Soybeans

Pythium is a soil-borne, fungal-like pathogen—several different species damage soybeans. The various Pythium species that infect soybeans have a broad host range, encompassing 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 seeds and seedlings before emergence and can also cause post-emergence damping-off under wet conditions. The characteristic symptom of most Pythium infections is soft, brownish-coloured, rotting tissue. Infected seeds appear to be rotted, and soil adheres to them. Seedlings exhibit water-soaked lesions on the hypocotyl or cotyledons, which develop into brown, soft rot. Pythium and Phytophthora have similar symptoms in seedlings and can only be distinguished by laboratory examination. Although Pythium primarily damages seeds and seedlings, roots of established plants can also be infected, and plants may be stunted, turn yellow, or wilt if infection is severe. Well-drained fields help reduce disease pressure, as does avoiding planting in cold, wet soils. Seed treatments are also effective against Pythium spp.

Rhizoctonia infects young seedlings, causing both pre- and post-emergence damping-off. This pathogen prefers wet, warm soil. In seedlings and older plants, a firm, rusty-brown decay, or a sunken lesion on the root or lower stem, is a characteristic symptom. Infections can be superficial and cause no noticeable damage, or they can girdle the stem, stunting or killing the plant.

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

Aphanomyces is a highly problematic root disease in many pulse crops, but soybeans are currently considered a non-host of Aphanomyces.

The soybean cyst nematode has not yet been found in Western Canada, but it is a significant economic disease of soybeans worldwide. Fields in areas frequently in soybean rotation should be carefully monitored. The soybean cyst nematode may not cause noticeable physiological symptoms even though yield loss occurs. Notable symptoms include stunted, slow-growing crops with little to no canopy closure and chlorotic foliage.

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 in colour. 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 the R3 to R5 growth stages to slow disease development and protect yield, they are usually not economically crucial in Saskatchewan.

Bacterial blight is another prevalent foliar disease; however, it seldom causes severe yield loss. Together with Septoria brown spot, it is usually the first foliar disease to occur on soybeans in most growing seasons. Infections can first happen on the cotyledons, and as plants grow, young leaves are most susceptible. Lesions are small, angular, and reddish-brown, with water-soaked margins and a yellow halo. As the disease progresses, lesions often merge to form large, irregularly shaped areas of necrotic tissue. Centres of older lesions usually fall out, and leaves appear tattered. Bacteria overwinter 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, while high temperatures slow or halt development.

Since it is a bacterial blight, foliar fungicides are ineffective. Some varieties appear more susceptible than others, but good crop rotation and tillage practices can reduce pathogen levels in the soil.

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

Wide-resistant varieties are available, as are foliar fungicides. However, it is usually not economically significant.

Powdery mildew is most prevalent in cooler seasons. It is characterized by white, powdery fungal growth that can cover all above-ground plant parts, particularly the upper leaf surfaces. It usually appears by mid- to late reproductive phases. Most commercial varieties are resistant, and unless it seems early (in July), there are traditionally no economic reasons to control it. Foliar fungicides are available if the infection is early and severe.

Frogeye leaf spot (Cercospora sojina) is becoming more prevalent in northern regions and in fields under continuous soybean rotation. It can be seedborne, and early infections 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 spots progress, the middle becomes gray to light brown, with dark red-brown margins. It can cause premature leaf drop and infected seeds if the pods get lesions. Infected seeds exhibit light to dark gray discoloured blotches and often show seed coat breakage.

Resistant varieties are available, and crop rotation is recommended to help reduce spore loads. 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 remain continuously wet for 12 hours or more for successful infection. The disease can be carried on infected seed, which may not exhibit any symptoms. Early infections cause damping-off when 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 exhibit irregularly shaped, red-to-dark-brown blotches during the early reproductive stages, and subsequently, black fungal bodies appear in the lesions. There is no known varietal resistance, but crop rotation can help reduce the incidence, and foliar fungicides are available for control.

