Diseases of soybean can be caused by numerous microorganisms (fungi, bacteria, viruses, and nematodes) that can damage plants, reducing vigor, quality, and yield. Seeds, seedlings, and older plants may all be affected by these diseases. Diseases can spread from plant to plant and cause damage over time. Plants under stress, such as from drought, excessive moisture, extreme temperature conditions, or chemical injury or other physical injuries, are more susceptible to disease. Symptoms of disease vary depending on the associated pathogen, and these symptoms may also be influenced by variety, environmental conditions, or physical injuries.

Disease management requires good use of the management tools available. Integrated pest management (IPM) incorporates the use of cultural and chemical management as well as the use of host resistance to reduce economic impacts of disease in soybean production. These tools aim to interrupt the interaction of the plant, the pathogen, and the environment that is suitable for disease development. In addition to reducing disease severity, these practices reduce the risk of fungicide resistance and race shifts of organisms.

Cultural Practices

Cultural practices, such as crop rotation, planting dates and density, and tillage, affect the development of plant diseases in several ways. Crop rotation can break the life cycle of many pathogens that cause disease, as the crops in rotation are hosts for specific pathogens. Adjusting planting dates can also minimize risk; for example, later planting dates of soybeans may reduce the risk of soilborne diseases because the pathogens causing root rot and damping-off prefer moist soil and cooler soil temperatures. Practices such as planting density or tillage methods may affect the microclimate of the canopy or root zone. Foliar pathogens require moisture for infection and reproduction; therefore, reducing humidity in the canopy by avoiding high populations or changing plant spacing to allow for more airflow between plants may reduce disease development. Tillage practices that reduce water within the root zone may also reduce the incidence of root-rotting organisms, such as Phytophthora sojae or Pythium spp. Cultural practices may also reduce the interaction of the pathogen with the soybean host. Tillage of crop debris may increase the degradation of surviving microorganisms or, in the case of deep tillage, remove the pathogens from the system.

Chemical Control

Chemicals are an important tool for the management of several fungal diseases and nematodes. Chemicals are most effective when applied preventively, and few have curative effects. Fungicides are used to manage soilborne diseases (in the form of seed treatments or drench treatments) and foliar diseases that may cause economic losses. Nematicides are also available for the reduction of nematode populations; however, these are often cost prohibitive for use in soybean production. Various fungicides provide different levels of control of a given pathogen (Table 7-1 and Table 7-2). Proper identification of the causal agent for a problem is crucial for selecting fungicides.

Host Resistance

Host resistance is the most effective and economical way to reduce disease severity in soybean. Disease and nematode resistances are available in a range of maturity groups and with different agronomic characteristics. More information on host resistance can be found from seed companies or through the NC State Official Variety Testing data and Variety Selection Tool.

Disease Scouting

Correct identification of causal agents is key to determining how to approach disease management. Proper and regular disease scouting is necessary to catch problems early. Disease often occurs randomly throughout a field in circular or elliptical patches. Scouting is best conducted in an irregular pattern, such as a zigzag or checkerboard pattern, to increase the likelihood of walking through a diseased area. If a particular region within the field has been frequently and repetitively affected, scouting that area first may allow for early implementation of disease management strategies. Scouting fields for diseases is good practice to determine if chemical strategies are necessary. Judicious use of fungicides, applied only when needed, can result in higher economic returns and reduce the risk of fungicide resistance developing.

Several economically important soybean diseases are present in North Carolina each year. These diseases may produce similar symptoms but require different management strategies. Accurate identification of these diseases is important for selecting the proper management strategy.

Seed and Seedling Diseases

Seedling diseases cause less than 1% of disease losses in North Carolina each year but are a chronic issue that may require management. Several soilborne fungi are responsible for losses, including Fusarium spp., Pythium spp., Rhizoctonia solani, and Phytophthora sojae. These fungi can reduce stands and limit yield potential.

Environmental factors. Seedling diseases are often more severe in poorly drained soils, especially when high rainfall or cold weather follows planting. Diseases of seeds and seedlings may occur in any soil type. Environmental factors that delay plant germination and emergence—such as poor seed quality, inadequate seedbed preparation, soil compaction, and improper planting depth—increase the likelihood of disease.

