Soybean Production Field Guide for North Dakota

From Fehr and Caviness1.

The soybean is a dicotyledonous plant that has epigeal emergence, meaning that during germination, the cotyledons (seed halves) are pulled through the soil surface (VE stage) by an elongating hypocotyl (Figure 1). The soil-penetrating structure, breaking through the soil surface, is the hypocotyl arch.

Figure 1. Soybean emergence

Once cotyledons have emerged, opened and turned green, they supply the new seedling with stored energy while capturing a small amount of light energy. The growing point is between the two cotyledons, and because it is above the ground, it could be killed by a spring frost or physical damage. This is in contrast with corn, wheat, field pea or lentil, in which the growing point is below the surface during the early development stages.

The first true vegetative leaves formed are the unifoliolate leaves. These two single leaves form directly opposite one another above the cotyledonary node (VC stage). All other leaves are trifoliolates and consist of three leaflets (V1 to Vn stages) and have an alternate arrangement on the stem (Figure 2).

Figure 2. The soybean plant V2 stage of development.

From Fehr and Caviness1.

Number of days between stages.

From Fehr and Caviness1.

1Fehr, W.R., and C.E. Caviness. 1977. Stages of soybean development. Spec. Rep. 80. Iowa State Univ. Coop. Ext. Serv., Ames.

Number of days between stages (0.0-0.5 relative maturity).

From Endres et al. Carrington Research Extension Center Annual Reports.

Extremes in growing conditions, such as temperature, rainfall and soils, can alter the development of soybean greatly. Many post-applied herbicides are labeled for application at certain soybean growth stages. To avoid herbicide injury (some herbicides), we highly recommend you identify development by growth stage and not use plant height, planting dates or row closure as a basis for application timing.

Additional information on soybean growth and development can be found in the NDSU Extension publication A1174, “Soybean Growth and Management Quick Guide.”

Soybean respond to day length (the increase of the night length) and heat units, so the actual calendar date a variety will mature is highly influenced by latitude; each variety has a narrow range of north-to-south adaptation. A model predicting when a variety will be mature is 9 available at the North Dakota Agricultural Weather Network (https://ndawn.ndsu.nodak.edu/soybeangrowing-degree-days.html). It will predict the soybean maturity date based on the selected maturity group, planting date and nearest weather station.

Soybean yield and quality are affected if a seasonending freeze occurs before a variety reaches physiological maturity. Dates of maturity are listed in the annual NDSU performance tables and provide a guide to predict physiologically mature. Usually harvest can commence seven to 14 days after the soybean crop is physiologically mature.

Varieties of maturity groups 00 (double zero), 0 (zero) and 1 are suitable for North Dakota. Maturity group 00 is very early and primarily is grown in the northern Red River Valley and the north-central area of North Dakota. Maturity group 0 is adapted to most North Dakota counties, while maturity group 1 is primarily suitable for southeastern North Dakota. These maturity groups are further subdivided. For example, a 0.1 maturity group is an early group 0 variety and a 0.9 is a late-maturity group 0 variety.

The best way to select a high-yielding variety is to use data averaged across several locations and years. Averaging across several years’ data will identify a variety that likely will yield well across different weather conditions.

Phytophthora root rot caused by the soilborne-fungus Phytophthora sojae is one of the most important disease problems of soybean in North Dakota. Phytophthora root rot tends to be more of a problem in the Red River Valley (RRV) and on poorly drained, heavy-textured soils, but the disease can cause significant stand reduction and yield loss in other areas when conditions are favorable for disease development. Management tools available to reduce Phytophthora root rot include selection of a resistant variety, use of a fungicide seed treatment, tile drainage and crop rotation.

Most varieties have Phytophthora root rot-resistance genes, and each gene confers resistance to a different race (or races) of Phytophthora. For example, a gene that may confer resistance to Race 3 may not confer resistance to Race 4, and vice versa.

Phytophthora sojae is a variable pathogen, and many races of the pathogen exist in North Dakota. No specific resistance gene guarantees control of the pathogen. Consequently, monitoring your fields for Phytophthora root rot every year is important. If the disease is widespread, the pathogen may have overcome the gene being used, and the gene may not be effective in future plantings.

Similarly, continually rotating effective genes is very important. Lack of a resistance gene or crop rotation can speed the development of new races. In some North Dakota fields, the pathogen already has become resistant to multiple genes. Fungicide seed treatments with activity against Phytophthora may help prevent early infection.

However, seed treatments do not provide season-long control, and over time the pathogen can become resistant to them. Consequently, fungicide seed treatments and resistance genes should be rotated. The most effective strategy would include the use of effective fungicide seed treatments, planting varieties with genetic resistance, water management (surface and subsurface drainage) and crop rotation.

Iron-deficiency chlorosis (IDC) is a major problem primarily in the eastern part of North Dakota and is caused by iron being less available in soil with a pH greater than 7 and the presence of soil carbonates. Iron-chlorosis symptoms are most common during the two- to seven-trifoliolate leaf stages.

Plants tend to recover and start to turn green again during the flowering and pod-filling stages. However, IDC during the early vegetative stages can reduce yield severely. Details on IDC are available in the NDSU Extension publication SF1164, “Soybean Soil fertility”.

For fields with historic IDC problems, select an IDC-tolerant variety of suitable maturity that is high yielding. Data on genetic differences for IDC tolerance is available in publication A-843, “North Dakota Soybean Variety Trial Results and Selection Guide.”

The soybean cyst nematode (SCN), Heterodera glycines, is a small parasitic roundworm that attacks the roots of soybean plants. Nematodes often are undetected because above-ground symptoms are uncommon until a 15% to 30% yield loss has occurred.

Soybean cyst nematode has been confirmed in many soybean-growing counties in North Dakota. Growers are strongly urged to test their soils for SCN eggs. If a positive sample for SCN is found, growers should begin managing SCN actively.

Crop rotation and resistance are the most important management tools against this disease. Two sources of resistance to SCN — PI88788 and Peking — can be found in varieties suitable for North Dakota. These sources are effective in the vast majority of the soybean fields in the state. However, the level of resistance in each variety is variable, so selecting the most resistant variety possible and monitoring the field for SCN is important. Due to the difference is resistance level between varieties, also rotate tolerant varieties in the same field.

For SCN management, a rotation out of soybean for two to three years is beneficial. Dry edible beans are susceptible to SCN and should not be used as a rotation crop for managing SCN. Nematode protectant seed treatments also are available and may help manage SCN; however, they are not a substitute for resistance and rotation.

Some soybean varieties have been developed for human consumption and have special food-processing characteristics. Tofu is a white curd that primarily is consumed in Asian countries. Special varieties have been developed that are high in protein and make smooth-textured tofu. These high-protein tofu types are lower yielding than the regular varieties that are sold to the elevator.

Natto is another human food product made from soybean. Natto is a fermented product made from whole soybean that is cooked. Natto varieties are very small seeded and tend to yield even less than the specialty varieties developed for the tofu market.

Growers should consult university publications on soybean variety performance to determine how much less these specialty varieties yield, compared with oilseed soybean. Based on the lower yield, a higher price per bushel needs to be obtained to economically justify growing these specialty soybean types. A contract should be arranged prior to growing these special types so that a market will be available.

Soybean varieties with modified oil content are commercially available. Different fatty acid compositions modify the type of oil the soybean plant produces in the seed. Low saturated fats are desirable because this type of oil is better for human health.

High oleic, low palmitic, low stearic and low linolenic acid content are all genetic modifications that produce more healthful oil for human consumption. There is no indication that these modifications reduce yield. However, yield of specific varieties with modified oil content should be evaluated to determine whether high yield has been incorporated with the modified oil content. These specialty varieties should be marketed as identity-preserved (IP).

NDSU research on various plant establishment management factors has been conducted in numerous field studies. The improved management options in column A are compared with traditional management options in column B and the average percent benefit was calculated. The number of trials is indicated in the last column.

NDSU Research Summary of Soybean Plant Establishment Factors.

1Seeding rate was compensated for the germination percent.
(Seeding rate / germination rate)*100. 

Soybean is a legume and receives N through the symbiotic relationship with the N-fixing bacteria species Bradyrhizobium japonicum. Carbohydrates and minerals are supplied to the bacteria by the plant, and the bacteria transform nitrogen gas from the atmosphere into ammonium-N for use by the plant. Soybean infection by N-fixing bacteria and symbiotic N fixation is a complex process between the bacteria and the plant. The right species of N-fixing bacteria must be present in the soil, either through inoculation of the seed or the seed zone at planting. Soybean is perfectly capable of attracting bacteria to the roots when N is required, and there is no need to apply supplemental “signaling” agents.

If soybean will be planted in a field for the first time, seed inoculation with Bradyrhizobium japonicum (soybean inoculum) is necessary. There are several inoculums types that can be used: peat-based, liquid-based or granular. Double inoculation is encouraged, using a granular inoculant and either a peat or a liquid.

If a field has been seeded to soybeans previously and nodulation was effective, inoculation is not necessary, but neither would it hurt soybean yield. Nitrogen fertilizer is not required by soybean if adequate inoculation is present. Higher soil nitrate levels in soils increase the severity of iron deficiency chlorosis (IDC) in soils where IDC supporting conditions prevail.

North Dakota soils are typically low in phosphorus (P). The soil test supported by NDSU recommendations is the Olsen sodium bicarbonate extraction method. North Dakota soybean P research within the past decade indicates an unlikely economic benefit to added P if the soil test is greater than 7 ppm. Rate of P is related to soil test from 7 ppm and below, and not to expected yield.

