Key management practices for organic soybean production are as follows:

• Choose varieties that have resistance to diseases and nematodes encountered in your fields.
• Choose moderate maturity groups that may encounter less pest and disease pressure (MG 5-7).
• For maximum yield plant from late April through late May.
• Use high plant populations, a seeding rate of 200,000 seed/acre, to compete with weeds.
• Use narrow row-spacing that still allows for cultivation to ensure quick canopy closure, especially in double-cropped beans.
• Rotate crops to reduce troublesome pests.

Variety selection is an excellent way to reduce problematic pests in organic soybeans, including seedling diseases, root rots, foliar diseases, and nematodes. Variety selection is also an excellent way to deal with nematode problems. Selecting varieties that are resistant to the species of nematode present in the field can limit the yield loss caused by these pests.

Farmers in North Carolina can successfully grow a wide range of maturity groups (MG2-8) despite most breeding efforts focusing on the MG5-7 range in the U.S. Southeast. There are many factors that influence soybean maturity group selection, including crop rotation, equipment availability, price premiums for early delivery, pest resistance needed in certain maturity groups, cultural benefits, and yield potential. In general, it is easier to find pest resistance for diseases and nematodes relevant to the Southeast environment in later-maturing varieties (>MG4), which is an important consideration for organic production. Recent results from conventionally managed plots in North Carolina indicate that under most production scenarios, a MG5 or MG6 variety will yield comparable to an earlier-maturing variety, especially at planting dates past mid-May. The yield advantage of using an earlier maturing variety (<MG5) is generally most pronounced in a high-yield situation where limited stress is encountered, which might not often be the case in organic soybean production in North Carolina. Early-maturing soybean varieties, such as Groups III and IV, avoid pests such as corn earworm, but can intensify weed management challenges and other disease and insect pressure. These maturity groups begin to lose leaves when summer weeds can still grow. Most producers who are growing these MGs are doing so to capitalize on greater yield gain in earlier-maturing varieties and premiums associated with early fall delivery. There are considerable risks with producing earlier-maturing varieties in North Carolina when they are planted before mid-May, most notably seed quality issues on the backend of the season, which can require aggressive mid-season pest management to help mitigate which is a challenge in organic production. If weed control has been an issue, Group V or later soybeans can be left unharvested until a killing frost defoliates weeds. In the coastal plain, a Group V or VI (or an earlier planting) will help avoid corn earworm (CEW) infestation during flowering. CEW is seldom a problem in the piedmont. It is also a good idea to choose at least two different varieties in order to spread out the seasonal workload and risk. The Official Variety Test Report, available online or through your local Extension office, is a good source of information on varieties. Unfortunately, there are fewer conventional or non-transgenic varieties available on the market compared to genetically modified varieties. Organic farmers must be aware that transgenic beans are not allowed in certified organic production, and therefore must choose alternate varieties.

Planting date is one of the most important management decisions for soybean production in North Carolina. Recent research indicates that generally planting from the third week of April to the third week of May will maximize soybean yield in North Carolina, although this varies slightly based on location in the state and the production situation. Planting date and variety (or maturity group) interact to impact soybean yield. The key is to match planting date and variety maturity to the soil so that the row middles are lapped with soybean plants about 3 feet tall by flowering time; planting earlier or planting a later-maturing variety can improve the likelihood of achieving this. In an organic farming system, avoiding pest problems is an important management technique. Planting early (before the end of May) with an early-to-mid-season variety can help the crop avoid insect and disease problems. Double-cropped soybeans, generally planted in mid-June, require the use of Group VI or VII or a Group V indeterminate variety to obtain sufficiently large plants by flowering.

The average row width for organic soybeans in North Carolina is 30 inches but can be up to 38 inches and as low as 20 inches. Narrow-row soybeans lap row middles sooner, which can facilitate more crop canopy competitiveness with weeds. The benefits of using a narrow row to achieve great canopy are generally more pronounced as planting date is delayed in double-crop planting situations, where the chances of hitting canopy closure are drastically reduced when using wider row spacing. While narrow-row soybeans will compete more effectively with weeds, row spacing should not be so narrow as to prevent between-row cultivation.