Symptoms. Evidence of seed or seedling diseases occurs usually with patchy emergence, or when seedlings are stunted or die just after emergence. Symptoms can be difficult to distinguish from herbicide injury, but the pattern of injury may indicate the causal agent. Seed and seedling disease symptoms often occur in irregular patterns and may be related to differences in soil type. Damages caused by herbicides are often associated with a pattern related to equipment, but soil type can also influence herbicide injury.

Rhizoctonia solani can damage seeds or damage seedlings prior to or just after emergence. On seedlings, R. solani can cause a red-brown decay at the soil line. In some cases, the damage can girdle the stem and kill the seedlings, or plants may be only superficially affected and outgrow infections.

Pythium spp. can affect seeds and seedlings prior to emergence or cause damping-off under cool, wet conditions following emergence. Tissues affected by Pythium spp. are soft and brown-colored, and symptoms are similar to those caused by Phytophthora sojae. Although they primarily affect seedlings, Pythium spp. can also cause root rot in established plants.

Phytophthora sojae can affect soybeans at any growth stage and causes a soft, brown rot of tissues affected by the pathogen. As plants affected by P. sojae mature, leaves yellow and plants frequently wilt and die. Soybeans affected later in the season will also present symptoms on the stem, as the infection makes its way up from the root and crown. The stem will appear dark-purple to black and have a distinct line on the stem and between internodes. Distinguishing Pythium and Phytophthora infections at the seedling stage is possible only in a laboratory.

Fusarium spp. can also affect soybeans at any growth stage. Infection by Fusarium spp. causes brown lesions on roots and damping-off of seedlings. Mature soybeans that are affected later in the season may present yellowing and wilting of the plant. Fusarium may also attack the taproot and promote adventitious root growth.

Seedling Disease Management. Seedling disease can be reduced by planting good-quality seed in well-drained soils. Rotating soybean with a grass crop such as corn or grain sorghum may help to reduce populations of soilborne diseases. In fields with poorly drained soils, planting dates should be postponed to when conditions for soybean germination and rapid growth are optimal. Seed treatments may be beneficial when planting in cool, wet soils. Several different seed treatments are labeled for soybean, with various combinations of chemistries available that control different causal agents of seedling diseases. Properly identifying and documenting the history of the disease-causing organism in fields with previous disease issues will inform more accurate choices in seed treatment selection.

Foliar Diseases

Foliar diseases of soybean reduce yields in North Carolina by about 1%. These diseases are largely managed by using resistant varieties or applying fungicides. In regions with high levels of rainfall and humidity, foliar disease epidemics may be more severe and warrant active management practices. Several bacteria and fungi cause foliar diseases of soybean, and each causal agent requires different management methods in North Carolina.

Frogeye Leaf Spot

(Cercospora sojina)

Frogeye leaf spot is primarily a foliar disease of soybean but is capable of being seedborne. Disease epidemics can become severe if infected seed is planted or if soybean is planted following a disease epidemic in the previous growing season. The fungus survives in infested crop residue, and continuous soybean fields may be more affected due to persistence of inocula.

Symptoms. Lesions on leaves may be circular or angular, gray spots with reddish-purple margins. These spots are first visible on the upper surface of the leaf. As lesions age, the centers become gray or light brown, sometimes with small dark spots in the center. Lesions may coalesce to form larger, irregular spots. In humid environments, the lesions on the underside of the leaf may develop a fuzzy appearance that can be seen with a magnifying glass; this is sporulation of the pathogen on the leaf. Heavy infestations may lead to premature defoliation of the plant. While young leaves are most susceptible to infections, lesions may not be visible until leaves mature because it takes several weeks for lesions to develop.

Under persistent heavy rainfall or humidity, stems and seeds may also become infected. Stem lesions appear as narrow, brown lesions that become light gray with dark margins as the lesions mature. Lesions on pods are initially circular and red-brown, then become gray with a dark margin.