Phosphorus and potassium recommendation (broadcast application) for soybean based on soil test.

1Use first number for the rate of K2O in soils with smectite:illite ratio less than 3.5; use the second number for the rate of K2O in soils with smectite:illite ratio greater than 3.5.

Potassium requirements for soybean are lower than they are for corn. However, in a corn and soybean rotation, fertilizing for higher nutrient levels is important. Although the critical soil level for K using the 1-N ammonium acetate extraction method is only 100 ppm for soybean, the critical level for K in corn is closer to 200 ppm. Therefore, adding K to replace what the soybean crop may remove will make fertilizing corn more efficient. In dry seasons, soybean has shown K deficiency symptoms even when soil test K is greater than 100 ppm. The recommendations for K fertilization of soybean based on soil test.

Foliar fertilizer for soybeans is not an effective or economic production strategy and should be avoided unless a severe nutrient deficiency is found in-season.

Sulfur deficiency is not commonly seen in soybean. It is most possible on sandy, low-organic-matter soils after heavy spring rains. Sulfate and thiosulfate sources are far superior to elemental S fertilizers of any kind. Sulfur is a spring fertilizer, and fall application is discouraged.

There are great amounts of iron (Fe) in North Dakota soils, but in eastern North Dakota, where rainfall is higher and calcium carbonate is abundant at the surface in many soils, soybean have a hard time obtaining it. Iron deficiency symptoms are commonly seen and yield reduction is almost always a result of the condition.

The best strategy for managing IDC is use of tolerant cultivars. NDSU IDC ratings can be found in the annually updated NDSU Extension publication “North Dakota Soybean Variety Trial Results and Selection Guide.”

Iron foliar sprays are not effective in correcting a deficiency. The best application to reduce IDC is ortho-ortho-EDDHA Fe chelate applied with water in-furrow at seeding. The chelate ortho-ortho-EDDHA succeeds in delivering Fe to the plant root early in the season, but after conveying its original Fe, it also has the ability to return to the soil solution, capture additional Fe and deliver it to the plant root with the soil water stream (Goos and Lovas, unpublished data, 2012). The amount of ortho-ortho EDDHA in relation to ortho-para EDDHA is very important in an iron chelate product. Both forms can be in a product, although the form effective in reducing IDC is only the ortho-ortho-EDDHA. Recent research at NDSU has shown that the response of soybeans to EDDHA fertilizer is directly proportional to the percentage of ortho-ortho EDDHA Fe in the product. The chemical structures of ortho-ortho-and ortho-para-EDDHA are shown in Figure 3.

Figure 3. Ortho-ortho EDDHA (left); ortho-para EDDHA (right).

1. Screen fields through past history of IDC or with soil testing to choose fields with lower carbonates near the soil surface and with soluble salts with EC less than 2 mmohs/cm.
2. Choose soybean cultivars with high IDC tolerance from NDSU soybean variety trial data.
3. Seed a companion crop of oats, barley, or spring wheat at the time of seeding, particularly if soil nitrate-N in greater than 80 pounds N per acre.
4. Use a high ortho-ortho-EDDHA, or similarly effective iron fertilizer, in-furrow at seeding at recommended rates.
5. If possible, choose herbicides with low phytotoxicity to soybean. However, controlling weeds is very important and weed control has priority over safety if weed population is high.

Soybean plant deficiencies of zinc, manganese, boron, molybdenum, nickel, chloride or copper have not been observed in North Dakota. There is no need to apply these nutrients on North Dakota soybean fields.

Soybean is one of the most sensitive crops to salt-affected soils. The relationship of relative yield and soil salt levels measured by electrical conductivity (EC) in North Dakota research is shown in Figure 4. Relative tolerance of soybean cultivars to soil EC is roughly related to cultivar IDC tolerance.

Figure 4. Relationship of salinity (1:1 soil: water EC) and relative soybean yield.

Details on identifying and managing saline soils for soybean and other crop production are available in the NDSU Extension publication “Managing Saline Soils in North Dakota.”

For more details on soybean plant nutrition, see NDSU Extension publication SF1164, “Soybean Soil Fertility.”

Soybeans are shallow rooted. Typically, roots grow laterally 8 to 12 inches and downward to a depth of 3 feet or more. Root distribution is concentrated near the soil surface. About 90% of the roots will be found in the top 2 feet, which is considered the effective rooting depth for irrigation purposes. During the course of a growing season, about 10% of the water used by the soybeans will be drawn from the soil below 2 feet.

Daily soybean evapotranspiration (ET) or water use depends on the day length, variety, stage of growth, local weather conditions, available soil water, disease pressure and soil fertility. The frequency and amount of irrigation depends on the growth stage (which determines the daily crop water use), the water-holding capacity of the soil in the root zone and the prevailing weather conditions.

Average soybean water use rates will increase from about 0.05 inch per day soon after emergence to more than 0.27 inch per day during pod development (Figure 5). The soybean water use amount includes the evaporation from the soil surrounding the plants. Water use is presented as a depth measurement because of the assumption that the soybean root mass will remove soil water from under every square foot of soil surface in the field.

Figure 5. Average and seasonal variation of daily water use rates (in inches) for soybean.

The depth and water-holding capacity of the soil have a great influence on when and how often irrigation is required. Soil texture determines the amount of available water in the root zone, although other factors such as organic matter and soil compaction will modify these numbers. Note that the greater the water-holding capacity of the soil in the root zone, the less frequent the irrigation applications should be. The values shown in the Table are a good starting point, but more field specific water holding capacities can be obtained by using the Natural Resources Conservation Service’s (NRCS) online Websoil Survey (https://websoilsurvey.nrcs.usda.gov/app/HomePage.htm).

Approximate available soil water-holding capacities for various soil textural classifications.

Knowing the soil texture and water-holding capacity of the dominant soil type in a field and using that information to make irrigation decisions is important. However, if different soil types are in the same field, irrigation scheduling should be adjusted based on the most drought-prone soil type.

Having a soil profile that is near field capacity at planting, which occurs naturally with normal fall and spring rainfall, together with some winter snowmelt, is highly desirable. Stored soil water lower in the root zone serves as a supplement during high water-use periods.

Soybeans planted on shallow soils (8 to 12 inches of top soil) underlain by coarse sand and gravel will have a reduced root zone. That means less soil water is stored and available to the plants. Sometimes, this requires applying less per irrigation (0.75 inch) more frequently, compared with deeper soils with greater total water-holding capacity.

During the period prior to flowering (R1 growth stage) and the period after the majority of the seeds are mature (R7), soybeans are relatively drought-tolerant. They can withstand 50% to 60% soil water depletion without a significant impact on yields. However, during the flowering and pod-development period, soil water levels in the root zone should not be depleted more than 50% (preferably 40%) to achieve maximum yields.

The first irrigation should be applied when the soil water is between 50% and 60% depleted after emergence. With normal rainfall, this should be when the soybeans are near flowering (late June to early July). After flowering, irrigate before the soil water profile reaches 50% depletion.

Most center pivots should be set to apply from 0.75 to 1 inch of water per revolution, which may require 2½ to 3½ days. During periods of high temperatures and wind, monitoring the soil water profile frequently is critical because keeping up with the soybean water use may be difficult.

Some tips for good irrigation management of soybeans:

• Soybeans are unique in that leaf-rolling is a visual indicator of water stress because the underside of the leaf is a lighter green than the upper surface. Usually, water stress will first occur in areas of the field with shallow topsoil.
• White mold (sclerotinia) risk increases with irrigated soybean. During flowering, an extended period with wet soil surface and crop canopy may require use of foliar fungicides to suppress disease infection and development.
• Irrigations can be terminated when at least 80% of the pods show yellowing and are mostly ripe. Another indicator is when 50% of the leaves have turned yellow.

Soil water levels in the root zone are the criteria used to determine when to start and stop irrigations. Managing irrigations around rain events can sometimes be challenging. Using soil moisture sensors or the Checkbook method is important.

Several soil water monitoring methods and/or instruments are available to estimate the soil water level at a particular time and location in the field. The “soil feel” method is the most widely used. It involves using a soil probe to obtain a soil sample from specific depths in the root zone, and the amount of soil water is estimated by squeezing the soil in the palm of your hand. Accuracy of the soil feel method improves with experience.

Soil water also can be measured using tensiometers, soil water blocks and other devices. When these are used, a measurement site typically will have two sensors, one at 9 to 12 inches and one at 18 to 24 inches below the soil surface. The soil water level is estimated by reading a gauge on the device or with a portable meter. These devices only indicate the soil water status at the installation location, so a large field may require soil water sensors at several sites.

Measuring soil water for irrigation scheduling requires a high level of dedication by the irrigation manager during the growing season. Informed irrigation decisions require reading soil water measurements two or three times per week. Measurements must be taken at several locations in the field and the readings recorded. Lately, companies now offer soil water measuring devices that can be accessed remotely, often with a smart phone app. Also, many companies offer field monitoring services based on remote sensing, either by satellite, drones or planes.

Another form of irrigation scheduling requires daily soybean water-use estimates. This method, sometimes called the “crop water use replacement method,” is based on obtaining daily estimates of soybean water use and measuring rainfall amounts. Irrigations are scheduled to replace the amount of soil water used by the soybeans minus the amount of rain received since the last irrigation. Estimations of average water use for soybeans based on maximum daily temperature are shown in the table.

