NC State Extension Publications

 

Diseases of soybean can be caused by numerous microorganisms (including fungi, bacteria, viruses, and nematodes) that can damage plants, reducing vigor and yield. Seeds, seedlings, and older plants may all be affected by disease-causing microorganisms. Disease can spread from plant to plant and cause damages over time. Plants under stress, such as from drought, excessive moisture, extreme temperature conditions, 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 Practices

Disease management requires good stewardship 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 pathogen development. In addition to reducing disease incidence, these practices reduce the risk of fungicide resistance and race shifts of organisms.

Cultural Practices

Cultural practices affect plant disease in several ways. Practices such as planting density or tillage practices may affect the microclimate of the canopy or root zone. Foliar pathogens require moisture for infection and reproduction, and reducing humidity in the canopy by avoiding high populations or changing plant spacing to allow for more airflow between plants may reduce their incidence. Tillage practices that reduce water within the root zone may also reduce the incidence of root rotting organisms, such as Phytophthroa 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 preventatively, 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 (Tables 8-1 and 8-2, below). Proper identification of the causal agent for a problem is crucial for selecting fungicides.

Host Resistance

Host resistance is the most effective and economic way to reduce disease incidence. Disease and nematode resistances are also 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 Official Variety Trial data from North Carolina State University.

Disease Scouting

Proper identification of causal agent requires proper disease scouting, and fields should be scouted regularly to observe problems early. Disease often occurs randomly throughout a field in circular or elliptical patches. As opposed to simply walking a straight line, scouting is best conducted in an irregular pattern, such as a zig-zag or letter shapes, to increase the likelihood of walking through a disease focus. If a particular region within the field is perennially affected, scouting that area first may allow for early action for disease management strategies.

Problem Diseases

Several economically important diseases are present in North Carolina each year. Several diseases present similar symptoms, described below, 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 active 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. Seed and seedling diseases may also occur in any soil type. Environmental factors that delay plant germination and emergence increase the likelihood for disease, such as poor seed quality, inadequate seedbed preparation, soil compaction, and improper planting depth.

Symptoms. Evidence of seed or seedling diseases occurs usually with patchy emergence, or when seedlings are stunted/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. Where herbicide injury is present, weed control near affected seedlings is excellent and adjacent plants will also show signs of damage. Damages caused by herbicide are often associated with a pattern related to equipment, but soil type can also influence herbicide injury.

Rhizoctonia solani can damage seeds prior to, or seedlings just after, emergence. On seedlings, Rhizoctonia can cause a red-brown decay at the soil-line (sore shin). In some cases, the damage can girdle the stem and kill the seedlings, or plants may only be 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 are soft and brown-colored, and symptoms are similar to those caused by Phytophthora sojae. Although it causes most damages to seedlings, Pythium can also cause root rots of established plants.

Phytophthora sojae causes a soft, brown rot of tissues affected by the pathogen. As plants affected with Phytophthora mature, leaves yellow and plants frequently wilt and die. Distinguishing between Pythium and Phytophthora infections is only possible in a laboratory.

Fusarium spp. can cause brown lesions on roots and damping off of seedlings. Fusarium may also attack the taproot and promote adventitious root growth above the root damage.

Seedling Disease Management. Seedling disease can be reduced by planting good-quality seed in well-drained soils. Rotating with a grass crop such as corn or grain sorghum may help to reduce populations of soilborne diseases. In fields with poorly draining soils, planting dates should be pushed back to when conditions for soybean germination and rapid growth are optimal. Seed treatments may be beneficial (Table 5-3) when planting in cool, wet soils. Several different seed treatments are labeled for soybean, with several combinations of chemistries available that control different seedling disease causal agents. Properly identifying the disease-causing organism will allow for more accurate choices in seed treatment selection.

Foliar Diseases

Foliar diseases of soybean reduce yields in North Carolina by about 1%, and are largely managed by using resistant varieties or applications of 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.

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 inoculum.

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. 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 and become gray with a dark margin.