Management. Planting resistant varieties or certified disease-free, high quality seed is the best mechanism for preventing disease outbreaks. Continuous soybean rotations also promote significant buildup of residues and should be avoided. Following a year with frogeye leaf spot infestation, plant a resistant variety. Applying foliar fungicides (Table 7-2) between the growth stages of R1 and R5 may be warranted where there are favorable environmental conditions and susceptible varieties. FRAC (Fungicide Resistance Action Committee) Group 11, also referred to as QoI or strobilurin fungicides, are commonly used; however, strobilurin-resistant Cercospora sojina populations have been detected in several North Carolina counties, so using a premix fungicide with multiple modes of action may help prevent losses of control.

Cercospora Blight and Purple Seed Stain

(Cercospora kikuchii)

Cercospora leaf, stem, and pod blight is one of the most common foliar diseases in North Carolina, though it often accounts for only 0.3% of soybean yield losses. It can, however, cause significant damage under warm, wet conditions. When seed becomes infected, it causes purple seed stain, resulting in reduced seed quality and potential reduced value of the crop.

Symptoms. Initial signs of infection are found in the upper canopy on young leaves during pod-filling stages and into maturity. Affected leaves show irregularly shaped patches of purple to bronze discolorations, typically only on the leaf surface, resembling sunburn and leaf scorch. Severe infections may result in premature defoliation of the uppermost leaves, while lower leaves remain green and attached to the plant.

When seeds are affected, a purple to pink discoloration may occur in small spots or on the entire surface of the seed. Infected seed may also be asymptomatic. Seedlings from infected seed will show symptoms of the disease, causing cotyledons to turn purple, shrivel, and drop. Stem lesions may also form on seedlings, which can girdle and kill the seedling. Seedlings that survive produce stunted plants and limit yield potential.

Management. A combination of cultural practices and host resistance will provide the best control of this disease. Promoting airflow and good soil drainage may help reduce incidence of cercospora blight. Crop rotation and residue management, such as tillage to promote rapid decomposition of crop residue, will also help reduce inoculum buildup from previous soybean crops. Choose more-resistant varieties and plant certified, disease-free seed. Fungicides are not generally recommended, and no fungicides are labeled with high efficacy (Table 7-2).

Septoria Brown Spot

(Septoria glycines)

Septoria brown spot causes 0.5% of yield losses in North Carolina soybean production and can be found throughout the growing season. Warm, wet weather favors disease development, and dense plantings favor disease development in the lower canopy.

Symptoms. Lesions, which are small, irregularly shaped, and dark brown, can be found on all leaf surfaces. Adjacent lesions can coalesce to form large, irregularly shaped brown spots. Infected leaves quickly turn yellow and drop. Disease typically begins in the lower portion of the canopy and, under favorable conditions, progresses into the upper portions of the canopy.

Management. Crop rotation and tillage reduce inoculum availability. Other legume species are also hosts, so crop rotations should include nonleguminous plants like corn or small grains. Foliar fungicide applications may be warranted under conducive environmental conditions. It is important to rotate chemicals with different modes of action each production season (Table 7-2). Applications made between R3 and R5 may slow the rate of development to protect yield.

Soybean Rust

(Phakopsora pachyrhizi)

Although soybean rust can cause defoliation and significant yield loss, the pathogen does not overwinter in North Carolina. The alternate host for soybean rust is kudzu; however, environmental conditions during the winter months in North Carolina generally prevent overwintering from occurring. Each year, inoculum disperses northward, and inoculum is tracked using the Soybean Rust IPM Pipe. Cool, wet weather and prolonged leaf wetness (more than six hours) allow for spore germination, and conditions with high humidity encourage disease development after establishment.

Symptoms. Rust lesions are tan to reddish-brown and may occur on any green surface of the plant. Lesions contain one to three pustules that are raised on the leaf surface. Lesions may have an angular shape and may be confused with bacterial pustule lesions. Placing leaves in a bag with a moistened paper towel for 24 hours may cause pustules to erupt and expose spores. Infected plants may defoliate early and have smaller seeds. If you suspect that rust may be present in your field, contact your county Extension agent to send samples to the Plant Disease and Insect Clinic for confirmation.

Management. No rust-resistant soybean varieties are commercially available in North America. When rust is found within 100 miles of a field, foliar fungicides may be needed to manage disease. To adequately control rust, fungicides need to be applied prior to or soon after first infection. Foliar fungicides (Table 7-2) applied between R3 and R5 may reduce the level of diseases that lead to yield loss.