Average soybean water use based on maximum daily air temperature and week after emergence (inches per day).

During the growing season, better soybean water use estimates can be obtained from the North Dakota Agricultural Weather Network (NDAWN) website: https://ndawn.ndsu.nodak.edu/crop-water-use.html. These estimates a calculated using the weather data from each weather station. Select the station closest to your field location.

The best choice of tools for irrigation scheduling is a combination of in-field soil water measurement and a recorded daily soil water accounting procedure called the “checkbook” method. Three different checkbook tools are available at www.ndsu.edu/agriculture/ag-hub/ag-topics/crop-production/irrigation-tiling-drainage/irrigation-scheduling.

The checkbook method has been used successfully for many years in Minnesota and North Dakota. The checkbook method is based on daily soybean water use and the soil water-holding capacity. Along with rainfall measurement, these parameters help predict the time and amount of water needed to replenish the root zone to maintain proper soil water levels. One advantage of the checkbook method is that you don’t need to be in the field to fill in the numbers, and it allows you to “look ahead” to determine when the next irrigation may be needed. Modern weather prediction of possible rain events also helps.

In soybean, field scouting to assess insect populations is based on the number of insects per foot of row, insects per plant, sweep net sampling or the level of defoliation.

Insects per foot of row are determined by shaking plants over the inter-row space, on which a strip of cloth has been laid. Count the total number of insect pests per foot of row that fall on the cloth.

The percent of defoliation is determined by estimating the amount of leaf tissue loss based on visual inspection of randomly selected plants. Examples provided (Figure 7) are guidelines for estimating loss for individual leaflets. Actual defoliation estimates made for pest management decisions are based on estimated leaf area lost from the entire plant.

The growth stage of the soybean plant is important when making pest management decisions. Under most conditions, moderate defoliation early in the season has little effect on final bean yield. As plants reach the flowering and pod-filling stages, defoliation poses a greater threat to yield.

• From vegetative to pre-bloom stages, the soybean plant can sustain a 30% leaf loss.
• From flowering to pod-set (reproductive), the plant can tolerate only a 20% defoliation level.
• If pod feeding insects are present in large populations and pod clipping is occurring, treat aggressively when pod injury reaches 10%.
  

  Figure 7. Soybean defoliation levels

Armyworms are greenish-brown with longitudinal stripes. Full-grown larvae are smooth, striped and almost hairless. Armyworms feed for three to four weeks. When full grown, larvae are 1½ to 2 inches long.

Armyworm larvae have six growth stages, or instars. The final instar lasts about 10 days, and they consume large amounts of plant material during that time.

Armyworms are inactive during the day, resting under plant trash, and clumps of grass or lodged plants. They feed at night or on cloudy days, crawling up on plants and consuming foliage. Due to their habit of feeding at night, armyworms may go undetected until significant damage has occurred.

Armyworms do not overwinter in the region. Moths migrate from southern states in late spring and early summer. This helps explain the sporadic infestations that occur. When moths arrive, they prefer to lay their eggs in moist, shady areas, usually where grasses have lodged. Infestations that develop within soybean fields are often due to grassy weed problems.

Armyworms are more of a problem in small grains and corn. Damage to soybean can occur when the armyworms’ usual host plants become exhausted due to feeding or dry conditions. When their food is depleted in the hatching site, the armyworms may move in large numbers or “armies,” eating and destroying plants or crops in their path.

Threshold

Control of armyworms is recommended when 25% to 30% of the foliage is destroyed or if significant injury to pods is evident. Most often in soybean, infestations are due to migrating armyworms. Under these circumstances, treatment of a couple of swaths ahead of the migrating armyworms to establish a barrier strip and prevent further migration and injury, may be all that is needed.

This beetle can vary from yellow to reddish brown and may have three to four black spots with a black border on the wing covers. Adults emerge from overwintering and move into bean fields as the seedlings emerge. The white larvae develop in the soil, feeding on the roots and nodules. New adults emerging from mid-July to August feed on foliage and pods.

Feeding injury to leaves appears as small round holes between the leaf veins. Late-season feeding on the foliage and pods by the new adults that emerge in August can be more important than early season feeding, especially if viruses are present. This may increase the risk of virus transmission and cause secondary fungal and bacterial infections (rotting and discoloration). Bean leaf beetles are the vector of bean pod mottle virus.

Threshold

Bean leaf beetle populations have been increasing in North Dakota over the past years. It is now found in southeast, east central and north central areas of North Dakota. A sweep net can be used to determine if bean leaf beetles are present. Treatment is recommended when three to seven beetles per sweep are found. Treatment thresholds based on defoliation are 30% defoliation during early vegetative to pre-bloom, 20% defoliation during bloom to pod set to fill and only 10% if pod feeding or clipping is occurring.

Several cutworm species affect soybeans. The dingy cutworm, Feltia jaculifera (Guenée), overwinters as a partially grown larva and is one of the first cutworm species to cause problems during crop emergence from early to mid-May. The moth of the dingy cutworm is known to lay her eggs on sunflower heads from mid-July through September. Soybean and other crops following sunflower in rotation are at greatest risk of injury by this cutworm.

Other cutworms — the redbacked, Euxoa ochrogaster (Guenée) and the darksided, Euxoa messoria (Harris) — overwinter as eggs, which hatch in mid to late-May. Eggs are laid in the fall and survive in weedy, wet and reduced-tillage areas. Feeding injury by these cutworms normally occurs in late-May to mid-June.

Most damage by cutworms occurs when soybean plants are in the early stage of development. Damage consists of young plants being chewed off slightly below or at ground level. Some climbing cutworms feed on foliage causing defoliation. Cutworms feed primarily at night. When checking soybean fields for cutworms during the day, dig down into the soil an inch or two around recently damaged plants; there you can find the gray to brown larvae.

Threshold

Economic thresholds for cutworm treatment decisions are not well-established. Treatment guidelines used include when one cutworm (larva) or more is found per 3 feet of row and larvae are small (less than ¾ inch long). Another guideline is when 20% of plants are cut or when gaps of 1 foot or more exist in the plant row. When making a final decision, consider if surviving soybeans are able to compensate for early stand reductions because of the plant’s long growth period and branching ability.

• Green Cloverworm, Cabbage Looper, Velvetbean Caterpillar, Thistle Caterpillar and Alfalfa Webworm: Populations of these foliage-feeding caterpillars (larvae) are considered occasional insect pests in North Dakota. To sample for larvae (caterpillars), use a drop cloth or vertical beat sheet placed between two rows of plants. Dislodge the larvae from the plants and count them on the cloth or collection tray to arrive at an estimate of the number per row feet. Treatments also can be based on the average defoliation (see Estimating Defoliation Damage section near the beginning of Insect chapter).
• Green cloverworm [Erebidae: Hypena scabra (Fabricius)]: These larvae are green with two narrow white stripes down the side. When mature, larvae are 1¼ inches long, and have only three pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, larvae arch the middle of the body, or “loop.” Young larvae scrape leaf tissue, creating a transparent skin or “window” on the leaf surface. Older cloverworms eat holes in the leaves.
• Cabbage looper [Noctuidae: Trichoplusia ni (Hübner)]: These larvae are light to dark green with lighter-colored stripes along the side and on the top, running the length of the body. When mature, larvae are 1½ inches long, and have only two pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, the larvae also arch the middle of the body or “loop.” These larvae feed on leaves on the interior and lower portion of the plant. As defoliation occurs, larvae feed higher in the plant. Feeding injury is similar to the cloverworm.
• Velvetbean caterpillar (Noctuidae: Anticarsia gemmatalis Hübner): This insect does not overwinter in the region. Instead, moths migrate from southern locations. Larvae have dark lines bordered by lighter-colored, narrower lines running the length of the body. The background color ranges from a pale yellow-green to brown or black. Velvetbean caterpillars have four pairs of fleshy prolegs to distinguish them from the cloverworm and the looper. Young larvae feed on the underside of leaves in the upper portion of the plant. Older larvae consume the entire leaf, except for the leaf veins.
• Thistle caterpillar [Nymphalidae: Vanessa cardui (Linnaeus)]: This insect is the larva of the butterfly known as the painted lady. This butterfly does not overwinter in the region but migrates from southern locations each spring. These caterpillars are brown to black, with yellow stripes along each side of the body. They are covered with spiny scoli (fleshy structures) that give the caterpillar a prickly appearance. Full-grown larvae are about 1½ inches long. Larvae feed on the leaves, webbing them together at the feeding site.
• Alfalfa webworm [Crambidae: Loxostege cereralis (Zeller)]: Larvae are 1 inch long when fully grown, and greenish to nearly black, with a light stripe that runs down the middle of the back. They have three dark spots, each with hairs, on the side of each segment. Larvae feed for about three-plus weeks. Infestations are characterized by light webbing over the leaves. Beneath the web is where the larvae feed, consuming the leaves. These larvae move very rapidly, forward or backward, when disturbed.

Threshold

Control of these foliage-feeding caterpillars normally is not warranted until greater than 30% of the foliage is destroyed prior to bloom, or when 20% of the foliage is destroyed after bloom, pod set or fill has been reached. This usually requires an average infestation of four to eight larvae per row foot.

