Diseases of Field Crops: Soybean Diseases

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Pub 811: Agronomy Guide >Diseases of Field Crops> Soybean Diseases

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Seedling Diseases

Seed Rot, Seedling Blight, Root Rot

Incidence and Management Strategies: See General Seed Rots and Seedling Blights,

Disease Cycle: In Ontario, five fungi are most often associated with early-season emergence problems in soybeans. These include Pythium and Phytophthora (called "water moulds"), Phomopsis, Fusarium and Rhizoctonia. Typical "damping-off" symptoms can be caused by one or more of these organisms. Although these organisms can be seed-borne, they are present in most fields to some degree. Seedling diseases are prevalent under cool wet conditions that keep the soil temperatures below 13°C. These organisms often survive as saprophytes, living on dead plant material, or as dormant mycelium or spores. Root exudates from germinating seedlings or growing roots stimulate the inactive fungi.

Lesions that appear water-soaked with brown or purple roots or lower stems are often the result of infection by Pythium, Phomopsis or Phytophthora. A reddish or brown lesion near the soil line is characteristic of Rhizoctonia or Fusarium (see Plate 120), respectively. Growth and vigour are often reduced in those plants that do survive.

Plate 120. Fusarium root rot causes a brown discolouration of the internal root tissue.

Phytophthora Root Rot (Phytophthora sojae)

Incidence: Phytophthora root rot is a potential problem in heavy clay soils. It remains one of the most destructive diseases of soybeans in Ontario. In problem fields, increased frequency of soybeans in the rotation increases the disease pressure

Appearance: The disease can affect soybeans at any stage of development but is often most damaging when it occurs early in the season. Plants infected at the primary leaf stage display typical "damping-off" disease symptoms. Seeds may fail to emerge, or infected seedlings are killed shortly after emergence. Infected areas of the stem are water-soaked or "bruised" and disintegrate easily (soft rot) (see Plate 121). Since it causes a "wet rot," it is difficult to distinguish phytophthora root rot from pythium root rot at this stage. Both diseases cause taproot and lateral root pruning or rotting, resulting in yellowing of the leaves, wilting and even death. Infected plants are easily pulled from the ground, since the plants are not well anchored. Older plants can be affected any time before maturity. A purple or dark-brown discolouration of the stem may extend from the roots (just below the soil line) to the lower nodes of wilted plants. Dead plants may appear a few in a row or as patches in low areas of fields. Leaves will often remain attached to the plant even after death.

Disease Cycle: Cool, wet weather favours disease development. Low, poorly drained areas and slow-drying areas of the field are most prone to the disease. Heavy clay soils, reduced tillage and monoculture of soybeans may increase the damage due to the disease. Phytophthora and Pythium are unique organisms, in that they produce mobile spores that can swim in the water film between soil particles to locate soybean roots. The fungus colonizes the root tissue and will plug the water-conducting tissues of the plant, resulting in wilting of the plant.

Management Strategies: Control of phytophthora root rot requires a combination of:

• soybean variety selection
• seed treatment
• good soil management

Soybean varieties with resistance or tolerance to phytophthora root rot are available. Some varieties have both resistance and tolerance. For variety profiles, consult soybean seed company reps, or the Ontario Soybean Performance Trials, available on the Ontario Soybean Growers website at www.soybean.on.ca.

Resistant varieties. The Phytophthora fungus is present in soils as a series of races. Resistance in any one soybean variety is effective against some but not all of the races. Root rot is controlled in a particular field when the variety grown is resistant to all of the Phytophthora races that happen to be present in that field. However, resistance will "break down" with the appearance of another race, to which the variety is not resistant. If this occurs, switch to a variety that is resistant to the new race or use a tolerant variety or a different gene for resistance. Grow varieties with different resistance genes in rotation. To determine which races may be present in a field, plant strips of several varieties with known race resistance.

Tolerant varieties. Some disease develops in these varieties when grown in infested soils regardless of which races of Phytophthora are present. Yields of tolerant varieties are not usually seriously reduced by the disease, but plants are not immune. Therefore, under extremely favourable conditions for disease development, plant injury can occur.