Bacterial Blight

(Pseudomonas syringae)

Pseudomonas syringae causes angular leaf spot and bacterial blight, which usually result in minimal damage in North Carolina. "Angular leaf spot" and "bacterial blight" are often used interchangeably to describe the same disease, which is a bacterial infection that affects various plants, including cotton. The name angular leaf spot refers to the shape of the lesions caused by the disease on the leaves, while bacterial blight refers to the disease as a whole. More severe outbreaks are associated with heavy rainfalls and winds. Infections occur through natural openings in the plant or through wounds, such as those caused by hail or cultivation. High temperatures often slow or stop the development of the disease.

Symptoms. Young leaves are most susceptible to infection, and leaf spots may appear on any portion of the plants. Lesions are often small, angular, and brown to red-brown, with a water-soaked margin. A yellow halo often surrounds margins. Lesions will often grow together to produce large, irregularly shaped necrotic areas of the leaf, the centers of which frequently fall out. On stems and petioles, lesions are large and black but do not share the angular margins of leaf infections.

Management. While complete resistance to bacterial blight is not currently available in commercial varieties, difference in tolerance has been observed in the field. Selecting a variety with some tolerance to bacterial blight may reduce incidence where there is a history of angular leaf spot and bacterial blight. Crop rotation and tillage to reduce the survival of inocula may aid in limiting infections.

Stem and Root Rots

Vascular diseases are the most common disease in North Carolina soybeans. These diseases occur early in the growing season but show symptoms from mid-to-late summer when hot, dry conditions are persistent. These diseases are often a complex of pathogens and may be caused by other problems, such as inadequate fertility, soil compaction, poor drainage, or nematode or insect damage. Management of these disease pathogens often relies on cultural practices or the use of resistant varieties to prevent significant losses of yield, as previous research has shown that fungicides have sporadic efficacy. Accurate diagnosis of the causal agent is important for selecting proper management tools for the disease.

Sudden Death Syndrome

(Fusarium virguliforme)

Sudden death syndrome is usually of minor importance in North Carolina soybean production. Disease symptoms are typically more severe under high-yield environments and are associated with nematode damage.

Symptoms. Sudden death syndrome symptoms resemble damage caused by other pathogens and insects, including phytophthora root rot, charcoal rot, stem canker, brown stem rot, southern blight, and, occasionally, dectes stem borer damage. Early symptoms, if present, include interveinal yellow (chlorotic) spots on leaves, beginning at R3 or later growth stages. As the soybean plant matures, forming pods, the yellow spots may coalesce, while veins remain green. The yellow between veins will become brown and necrotic as the tissue dies; this often leads to premature defoliation, although the petioles often remain attached to the stem. Pods may also be aborted. Roots exhibit rot, and affected plants can be easily pulled from soil. When split open, the vascular tissue of roots will be reddish-brown to brown; this discoloration usually does not extend more than 1 or 2 inches above the soil line. Occasionally, the fungus will form white masses on the roots that become blue to blue-green as the fungi mature.

Management. Proper identification of the causal agent is important for proper management, and plants with suspected sudden death syndrome should be sent to the Plant Disease and Insect Clinic for verification of diagnosis. Soybean varieties with resistance to sudden death syndrome should be considered in fields with a history of the disease. Disease tends to be more severe under reduced tillage. Late planting may also suppress disease development.

Charcoal Rot

(Macrophomina phaseolina)

Charcoal rot is an infection of the root that has the potential to limit yields when weather conditions are hot and dry.

Symptoms. Disease symptoms usually appear after the onset of reproductive stages. Affected plants produce slightly smaller leaflets and have reduced vigor. Leaves become yellow, then wilt and turn brown. Brown leaves remain attached to the petioles. When stems of infected plants are dissected, speckled black streaks will be evident within the vascular tissue of the stem near the crown and soil line. Light-gray to silver discoloration may be visible on the root, and the black specks or microsclerotia (the overwintering structures) will be present in tissues of the stem and taproot.

