Soybeans

Pythium is a soil-borne, fungal-like pathogen. Several different species damage soybeans. The various species of Pythium that infect soybean have a wide host range that can include many other crops. Pythium tends to be favoured by cool and moist soil, but some species may do more damage in warm soils.

Pythium can attack rotting seeds and seedlings prior to emergence and can cause post-emergence damping-off under wet conditions. The characteristic symptom of most Pythium infections is soft, brownish-coloured, rotting tissue. Infected seeds appear rotted and soil sticks to them. Seedlings have water-soaked lesions on hypocotyl or cotyledons that develop into brown soft rot. Pythium causes symptoms like Phytophthora in seedlings and can only be distinguished by laboratory examination. Although Pythium causes most damage to seeds and seedlings, roots of established plants can be rotted, and plants may be stunted. Older plants become yellow, stunted, or wilted if infection is severe. Saturated soils are critical for infection of all Pythium species. Well drained fields help reduce disease, as well as avoiding planting in cold, wet soils. Seed treatments are also effective against Pythium spp.

Rhizoctonia infects young seedlings causing pre- and post-emergence damping off. In seedlings and older plants, a firm, rusty-brown decay, or sunken lesion on the root or on the lower stem is a characteristic symptom. The infections can be superficial and cause no noticeable damage, or they can girdle the stem and stunt or kill plants.

Fusarium is a common pathogen that can damage seeds and seedlings. It causes light to dark brown lesions on roots that may spread over much of the root system and may appear shrunken. Fusarium may attack the tap root and promote adventitious root growth near the soil surface and may degrade lateral roots.

Aphanomyces is a root disease that has been extremely problematic in many pulse crops, but soybeans are currently considered a non-host of Aphanomyces. In general, these diseases can be reduced by planting good-quality seed in well-drained, non-compacted fields. Delaying planting until soils are warmer than 10°C and relatively dry, to allow for rapid emergence and growth, can be beneficial. Crop rotation and tillage may be of some benefit. Genetic resistance in soybean varieties is available for managing Phytophthora infections.

The soybean cyst nematode has not yet been found in Western Canada but is a major economic disease of soybeans worldwide. Fields in areas frequently in soybean rotation should be carefully monitored. The soybean cyst nematode may not develop obvious symptoms even though yield loss occurs. Noticeable symptoms include stunted, slow growing crops that have little-to-no canopy closure and chlorotic foliage. Infected plants have poorly developed root systems and fewer nodules formed. Roots need to be dug up and soil carefully checked for white female nematodes starting about six weeks after crop emergence. To get an idea of the amount of soybean cyst nematode in the soil, a soil sample should be sent to a diagnostic lab. Plant damage occurs from both the direct effects of the nematode feeding, as well as indirectly from wounds that allow openings for other soil pathogens. Once established in a field, it is impossible to eliminate. Resistant varieties exist to manage soybean cyst nematode but are not resistant to all populations. Most contain only one source of genetic resistance, so rotating sources of resistance can help prevent the development of more damaging soybean cyst nematode populations. Cleaning equipment to avoid moving it from field to field is a good practice, as well as longer rotations between host crops to help decrease numbers.

Septoria glycines or brown spot is a common foliar disease of soybeans that develops soon after planting and is usually present throughout the growing season. Yield losses depend on how far up the canopy the disease progresses during grain fill.

It is typically a milder disease during vegetative growth stages, with infected plants exhibiting purple lesions on unifoliate leaves. Lesions on later leaves are small, irregularly shaped, and dark brown. Infected leaves will turn yellow and drop. Lesions on stems, petioles, and pods are not as common but are similar in appearance. While foliar fungicides can be applied during R3 to R5 growth stages to slow rate of disease development and protect yield, it is usually not of economic importance in Saskatchewan.

Bacterial blight is another very common foliar disease, however, it seldom causes serious yield loss. Together with Septoria brown spot, it is usually the first foliar disease to occur on soybean in most growing seasons. Infections can first occur on the cotyledons, and as plants grow, young leaves are most susceptible. Lesions are small, angular, and reddish-brown with water-soaked margins, surrounded by a yellow halo. As the disease progresses, lesions often grow together to produce large, irregularly shaped dead areas. Centres of older lesions often fall out, leaving leaves appearing tattered. On stems and petioles lesions are large and black. Bacteria survive the winter in crop residue and seed. They are spread with rain and wind, and infection occurs through wounds on the plant, making outbreaks common after rainstorms with high winds. Cool, rainy weather favours disease, and high temperatures will slow or stop development.

Since it is a bacterial blight, foliar fungicides are not effective against it. Some varieties appear more susceptible than others, but good crop rotation and tillage to reduce survival of the bacteria, will help decrease pathogen levels.