Management. Planting resistant varieties or certified disease-free, high quality seed are the best mechanism for preventing disease outbreaks. Continuous soybean rotations also promote significant buildup of residues, and should be avoided. If planting soybean following a year with frogeye leaf spot infestation, plant a resistant variety. Foliar fungicides (Table 8-2) at R2 and R5 may be warranted where there are favorable environmental conditions and susceptible varieties. Strobilurin-resistant Cercospora sojina populations have been detected in several counties of North Carolina, and using a pre-mix fungicide with multiple modes of action may help prevent losses of control.

Cercospora Blight and Purple Seed Stain
(Cercospora kikuchii)

Cercospora blight accounts for 0.3% of yield losses caused by foliar pathogens in North Carolina and can cause significant damages under warm, wet conditions. When seed become infected, causing purple seed stain, the value of the crop may be reduced.

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. Severe infections may result in premature defoliation of the uppermost leaves, while lower leaves remain green and attached to the plant.

When seed are affected, a purple to pink discoloration may occur in small spots to the entire surface of the seed. Infected seed may also show no symptoms of infection. Resulting 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 air flow 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 build-up 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 8-2).

Bacterial Blight
(Pseudomonas syringae)

Pseudomonas syringae causes angular leaf spot and bacterial blight, which usually cause minimal damages in North Carolina. 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 infections, 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, and 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 available commercially, 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 inoculum may aid in limiting infections.

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 are small, irregularly-shaped and dark brown, which 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 non-leguminous plants like corn or small grains. Foliar fungicide applications may be warranted under conducive environmental conditions, and the efficacy of different modes of action are important to rotate each production season (Table 3). Applications made between R3 and R5 may slow the rate of development to protect yield.

Soybean Rust
(Phakospora 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 with prolonged leaf wetness (more than 6 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 North Carolina State University Plant Disease and Insect Clinic for confirmation.

Management. There are no soybean rust resistant varieties commercially available in North America. When rust is found within 100 miles of a field, foliar fungicides may be necessary to manage disease. To adequately control rust, fungicides need to be applied prior to or soon after first infection. Foliar fungicides (Table 8-2) applied between R3 and R5 may reduce the level of disease.

Stem and Root Rots

Vascular diseases are the most common disease for North Carolina producers. 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/insect damages. Disease management of these pathogens often relies on managing these damages or the use of resistant varieties to prevent significant losses of yield. 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 to North Carolina soybean production. Disease symptoms are typically more severe under high yield environments and are associated with nematode damages.

Symptoms. Sudden death syndrome symptoms resemble damages caused by other pathogens and insects including black root rot, phytophthora root rot, charcoal rot, stem canker, brown stem rot, southern blight, and occasionally dectes stem borer damage. Early symptoms, if any are present, are chlorotic yellow spots on leaves between leaf veins beginning at R3 and later growth stages. As the soybean plant progresses to forming pods, yellow spots may coalesce with veins remaining green. The yellow between veins will become brown and necrotic as the tissue dies. Pods may be aborted, and plants may defoliate early. Petioles may be retained on plants that have defoliated prematurely. Roots exhibit a root rot, and plants can be easily pulled from soil. The vascular tissue of roots when split open are discolored reddish-brown to brown that usually does not extend beyond an inch or two above the soil line. Occasionally, the fungus will form white masses on the roots that become blue to blue-green as they mature.

Management. Proper identification of the causal agent is important for proper management, and suspected sudden death syndrome plants should be sent to the North Carolina State University 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 has the potential to limit yields when weather conditions are hot and dry. The disease affects the roots of soybean plants.

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 are split, black streaks are evident in the woody portion of the stem. Light gray to silver discoloration may be visible on the root, and black specs (microsclerotia) will be in tissues of the stem and tap root.

Management. Resistant varieties are not available, but soybean varieties vary in susceptibility. Crop rotation with small grains can reduce inoculum in the field. Corn is also a host and is not a good rotation to reduce disease. Avoid excessive seeding rates to reduce competition for moisture. Supporting plant health may reduce the impact of disease 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. Crop rotation with a non-host crop such as corn and 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 and managing weeds can suppress disease. Foliar fungicides can reduce disease severity, and are most effective when applied immediately before infection.