Maximum soybean yield potential is achieved once a soybean canopy has lapped the row middles and reached a height of at least 3 feet before flowering. In conventional production this can be consistently achieved with plant densities of 75,000 to 100,000 plants per acre. However, this population recommendation is for production where herbicides are used and there is minimal-to-no weed competition. In contrast to conventional soybean production, weed control is the largest challenge for organic soybean production, and higher soybean seeding rates is one tactic that can improve weed control. A higher soybean plant population produces a thicker soybean canopy early in the season when weed control is critical. A seeding rate as high as 225,000 seeds per acre for organic soybean results in better weed control, higher yield, and the highest economic return compared to seeding rates of 175,000, 125,000, and 75,000 seeds per acre. With some varieties, lodging can become a concern with rates higher than 200,000 seeds per acre. While a thick plant stand will trap moisture in the canopy, which can create a more favorable environment for many diseases, the weed management benefits of using a higher seeding rate consistently outweigh disease risk in our environment. Higher populations in organic soybean production are also recommended when blind or broadcast cultivation from implements like the rotary hoe or flex-tine harrow are used. These secondary tillage implements pass over the crop rows, often reducing the plant stand by 10% to 20%.

Seeding rate will depend on the planter capability, seed germination, and soil condition. Proper calibration of the planter is important, as well as planting in ideal soil conditions (the soil should be warm and moist but not wet).

Soybeans yielding 50 bushels per acre will remove about 188 pounds of nitrogen per acre, 41 pounds of phosphate per acre, and 74 pounds of potash per acre from the soil. However, manure and compost applications are usually unnecessary because soybeans are nitrogen-fixing legumes and the crop can make use of any nutrients applied to, but not removed by, previous crops. If soybeans were not grown in previous years, soybeans should be inoculated with species of Bradyrhizobium bacteria specific for soybeans. See Chapter 9 of this guide for more information about organic soil management.

Organic weed management is more challenging in soybeans than in corn since soybean foliage does not generally overlap and shade the row middles until later in the season. Generally, narrow rows (down to 20 inches) and increased plant population can help the crop compete more effectively against weeds. When managing weeds in soybeans, consider also that planting soybeans at different times will result in the plants competing against different sets of weed species. Weeds that emerge during the first four to five weeks after planting will cause the most damage in terms of yield reductions. Weeds that emerge after this time will have little effect on yield, although they may make harvest more difficult and will set seed. The goal should be to keep the field clean through the first four to five weeks after planting. Clean cultivation is used on most organic soybean acreage in the state. A blind cultivator, such as a flex-tine harrow or rotary hoe, is used before soybean emergence and approximately every five days afterwards. Anywhere from two to five blind cultivations occur before between-row cultivation begins. A frequent problem is missing blind cultivation passes due to wet weather. Unfortunately, near-row weeds missed during wet weather often remain until the end of the season. Taller crops, such as corn, can endure lots of soil throwing and between-row cultivators can be set to bury young weeds. However, soybeans can only tolerate small amounts of burial due to how low pods are set on the stem. One tactic is to plant into moisture when the weather forecast is clear so that at least one or two blind cultivations can occur on schedule. See Chapter 7 of this guide for more information on managing weeds in organic production.

Differences caused by variety selection, planting date, cultural techniques, site, and season cause great variations in soybean plant attractiveness to insect pests. If organic soybean farmers recognize these differences, they can manage the crop for reduced insect pest numbers, or, when this is not possible, predict which fields are attractive and may need more attention to prevent yield loss. Organic growers can normally rely on three factors to limit insect damage: reducing soybean attractiveness to pests, increasing beneficial insects that reduce pest numbers, and supporting the plant’s ability to compensate for insect damage (tolerance). Important tactics used to reduce insect damage include the following five strategies:

Crop Rotation

Rotation helps reduce levels of pests like grape colaspis and often improves crop health. Using a rotation of a non-legume for at least two years allows soybeans to avoid pests that may have previously been present in the field.

Tillage

For pests associated with the seed or soil, or for those that harbor in stubble or residue, tillage can be an effective management method. For example, the Dectes stem borer harbors in soybean crowns after harvest over the winter. Research has demonstrated that burying stubbles to a depth of 2 inches can reduce larval survival and adult emergence. Mortality is higher on poorly drained land or when conditions are relatively moist. Seedcorn maggots, which can consume germinating seeds, can be reduced using tillage.