• Green Cloverworm, Cabbage Looper, Velvetbean Caterpillar, Thistle Caterpillar and Alfalfa Webworm: Populations of these foliage-feeding caterpillars (larvae) are considered occasional insect pests in North Dakota. To sample for larvae (caterpillars), use a drop cloth or vertical beat sheet placed between two rows of plants. Dislodge the larvae from the plants and count them on the cloth or collection tray to arrive at an estimate of the number per row feet. Treatments also can be based on the average defoliation (see Estimating Defoliation Damage section near the beginning of Insect chapter).

• Green cloverworm [Erebidae: Hypena scabra (Fabricius)]: These larvae are green with two narrow white stripes down the side. When mature, larvae are 1¼ inches long, and have only three pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, larvae arch the middle of the body, or “loop.” Young larvae scrape leaf tissue, creating a transparent skin or “window” on the leaf surface. Older cloverworms eat holes in the leaves.

• Cabbage looper [Noctuidae: Trichoplusia ni (Hübner)]: These larvae are light to dark green with lighter-colored stripes along the side and on the top, running the length of the body. When mature, larvae are 1½ inches long, and have only two pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, the larvae also arch the middle of the body or “loop.” These larvae feed on leaves on the interior and lower portion of the plant. As defoliation occurs, larvae feed higher in the plant. Feeding injury is similar to the cloverworm.
• Velvetbean caterpillar (Noctuidae: Anticarsia gemmatalis Hübner): This insect does not overwinter in the region. Instead, moths migrate from southern locations. Larvae have dark lines bordered by lighter-colored, narrower lines running the length of the body. The background color ranges from a pale yellow-green to brown or black. Velvetbean caterpillars have four pairs of fleshy prolegs to distinguish them from the cloverworm and the looper. Young larvae feed on the underside of leaves in the upper portion of the plant. Older larvae consume the entire leaf, except for the leaf veins..
• Thistle caterpillar [Nymphalidae: Vanessa cardui (Linnaeus)]: This insect is the larva of the butterfly known as the painted lady. This butterfly does not overwinter in the region but migrates from southern locations each spring. These caterpillars are brown to black, with yellow stripes along each side of the body. They are covered with spiny scoli (fleshy structures) that give the caterpillar a prickly appearance. Full-grown larvae are about 1½ inches long. Larvae feed on the leaves, webbing them together at the feeding site.
• Alfalfa webworm [Crambidae: Loxostege cereralis (Zeller)]: Larvae are 1 inch long when fully grown, and greenish to nearly black, with a light stripe that runs down the middle of the back. They have three dark spots, each with hairs, on the side of each segment. Larvae feed for about three-plus weeks. Infestations are characterized by light webbing over the leaves. Beneath the web is where the larvae feed, consuming the leaves. These larvae move very rapidly, forward or backward, when disturbed.

Threshold

Control of these foliage-feeding caterpillars normally is not warranted until greater than 30% of the foliage is destroyed prior to bloom, or when 20% of the foliage is destroyed after bloom, pod set or fill has been reached. This usually requires an average infestation of four to eight larvae per row foot.

In the northern Great Plains, grasshopper egg hatch normally begins in late April to early May. Most grasshoppers emerge from eggs deposited in uncultivated ground. Soybean growers should expect to find grasshoppers feeding first along bean field margins adjacent to non-crop sites, where the nymphs are hatching. Later infestations may develop when grasshopper adults migrate from harvested small grain fields. Grasshoppers will feed on leaves and pods, chewing holes in them. Soybean fields often become sites for significant egg laying from migrating grasshopper adults.

Threshold

The threatening rating is considered the action threshold for grasshoppers. Grasshopper control is advised whenever 50 or more small nymphs per square yard can be found in adjacent, non-crop areas, or when 30 or more nymphs per square yard can be found in the field. When 20 or more adults per square yard are found in field margins or eight to 14 adults per square yard are occurring in the crop, treatment would be justified. Because estimating the number of grasshoppers per square yard is difficult when population densities are high, pest managers can use a 15-inch sweep net and count grasshoppers collected from four 180-degree sweeps and use that value as an estimate for the number of adult (or nymph) grasshoppers per square yard.

Many of the grasshopper infestations in soybean will be heaviest on field margins. Treating these areas early in the season during outbreaks may lessen the total number of grasshoppers successfully entering a field.

Soybean is most sensitive to defoliation during pod and seed development (growth stages R4 to R6). During this time, plants can tolerate up to only 20% defoliation.

Of greater concern is direct feeding damage to pods and seeds. Grasshoppers are able to chew directly through the pod walls and damage seed. If grasshoppers injure more than 10% of the pods, a prompt insecticide application is recommended.

The adult is about 1/8 inch long, pale green and wedge-shaped. Adults are very active, jumping or flying when disturbed. Nymphs are wingless. Adults and nymphs run backward or sideways rapidly when disturbed.

Nymphs feed on the underside of the leaf, usually completing their growth on the leaves near where they hatched. Potato leafhoppers migrate into North Dakota from southeastern U.S. When large numbers of adults may appear early in the season, frequent scouting is necessary.

Soybean with moderate to dense pubescence, or plant hairs, are tolerant to leafhopper infestations. The short plant hairs form a barrier that discourages leafhoppers from feeding and laying eggs on plant tissue. When feeding does occur, damage by leafhoppers is referred to as “hopper burn.” Foliage becomes dwarfed, crinkled and curled. Small triangular brown areas appear at the tips of leaves, gradually spreading around the entire leaf margin. Potential damage to soybean by potato leafhopper is not fully understood. Damage is more likely when drier growing conditions occur.

Threshold

The threshold for spray decisions is an average of five leafhoppers per plant in the vegetative stages, and nine leafhoppers per plant in early bloom stages.

Soybean aphid was first detected in North Dakota in 2001 and has become a major insect pest.

Foliar insecticides are the primary management tactic for aphid control. However, multiple years of research have shown that natural enemies, including predators (lady beetles) and parasitic wasps, often can keep soybean aphids below the economic threshold in non-outbreak years. Another nonchemical management tactic that shows promise for controlling soybean aphid is the use of genetically based aphid-resistant soybean varieties.

Soybean aphid is small, about 1/8 inch, lime green with black cornicles (“tail-pipes”) and a pale-colored cauda (tail projection). Nymphs (or young) are smaller yet.

Aphids have piercing-sucking mouthparts and feed on plant sap. When infestations are large, infested leaves are wilted or curled. Aphids excrete honeydew, a sweet substance that accumulates on surfaces of lower leaves and promotes the growth of sooty mold.

Soybean aphids colonize tender leaves and branches from early vegetative through reproductive plant stages. Later, as vegetative plant growth slows, the aphids slow their reproductive rate, move down to the middle and lower part of the plant, and feed on the undersides of leaves. Toward the end of the season, the colonies again begin to increase in number rapidly. These increases are followed by a migration to the overwintering host, buckthorn. Soybean aphids overwinter as eggs near the buds of buckthorn.

Threshold

The guidelines for making soybean aphid treatment decisions include:

• Begin scouting soybean fields at the V3 to V4 stage to determine if soybean aphids are present in fields. No treatment is recommended at this time and is discouraged so that insecticides do not reduce the presence of predators and parasitic wasps.
• The critical growth stages for making most soybean aphid treatment decisions are the late-vegetative to early reproductive stages (Vn to R3). Assessing aphid populations at these times is critical.
• The economic threshold from R1 (first flower) through R5 (beginning seed) is an average of 250 aphids per plant and when populations are increasing actively in 80% of the field. At R6 (full seed), no insecticide treatment is recommended. Research trials throughout the north central states have not demonstrated a yield benefit for soybean aphid management at the R6 and later maturing stages.

Soybean aphid with pyrethroid resistance was first documented in North Dakota in 2017. Reported failures of pyrethroid insecticides for control of soybean aphids were widespread and occurred primarily in the eastern counties of North Dakota. The source of our pyrethroid-resistant soybean aphids in North Dakota could be from migrating resistant soybean aphids that flew into North Dakota from neighboring states or from overwintering resistant soybean aphids. For pest management, growers should assume that most of our soybean aphid populations in eastern North Dakota are resistant to pyrethroids, and use other insecticide groups for aphid control.

To reduce development of insecticide resistance in soybean aphids, Extension entomologists recommend:

• Scout fields regularly and start in mid-June.
• Use the economic threshold to aid in decision-making, prevent unnecessary insecticide applications and conserve natural enemies.
• Rotate the mode of action (or insecticide class) if more than one application is necessary in a season.
• Do not use the same mode of action (or insecticide class) repeatedly year after year.
• Avoid using the lowest rate of insecticide on the label. Use high rates to prevent the development of insecticide resistance.
• Do not use premix insecticides containing two insecticides of the same or two different modes of action, because premixes have lower amounts of active ingredient per insecticide could promote the development of insecticide resistance.

For more information, please consult the NDSU Extension publication “Management of Insecticide-resistant Soybean Aphids E1878” (www.ndsu.edu/agriculture/ag-hub/publications/management-insecticide-resistant-soybean-aphids).

Seedcorn maggots attack soybean seed, preventing sprouting or weakening the seedlings. The tiny, yellowish-white maggot is found burrowing in the seed, the emerging stem or the cotyledon leaves. Damage to the seedlings results in a condition called “snakeheads,” or plants without cotyledon leaves.

Adult flies emerge in the spring when soil temperatures reach 50 F. They deposit eggs in soil with abundant organic matter and decaying crop residue, or on the seed or seedling. Injury from seedcorn maggots is usually most severe during wet, cold springs and in fields with high organic matter soils. When cool, wet conditions occur during planting, the slow germination and emergence of the seedling extends the period of time it is vulnerable to the maggot’s feeding.