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, initial 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. The vascular tissue near the canker also becomes discolored when lengthwise section of the stem is cut. Small black spots (stroma) may also appear on the dead tissue within the canker. The stroma are compact masses of fungal hyphae in which perithecia (the sexual fruiting structures of the fungus), will appear given adequate moisture. Lesions may expand down towards and below the soil line and may be confused with other root rots. Foliar symptoms appear at the onset of flowering and begin as interveinal chlorosis that become 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. Submission to the Plant Disease and Insect Clinic may be necessary for proper diagnosis.

Management. The best management method for stem canker is to plant resistant soybean varieties. Rotation with a non-host 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 high risk for stem canker infections due to the long duration of survival of the fungus on crop debris. There is not yet data to support the efficacy of foliar fungicides for the management of stem canker.

Phomopsis Stem and Seed Decay
(Phomopsis longicolla)

Phomopsis can affect stems, petioles, pods, and seed. Disease is usually associated with warm, humid conditions when soybean plants are maturing. The severity of 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 mold.

Management. Cultural practices that favor high humidity, such as high populations. Select high quality, certified seed to avoid infected seed. Crop rotation with non-host crops, such as corn, and tillage will reduce the survival of Phomopsis. Foliar fungicides may protect seed from new infections but may not affect yield. Harvesting early maturing varieties first may help reduce the incidence of seed rot.

Fusarium Wilt
(Fusarium spp.)

Fusarium 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 stems below the soil line. Scorching of upper canopy leaves and chlorosis of middle and lower canopy leaves can be observed in infected plants. Severely affected plants with compromised root systems may wilt and die.

Management. Seed treatments (Table 5-3) may help reduce seedling disease caused by Fusarium infections. 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. Planting soybeans with a history of Fusarium wilt when soil temperatures are warmer may help to establish stands 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 of older plants may vary based on variety. Leaves typically become chlorotic between the veins and plants wilt and die. Varieties that are tolerant to resistant may appear stunted. Characteristically, a dark brown 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 observed. 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 also 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 where 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.

Soybean Mosaic Virus

Soybean mosaic virus (SMV) is the most common 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, and management of other plants will not reduce SMV incidence. The only management available for SMV is the use of resistant soybean varieties and planting 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 trifurcate). Virus transmission through 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,” where 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. Like SMV, high temperatures limit symptom expression, and cool temperatures enhance leaf symptom development.

Management. Controlling insect vector and weedy host populations has not been verified for efficacy in limiting virus incidence. Practical management is reliant on using host resistance or tolerance to BPMV.

Tobacco Ringspot Virus

Tobacco ringspot virus (TRSV) is common to North Carolina, but 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 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 the petioles may remain attached with dark brown to black lesions.

Management. The most practical management of this disease is to plant fields away from pastures or borders that may harbor weeds that are infested with TRSV.

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. Yellowing spots are typically limited within 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 southeast United States. Yield losses are generally negligible because it typically appears only scattered throughout the field. Bean leaf beetle and spotted cucumber beetle can vector this virus, and likely uptake the virus feeding on various weed species.

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

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

Nematodes

Plant-parasitic nematodes are microscopic roundworms that feed on plant tissues to survive. Nematodes that affect soybean survive on root tissues and move through water films between soil particles to find a host. There are several different feeding strategies of nematodes, and many survive within the root of the soybean or outside of the root system. Different feeding strategies and life cycles may influence the damage caused by nematodes. In addition to direct damages caused by nematodes, secondary diseases by fungi or bacteria that gain entry through wounds caused by nematodes may also limit the yield potential of soybean fields.

Proper management of nematodes requires adequate diagnosis of the species causing damage. If you suspect nematode damage, take systematic soil samples in the fall when nematode numbers are high. Pull 20 to 30 soil cores 6 to 8 inches deep from 4 to 5 acres. Send soil in a plastic bag and the appropriate box, which can be obtained from the county Extension agent, and send soil sample with the appropriate form and payment to the North Carolina Department of Agriculture & Consumer Services, Agronomic Services, Nematode Advisory and Diagnostic Lab.