Any soil management practice that reduces soil compaction or waterlogging will decrease the incidence of phytophthora root rot. On clay soils where the disease may be a problem, the following procedures are recommended:

• Choose a variety with a low percentage of infected plants (field tolerance) and a good resistance gene (Rps1c, Rps1k or Rps 8). See the Ontario Soybean Variety Performance Trial Report on the Ontario Soybean Growers website (www.soybean.on.ca).
• Rotate with corn and wheat. A short rotation will result in a higher population and an increase in the number of races present in the field.
• Do not work the field when the soil is wet.
• Use good soil management practices to improve soil structure and drainage (rotation, manure, cover crops, reduced tillage, etc.).
• Tile inherently slow-draining fields.
• A small amount of tillage will help warm soil and increase surface drainage.
• Plant when soil temperatures are above 13°C.
• Inspect each soybean field for dead plants in late July or early August to determine whether the variety has enough resistance/tolerance to provide adequate protection under local conditions.

Plate 121. Phytophthora root rot causes water-soaked lesions on seedlings and purple or dark-brown discolouration of the stem. The symptoms begin at the soil line and progress into the lower nodes.

Rhizoctonia Root Rot (Rhizoctonia solani)

Incidence: Rhizoctonia root rot has been found in most of the soybean-growing areas of the province. In most fields, stand losses range from less than 5% to over 50% in severe cases. The disease has been increasing in importance and can result in substantial yield losses. It is most prevalent on seedling and young plants, causing a root and stem rot, particularly during prolonged wet periods.

Appearance: Rhizoctonia root rot causes pre-emergence (seed rot) and post-emergence (seedling blight) damping-off on affected seedlings. A characteristic reddish lesion is produced on the stem at or just below the soil line (see Plate 122). These firm, dry, brick-red lesions can join, forming a sunken girdling of the stem that may move down the taproot, pruning roots along the way. Above-ground plant symptoms are very similar to plants infected with phytophthora root rot. Affected plants are pale yellow, which is often confused with nitrogen deficiency symptoms or poor nodulation. Severely infected plants may lose their leaves. Wilted and/or dead plants often occur in small patches. Stem lesions girdle the stem and weaken the plant, often causing infected plants to break at the soil line under stormy conditions. Stressful growing conditions favour this disease. Rhizoctonia root rot is most damaging when cool, wet conditions in the spring are followed by hot (25°C-29°C), dry conditions.

Disease Cycle: This disease occurs on all soil types and environmental conditions. The fungus is primarily a soil inhabitant and survives as resting mycelium or sclerotia. Disease severity will be greatest in fields that have a history of the disease. Over time, small infected areas increase in size.

Management Strategies: Few management options exist since no resistant and few tolerant varieties are presently available. Crop rotation with corn and small grains can help minimize the disease. Maintain good soil drainage and avoid planting under cool, wet conditions. Fungicide seed treatments offer some measure of protection and increase emergence.

Plate 122. Rhizoctonia root rot causes reddish lesions on the stem at or just below the soil line.

Leaf and Stem Diseases

Septoria Brown Spot (Septoria glycines)

Septoria brown spot is a fungal disease that normally does not cause major yield losses in Ontario. However, losses of 5%-10% have been observed in the province in very susceptible varieties that have been infected early and have been under prolonged stress conditions.

Appearance: Symptoms first appear on the primary unifoliate leaves shortly after trifoliolate leaves have developed. Disease symptoms begin as small, dark brown, irregular spots (1-2 mm (1/16 in.) in diameter) with or without a yellow halo, which develop on upper and lower surfaces of lower leaves. Lesions may enlarge and coalesce, and frequently they are concentrated along the leaf veins or at the leaf margin (see Plate 123 ). Rapid yellowing and senescence (death) of infected leaves occurs. Symptoms may be difficult to distinguish from those of bacterial blight, soybean rust and downy mildew. One way of distinguishing the disease is that characteristic brown pycnidia (spots) are imbedded in the dead tissue of older lesions.