Threshold

When conditions are wet and cool, or when planting into high crop residue conditions, insecticide seed treatments provide the best defense against maggot injury.

Adult spider mites are tiny (less than 0.2 inch) and greenish white to orange red, with two dorsal spots and four pairs of legs. Nymphs are smaller than adults and have three to four pairs of legs. Magnification is often necessary to see mites.

Host plants for spider mites include soybean, dry bean, alfalfa, corn, vegetables, ornamentals and trees. Mites overwinter as eggs on vegetation. The life cycle of spider mites can be completed in only five to 14 days, with the fastest development rates occurring above 91 F.

Each female lives for 30 days and produces about 300 eggs during her lifetime. In hot, dry weather, natural fungal diseases of mites are slowed and populations can increase from a few individuals to millions within a few generations. Mites thrive on the stressed plants that are nutrient-rich.

Leaf injury symptoms appear as stippling first and then progress to yellowing, browning or bronzing as feeding injury increases, and eventually leaf drop. Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant. In severe cases, premature leaf senescence and pod shattering, and even plant death can occur.

When severe mite infestations occur during late vegetative and early reproductive growth, a 40 to 60% yield loss between treated and untreated soybean has been demonstrated in other Midwest states. Be sure to scout during full pod (R4) through beginning seed (R5) stages because these crop stages are the most important contributors to soybean yield. Spider mites can cause yield reduction as long as green pods are present.

When scouting for spider mites, look on the underside of leaves and lower foliage at the field edges first for tiny mites and fine spiderlike webbing. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below leaves, then tap them to dislodge the mites. The mites appear as tiny dust specks; however, they will move slowly after being knocked off the leaf. Dislodged predatory mites will move faster than the two-spotted spider mite.

Another sampling measure involves pulling up plants and examining the underside of the leaves from the bottom of the plants upward. When spider mites need to move due to diminishing food supply, they climb to the top of plants and are dispersed by the wind through “ballooning,” so they can spread quickly within a field or to adjacent fields.

Infestations typically are first noted near field edges and fields near alfalfa (a preferred host). Products labeled for mite control often do not give adequate control and the population of mites may rebound quickly to pretreatment levels or higher. When rain and humidity are present, natural infections of fungal pathogens can reduce mite populations.

Conditions that are good for the development of the pathogen are temperatures lower than 85 F, with at least 90% relative humidity for 12 to 24 hours. Mites usually become a problem when hot, dry weather occurs. When a production area has low rainfall, the region can become a “hot spot” for mite injury and a source of mites migrating to neighboring areas.

Threshold

Deciding whether to treat is difficult. No specific threshold has been developed for two-spotted spider mite in soybean. Sample plants at least 100 feet into the field. Walk in a “U” pattern and sample two plants per location at 20 different locations. Assess mite damage using the following scale from University of Minnesota Extension:

0 - No spider mites or injury observed

1 - Minor stippling on lower leaves; no premature yellowing observed

2 - Stippling common on lower leaves; small areas or scattered plants with yellowing

3 - Heavy stippling on lower leaves, with some stippling progressing into middle canopy; mites present in the middle canopy, with scattered colonies in upper canopy; lower-leaf yellowing common; small areas with lower leaf loss may occur (spray threshold)

4 - Lower leaf yellowing readily apparent; leaf drop common; stippling, webbing and mites common in the middle canopy; mites and minor stippling present in the upper canopy (economic loss)

5 - Lower leaf loss common; yellowing or browning moving up plant into the middle canopy; stippling and distortion of upper leaves common; mites present in high levels in the middle and lower canopy

If spider mites are above threshold when significant pod or seed fill remains, an organophosphate insecticide (Dimethoate) is recommended versus a pyrethroid insecticide. Pyrethroids (Asana, Baythroid, Mustang Max, Warrior) tend to flare (increase) mite populations seven to 10 days after application. Reasons for an increase in mite populations include: disruption of the natural enemies that control spider mites (predatory mites, thrips); increased movement of mites out of fields and increased reproductive rates of female mites. The only pyrethroid insecticide that will control spider mites is bifenthrin (Bifenture, Tundra, Brigade, other bifenthrin generics). Other insecticide/miticides registered for use in soybean include Agri-Mek SC (abamectin) and Zeal SC (etoxazole).

Early detection facilitates timely and effective rescue treatments. Current insecticides for soybean provide short-term protection, about seven days residue for mites and do not kill mite eggs. Fields will need to be re-monitored continually for resurging populations. If multiple applications are needed for spider mite control, be sure to rotate to a different mode of action with each subsequent application to prevent resistance.

The efficacy of an insecticide can be improved significantly with sufficient coverage (greater than 18 gallons per acre of water) and application at high pressure to penetrate foliage. Insecticides labeled for mite control have a 18-day (bifenthrin) to 28-day (abamectin) preharvest interval. Consequently, if infested fields still have green seeds but seeds are filling, accepting some yield loss from mites and not treating may be better than treating and being unable to harvest. Check product labels for current preharvest intervals and other use restrictions and plan accordingly.

When scouting soybean fields, consider all insect pests (soybean aphid and spider mites) that are present and their population levels. If the heat and drought stress continue, the risk for spider mites increases and the risk for soybean aphids is reduced (increased mortality and decreased reproductive rate due to hot temperatures greater than 90 F). If heavy rains occur, mite and aphid populations can both collapse.

Mite infestations often are concentrated early in field edges, and spot treatment can be feasible and more economical. However, under dry conditions, mites will quickly spread throughout the field, and spot treatments are not effective in controlling mites and unlikely to prevent the infestation from spreading. Early detection facilitates timely and effective rescue treatments.

Wireworms are most likely to be problems when soybean follows pasture, CRP or grassland. Infestations often are found in coarse-textured soils (sandy loam) where moisture is abundant, perhaps in low spots of fields.

Thresholds

Estimating wireworm infestations is difficult and labor intensive. Two methods are used by pest managers:

• Soil Sampling: Sample 20, well-spaced, 1-square-foot sites to a depth of 4 to 6 inches for every 40 acres being planted. If an average of one wireworm per square foot is found, insecticide treatment would be justified.
• Solar Bait Trap: In September, establish bait trap stations for two to three weeks before freeze-up. Place bait stations randomly through the field, but represent all areas of the field. You should have 10 to 12 stations per 40-acre field. Place 1 cup of wheat and 1 cup of shelled corn in a 4- to 6-inch-deep hole. Cover grain with soil and then an 18-inch-square piece of clear plastic. Dig up the grain after two weeks in the field. If an average of one or more wireworm larvae is found per station, treatment would be justified.

Seed Treatment: Insecticide seed treatments or at-plant soil insecticides should be applied for managing wireworms in soybean.

Two gall midge flies are associated with soybean: the soybean gall midge, Resseliella maxima Gagné, and the white-mold gall midge, Karshomyia caulicola (Coquillett). The soybean gall midge is a new economic insect pest of soybean, which first was reported causing yield losses in Nebraska, Iowa and South Dakota in 2018. In 2022, the first infestation of soybean gall midge in North Dakota was possibly detected in Sargent County. However, DNA confirmation of the collected midge larvae are still pending. In contrast, the white mold gall midge in a non-pest and larvae only feed on the white mold of soybean. For more information, see the NDSU Extension publication E2006, “Soybean Gall Midge and White-mold Gall Midge in Soybean.”

Consult the IPM Crop Survey website for yearly soybean insect pest maps: www.ndsu.edu/agriculture/ag-hub/ag-topics/crop-production/diseases-insects-and-weeds/integrated-pest-management/soybeans-ipm

Identification

Gall midges are in the fly family Cecidomyiidae and are similar in appearance, and requires close microscopic examination of the terminal abdominal segments of larvae or DNA testing to confirm its species identification.

Soybean gall midge – Young larvae (first and second instars) are white and smaller, whereas the mature third instar larvae are orange to reddish orange and about 1/12 inch in length. Larvae of soybean gall midge feed on plant liquids by excreting enzymes that digest the plant tissues, sometimes causing galls.

Adults are light to dark brown, small, about 1/8 inch in body length, and mosquito-like flies with an orange abdomen. Their characteristic markings are the white and black banding on the antennae and legs, and mottled wings.

Life Cycle – Soybean gall midge likely has two generations per year in northern states. The larva overwinters inside of a larval cocoon in the soil. Larvae pupate during the spring. The first generation of soybean gall midge adults emerge from mid-June through early July in Minnesota. Adults only live three to five days, and do not feed on soybeans. Females lay their eggs in crevices or cracks on the soybean stem. Once eggs hatch, larvae begin feeding under the epidermis of the soybean stem and pass through three instars. Numerous larvae sometimes can be found in one soybean stem. Larvae drop off the plant to the soil to form larval cocoons and then pupate. The cycle repeats for the second generation until the fall, when larvae will overwinter as cocoons in the soil.

Crop Damage – Soybean gall midge feeds beneath the epidermis near the base of the stem of soybeans. Sometimes the base of the stem is necrotic (dark), swollen, deformed and gall-like. Soybean stems heavily infested by soybean gall midge are stunted, wilted, lodged or dead. Significant yield losses have been recorded in states with severe infestations and are most common at the field edges. Although current research shows soybean is the preferred host of soybean gall midge, additional hosts are dry bean, lima bean, clover and rarely alfalfa.