Management. Resistant varieties are not available, but soybean varieties vary in susceptibility. Crop rotation with small grains can reduce inoculum buildup in the field. Corn is also a host, so it is not a good rotation to reduce disease. Lowering populations of seeding rates can reduce competition for moisture and minimize risk for disease development. Supporting plant health may reduce the impact the disease has on yield.

White Mold

(Sclerotinia sclerotiorum)

White mold has a wide host range and may affect several rotational crops of soybean. White mold is more prevalent in cooler, wet seasons. Under high pressure, disease can reduce yield and quality of soybeans.

Symptoms. White mold is characterized by fluffy, white growth on soybean stems, with lesions initially developing at nodes from R3 to R6. Lesions expand above and below nodes, and may girdle the whole stem. Black sclerotia eventually develop within the white mycelium on stem lesions and inside the plant. Severe infections can result in wilting, lodging, and plant death. Dead leaves remain attached to the stem.

Management. Rotate soybean with a nonhost crop such as corn or small grains. Crops should not be rotated with beans, peas, sunflowers, or cole crops. Increased tillage promotes inoculum release and viability. Cultural practices that favor high humidity, including high population plantings and narrow row widths, can increase disease incidence. Weeds may harbor the disease, so managing weeds can suppress disease. Foliar fungicides can reduce disease severity and are most effective when applied immediately before infection.

Southern Stem Canker

(Diaporthe phaseolorum var. meridionalis)

Southern stem canker is endemic throughout the South. The fungus survives in infested residue or in the soil for several years and is splash-dispersed to the lower stems of soybeans early in the growing season.

Symptoms. Symptoms are typically observed during the reproductive phases of the plant; however, infections typically occur during early vegetative growth. Initial stem symptoms are characterized by small, reddish-brown lesions on the main stem at the base of a lower node. As the lesion expands and elongates, it becomes sunken, with dark-brown to black margins. To discover symptoms, dissect the lesion to look for the presence of discolored vascular tissue where the canker is located. Under favorable (humid) conditions, small black spots (stroma) may also appear on the dead tissue within the canker. Lesions may expand down toward and below the soil line and can be easily confused with other root rots. Foliar symptoms appear at the onset of flowering and begin as interveinal chlorosis that becomes necrotic as the disease progresses. Dead leaves often remain attached to the stem. The symptoms of this disease are similar to other common diseases in North Carolina and can be easily misdiagnosed if not lab verified. Submission to the Plant Disease and Insect Clinic may be necessary for proper diagnosis.

Management. The best management method for southern stem canker is to plant resistant soybean varieties. Rotation with a nonhost crop, such as corn, wheat or sorghum, may help reduce the amount of inoculum available to infect the next soybean crop. No-till or conservation tillage may promote the survival of the stem canker fungus. Fields with a history of stem canker are at a higher risk for stem canker infections due to the long duration of survival of the fungus on crop debris. There are no data to support the efficacy of foliar fungicides for the management of stem canker.

Phomopsis Stem and Seed Decay

(Phomopsis longicolla)

P. longicolla can affect stems, petioles, pods, and seed. Disease is usually associated with warm, humid conditions when soybean plants are maturing. The disease is also more severe if harvest is delayed.

Symptoms. Lesions on stems appear similar to those caused by the stem canker pathogen, with gray middles and darkened margins. Black specks (fruiting bodies) within mature lesions of stems form in linear rows. Infected seeds are cracked, shriveled, and often covered in chalky, white fungal tissue.

Management. Cultural practices that reduce humidity levels within the canopy include wider row spacing and lower plant populations. Select high-quality, certified seed to avoid infection. Rotation with nonhost crops, such as corn, and tillage will reduce the survival of P. longicolla. Foliar fungicides may protect seed from new infections but may not affect yield. Harvesting early-maturing varieties first may help reduce the incidence of damaged seed.

Fusarium Wilt

(Fusarium spp.)

Fusarium wilt is a widespread disease in the United States and may have a significant impact in years when plants are significantly weakened. Damage caused by herbicides, high soil pH, nematode feeding, and nutritional disorders may predispose plants to this disease. Disease may also become worse under dry conditions, due to compromised root systems of the plants.