Disease Cycle: The fungus does over-winter on crop debris and can be spread by infected seed. In most cases, seed infection is low in commercial seed. It can be a problem in seed that has not been cleaned or has been kept for a number of years. Initial infections on primary leaves and cotyledons produce secondary inoculum that infects upper leaves as they develop. Humidity and moisture are important for brown spot development and spread (through splashing). The fungus produces a toxin that contributes to yellowing.

Management Strategies: The disease is more cosmetic than damaging, but development early in the season can lead to significant defoliation of the plants. There are differences in soybean varieties, but none are completely resistant. A good rotation with non-host crops, such as wheat and corn, will lower disease levels. Fungicides are not normally economical.

Plate 123. Septoria brown spot symptoms begin early in the season with varying sizes of brown spots beginning on the lower leaves. Infected areas turn yellow quickly, and leaves will fall to the ground.

Soybean Cyst Nematode (Heterodera glycines)

Incidence: Since it was first identified in Ontario in 1988, soybean cyst nematode (SCN) has been identified in most counties west of Toronto and more recently in parts of Eastern Ontario.

The disease can be managed effectively, but the first step is identification and awareness. Losses to SCN in Ontario have ranged from 5%-100%. Unfortunately, by the time SCN symptoms become visible on the plants, the producer has lost significant yield potential. Once SCN is in a field, eradication is impossible.

Appearance: These microscopic, worm-like nematodes damage the root system and prevent the uptake of water and nutrients. In many cases, SCN symptoms may not be obvious in a field until populations build significantly. At this point, typical above-ground SCN symptoms include yellowing of the leaves, stunting of plants and early maturity, particularly on lighter soils under dry conditions (see Plate 124). Damage often occurs in circles and is often confused or misdiagnosed as nutrient deficiency, flooding, herbicide injury, compaction, drought or root rot damage. Yellowing of the leaf margins can resemble potassium deficiency symptoms. However, the addition of potassium will not reduce the damage from SCN or eliminate symptoms. Never pull up a plant to check for SCN since too much root will be lost, and the nematodes will be stripped off. Instead, use a shovel and dig up the plant along with the soil surrounding the roots.

Below-ground SCN symptoms include dwarfed, stunted and discoloured roots (due to root rot pathogens) with few nitrogen-fixing nodules. However, the most obvious sign of SCN infection is the presence of the adult female "cysts" on the roots - white to yellow-brown cysts less than 1 mm in diameter (pin-head) (see Plate 125). Nematode injury symptoms (including plant death) are most obvious under stressed growing conditions, especially hot, dry environments. Under good growing conditions, the visual damage from soybean cyst nematode may go unnoticed. In contrast, under high-stress conditions, even low SCN numbers cause considerable visual damage and high yield loss.

SCN infection symptoms may not be obvious and yield reductions of 25%-30% on susceptible fields can occur without visual (above-ground) symptoms. Areas of the field where above-ground SCN symptoms will most often occur include entrance points for equipment into the field, equipment and vehicle storage areas, tops of knolls, compacted headlands and along the fencerow where wind-blown soil tends to accumulate.

Disease Cycle: The life cycle of SCN has three major stages: egg, juvenile and adult. The cycle begins when eggs hatch to release worm-shaped juveniles in the soil. This is the only stage when SCN can infect soybean roots. Once they have penetrated the roots, the young nematodes migrate to the water and nutrient-conducting tissue (vascular system) and establish a feeding site. At this stage, the female nematode begins to swell and eventually breaks through the root surface. Adult females that remain attached to the root to feed produce eggs in a mass, or egg sac, outside of the body. As the life cycle nears completion, eggs also develop within the female's body cavity, called a "cyst." Initially white, the cysts turn yellow and brown as females mature. Each cyst can contain 100-300 eggs. The number of cysts per plant varies from a few to many hundreds. In infected soil, cysts are distributed throughout the root zone and can survive for 10 or more years. The entire life cycle takes approximately 4 weeks when soil temperatures are 25°C.