Scouting – Adults are not readily observable in the field due to their cryptic appearance, small size and short life span. Larval-infested stems are easier to find near the field edges of soybean fields or in newly planted soybean fields that are close to last year’s infested fields, and during the R2 (full bloom) to R8 (maturity) growth stages of soybean.

Scout by walking a transect in the first four rows near the field edge, and focus in areas where dense vegetation occurs along the field edge. Examine 10 consecutive plants at 10 sampling sites per field (total of 100 plants per field). Sampling sites should be spaced more than 50 feet apart.

At each sampling site, examine plants for the presence of necrosis and brown or dark discoloration at the base and lower portion of each stem. If necrosis is observed, pull up the soybean plant and peel back the outermost layer of the stem (epidermis) on the necrotic area to look for small white or orange larvae.

Growers are encouraged to scout for soybean gall midge. Additional scouting will help further detect this economic insect pest of soybean and determine its infestation levels in North Dakota.

If you happen to find white or orange larvae in the stems of soybean plants, you need to confirm whether it is the soybean gall midge or white-mold gall midge. Collect more than 10 larvae and place them in alcohol vials, or collect two to three plants with larvae and place them in a plastic bag. Notify and send collected samples to the Extension agent in your county or to NDSU Extension Entomology for further identification.

Because the soybean gall midge is a newly discovered insect pest, entomologists have been studying different IPM strategies. Studies on planting dates, host plant resistance, crop rotation, tillage and insecticide control for this pest are being conducted in states with economic populations. For more information, see the websites on soybean gall midge that summarizes the IPM research:

• Soybean Gall Midge, Soybean Research and Information Network

https://soybeanresearchinfo.com/soybean-pest/soybean-gall-midge/

• Midwest Soybean Gall Midge Discussion Series, University of Minnesota Extension
https://extension.umn.edu/courses-and-events/midwest-soybean-gall-midge-discussion-series

• Soybean Gall Midge Alert Network, University of Nebraska Extension
https://soybeangallmidge.org/

In general, the most significant diseases in our region, such as root rots and the soybean cyst nematode, are found underground.

The soybean cyst nematode (SCN), Heterodera glycines, is the most important pathogen of soybean in the U.S. This nematode is a microscopic roundworm that infects soybean roots. Extensive losses occur throughout infested areas. Losses greater than 50% have been measured in infested fields in North Dakota.

Nematodes, like other soilborne pathogens, easily spread from field to field by equipment contaminated with infested soil, wind-blown soil, overland flooding or birds carrying it. In addition, SCN can be carried in soil “peds,” small clumps of soil sometimes found associated with soybean seed.

In 2003, SCN was confirmed in North Dakota (Richland County). SCN has been confirmed in many North Dakota and northern Minnesota counties and is expected to continue to spread.

Symptoms

Infection by SCN often will not result in obvious above-ground symptoms unless plants are stressed or the nematode densities are high. Above-ground symptoms include stunting and yellowing or unthrifty plants. The roots can appear dark and decayed, and they have few, if any, N2-fixing nodules.

A diagnostic characteristic is the presence of white to yellowish lemon shaped female nematode cysts (about 1 millimeter in diameter) on the roots. These can be observed if the plant is removed from the soil carefully with a shovel and the soil is gently washed or shaken off the roots.

The severity of the symptoms is directly related to the amount of SCN in the soil. Warm, dry growing seasons tend to increase severity of the above-ground symptoms, while cool, wet years tend to decrease severity. Losses from SCN are usually much greater during droughts. Severity also tends to be higher in sandy soils than heavy-textured soils, and in soils with a higher pH.

Management

Effective management of SCN is accomplished by actively managing SCN using multiple management tools.

Maintain Fields Free of SCN

In fields where SCN has not yet been confirmed, reducing the likelihood of introduction is critical. In areas where SCN has been confirmed, limiting its spread to new areas in the field or neighboring fields can reduce the chance of widespread yield loss in the future.

Reduce the likelihood of spread to new fields by cleaning any equipment that was used in an infested area. Reduce the potential spread of SCN from infested fields to adjacent clean fields through reduced-tillage operations and practices that limit wind erosion and movement of soil.

Identify SCN

Management begins by confirming the presence of H. glycines in the field. Visual confirmation of SCN white females (lemon-shaped cysts) on soybean roots in the mid- to late growing season can serve as confirmation of SCN. However, these are hard to identify without experience.

Take the Test – Beat the Best

The most effective method to identify SCN in fields is to soil-sample and test your fields for SCN. Soil sampling should concentrate on the top 6 to 8 inches of the soil and close to plant roots.

The most effective sampling strategy varies by situation. In fields where SCN has not been confirmed, sampling should concentrate on areas of the field where the nematode may have been introduced, such as field entrances, shelter belts and previously flooded areas. In fields where SCN sampling is being done to monitor egg levels, gridded sampling may be most effective. Sampling can be done in the fall or spring.

Growers can sample the soils in their fields and send the samples to public (for example, the NDSU diagnostic laboratory in Fargo, ND) or private laboratories where egg counts can be determined. Soil test results are used to determine if a field is infested, if an extended rotation is necessary or if a resistant variety should be used.

If SCN is confirmed, active management should begin. SCN reproduces quickly and to very high egg levels in this northern soybean-growing area; thus, once a field is infested, growing susceptible varieties may increase egg levels very quickly. Thus, management should begin immediately.

Crop Rotation

Crop rotation is an effective method to reduce egg densities in soil. Rotation to nonhost crops for one year commonly reduces populations, but two or three will increase the effectiveness of rotation. Many factors affect the reduction in egg densities, and the results in each field will vary. Data from North Dakota indicates that in heavily infested fields following rotations to nonhost crops for four to five years, egg densities in some fields were still high.

Nonhost crops include corn, sugarbeet, alfalfa, potato, small grains and sunflower. Dry edible bean, certain lupines and crambe are hosts of SCN. Consult a list of susceptible crops before growing specialty crops or cover crops in SCN-infested fields. Weed control is important because SCN will reproduce on a wide range of weeds.

Resistant Varieties

Resistant soybean varieties are an effective tool to manage SCN. Most varieties have the same source of genetic resistance (PI88788). Effectiveness of PI88788 in varieties will vary, with some varieties being more resistant than others.

For severe infestations of SCN, selecting a variety with robust resistance, if known, is important. Long-term rotation strategies that also include soybean varieties with an additional source of resistance (such as Peking resistance) are likely to be most effective and help slow nematode adaptation to the genetic resistance.

Seed Treatments

Seed treatments may provide another tool to help manage SCN, but do not take the place of genetic resistance or crop rotation. Nematicide seed treatments for SCN became widely available in the late 2010s, and new products continue to be labeled. At the time of this printing, more than a half-dozen nematicide seed treatments are available. They range from biologicals to resistance inducers, fungicides and nematicides. Consequently, effectiveness among them likely varies and may be influenced by many in-field factors. Consult the most up-to-date efficacy information when considering a seed treatment for SCN.

Phytophthora root rot is a major disease of soybean, especially in areas where soybean has been cultivated for many years. The disease is caused by the fungal-like pathogen Phytophthora sojae. Yield losses can be substantial, and entire fields have been destroyed by the disease.

The disease is common in the Red River Valley. The pathogen survives in soil as spores called oospores, which are produced in infected plants. When the soil water content is high, the spores germinate and infect the roots. Infection and disease development can occur at any stage of plant development.

Disease is most common in heavy, compacted clay soils and fields that have been saturated with water or flooded. Drenching rains, especially near the time of planting, favor disease development. Reduced tillage, especially no till, is reported to increase damage.

The pathogen does not infect other crops grown in this region. Only three Lupinis spp. and soybean are natural hosts. Many pathotypes (similar to races) of P. sojae overcome many resistance genes, and as more acreage is cropped to soybean and more resistance genes are deployed, we expect that the pathotype frequency will change.

Symptoms

Symptoms include seed rot and pre and postemergence damping off and wilting of young plants. These are common in flooded soils and often are misidentified as water damage. On older plants, leaves may become yellow and plants will wilt, with wilted leaves remaining on the plant. The lateral and tap roots are destroyed. A brown discoloration often appears on the lower portion of the stem near the soil. Disease is usually patchy in the field, often occurring in low or flooded areas.

Management

Planting resistant varieties is an important method to manage Phytophthora root rot. However, no available resistance gene can manage all the pathotypes (races) of Phytophthora found in our region. We recommend that growers choose a resistance gene and then monitor the field for Phytophthora root rot. If the disease is significant in the field, select a variety with a different resistance gene the next time the field is planted. Many varieties also have tolerance and partial resistance to Phytophthora root rot. This type of resistance can be effective against all pathotypes. These varieties may not lose yield under low to moderate disease pressure but can be damaged severely under high disease pressure.

Crop rotation is not generally an effective method to reduce disease because the oospores are long-lived in soil. Reducing saturated soil conditions – for instance utilizing subsurface (tile) drainage – is a method to reduce Phytophthora root rot in the field.

Fungicide seed treatments will help protect plants from infection for the first few weeks after planting. Metalaxyl and mefenoxam have efficacy against Phytophthora and have been used for many years in our region. Recently, additional efficacious fungicide seed treatments, such as ethaboxam and oxathiapiprolin, have become available. Ethaboxam and oxathiapiprolin have different modes of action than metalaxyl and mefenoxam. Rotation of chemical products with different modes of action will help prevent the development of fungicide resistance.