Symptoms. Infected plants have discolored, brown vascular tissue in the roots and in the crown near or below the soil line. Scorching of leaves in the upper canopy and chlorosis of leaves in the middle and lower canopy can be observed in infected plants. Severely affected plants with compromised root systems may wilt and die.

Management. Seed treatments may help reduce seedling disease caused by Fusarium spp. Reducing the impact of stress, such as avoiding injury from nematodes, herbicides, or excessively wet or dry soils, may help to reduce the incidence of fusarium wilt. In fields with a history of fusarium wilt, planting when soil temperatures are warmer may help to establish stands more quickly and reduce early infections.

Phytophthora Root Rot

(Phytophthora sojae)

Root rot caused by Phytophthora sojae is most severe in poorly drained soils. This pathogen can cause damping-off and seedling disease but may also cause losses later in the season if warm temperatures (above 60°F) and heavy rainfalls occur.

Symptoms. Symptoms on older plants may vary based on variety. Leaves typically become chlorotic between the veins, and plants wilt and die. Varieties that are tolerant or resistant may appear stunted. Characteristically, a dark-brown or black lesion of the lower stem that extends upward from the taproot will often girdle the stem, which may stunt or kill the plant.

Management. Use of resistant varieties in fields with poor drainage or a history of P. sojae infections may help reduce disease incidence. Continued soybean production may increase disease severity, but rotation may not reduce inoculum that is already in a field due to the long survival of spores. Seed treatments that contain mefenoxam or metalaxyl may help to reduce incidence of seedling infections in fields with a history of disease or poor soil drainage.

Soybean Viruses

Several viruses can affect soybean production in the Southeast on an annual basis; however, yield losses are generally low. Despite low incidence statewide, individual fields may be significantly impacted if heavily affected by viruses.

Viruses are microscopic and are made up of DNA or RNA enclosed in a protein coat. These viruses require a living host to reproduce, and several important viruses are transmitted by insect vectors. Although it is not possible to directly observe the virus without an electron microscope, tests for several viruses are available to accurately identify the virus. See the Soybean Research and Information Network for information about soybean viruses. Test kits may be purchased from Agdia, and samples may be submitted to the Plant Disease and Insect Clinic.

Soybean Mosaic Virus

Soybean mosaic virus (SMV) is the most common soybean virus encountered within North Carolina. This virus can be seedborne or transmitted by aphids. Yield losses to SMV are generally related to the time of infection. For example, when SMV is transmitted through seed, losses may be as high as 30%.

Symptoms. SMV causes raised areas or puckering of the leaf surface and stunting of the plant. Seeds can be mottled; however, seed mottling can be caused by other factors. Symptom severity is related to the virus strain, soybean variety, and when the plant was infected. Symptom expression is also related to temperatures. High temperatures limit symptom expression, whereas cool temperatures enhance leaf symptom development.

Management. Although SMV is transmitted by aphids, controlling the aphid vector is neither practical nor reliable as a means of preventing the virus. Wild hosts of SMV are rare, so management of other plants will not reduce SMV incidence. The only management available for SMV is planting resistant soybean varieties and high-quality, disease-free seed.

Bean Pod Mottle Virus

Bean pod mottle virus (BPMV) affects soybean, snap bean, and other legumes. BPMV is less common than SMV, but is capable of severely lowering yields in infected fields. The magnitude of yield loss is related to the time of infection, and early infection can result in severe losses. BPMV is transmitted by several leaf-feeding beetles, including the bean leaf beetle (Cerotoma trifurcata). Virus transmission via seed is very low (less than 0.01%), and overwintering beetles likely acquire the virus from wild legume species.

Symptoms. BPMV causes leaf mottling and distortion, stunting of the plant, and mottling of the seed. BPMV has also been implicated in “green stem syndrome,” in which the stems of mature plants remain green and leathery, which can lead to additional yield losses. Symptom severity is related to the virus strain, soybean variety, and the time of infection. As with SMV, high temperatures limit symptom expression, and cool temperatures enhance leaf symptom development.

Management. Controlling populations of insect vectors and weedy hosts has not been verified for efficacy in limiting virus incidence. Practical management is reliant on planting varieties that are resistant or tolerant to BPMV.