Management Strategies: The following practices will decrease the likelihood of this pest causing significant economic losses:

• Plant certified or good-quality, clean seed that is free of soil peds.
• Wash soil off farm equipment when moving it between infested fields or farms.
• Use proper soil conservation practices to reduce soil movement between fields.
• Practice prudent weed control. Many weeds, particularly annual weeds such as purple deadnettle, henbit and field pennycress, may serve as hosts to SCN.
• If SCN has been diagnosed in a field, use SCN-resistant soybean varieties. SCN will significantly reduce yields. Therefore, when selecting soybean varieties for fields with a history of SCN, ensure that the variety has SCN resistance (PI 88788, Peking, PI 437654). This is especially true for new technologies or traits such as herbicide resistance.

• SCN resistance is not 100% effective, and a few cysts can be found on the roots. The better the resistant variety and the less diverse the SCN field population, the fewer cysts on the roots. Avoid continuous use of the same resistant variety since it will pressure the SCN population to adapt and shift in the field, thereby making the variety ineffective in combating this pest. Consult the Ontario Soybean Variety Performance Trial Report for SCN-resistant varieties, resistance genes and their performance on infested soil.
• Establish a rotation with non-host crops such as corn, wheat, alfalfa, oat, clover, vegetable crops, such as tomatoes and some cover crops (see Cover Crops, on page 146) which benefit by reducing SCN populations and improving yields. It is not advisable to substitute dry edible beans into the rotation instead of soybeans since these crops are also hosts for SCN. See Table 14-1, Potential Risk of Yield Loss for Various SCN Population Levels.
  o Monitor SCN population in the soil by soil sampling every 3-6 years. Send samples for nematode analysis to any of the laboratories listed in Appendix E, Soybean Cyst Nematode-Testing Laboratories. Ask for both an egg count and a total cyst count.

Plate 124. Plants infected with soybean cyst nematode can be stunted, with yellowish leaves, frequently in circular patterns in the field.

Plate 125. Soybean root with SCN cysts. Yellow-brown, lemon-shaped cysts (pin-head size) are produced on the roots of plants infected with soybean cyst nematode.

Powdery Mildew (Microsphaera diffusa)
Downy Mildew (Peronospora manshurica)

Incidence: Both diseases are most noticeable when conditions are wet or humid. Although powdery mildew and downy mildew occur in most fields, they are considered minor diseases and economically insignificant.

Table 14-2. Symptom Expression on Roots, Stems and Leaves for Brown Stem Rot, Stem Canker and Sudden Death Syndrome on Soybeans

Appearance: Powdery mildew appears as a white powdery growth of the mildew fungus on the upper surface of the leaf (see Plate 126). The soybean seeds do not become infected.

Downy mildew appears as yellow-to-brown spots on the leaves during late July through September ( see Plate 127). In moist weather, a pale blue-to-grey, downy growth of the mildew fungus appears on the lower leaf surface, directly under these spots. Severely affected leaves may drop prematurely. Whitish growth of the fungus may encrust the seeds, affecting even healthy pods. Planting infected seed may result in diseased seedlings.

Disease Cycle: Powdery mildew develops on the leaves, usually in August and September. Outbreaks arise when disease symptoms begin in early July and the environmental conditions remain cool, cloudy and humid through to pod fill. Downy mildew survives in infected leaves and on seed. Air-borne spores blown into Ontario from the U.S. are the most common cause of infection

Management Strategies: Removal of crop residue and rotation with non-host crops, such as corn and wheat, will help prevent both diseases. Fungicidal seed treatments will reduce seed-borne downy mildew.

Plate 126. Powdery mildew appears as a white powder on the upper surface of the leaf.

Plate 127. Downy mildew appears as yellow-to-brown spots on the leaf with pale blue-to-grey mildew on the lower leaf surface.

Brown Stem Rot (Phialophora gregata)
Brown Stem Rot (Diaporthe phaseolorum)
Sudden Death Syndrome (Fusarium verguliforme)

Incidence: All three diseases are found in all soybean-growing areas of Ontario but are more common in southwestern counties. Yield losses range from a few bushels to significant portions of the field being killed (especially for sudden death syndrome).

Appearance: To aid in correctly identifying these diseases, see Table 14-2, Symptom Expression on Roots, Stems and Leaves for Brown Stem Rot, Stem Canker and Sudden Death Syndrome on Soybeans.