Pod and stem blight (Phomopsis longicolla) causes seed decay, stem canker, and pod and stem blight. Fungi survive the winter in infected seeds and crop residues. Infection can occur early in the growing season, even asymptotically. All parts of the plant are susceptible to infection. Disease is favoured by warm, humid weather during maturation and by delayed harvest 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 pod infection. The seeds will appear cracked, shrivelled, and covered with chalky, white mould. An infected seed, if planted, will rot or have blight. Ensuring seed supply is disease-free is important in managing pod and stem blight. Foliar fungicides are registered for in-season control, which may help protect quality but may not significantly impact yield.

Many other diseases may also infect soybeans in Saskatchewan as the crop becomes established; however, they are currently seldom observed.

Several different viruses can be transmitted to soybeans from feeding insects. Viruses cannot be targeted with foliar fungicides; therefore, proper pathogen identification is crucial for determining the appropriate management strategy. Some viruses affecting soybeans include alfalfa mosaic virus and soybean mosaic virus, both of which are spread by aphids. A beetle spreads the bean pod mottle virus. Visual symptoms closely mimic herbicide drift damage, and a diagnosis based solely on them is challenging. Foliar insecticides may help control bean pod mottle beetle populations. Unfortunately, they are generally ineffective at reducing the transmission of alfalfa mosaic virus and soybean mosaic virus by aphids.

The adult seed corn maggot resembles a common house fly and emerges from eggs in the soil in late May. The adult then lays eggs in cracks in moist soil, which hatch after two to four days when the temperature is above 10°C. Larvae are 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 are delayed. Therefore, ensuring planting as early as possible into a warm, shallow seedbed can help limit damage. Sprays are ineffective, and seed treatments are the only registered controls to protect against the seed corn maggot.

Cutworms are the larvae of several different types of moths. They overwinter in the soil as pupae, emerging as larvae in the spring to feed on young plants. There are many cutworm species in Saskatchewan which affect a variety of crops. Soybean stems can be cut off below the soil surface by cutworms. Soybeans lack underground growing points and therefore cannot regrow after being damaged by cutworms. Crops should be scouted regularly in early spring. If dead or wilted plants are observed, the soil around and up the row from the plant should be scouted for cutworms. Most cutworm species feed at night and remain up to two inches below the soil surface during the day.

Two-spotted spider mites are yellow-orange in colour, with two distinct black spots on each side of the body. They commonly overwinter in alfalfa fields and make their way into soybean crops on the wind in mid-to-late summer. Two-spotted spider mites are tiny and difficult to see. Plant damage is usually evident before the mites are even noticed. Shaking symptomatic leaves over a white piece of paper to look for small, dark specks, or using a magnifying glass can help identify mites. Eggs may be observed on leaf undersides, characterized by a cover of webs.

Spider mites use their piercing and sucking mouthparts to extract cell contents, rendering 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 like general yellowing from a distance. Since mites often overwinter in grasses and clovers, an edge effect may occur from the mites moving in from the edges of the field. There may be 7–10 generations per season, and in severe infestations, yield losses can reach 40–60%.

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

Thistle caterpillars are sporadic pests in soybeans. They do not overwinter in Saskatchewan; instead, they migrate northward from the southern U.S. Their host crops include soybeans, Canada thistle, and sunflowers. Caterpillars grow to be two to three 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 golden/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. Instead, the focus is on the severity of defoliation. At vegetative stages before flowering, defoliation should be >30% to warrant control; at flowering and in reproductive stages, the threshold for control measures is 20% defoliation.

Green cloverworms are only an occasional pest of soybeans. They do not overwinter in Saskatchewan; instead, they migrate north in the spring. They may have one or two generations per year. These caterpillars are green, with two narrow, white stripes running down their sides. 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. The worms move by arching the middle of their bodies, a movement known as 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 mistaken for grasshopper defoliation or damage caused by other worms. Gently shaking damaged plants over a sheet to dislodge worms can help identify the culprit. While easy to mistake for alfalfa loopers because of their similar looping movement, green cloverworms can be distinguished by their vigorous wiggling when disturbed.

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