Soybean Cyst Nematode
(Heterodera glycines)

Soybean cyst nematode (SCN) has been the most serious disease in North Carolina. It is found in all soybean producing regions of the state, and severe losses are especially common in sandy coastal plain soils. Cool to moderate conditions with adequate moisture tend to enhance SCN reproduction and favor spread. At the end of the growing season, the female nematodes form cysts that allow their egg masses to overwinter until a susceptible host is available, contributing to inoculum in future growing seasons.

Symptoms. Symptoms of SCN can be easily confused with herbicide injury or nutritional disorders. SCN is characterized by irregular patches of stunted soybeans, yellow soybeans, or both. A decline of yields can be observed over several years in fields affected with SCN. Failures in weed control may also be indicative of SCN infections. It may also be possible to observe cysts on roots of affected plants, but they are not always visible.

Management. Reducing other production limitations, such as soil fertility and hard pan management, may curb the impact of nematode infections. Fields that have been planted in continuous, susceptible soybean varieties for several years will increase populations of nematodes, and rotation with non-host crops (corn, cotton, tobacco, wheat, peanut) are recommended. Damages of SCN tend to be greater in early versus late planted soybean, but yields from later planted soybean tend to be somewhat lower than those planted early. No-till practices can also reduce SCN populations as long as weeds can be adequately controlled. Several nematicides are labeled for use in soybean but are often not economical for use in soybean and are not recommended. Several seed treatments are labeled to have activity on nematodes, but their efficacy in North Carolina have not yet been tested. Resistant varieties, where available, are the most economical means of managing SCN; however, a variety with resistance to the race in the affected field must be selected.

Root Knot Nematode
(Meloidogyne spp.)

Root knot nematode can cause significant damages in fields with high populations. There are several different root knot nematode species found in North Carolina, though their distribution is varied throughout the state. Root knot nematode has a wide host range and affects numerous field crops and weeds species common to North Carolina.

Symptoms. Reduced vigor and stunting of plants is a first indication of root knot nematode pressure. Under hot, dry conditions, infected plants wilt more rapidly than unaffected plants. Severely affected plants show stunting and chlorosis due to nutrient and water deficiency. These plants may die before maturity and fail to produce seed. Roots of affected plants have irregular deformations (galls) that contain the adult female nematodes. As opposed to beneficial root nodules, nematode galls cannot be removed from the roots and are not perfectly round.

Management. Reducing other production limitations, such as soil fertility and hard pan management, may curb the impact of nematode infections. Because root knot nematode species have wide host ranges, including cotton, peanut, sweetpotato, and corn, crop rotations are not generally a good management technique to limit roto knot nematode populations. Destroying crop debris, especially roots, and weed species help limit the survival of root knot nematode in the soil. Several nematicides are labeled for use in soybean, but are often not economical for use in soybean and are not recommended. Several seed treatments are labeled to have activity on nematodes, but their efficacy in North Carolina have not yet been tested. Resistant varieties, where available, are the most economical means of managing root knot nematode.

Fungicide Efficacy for Control of Soybean Diseases

Seedling Diseases

The members of the Identification and Biology of Seedling Pathogens of Soybean project funded by the North Central Soybean Research Program and plant pathologists across the United States have developed the following 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 8-1 were determined by field-testing the materials over multiple years and locations by the members of this group, and include ratings summarized from national fungicide trials published in Plant Disease Management Reports (and 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 and/or yield increases obtained from applying the active ingredient.

The list includes the most common products available as of the release date of the table. It is not intended to be a list of all labeled active ingredients and products. Additional active ingredients may be available, but have not been 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 use 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 applicator’s and user’s legal responsibility to read and follow all current label directions. Reference 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 group, or by the North Central Soybean Research Program. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.


Table 8-1. Fungicide efficacy for the control of soybean seedling diseases.
Fungicide active ingredient Pythium sp.1. Phytophthora root rot Rhizoctonia sp. Fusarium sp.1,3 Sudden death syndrome (SDS) (Fusarium virguliforme) Phomopsis sp.