Brown Stem Rot (BSR): Symptoms of the disease generally develop in August during pod fill. Upper leaves develop yellow and necrotic areas between the veins similar to sudden death syndrome. Plants wilt suddenly, and pods are poorly filled. The disease is more prevalent under minimum tillage.

Stem Canker: This disease may cause seedling damping off and wilt but commonly affects soybean plants after flowering. Plants wilt suddenly, and leaves and petioles droop, resembling symptoms of phytophthora root rot. Brownish-red lesions appear on the exterior of diseased plants at lower nodes (see Plate 128). The pith of diseased plants is generally brown near the nodes. The fungus can also cause a stem or tip dieback late in the growing season. The fungus can cause seed mould similar to phomopsis seed mould. Stem canker overwinters in crop debris and is more prevalent in minimum tillage situations.

Sudden Death Syndrome (SDS): Infected plants wilt and die very quickly in July and August. Interveinal chlorosis and necrosis of the upper leaves (see Plate 129) and defoliation may occur. Petioles are generally retained. Wet soils and warm temperatures are conducive to disease development. A slight brownish discolouration occurs in crowns of affected plants. The disease is frequently, but not always, associated with soybean cyst nematode.

Disease Cycle:

• All of these fungi survive long periods in crop residue in the soil.
• Brown stem rot infects early in the growing season but does not appear until a month before harvest. Conditions during pod-fill affect disease development. Development is favoured when conditions during pod-fill are cool and wet followed by hot and dry.
• Stem canker prefers moderately warm, wet weather and occurs from mid-July to maturity.
• Sudden death syndrome begins showing symptoms from flowering to maturity and prefers cool, moist soil conditions. Well-fertilized or vigorously growing fields are most likely to show the sudden death syndrome symptoms.

Management Strategies: Crop rotation with corn and cereals will reduce the incidence of disease. These diseases occur most often on reduced tillage fields. Incorporation or removal of infested residue will reduce the risk of these diseases. A few resistant or tolerant soybean varieties are available.

Plate 128. Stem canker causes plants to wilt suddenly. The plants have brownish-red lesions near lower nodes.

Plate 129. Plants infected with sudden death syndrome wilt and die quickly. Interveinal chlorosis and necrosis of upper leaves may occur.

White Mould (Sclerotinia sclerotiorum)

This disease also affects dry edible beans, canola and sunflowers. See White Mould and Plates 161-165.

Asian Soybean Rust (Phakospora pachyrhizi)

Incidence: Asian soybean rust is a new and invasive fungal disease of soybeans in North America. The threat to the Canadian soybean producer from this destructive disease increases as the disease continues to spread and over-winter in the southern U.S. The confirmation of soybean rust in Ontario during the 2007 growing season shows that although soybean rust does not overwinter in Ontario, the pathway exists and Ontario soybeans can become infected.

Appearance: The most common symptoms are small tan-to-dark brown or reddish-brown lesions, 2-3 mm (1/8 in.) in diameter. Although most often found on the underside of the leaves, they can also occur on petioles, pods and stems. These lesions are raised (pustules), which is where the spores are produced (see Plates 130 and 131). The tan lesion types will produce more spores than the reddish-brown lesions. Infected leaves will have a mottled appearance. Infection often begins on the lower leaves and moves up the plant. The leaves eventually turn yellow and fall off. It is the loss of photosynthetic tissue, premature defoliation and death that can severely decrease yields. Soybean rust can easily be confused with septoria brown spot, downy mildew or bacterial blight, which are common in Ontario.

Disease Cycle: Soybean rust is a parasite that requires living soybean plants to survive. This is good news to Ontario soybean producers, since the disease will not overwinter in Ontario. Although soybean rust cannot survive the harsh Ontario winters, spores routinely arrive each growing season from their over-wintering locations in the southern U.S. The viability of these spores depends on many factors. The most critical are crop growth stage and the environmental conditions during the time spores are deposited. Long periods of leaf wetness are needed for spore germination, as well as temperatures between 15°C and 30°C and a high relative humidity.

Management Strategies: Currently there is no effective resistance to soybean rust in commercial soybean varieties grown in North America. Soybean rust management depends on scouting, early detection and the use of foliar fungicides, until resistant varieties become available (see Plate 132). Monitoring or predicting the risk of soybean rust has been aided by the comprehensive North American soybean rust "sentinel plot network," which is available on the Ontario Soybean Growers website at www.soybean.on.ca.