Soil Fertility and pH Maintenance

Thin plant stands often have more corn earworms, but good growth can compensate for injury from this pest as well as others. Reducing plant stress from low pH, poor fertility, or inadequate moisture will enable plants to better tolerate insect feeding.

Variety Selection and Early Planting

High caterpillar populations can often be avoided by early planting of an early-maturing variety (such as varieties from maturity group IV). These plantings will bloom and harden-off before the corn earworm moth flight from corn fields, and the plants will be unattractive to the moths. Also, early maturity can greatly reduce soybean looper, velvetbean caterpillar, and late stink bug infestations. Percentage loss from insects is generally between 5% and 25% but can be as high as 50%. In rare situations, stink bugs can be trap-cropped by early-maturity fields, leading to greater damage. This occurs when soybeans are the only attractive feeding host in the environment. Occasionally, this will be an early-planted soybean field in the middle of the growing season. More often, later-planted and later-maturing soybean fields are the attractive crop hosts for stink bugs in the late season. Early-planted fields are generally more susceptible to colonization by Dectes stem borer and bean leaf beetles. However, planting soybean at the recommended rate and avoiding thin stands can reduce stalk girth and reduce the incidence of Dectes and another stem pest, the lesser cornstalk borer. Finally, beneficial insects often will colonize and establish in early-planted soybeans, helping to reduce the abundance of pests that arrive later in the season.

Variety selection can be important to manage certain insect pests. For example, the lesser cornstalk borer can be a pest on drought-prone and sandy soil. Research has demonstrated differences in injury due to this pest that ranged from 9% to 31% among varieties.

Narrow Rows

A complete canopy allows a higher level of biological control of insect predators, parasites, and diseases. In addition, the plants tend to have thinner stems in narrow rows, leading to reduced issues from Dectes and lesser cornstalk borers.

Remedial Management

Group V or later-maturing varieties that are planted after late May can become infested by corn earworm moths moving from corn. These moths produce pod-feeding corn earworm larvae, and a high infestation may reduce yield by as much as 50%. Also, populations of leaf-feeding caterpillars (green cloverworm, soybean looper, and velvetbean caterpillar) may occasionally damage soybeans. These worms usually appear very late in the season. In cases where caterpillar pests cannot be avoided, insecticides approved for organic production, such as spinosads or Baculoviruses (NPV), may be successfully used. Bacillus thuringiensis (Bt) insecticides are not effective for corn earworm, armyworms, or soybean looper, but are effective for green cloverworm and velvetbean caterpillar. Scouting and the use of thresholds will indicate which fields are at risk. For scouting procedures for corn earworm, see the NC State Extension Soybeans website.

Corn Earworm and Tobacco Budworm

Corn earworm and tobacco budworm are the most important insect pests of soybean in North Carolina, through pod-feeding and sometimes foliar and flower feeding. Their biologies are very similar. In conventionally managed soybeans, tobacco budworm is more tolerant of certain conventional insecticides than corn earworm. However, from the point of view of the organic soybean producer, the management of these pests will be similar. These insects will often infest soybeans in late July or early August. Cultural management tactics are of the highest importance. Soybeans will be less attractive to these pests if blooms are not present and if there is little young or newly grown vegetation. Remedial management using a spinosyn or Baculoviruses (NPV) can be highly effective, especially if applied when the larvae are small.

Stink Bugs

The stink bug complex is an extremely important pest group in soybeans. These insects injure soybeans by feeding on developing seeds inside the pods. Although early-maturing fields can occasionally attract high densities of these pests, in general, earlier-maturing and earlier-planted soybeans are less susceptible to stink bug infestation than later-maturing and later-planted soybeans.

Soybean Looper

The defoliating soybean looper is a year-round resident in most areas of the South, but it migrates into North Carolina each year. Peak larval population densities occur in September and are most prevalent on late-planted or later-maturing soybeans in counties closer to the coast. Soybean can tolerate a relatively high amount of foliar feeding compared to other crops. Recent research has shown that both full-season and double-cropped soybeans can handle up to 17% defoliation across the entire canopy during their reproductive stages without experiencing a decrease in yield. Remedial management using a spinosyn or Baculoviruses (NPV) can be highly effective.