Tobacco Ringspot Virus

Tobacco ringspot virus (TRSV) is common in North Carolina, but soybean yield losses are generally insignificant. Although the virus can be spread by the dagger nematode, the virus does not move from the roots to the shoots and leaves. TRSV may also be spread by thrips, but no efficient vector control has been established.

Symptoms. Infected plants may show signs of stunting, leaf distortion, and characteristic browning and curling of the terminal branch. The most obvious symptoms are the abundance of buds and flowers and the lack of pods or poorly formed pods. The stems remain green, and petioles with dark-brown to black lesions may remain attached.

Management. The most practical management of this disease is to plant soybeans away from pastures or borders that may harbor TRSV-infected weeds.

Soybean Vein Necrosis Virus

Soybean vein necrosis virus (SVNV), a more recently identified virus first reported in Arkansas and Tennessee in 2008, is occasionally found in North Carolina. The long-term implications of this disease are presently unknown. SVNV is vectored by soybean thrips, and may affect several different weed species.

Symptoms. SVNV lesions begin as irregularly shaped, yellow spots along the leaf vein. As the lesions progress, they become red-brown and necrotic. Yellow spots are typically limited to the area of the major leaf veins. Vein discoloration and necrosis may also be seen on the undersides of leaves. Symptoms are easily confused with several other pathogens, including phyllosticta leaf spot and sudden death syndrome, or damage caused by ALS inhibitors or ACCase inhibitors. Proper diagnosis may require that samples be tested at the Plant Disease and Insect Clinic.

Management. Although it has not yet been tested for SVNV management, vector control of other viruses in soybean has been unsuccessful in managing disease, so there is no recommendation to manage soybean thrips populations for this virus. There are at present no remedial controls in response to this disease, and management recommendations will be updated as more research becomes available.

Cowpea Chlorotic Mottle Virus

Cowpea chlorotic mottle virus (CCMV) is occasionally found in the southeastern United States. Yield losses are generally negligible because it typically occurs only scattered throughout the field. Bean leaf beetle and spotted cucumber beetle can transmit CCMV, and they likely uptake the virus when feeding on various weed species.

Symptoms. CCMV causes a distinct mosaic that yellows and stunts the host plant.

Management. Some soybean varieties are resistant to this virus and may be necessary if a significant portion of a field has previously been affected.

Seedling Diseases

The members of the Identification and Biology of Seedling Pathogens of Soybean project, funded by the North Central Soybean Research Program (NCSRP), and plant pathologists across the United States have developed ratings for how well fungicide seed treatments control seedling diseases of soybeans in the United States. Efficacy ratings for each fungicide active ingredient listed in Table 7-1 were determined by field-testing the materials over multiple years and locations by the members of the pathogens project. The table includes ratings summarized from national fungicide trials published in Plant Disease Management Reports (formerly Fungicide and Nematicide Tests) by the American Phytopathological Society. Each rating is based on the fungicide’s level of disease control, and does not necessarily reflect efficacy of fungicide active ingredient combinations or yield increases obtained from applying the active ingredient.

The table includes the most common products available at time of publication. It is not intended to be a list of all labeled active ingredients and products. Additional active ingredients may be available but not evaluated in a manner allowing a rating. Additional active ingredients may be included in some products for insect and nematode control; however, only active ingredients for pathogen control are listed and rated.

Many active ingredients and their products have specific use restrictions. Read and follow all restrictions before applying any fungicide to seed or before handling any fungicide-treated seed. This information is provided only as a guide. It is the legal responsibility of the applicator and user to read and follow all current label directions. References in this publication to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name, is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind by members of the pathogens project or by the NCSRP. Growers using such products assume responsibility for their use in accordance with current directions of the manufacturer.

¹Efficacy categories: E=Excellent; VG=Very Good; G=Good; F=Fair; P=Poor; NR=Not Recommended; NS=Not Specified on product label; U=Unknown efficacy or insufficient data to rank product. ↲

²Products may vary in efficacy against different Fusarium and Pythium species. ↲
³Areas with mefenoxam- or metalaxyl-insensitive populations may see less efficacy with these products. ↲
⁴Listed seed treatments do not have efficacy against Fusarium virguliforme, the causal agent of sudden death syndrome. ↲

Please note: Efficacy ratings may be dependent on the rate of the fungicide product on seed. Contact your local Extension plant pathologist for recommended fungicide product rate information for your area.

