us/crops/soybean.

Before purchasing seed, selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Every product is not suitable for every situation, and correct application technique will ensure the best results.

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