Bean Leaf Beetle

Bean leaf beetles overwinter as adults and emerge over a three-month window during the spring in North Carolina. Adult beetles can injure soybeans by feeding on the foliage. The first full-season soybeans to sprout in an area will attract many of the strong-flying beetles. Soybean can tolerate a relatively high amount of foliar feeding compared to other crops and defoliation levels rarely exceed the threshold of 33% defoliation during the vegetative stages. There are two generations of bean leaf beetle per year in North Carolina. The second generation can sometimes severely defoliate soybeans during the late season, especially if soybeans are late-maturing.

Soybean producers are annually challenged with diseases that can lead to significant yield losses. Many different diseases have been reported on soybeans and can affect different parts of the plant at different growth stages throughout the season. Soybean diseases can infect the roots, stems, leaves, and pods.

The pathogens that cause root and stem rots are often soilborne or residue-borne pathogens. These same pathogens also cause seedling diseases that may result in stand loss. Common root and stem diseases of soybean that occur in North Carolina include Phytophthora root and stem rot, and stem canker. Common foliar diseases include Cercospora leaf blight, frogeye leaf spot, Septoria brown spot, soybean vein necrosis virus, and soybean rust. Nematodes are also a cause of major soybean diseases in North Carolina, the two most problematic being the soybean cyst nematode and root-knot nematodes.

Root and Stem Rots

Phytophthora root and stem rot (PRR) is caused primarily by the pathogen Phytophthora sojae. The pathogen causing PRR is a fungal-like organism that survives in the soil and requires adequate amounts of water to infect soybean roots and reproduce. Once the pathogen has established and colonized the root and stem tissue, the plant can no longer uptake water or nutrients, causing symptoms of wilting or, in severe cases, death of the plant. Therefore, one of the most effective management strategies is to maintain and improve soil drainage if needed. The pathogen can also survive in the soil for multiple years. While a crop rotation out of soybean for multiple years may reduce the pathogen population in fields, this strategy alone is not sufficient for controlling these diseases. Using resistant varieties is also an effective management tool. There are a number of different soybean varieties that are partially resistant to PRR that contain Rps (P. sojae) resistance genes.

Stem canker, another common stem blight of soybean, is caused by a complex of Diaporthe species. These fungal pathogens can survive in host residue for several years and tend to favor warm and wet weather for extended periods of time. Infection occurs when the pathogen is dispersed through wind or rain onto soybean stem tissue. As the disease progresses, the plant will begin to wilt as the pathogen continues to colonize the stem, inhibiting the uptake of water and translocation of nutrients. In severe cases, this can cause the plants to die. Stem canker is best managed using resistant varieties though crop rotation away from soybeans of two or more years is another very effective strategy.

Foliar Diseases

Cercospora leaf blight, caused by the fungal pathogen Cercospora kikuchii, is a foliar disease of soybean. C. kikuchii overwinters within soybean residue; warm and humid conditions promote sporulation of the pathogen within the infested residue. Rain splash and wind disseminate the spores on soybean leaves. The pathogen life cycle continues to reproduce throughout the season, producing spores to infect un-infected soybean foliage. As the disease progresses, the leaves will appear bronze or almost purple in color, limiting the plant’s photosynthetic activity and potentially causing yield loss (though significant yield losses caused by this disease are rare). C. kikuchii can also cause purple seed stain when the infection occurs on the pods, which can lead to reduced seed quality. Crop rotation away from soybeans for at least one year and planting resistant varieties are the most effective management strategies for Cercospora leaf blight and purple seed stain.

Frogeye leaf spot, caused by the pathogen Cercospora sojina, is one of the most yield-limiting diseases of soybeans, though the occurrence and severity may be sporadic from year to year depending on several environmental factors. Similar to the pathogen that causes Cercospora leaf blight, C. sojina also overwinters in soybean residue and reproduces in warm and humid environmental conditions. Frogeye leaf spot is named after the appearance of the symptoms on infected soybean leaves. The leaf spot lesions will often be small and round or irregularly shaped lesions with a gray discolored center surrounded by a thin reddish-brown outer margin. Similar to Cercospora leaf blight, frogeye leaf spot can also continue to reproduce from these lesions throughout the season. Signs of the sporulation of the pathogen can be seen, using a magnifying glass, in the center of the lesions that will have a gray and fuzzy appearance. Crop rotation and variety resistance, like management for the diseases listed above, are the best management strategies for protecting against frogeye leaf spot.