Plate 130. Leaves infected with Asian soybean rust have a mottled yellow appearance that often begins on the lower leaves and moves up the plant.

Plate 131. Asian soybean rust produces small, tan-to-dark brown or reddish-brown raised lesions where the spores are produced. A hand-lens will assist in distinguishing soybean rust from other foliar diseases.

Plate 132. Asian soybean rust can be managed effectively through scouting and a well-timed fungicide application. The left side of the field was untreated; the right side had a fungicide application.

Bacterial Blight (Pseudomonas syringae pv. glycinea)

Incidence: Most years, the impact of bacterial disease is minimal. Yield losses and seed quality problems can occur when environmental conditions are cool and wet for a prolonged period of the summer.

Appearance: Red or black lesions with a yellow halo and a shiny centre are produced on the leaves of infected plants (see Plate 133). Symptoms often disappear under dry, hot conditions. Infected seed often has a water-soaked discolouration, starting at the hilum, that can reduce seed viability and reduce germination.

Disease Cycle: The bacteria survives on seed and crop residue and is spread to the upper leaves primarily through rain splash, wind and plant injury (hail, insects, mechanical, etc). There are different physiological races in the province.

Management Strategies:

• crop rotation with crops such as corn and wheat
• removing the crop residue
• staying out of the field while the leaves are wet

There is some tolerance in varieties, but none are resistant to all physiological races.

Plate 133. Bacterial leaf blight infection produces a distinctive yellow halo around the lesion, and the leaves often have a ragged appearance (tear or rip).

Pod and Seed Diseases

Soybean Mosaic Virus

Incidence: Low levels of soybean mosaic virus (SMV) occur in most areas of the province. Food grade or specialty beans requiring blemish-free seed coats are at the highest risk of economic losses due to the soybean mosaic virus.

Appearance: Leaves of infected plants are distorted, wrinkled and puckered. They have a typical mosaic pattern that is most evident on younger leaves (see Plate 134 ). Infected plants may be stunted. Infected seeds have a characteristic brown or black discolouration extending in streaks from the hilum region (see Plate 135 ). Virus symptoms are often confused with hormonal herbicide injury. Plants infected with soybean mosaic virus are scattered in the field, and the affected area is generally smaller than if the cause was herbicide injury. There is no field pattern to the injury.

Disease Cycle: The virus survives from season to season in infected seed and is transmitted from plant to plant by aphids.

Management Strategies: Planting disease-free seed controls the disease in Ontario.

Plate 134. Soybean mosaic virus symptoms include distorted and puckered leaves. Plants may be stunted. Soybean aphids are vectors of this disease.

Plate 135. Soybean mosaic virus can cause brown or black discolouration of the seed and streaks near the hilum.

Bean Pod Mottle Virus

Incidence: Bean pod mottle virus (BPMV) has recently been identified in Ontario. This virus can affect soybean quality and, therefore, export potential.

Appearance: A common symptom of BPMV infection is uneven crop maturity or "green stem" in which stems and leaves remain green, even though pods have matured. Young leaves in the upper canopy often have a green-to-yellow mottling that may fade and then re-develop later in the growing season. In severe cases, malformed leaves and pods may be produced. Infected leaves show reduced turgidity resulting in curling. A reduction in pod set often occurs in infected plants that have undergone moisture stress during dry periods. Infected seed coats are mottled with brown or black streaks extending from the hilum.

Disease Cycle: Cool weather enhances disease development. Unlike soybean mosaic virus, BPMV does not spread very efficiently in seed. The bean leaf beetle is its primary vector, and the cucumber beetle may be a vector as well. The virus has a wide host range among legumes and will be transferred to bean leaf beetles that feed on infected legume plants. The virus can be spread by mechanical injury, especially under wet conditions.

Management Strategies: Plant disease-free seed or resistant varieties in areas with a history of the disease. Consider controlling bean leaf beetle adults when populations are high early in the season. For bean leaf beetle thresholds, see Bean Leaf Beetle

Frog-eye Leaf Spot (Cercospora sojina)

Incidence: Economic impact is usually minimal and is most frequent in the extreme southwest counties.