Foliar Diseases

The North Central Regional Committee on Soybean Diseases (NCERA-137) has developed the following information on foliar fungicide efficacy for control of major foliar soybean diseases in the United States. Efficacy ratings for each fungicide listed in Table 7-2 were determined by field-testing the materials over multiple years and locations by the members of the committee. Efficacy ratings are based upon the level of disease control achieved by the product and are not necessarily reflective of yield increases obtained from its application. Efficacy depends upon proper timing, rate, and method of application to achieve optimum effectiveness of the fungicide as determined by label instructions and overall level of disease in the field at the time of application. Differences in efficacy of fungicide products were determined by direct comparisons between products in field tests and are based on a single application at the labeled rate as listed in Table 7-2, unless otherwise noted. The table includes available systemic fungicides that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products.

¹Efficacy categories: NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; N= Not Labeled for use against this disease; U=Unknown efficacy or insufficient data to rank product efficacy. ↲

²Cercospora leaf blight efficacy relies on accurate application timing, and standard R3 application timings may not provide adequate disease control. Fungicide efficacy may improve with earlier or later applications; however, efficacy has been inconsistent with some products. Fungicides with a solo or mixed QoI or MBC mode of action may not be effective in areas where QoI or MBC resistance has been detected in the fungal population that causes cercospora leaf blight. ↲

³In areas where QoI-fungicide resistant isolates of the frogeye leaf spot pathogen are not present, QoI fungicides may be more effective than indicated in this table. ↲

⁴Harvest restrictions are listed for soybean harvested for grain. Restrictions may vary for other types of soybean (for example, edamame) and soybean for uses such as forage or fodder. ↲

⁵Multiple generic products containing this mode of action may also be labeled in some states. ↲

⁶Proline has a supplemental 2(ee) label for soybean, only for use on white mold in IL, IN, IA, MI, MN, NE, ND, OH, SD, and WI. A separate 2(ee) label for NY exists for white mold. ↲

⁷Rating is based on two applications of a 9 fl oz/A rate of Aproach at R1 and R3. ↲

Note: Many products have specific use restrictions for the amount of active ingredient that can be applied within a period of time or the amount of sequential applications that can occur. Please read and follow all specific use restrictions prior to fungicide use. This information is provided only as a guide. It is the responsibility of the pesticide applicator by law to read and follow all current label directions. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Growers using such products assume responsibility for their use in accordance with current directions of the manufacturer. Members or participants in the NCERA-137 group assume no liability resulting from the use of these products.

Fungicide Resistance

The development of fungicide resistance is a large concern for soybean producers due to the limited modes of action available and limited number of applications that are economical to use in the field. Fungicide resistance arises when a chemical is used in frequent rotation. For example, if a producer uses one chemistry multiple times within a season, or the same chemistry each year without rotation, fungicide resistance may develop. The continued use of a chemistry selects for the population that is resistant to it by killing the susceptible microbes and leaving the resistant microbes in the population. This selection is more rapid in sexually reproducing pathogens than in asexually reproducing pathogens, but resistance development has been observed in pathogens of both types of reproduction.

North Carolina soybean producers are already being affected by fungicide resistance. Strobilurin-resistant frogeye leaf spot has been observed in several North Carolina counties. The distribution of resistant populations of frogeye leaf spot in North Carolina is not presently known, and this development will be assessed over the next couple of years. To manage frogeye leaf spot, the use of multiple modes of action in a premix or avoiding the strobilurin (FRAC 11) fungicides is recommended to avoid losses of control in strobilurin-resistant populations.

To manage fungicide resistance for any pest, rotation of chemistry classes (FRAC group) each time a pesticide application is used or use of a mixed-mode-of-action pesticide is recommended. FRAC group codes are listed in Table 7-2 for each pesticide, and classes of fungicides should be rotated where possible. Where it is not possible to spray multiple times within a growing season, utilization of multiple modes of action in a single fungicide application are recommended. Premixed fungicides with multiple modes of action are available, and their efficacy is described in Table 7-2.