Downy mildew is a common disease of soybeans but seldom causes serious yield loss. The pathogen may also infect seed and reduce seed quality. The disease may affect seedlings but is more common in later vegetative and reproductive growth stages. Lesions occur on upper surfaces of leaves as irregularly shaped, pale green to light yellow spots. Older lesions turn brown with yellow-green margins. On the underside of the leaf, fuzzy, gray tufts may be seen growing from the lesions, especially when humidity is high. Infected pods show no symptoms externally, but inside the pod and seed may be covered with a dried, whitish fungal mass that appears crusty. Infected seeds can be smaller and appear dull white with cracks in the seed coat.

Many sources of resistance are available in varieties to help manage the disease, and foliar fungicides are available, but it is usually not an economic concern.

Powdery mildew is more prevalent in cooler seasons. It is characterized by white, powdery fungal growth that can cover all above-ground plant parts, particularly the upper surface of leaves. It usually does not appear until mid- to- late reproductive phases. Most commercial varieties are resistant, and unless it initiates early (July), there are not usually economic reasons to control it. There are foliar fungicides available if infection is early and severe.

Frogeye leaf spot (Cecospora sojina) is slowly becoming more prevalent in northern geographies, and in fields under continuous soybean rotations. It can be carried on the seed, and early infections resulting from infected seed will result in stunted seedlings. Later in the season, infections on leaves, stems (less common), and pods start as dark, water-soaked spots that vary in size. As they age, the central area becomes gray to light brown, with dark, red-brown margins. It can result in premature leaf drop and infect seeds if pods get lesions. Infected seeds have light to dark gray discolored blotches, and often seed coat breakage.

There are resistant varieties available, and crop rotation to help reduce spore loads is recommended. Foliar fungicides applied during late flowering and early pod set can improve seed quality and yield when disease is present.

Colletotrichum truncatum or Anthracnose stem blight is generally a later-season disease that is prevalent on maturing soybean stems, but soybeans are susceptible to infection throughout the growing season. Warm, wet weather favours infection, as the fungus is sensitive to drying, and must be wet continuously for 12 hours or more for successful infection. The disease can be carried on infected seed that may or may not show any symptoms. Early infections result in damping off if infected seed is planted. Leaves, pods, and stems may also be infected without showing symptoms. When symptoms are present, they include reddish veins, leaf rolling, and premature defoliation. Stems and petioles appear to have irregularly shaped red to dark brown blotches during early reproductive stages, and later black fungal bodies are present in the lesions. There is no known varietal resistance, but crop rotation can help reduce incidence, as well foliar fungicides are available for control.

Pod and stem blight (Phomopsis longicolla) is a disease that causes seed decay, stem canker, and pod and stem blight. The fungi survive winter in infected seed and crop residue. Infection can occur early in the growing season without causing symptoms. All parts of the plant, stem, petioles, pods, and seeds are all susceptible to infection. Disease is favoured by warm, humid weather when maturing, and if harvest is delayed due to wet weather.

The most characteristic sign of pod and stem blight is linear rows of black specks (pycnidia) on mature stems of soybeans. Poor seed quality may result from infection in pods. Seed infected is cracked, shriveled, and covered with chalky, white mould. Infected seed, if planted, will rot, or have blight. Ensuring seed supply is free of disease is important in managing pod and stem blight. Foliar fungicides are also registered for control in-season, which may help protect quality, but may not impact yield much.

Many other diseases may also infect soybeans in Saskatchewan as they become an established crop, but currently they are seldom observed. These include stem canker, various stem rots, soybean rust, sudden death syndrome, and bacterial pustule.

Several different viruses can be transmitted to soybeans from feeding insects. Viruses cannot be targeted with foliar fungicides, so proper identification of the pathogen is important to determine appropriate management. Some viruses affecting soybeans include alfalfa mosaic virus and soybean mosaic virus, which are both spread by aphids. The bean pod mottle virus is spread by a beetle. Visual symptoms closely mimic damage from herbicide drift, and diagnosis based on visual symptoms alone is difficult. Foliar insecticides may help control beetle populations for bean pod mottle, but generally are not effective in reducing transmission of alfalfa mosaic virus and soybean mosaic virus by aphids.

The adult seedcorn maggot looks like a common house fly and will emerge from eggs in the soil in late May. The adult then lays eggs in moist cracks in the soil that hatch after two to four days when temperatures are 10°C. Larvae is white, tough skinned, and roughly a quarter inch long. They will burrow into both the seed and the stem of developing soybean plants, resulting in thinned stands and wilted plants. Damage is worse when germination and emergence is delayed, so ensuring planting as early as possible into a warm, shallow seedbed to encourage fast emergence may help limit damage. Sprays are not effective and seed treatments are the only control registered to protect against the seedcorn maggot.