Appearance: Lesions are 1-5 mm (1/5 in.) in diameter with a tan centre and a dark red/brown border. Older lesions coalesce, and leaves may appear ragged or with a slight slit in the centre of the lesion (see Plate 136).

Disease Cycle: The pathogen overwinters in residue. Seed and leaf spotting occurs under hot, humid conditions, particularly on very susceptible varieties during flowering and pod development.

Management Strategies:

• crop rotation with non-hosts, such as corn or wheat
• use of non-infected seed
• use of a resistant variety

Foliar fungicides are not often economical unless disease starts early on a very susceptible variety.

Plate 136. Frog-eye leaf spot foliar lesions have a red/brown border with a tan centre that may disintegrate, leaving holes in the centre of the lesions.

Cercospora Leaf Spot
Purple Seed Stain (Cercospora kikuchii)

Incidence: This disease often appears late in the season and can cause leaf blighting and staining of the seed. Yield losses are often minimal, but a reduction in seed quality can occur due to staining.

Appearance: Leaves often have red to purple lesions, less than 1 cm (4/10 in.) in diameter, which become noticeable in August or early September. These lesions can join and form large infected areas that may extend along the midrib or lateral veins. Lesions can also be found on the petioles, stem and pods. Symptoms are often confused with sun scald or ozone damage. Infected seed has a distinctive purple discolouration (purple seed stain), varying from violet to pale purple to dark purple over part or all of the seed coat (see Plate 137). This discolourization is often confined to the upper two layers of the seed coat. The embryo is not discoloured or affected. In most cases, a 7%-13% reduction in emergence can occur in the field.

Disease Cycle: The fungus overwinters in seed, but the primary infection source is crop residues.

Management Strategies:

• using clean seed and a fungicide seed treatment
• crop rotation and removal of residue to reduce infection potential
• using a variety with greater tolerance

Plate 137. Distinctive purpling or staining of seed due to cercospora leaf blight infection (leaves often have a purple discolourization as well).

Phomopsis Seed Mould (Phomopsis longicolla)
Diaporthe Pod and Stem Blight (Diaporthe phaseolorum)

Incidence: Historically, phomopsis seed mould has been Ontario's most important soybean seed disease. It is most problematic when the weather conditions at harvest are warm and wet. Delaying harvest under these conditions will increase the incidence of this disease.

Appearance: Phomopsis seed mould is characterized by fine cracks that usually develop near the hilum of the infected seed (see Plate 138). A white or grey mould may be visible on the seed surface. The yield, grade, viability and vigour of the seed can be reduced. Yield losses occur because severely infected seeds remain small and light and may be lost during harvest and cleaning operations.

The second phase of the disease is referred to as "pod and stem blight." Although plants are infected early in the season, symptoms do not become apparent until after mid-season. Symptoms on the stems appear as small, black, raised dots or bumps (pycnidia) (see Plate 139). These bumps are arranged in rows or islands on the stem. On the pods, there are also numerous black dots, but they are not arranged in any particular pattern.

Disease Cycle: The fungus overwinters in seed and crop debris. Spores of the fungus are splashed onto developing plants early in the season. Warm, wet and humid weather during pod fill favours disease development.

Management Strategies: Whenever possible, grow full-season varieties that mature during the cool weather late in the growing season. Varieties that are short-season for an area tend to mature earlier when conditions are warmer and more favourable for seed mould development. Pod and stem blight can be controlled or reduced by integrating one or more of the following: planting disease-free seed

• planting later
• crop rotation
• plowing under soybean debris
• a well-timed harvest

Harvest soybeans destined for export or seed first. Seed treatment usually increases germination and emergence of seed. However, distorted seed with visible fungal growth often fails to germinate, even when treated.

Plate 138. Phomopsis seed mould causes fine cracks and mould, starting near the hilum, reducing quality and seed vigour.

Plate 139. Phomopsis pod and stem blight symptoms include small, black, raised dots or bumps arranged in rows or islands on the stem.