The fungus Rhizoctonia solani causes pre and postemergence damping-off and root rot of young and adult plants. When soil populations of Rhizoctonia are high and soil is warm and moist for the first month after planting, pre- and postemergence damping-off can reduce stands by 50% or greater. Generally, Rhizoctonia causes most damage in the seedling and early vegetative stages of plant growth, but damage has also been observed on older plants. The pathogen survives in the soil and is common in this region.

Symptoms

Seed decay and brown to reddish lesions on seedling stems and roots just below the soil line are symptoms. These lesions may girdle stems and kill the plant. On older plants, the pathogen causes a reddish-brown cortical root rot, which may extend into the base of the stem. Plants may appear unthrifty or may die. Root rot can greatly reduce nodulation.

Damage from Rhizoctonia solani commonly is observed in areas with a long history of soybean production with close rotations or during weather conditions not favorable for seed germination and rapid growth of seedlings. Various anastomosis groups (AG) of R. solani exist, some of which are semi-host specific. AG 2-2, AG-4 and AG-5 are most common on soybean, but AG-3 occasionally is found. AG 2-2 can be highly pathogenic on soybeans, especially at high temperatures. AG-4 and AG 2-2 are also common on sugarbeet. AG-3, generally found on potato, is weakly pathogenic on soybean.

Crop rotation practices may impact the amount of R. solani in the field, and therefore, affect the severity of the disease. Disease severity appears greater in plants showing iron deficiency chlorosis.

Management

Crop rotation to non-susceptible hosts such as small grain will reduce populations of Rhizoctonia in the soil. Avoid close rotations with sugarbeet if you have evidence of Rhizoctonia in the field. Close rotations with dry bean also may increase incidence of disease.

Some seed treatments and good seedbed preparation can reduce damping-off. Cultivating soil to hill up around stems promotes lateral root growth and may lessen the effect of root rot on older plants.

Fusarium root rot caused by a complex of Fusarium species can cause damping-off of seedlings and root rot on older plants. Infected seedlings can result in poor stands, late emergence or stunted plants.

Fusarium root rot often has been observed in association with stressed plants, such as in drought conditions or with herbicide damage. However, high populations of the pathogen in the soil may result in disease development under good growing conditions.

The pathogen may interact with other pathogens such as Rhizoctonia or the soybean cyst nematode to cause disease. Disease severity may be greater in plants showing iron deficiency chlorosis.

Symptoms

Infected seedling roots will show reddish or dark brown discoloration and decay. The disease at this stage may be misdiagnosed as Rhizoctonia root rot because symptoms are similar. Symptoms on older plants consist of reddish-brown lesions on roots. In advanced stages of the disease, the cortex decays, the roots are black and fissures develop in the dead surface tissues of the tap root, and the tap root may be rotted. Few nitrogen-fixing nodules may be on the roots.

Plants may appear stunted or unthrifty, and the leaves may be yellowing while the veins remain green for a short time. The leaves eventually become completely yellow, then die from the edges inward and fall from the petioles.

Management

Use high-quality seed, plant in warm, well-drained soils, reduce soil compaction and provide good fertility. Crop rotation can lower populations of the pathogen in the soil. Fungicide seed treatments may reduce damping-off by Fusarium. Most varieties appear to be susceptible to Fusarium root rot.

Damage to seedlings often occurs during weather conditions not conducive to rapid seed germination and plant emergence. Ridging soil around the base of the plants can promote root growth and reduce damage to root rot in older plants.

Sudden death syndrome (SDS), caused by the fungal pathogen, Fusarium virguliforme, is a severe disease of soybean that is expanding in North Dakota. At the time of printing, the disease has only been confirmed in Richland and Cavalier Counties in ND, but likely occurs in other areas. Yield losses from SDS can be severe when symptoms develop during early flowering.

Symptoms

The disease usually first appears in patches in fields, which expand in subsequent years. SDS symptoms are generally first noticed on the leaves at or just after pod-fill stages begin. Initial symptoms appear as scattered circular to irregular-shaped interveinal yellow spots that produce a mottled appearance to the leaves. The yellow tissue then dies and turns brown. The upper leaves are the first to defoliate, leaving petioles. Complete defoliation can occur when the disease is severe. Flower and pod abortion can occur. Plants showing severe leaf symptoms commonly have decay of roots and plants can easily be pulled from the ground. In favorable environments, a bluish fungal growth may be observed on surface of roots.

Notably, foliar symptoms of SDS are very similar to those caused by brown stem rot, and careful examination of the stem symptoms is needed to differentiate the two diseases. We recommend cutting the lower stem longitudinally to help differentiate SDS from brown stem rot. Commonly, SDS will have a healthy (white) pith, whereas the pith will turn brown in plants suffering from brown stem rot.

Disease development is associated with wet conditions early in the growth of soybean plants, and warm temperatures and heavy rainfalls during and after flowering. Severity of the diseases is often linked to the presence of soybean cyst nematode.

Management

Selection of a soybean variety with reduced susceptibility to SDS may be prudent, if available. A few fungicide seed treatments are available and can be an effective tool for reducing yield loss to SDS. Consult the latest efficacy information when considering a fungicide seed treatment. If SCN is present in fields with SDS, management of the nematode may help reduce SDS severity.

Dry edible bean is a host of the SDS pathogen; infected dry edible bean plants express only the root rot symptoms, and do not express the classic foliar SDS symptoms.

Because SDS is favored by high water content in the soil, practices that encourage drainage (including subsurface drainage) will help minimize disease development. Reducing soil compaction can reduce severity of SDS.

White mold of soybean is a common disease caused by the fungus Sclerotinia sclerotiorum. It can cause significant yield reductions, particularly when soybean is planted in infested soil, the plant canopy is dense and prolonged periods of cool and wet weather occur. The disease rarely is severe when hot and dry periods persist in July and August.

Sclerotinia overwinters as sclerotia in soil and can survive for many years. When sclerotia are near the soil surface, they can germinate to form small mushrooms called apothecia. Airborne ascospores are released from the apothecia, and the infection process begins when ascospores begin to digest/infect flower tissue. This commonly occurs after the plant canopy has closed, which creates a favorable microclimate of cooler and wetter plants. Once the infection is established, the pathogen easily spreads into healthy green tissue.

Symptoms

Symptoms are commonly first observed as a cottony, white mycelial (fungus threads) growth on stems, leaves or pods. Lesions develop on main stems and lateral branches, and eventually may appear bleached and sometimes shredded from advanced decay. Sclerotia, which resemble the size and color of rat/mouse fecal matter, form in and on diseased tissue. Seeds in diseased pods may be shriveled and infected by the fungus or replaced by sclerotia. When a field with white mold is harvested, the seed is usually contaminated with sclerotia.

The pathogen has an extensive host range of more than 370 plant species and causes diseases on many crops such as sunflower, dry bean, canola, alfalfa, buckwheat, lupine, mustard, potato, Jerusalem artichoke, safflower, lentil, flax, field pea and many vegetables. Many common broadleaf weeds such as marsh elder, lambsquarters, pigweed, Canada thistle and wild mustard also are hosts.

Management

Many management tools are available for white mold control, including crop rotation, planting less susceptible varieties, avoiding planting on soils heavily infested with Sclerotinia, reducing the planting rate, wider row spacing and timely fungicide application. However, when the environment highly favors white mold development (cool temperatures and frequent, prolonged rains) the disease still may cause problems even if multiple tools are used.

Soybean fields should be monitored for disease incidence. Check the seed hopper at harvest for the presence of sclerotia. As the disease begins to increase in a field, the rotation time to non-susceptible crops such as small grains and corn should be increased.

Crop rotation will reduce populations of sclerotia in soil but will not eliminate the pathogen. Careful consideration should be given to avoid rotating with crops that are particularly sensitive, such as sunflower, dry edible bean and canola.

Although common soybean varieties adapted for this region are susceptible to white mold, some varieties are less susceptible than others. For example, short-maturity varieties typically are less impacted by white mold. Information on variety susceptibility may be available from seed companies.

Do not use seed from a white mold-infected crop. Seed quality could be low, and sclerotia may be introduced into the field along with the seed. Also, maintain good control of broadleaf weeds because they can be hosts of Sclerotinia and can make the microclimate more favorable for white mold. When growing a susceptible crop under irrigation, avoid practices that favor a dense canopy and free water on the plant during flowering because these will create ideal conditions for disease development.

Many fungicides are labeled for management of white mold on soybean. In favorable environments, fungicides may reduce disease. Efficacy varies, so consulting fungicide efficacy data before selecting a fungicide is important.

Optimization of the fungicide application will greatly impact efficacy, and several factors should be considered, including timing and droplet size. The pathogen needs senescing flowers to begin the infection process and lesion development takes time, so applications prior to R1 or after R3 often miss the critical window when plants are most sensitive to economically important infections. Environment has a strong influence on infection and disease development, with cool temperatures and frequent rainfall during bloom, and saturated soil conditions prior to bloom, being most favorable. Microclimate also strongly influences infection, and canopy closure and factors that influence it are very important in determining the risk of infection and optimal application timing. Droplet size is a very important consideration for optimal application, and research shows that droplets maximizing fungicide coverage vary according to canopy closure; with smaller droplets being most effective prior to canopy closure and larger droplets being more effective at canopy closure.