Cutworms are the larvae of several different types of moths and overwinter in the soil as pupae. Larvae emerge from the pupae in the spring and feed on new plants. There are many different species of cutworms in Saskatchewan, affecting and eating many different types of crops. Soybeans are no exception and can be cutoff below the soil surface by cutworms. Soybeans do not have growing points below-ground and therefore cannot regrow after being cut off by cutworms. Crops should be scouted regularly in early spring, and if dead wilted plants are observed, the soil around and up the row from the plant should be examined for the presence of cutworms. The feeding behavior will vary depending on species, but most feed at night and will be below the soil surface, up to a depth of 2 inches (in) during the day.

Two spotted spider mites are more closely related to spiders than to insects, and will overwinter in soil, debris, and grass. They commonly overwinter in alfalfa fields and are carried by winds into soybean fields in mid-to-late summer. They are extremely small and difficult to see on the plants, and damage is usually noticed before the mite. Shaking leaves showing symptoms over a white piece of paper and looking for small dark specs coming off helps to identify mites. Also look carefully at the undersides of leaves for eggs, they may be noted by minute webbing covering them. Two spotted spider mites are yellow orange in colour, with two distinct black spots on each side of the body. The use of a 10 times magnifying lens aids in identification and verification of a mite infestation.

Spider mites suck the contents of the cell out using piercing, sucking mouth parts, and render the cell non-functional. Cells then turn yellowish and eventually brown, leaving a mottled, sandblast effect on the leaf when observed up close, which may look just like a general yellowing from farther away. Since mites often overwinter in grasses and clovers, you may notice an edge type effect from the mites moving in from edges of the field. Once in a crop, there may be seven to 10 generations per season, and in severe infestations, yield losses from 40–60%.

There is limited information for economic thresholds, but recommendations based on visual damage suggest an insecticide application if 25% of the leaves show discolouration prior to seed set, or 10% of the leaves if pods have set. Since spider mites often move in from field edges, growers could consider the potential for spot treatment if populations appear to be severe. Natural predators such as the predatory spider mite, minute pirate bug, and mite destroyer beetle can help keep populations in check. As well, Neozygites floridana is a fungal agent that can infect and kill spider mites.

Thistle caterpillars are sporadic pests in soybeans. They do not overwinter in Saskatchewan and migrate northward from the southern U.S. Their host crops include soybeans, Canada thistle, and sunflowers. Caterpillars grow to be 2 to 3 centimetres (cm) long and are brown and black with yellow stripes down each side. They are covered in branched spiny hairs, giving them a bristly appearance. They are often found where they feed on leaves, inside a webbed area formed by cupping leaf margins together. Caterpillars form a goldish/brown chrysalis that hangs from the plants, and the painted lady butterfly will emerge. Several generations of thistle caterpillars can occur in one season. Like grasshoppers, there are no established economic thresholds based on the number of caterpillars, rather a focus on the amount of defoliation that is occurring. At vegetative stages prior to flowering, defoliation should be greater than 30% to warrant control. When flowering and in reproductive stages, 20% defoliation is considered a threshold.

Thistle caterpillars are sporadic pests in soybeans. They do not overwinter in Saskatchewan and migrate northward from the southern U.S. Their host crops include soybeans, Canada thistle, and sunflowers. Caterpillars grow to be 2 to 3 centimetres (cm) long and are brown and black with yellow stripes down each side. They are covered in branched spiny hairs, giving them a bristly appearance. They are often found where they feed on leaves, inside a webbed area formed by cupping leaf margins together. Caterpillars form a goldish/brown chrysalis that hangs from the plants, and the painted lady butterfly will emerge. Several generations of thistle caterpillars can occur in one season. Like grasshoppers, there are no established economic thresholds based on the number of caterpillars, rather a focus on the amount of defoliation that is occurring. At vegetative stages prior to flowering, defoliation should be greater than 30% to warrant control. When flowering and in reproductive stages, 20% defoliation is considered a threshold.

Green cloverworms are only an occasional pest of soybeans. They do not overwinter in Saskatchewan, migrating north in the spring. They may have one or two generations per year. These caterpillars are green, with two narrow, white stripes down the side. When mature, the worms are 2–4 cm long. These caterpillars have only three pairs of fleshy prolegs on the abdomen, plus a pair of prolegs on the back segment. When moving, the worms move by arching the middle of the body, or looping. Young worms scrape leaf tissue, creating a transparent skin, or window on the leaf surface. Older cloverworms eat holes in the leaves. Damage can be confused with grasshopper defoliation, or damage from other worms. Shaking plants over a sheet to dislodge worms can help identify what is present. While they may be confused with an alfalfa looper because they loop when they walk, they are different in that they will wiggle vigorously if disturbed.

When cloverworms are actively feeding, % defoliation can be used to determine if control is required. At pre-bloom stage, 40% defoliation, at bloom-pod fill, 20% defoliation, and at pod fill harvest, 35% defoliation is tolerated. An average infestation of four to eight larvae per row foot typically causes 20–30% defoliation.