Soybean rust is a possible problem, and if present, will require much more intensive management to make organic soybean production viable. Soybean rust is caused by the pathogen Phakopsora pachyrhizi. Depending on the timing of its onset, the disease can cause significant yield loss; however, due to the environmental conditions and the unique life cycle of this pathogen, we infrequently experience severe yield loss in North Carolina. The pathogen primarily overwinters in the south and is blown north throughout the growing season through weather events. Symptoms appear as reddish-brown pustules on soybean leaves, and similar to the foliar diseases mentioned above, can reproduce prolifically under the conducive conditions of long periods of humid, warm weather and frequent rain events. Soybean rust is a disease that has the potential for causing severe economic damage in North Carolina soybean crops as it can cause premature defoliation and must be considered when managing soybean disease. To manage soybean rust potential in organic soybeans in North Carolina, select early-maturity groups and plant early to get the plants out of the fields in time to avoid the rust inoculum. Do not, however, create such an early-maturing soybean crop that yields are substantially reduced. If you are at risk of Asian Soybean Rust, consult OMRI’s listing of sprays labeled for use in soybeans.

Soybean ipmPIPE and the USDA’s page on soybean rust are excellent sources for further information on soybean rust.

Plant-Parasitic Nematodes

The best way to avoid nematode damage is to plant varieties that are resistant to the nematode present in the field. These varieties can be found on NC State’s NC Soybean Variety Information page or from N.C. Cooperative Extension agents. Conventional nematicides are prohibited in organic agriculture.

Soybean Cyst Nematode (SCN), caused by the nematode Heterodera glycines, is a plant-parasitic nematode found in all soybean growing regions and is the most yield-limiting disease of soybean in the U.S. The nematode penetrates soybean roots and continues to feed and reproduce within the roots. This causes extensive damage to the root tissue, reducing the uptake of water and nutrients. SCN gets its name due to the reproductive stage of its life cycle where small yellowish colored cysts erupt throughout the root tissue. These cysts can survive for many years in the soil and contain as many as 200 eggs per cyst. The most effective management strategy is to use an integrated approach of different crop rotations and variety resistance. Resistant varieties are available and can vary in effectiveness due to the different populations of SCN, but do not provide complete immunity. Historically, these different populations were characterized as races but are now referred to as HG types, in reference to the nematode’s scientific name. HG types, while similar to race classification, is a more robust way to classify the different SCN types based on the tests used for classification.

Several root-knot nematode species have been reported to infect soybeans in North Carolina, including Meloidogyne incognita (southern root-knot), Meloidogyne enterlobii (guava root-knot), Meloidogyne javanica (Javanese root-knot), Meloidogyne hapla (northern root-knot), and Meloidogyne arenaria (peanut root-knot). The most abundant species of root-knot nematode in North Carolina is M. incognita (southern root-knot). Symptoms of root-knot nematode infection may vary though are often associated with non-uniform stands, stunting, wilting, and chlorotic (yellow) patches. The most common symptom of root-knot is the galling found on the roots. These galls are enlarged female nematodes within the root tissue that contain egg masses inside the galls. Crop rotation of at least two years may help reduce soybean cyst nematode populations but may not be as useful when dealing with root-knot nematode because they have multiple host plants. The most effective management strategy is planting resistant varieties if there is a history of root-knot nematode in the field. If nematode damage is suspected, collect samples from the field (during fall is best) and send them to the NCDA&CS laboratory (1040 Mail Service Center, Raleigh, NC 27699-1040, 919-733-2655) for nematode assays. NCDA&CS will identify a nematode population and species, if it is present. The Agronomic Division of NCDA&CS also has nematode management and assay information online.

Acknowledgment of Previous Contributing Authors

Jim Dunphy, Crop Science Extension Specialist, NC State University

George Place, Crop Science, Research Associate, NC State University