Azoxystrobin

P-G

NS

VG

F-G

NR

P

Carboxin

U

U

G

U

NR

U

Chloroneb

U

P

E

P

NR

P

Ethaboxam

E

E

U

U

U

U

Fludioxonil

NR

NR

G

F-VG

NR

G

Fluopyram

NR

NR

NR

NR

VG

NR

Fluxapyroxad

U

U

E

G

NR

G

Ipconazole

P

NR

F-G

F-E

NR

G

Mefenoxam

E2

E

NR

NR

NR

NR

Metalaxyl

E2

E

NR

NR

NR

NR

PCNB

NR

NR

G

U

NR

G

Penflufen

NR

NR

G

G

NR

G

Prothioconazole

NR

NR

G

G

NR

G

Pyraclostrobin

P-G

NR

F

F

NR

G

Sedaxane

NR

NR

E

NS

NR

G

Thiabendazole

NR

NR

NS

NS

P

U

Trifloxystrobin

P

P

F-E

F-G

NR

P-F

1 Products may vary in efficacy against different Fusarium and Pythium species.
2 Areas with mefenoxam or metalaxyl insensitive populations may see less efficacy with these products.
3 Listed seed treatments do not have efficacy against Fusarium virguliforme, causal agent of sudden death syndrome.
Efficacy 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.
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 the table were determined by field-testing the materials over multiple years and locations by the members of the committee. Efficacy ratings are based upon level of disease control achieved by product and are not necessarily reflective of yield increases obtained from product application. Efficacy depends upon proper application timing, rate, and application method to achieve optimum effectiveness of the fungicide as determined by labeled instructions and overall level of disease in the field at the time of application. Differences in efficacy among fungicide products were determined by direct comparisons among products in field tests and are based on a single application of the labeled rate as listed in the Table 8-2, unless otherwise noted. The table includes systemic fungicides available that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products.


Table 8-2. Efficacy of foliar fungicides for the control of foliar soybean diseases1.

Fungicide(s)

Aerial web blight

Anthracnose

Brown spot

Cercospora leaf blight2

Frogeye leaf spot3

Phomopsis / Diaporthe (Pod and stem blight)

Soybean rust

White mold4

Harvest restriction5

Class

Active ingredient (%)

Product / Trade name

Rate/A (fl oz)

QoI Strobilurins

Group 11

Azoxystrobin 22.9%

Quadris 2.08 SC

Multiple Generics6

6.0 - 15.5

VG

VG

G

P

P

U

G-VG

P

14 days

Fluoxastrobin

40.3%

Aftershock 480 SC

Evito 480 SC

2.0 – 5.7

VG

G

G

P

P

U

U

NL

R5

beginning seed

30 days

Picoxystrobin

Aproach

2.08 SC

6.0 -12.0

VG

G

G

P

P

U

G

G-VG9

14 days

Pyraclostrobin 23.6%

Headline 2.09 EC/SC

6.0 - 12.0

VG

VG

G

P

P

U

VG

NL

21 days

DMI Triazoles

Group 3

Cyproconazole 8.9%

Alto 100SL

2.75 – 5.5

U

U

VG

F

F

U

VG

NL

30 days

Flutriafol

11.8%

Topguard 1.04 SC

7.0 – 14.0

U

VG

VG

P-G

VG

U

VG-E

F

21 days

Propiconazole 41.8%

Tilt 3.6 EC

Multiple Generics6

4.0 - 6.0

P

VG

G

NL

F

NL

VG

NL

R5

beginning seed

Prothioconazole

41.0%

Proline 480 SC7

2.5-5.0

NL

NL

NL

NL

G-VG

NL

VG

F

21 days

Tetraconazole

20.5%

Domark 230 ME

4.0 – 5.0

NL

VG

VG

P-G

G-VG

U

VG-E

F

R5 beginning seed

2,6-dinitro-anilines

Group 29

Fluazinam

40.0%

Omega 500 DF

0.75-1.0 pints

NL

NL

NL

NL

NL

NL

NL

G

R3 (beginning pod)