Septoria brown spot, caused by Septoria glycines, is a common leaf disease that may develop throughout the season. How frequently economic loss occurs in North Dakota and Minnesota is unclear, but in most of the Midwest, little evidence is available to suggest that Septoria brown spot causes much yield loss, except under the most extreme circumstances.

Symptoms

The disease first appears as pinpoint brown spots on the leaves on the lower leaves of the plants. These spots may remain small or enlarge up to 3/16 inch, becoming irregular and angular and reddish brown to dark brown with age.

Severely diseased leaves turn yellow and fall off, with defoliation beginning lower on the stem and progressing up the plant. Brown, irregularly shaped spots may develop on the stems, petioles and pods.

Septoria brown spot development is favored by warm, humid weather. Hot, dry conditions will arrest disease development, but it may resume if conditions become wet. Rainy weather is especially favorable because Septoria spreads by splash-dispersed spores. Disease development also is favored in areas with poor drainage. The brown spot fungus survives on soybean crop residue and may be seed-borne.

Management

Active management of this disease in most years is unnecessary, and management techniques utilized for other diseases (for example, crop rotation) likely are sufficient. Fungicides are available, but disease pressure warranting their use in our area is unlikely.

Additionally, bacterial blight is more common in some areas than brown spot (especially in the early part of the season) and the symptoms of both pathogens can be confused. Correct identification of the pathogen is essential.

Bacterial blight is a common foliar disease in North Dakota, but it is unlikely to cause significant yield loss. Bacterial blight (caused by Pseudomonas syringae pv. glycinea) favors cool, humid weather.

The bacteria blight pathogen can be seed-borne and can survive on soybean crop residue. Bacteria readily enter wounds in the leaf, and rapid spread may occur following late-spring or early summer rain storms, hail or cultivation when the plants are wet.

Symptoms

The disease typically begins in the upper half of the plant as small, greasy green, angular, water soaked spots that later turn yellow and reddish brown. The spots are surrounded by a narrow yellow border. As the spots coalesce, portions of the leaf tissue fall out, and the leaves become torn and ragged.

Infected young leaves may be distorted and stunted. Severely diseased leaves may drop off. Occasionally, large black spots may develop on stems, petioles and pods. Seeds in infected pods may become slimy. Hot, dry weather will slow or stop the development of bacterial blight.

Management

Do not use seed from a diseased field. Use crop rotation. Do not cultivate when the plants are wet. Some varieties are less susceptible. Because bacterial blight is caused by a bacterial pathogen, fungicides are not useful for control.

Downy mildew, caused by Peronospora manshurica, develops primarily in years with extended periods of cool, humid weather. Yield loss rarely has been reported, but localized outbreaks in northern North Dakota counties were thought to have reduced yield by up to 5% to 10% in some fields in the 2020s.

Symptoms

Symptoms include yellow-green to yellow spots on the upper leaf surface and a purplish or grayish downy fungal growth on the lower leaf surface, opposite the yellow-green patches on the upper leaf surface. The yellow spots turn brown later in the season.

Pod infection may result in seeds that are dull white, cracked or covered with a white crust of overwintering oospores. If these white or encrusted seeds are planted, a small percentage of the emerging seedlings may be infected systemically with the downy mildew fungus, resulting in stunted plants. Leaves of systemically infected plants will have areas of green-yellow tissues along the main veins and the leaf edges will be curled downward.

Management

Use crop rotation. Use a seed treatment if planting seed from an infected field or seed that has a white crust on it. Few fungicides are labeled for downy mildew and little efficacy data exists.

Frogeye leaf spot, caused by the fungal pathogen Cercospora sojina, was first identified in North Dakota in 2020. The implications for yield loss in North Dakota are unclear, but the disease causes significant yield loss in the central corn/soybean belt and mid-southern states.

Symptoms

Frogeye leaf spot is commonly observed on leaves as small, dark water-soaked spots that develop into small circular to irregular lesions (approximately 1/4 inch or less). Lesions will develop a narrow purple-red ring surrounding a tan to gray center. During high humidity, a fuzzy gray mold (mycelium) may be visible in the centers of the lesions on the underside of the leaf.

Management

Crop rotation, tillage that encourages residue decomposition, genetic resistance if available, and disease-free seed may all contribute to disease prevention or management. Foliar fungicides are commonly used in states where frogeye leaf spot is yield limiting. However, the pathogen is prone to develop resistance to fungicides, making rotation (or mixtures) of fungicide mode of action imperative. In 2020, 23% (of over 300) of the pathogen isolates collected throughout 12 southeastern North Dakota counties, were resistant to FRAC 11 (i.e., QoI or strobilurin) fungicides. Should fungicides be needed, multiple modes of action should be used.

This disease, caused by Diaporthe sojae and D. longicolla, is common in southern and central Minnesota but thought to be somewhat less so in North Dakota and northwestern Minnesota.

Symptoms

Symptoms include rows of raised black fruiting bodies that develop on the stem, and a random pattern of raised fruiting bodies that develop on the pods. Infected stems often are killed. Infected seeds are shriveled and cracked and may be covered with white fungal growth.

The pod and stem blight fungus survives on infected soybean crop refuse and can be seed-borne. It favors wet weather and crop injury as the crop nears maturity. If infected seeds are planted, plants may die on emergence.

Management

Use crop rotation. Plant high-quality seed that is nearly free of the pod and stem blight pathogen or use a seed treatment. Harvest promptly at maturity. Maintain adequate K levels in the soil.

Stem canker is relatively common in southern Minnesota, but how frequently it occurs in North Dakota is unclear. Stem canker is caused by two different pathogens, Diaporthe caulivora and D. aspalathi. Between the two pathogens, only D. caulivora has been reported in North Dakota at the time of printing.

Symptoms

Early symptoms of stem canker are reddish-brown lesions that appear at the base of the leaf petiole or branches. Lesions may develop into sunken dark brown cankers with small black raised structures on the surface (perithecia). Plant parts above the lesion may die. Stem canker favors wet weather, and symptoms may be most likely observed beginning in mid-July following wet springs.

Management

Resistant varieties (if available) and crop rotation may be useful to mitigate stem canker where it has been a problem.

Brown stem rot (BSR) occurs in North Dakota is capable of causing yield loss in our region. BSR is more likely to be severe in the presence of SCN. Infection occurs through the roots and develops slowly until pods are filling.

Symptoms

Symptoms usually do not appear until late in the season. The most obvious symptoms develop inside the lower stem. When the stem is split open with a knife, the pith (central tissues) is brown. The internal browning may extend several inches above the soil or over most of the stem.

Leaf symptoms, which develop sporadically, consist of a yellowing followed by browning of tissues between the main veins. The veins remain green. The foliar symptoms (when they occur) are similar to those caused by sudden death syndrome.

The best time to assess for brown stem rot is the R5 to R6 stage, when seeds are beginning to develop in pods at the four uppermost nodes. Any time that a field suddenly turns brown late in the season, rather than yellow green, the lower stems should be split and examined for brown stem rot.

The brown stem rot fungus survives several years in soybean crop residue. The disease develops during cool or moderate temperatures. The greatest damage occurs when cool and wet weather occurs during the early reproductive stage and is followed by hot and dry weather.

Management

Resistant varieties may be available. Use crop rotation, planting nonhost crops for three years. Manage SCN. Small grain and corn are not hosts.

Charcoal rot occurs in North Dakota and northwestern Minnesota. Charcoal rot is caused by the fungal pathogen Macrophomina phaseolina. The pathogen has a very large host range, which includes corn, dry edible bean and sunflower.

Yield losses are most likely when plants are under water stress midway to late in the season. Hence, drought, high temperatures and sandy soils favor charcoal rot.

In 2017 and 2018, severe outbreaks of charcoal rot occurred in localized areas of eastern North Dakota. Yield losses to the most severely infected fields in 2018 likely exceeded 50%. Yield losses to charcoal rot in North Dakota can occur, particularly in warm and dry growing seasons.

Symptoms

Charcoal rot often is observed first as a general loss of vigor in small, drought-prone, areas in a soybean field. Plants in those areas may begin yellowing and appear to be maturing up to three or four weeks ahead of green areas in the field. Plants surrounding those areas begin pre-maturely senesce soon after, giving the misleading appearance of the disease “spreading.”

Symptoms include a light gray to silver discoloration at the base of the stem. If the epidermis is scraped off at the base of the stem, charcoal-colored microsclerotia are visible. Microsclerotia are very small, and may appear as tiny black specs or as if the stem were covered in charcoal dust. The pathogen survives as microsclerotia.

Management

Crop rotation and planting nonhost crops are important management tools. If irrigating, keep soil water content reasonably high at the end of the season.

Virus diseases occur but have not been a serious problem in North Dakota.

The two most common viruses that have been found in North Dakota are soybean mosaic virus (SMV) and bean pod mottle virus (BPMV). Numerous other viruses are known in soybean. The identification and confirmation of a virus disease requires special techniques. Identifying a virus based on symptoms is very difficult.

Virus symptoms vary greatly but may consist of stunting, fewer pods, leaf mosaic (light and dark green areas), puckering, blistering, distortion, chlorosis or necrosis. Plants can be infected without showing symptoms. Seed mottling can occur, which is detrimental to the quality of food beans.

The severity of disease and the effect on yield is greatly affected by the plant stage at infection, the environmental conditions and the susceptibility of the variety. Yield losses can be substantial under heavy disease pressure. Seed can transmit SMV and the virus is vectored by aphids (Aphis glycines). The vector of BPMV is the bean leaf beetle (Cerotoma trifurcata).