SDHI Carboxamides

Group 7

Boscalid

70%

Endura 0.7 DF

3.5 – 11.0

U

NL

VG

U

P

NL

NL

VG

21 days

Mixed mode of action

Azoxystrobin 18.2%

Difenoconazole 11.4%

Quadris Top 2.72 SC

8.0 – 14.0

U

U

G-VG

P-G

VG

U

VG

NL

14 days

Azoxystrobin 19.8%

Difenoconazole 19.8%

Quadris Top SBX 3.76 SC

7.0-7.5

U

U

U

U

G-VG

U

U

U

14 days

Azoxystrobin 7.0%

Propiconazole 11.7%

Quilt 1.66 SC

Multiple Generics6

14.0 – 20.5

U

U

G

F

F

U

VG

NL

21 days

Azoxystrobin 13.5%

Propiconazole 11.7%

Quilt Xcel 2.2 SE

10.5 - 21.0

E

VG

G

F

F

U

VG

NL

R6

Bensovindiflupyr 10.27%

Azoxystrobin 13.5%

Propiconazole 11.7%

Trivapro A 0.83

+ Trivapro B 2.2 SE

A = 4.0

B = 10.5

E

U

VG

U

VG

U

U

NL

14 days

R6

Cyproconazole 7.17%

Picoxystrobin 17.94%

Aproach Prima

2.34 SC

5.0-6.8

U

U

VG

P-G

G

U

U

NL

14 days

Flutriafol 19.3%

Fluoxastrobin 14.84%

Fortix SC

Preemptor SC

4.0-6.0

U

U

G

U

G

U

U

U

R5

Mixed mode of action

Cyproconazole 7.17%

Picoxystrobin 17.94%

Aproach Prima

2.34 SC

5.0-6.8

U

U

VG

P-G

G

U

U

NL

14 days

Flutriafol 19.3%

Fluoxastrobin 14.84%

Fortix SC

Preemptor SC

4.0-6.0

U

U

G

U

G

U

U

U

R5

Pyraclostrobin 28.58%

Fluxapyroxad 14.33%

Priaxor

4.17 SC

4.0 – 8.0

E

VG

E

P-G

P-F

U

VG

P

21 days

Pyraclostrobin 28.58%

Fluxapyroxad

14.33%

Tetraconazole

20.50%

Priaxor D

4.17 SC

1.9 SC

4.0 (each component)

U

U

VG

U

G-VG

U

U

P

21 days

R5

Trifloxystrobin 11.4% Propiconazole 11.4%

Stratego 250 EC

10.0

G-VG

VG

G

F

VG

--6

VG

NL

21 days

Trifloxystrobin 32.3% Prothioconazole 10.8%

Stratego YLD 4.18 SC

4.0-4.65

VG

VG

VG

F

VG

--6

VG

NL

21 days

1 Efficacy categories: NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; NL = Not Labeled for use against this disease; U = Unknown efficacy or insufficient data to rank product efficacy
2 Cercospora 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.
3 In 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.
4 White mold efficacy is based on R1-R2 application timing, and lower efficacy is obtained at R3 or later application timings, or if disease symptoms are already present at the time of application.
5 Harvest restrictions are listed for soybean harvested for grain. Restrictions may vary for other types of soybean (edamame, etc.) and soybean for other uses such as forage or fodder.
6 Multiple generic products containing this mode of action may also be labeled in some states.
7 Proline has a supplemental label (2ee) for soybean, only for use on white mold in IL, IN, IA, MI, MN, NE, ND, OH, SD, WI. A separate 2ee for NY exists for white mold.
8 Stratego YLD has a supplemental label (2ee) for white mold on soybean only in IL, IN, IA, MI, MN, NE, ND, OH, SD, WI.
9 Rating 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 about 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. Persons 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

Fungicide resistance development is a large concern for soybean production 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 pre-mix or avoiding the strobilurin (FRAC 11) fungicides is recommended to avoid losses of control due to 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 8-2 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. Pre-mixed fungicides with multiple modes of action are available, and their efficacy is described in Table 8-2.

Author:

Assistant Professor & Extension Specialist
Entomology and Plant Pathology

Publication date: Nov. 21, 2017
Last updated: Jan. 3, 2018
AG-835

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

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