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Soybean – Diseases

Disease Prevention

Soybeans are susceptible to many different diseases. Fungus, bacteria, and viruses can all negatively affect soybean growth. In addition to disease, nematodes feeding on roots can cause above-ground symptoms similar to root rot diseases. Other non-infectious disorders can also mimic disease symptoms, so careful scouting and management of the crop is important to correctly identify the pathogen affecting the crop. As there are very few acres of soybeans in Alberta and soybeans are still a relatively new crop, many of the diseases have yet to establish at high incidence. As soybeans become more established in Alberta, the incidence of some of these diseases may increase as well.

Many management practices can reduce the risk of some diseases. Cultural control methods can play a large role in reducing disease pressure for the growing season. Growers should consider:

  • Seed Source and Handling – Planting clean seed that is disease free. Many diseases can be carried on the seed such as downy mildew, cercospora leaf blight, bacterial blight, and white mould. Seed should be handled gently both in storage and seeding operations to avoid damage to the seed coat.
  • Variety Selection – Many sources of resistance are available for a number of different diseases. If a particular disease is of concern, checking the ratings when selecting a variety is good practice.
  • Weed Control – Keeping the crop weed free will help reduce canopy cover and provide a drier canopy, as some weeds can host pathogens further increasing disease pressure.
  • Proper Fertilization – Balancing nutrition is important to avoid nutrient deficiencies that may stress the plant and make it more susceptible to disease.
  • Crop Rotation and Tillage – Proper crop rotation and burial of infested crop residue using conservation tillage practices can reduce pathogen levels of some diseases. Avoiding compaction and breaking up hardpans can also help reduce stress on roots, helping to decrease disease incidence and severity.

Seed treatments will help protect seed and seedlings from seed-borne and soil-borne pathogens, to help ensure good establishment of a strong, vigorous crop. Foliar fungicide applications can be used to target identified diseases during the growing season at the correct timing, to maximize effectiveness and protection of yield.

Scouting your field for diseases

Scouting is one of the most powerful practices to effectively manage crops. It allows early detection of disease symptoms, provides opportunity for mitigation of spread and minimizes the impact on yield.

  • Develop a systematic approach beginning soon after crop emergence.
  • ALWAYS bring along a shovel to dig up plants, pulling plants will not work when assessing root and nodule health.
  • Scout weekly beginning after the crop emerges until maturity, first looking at plant stand, then root/seedling diseases, and as the crop develops for foliar diseases.
  • Take photos, mark problem areas in the field, collect whole plants (leaves, stems and roots) for identification of the disease (put in plastic Ziploc, label with date and location and keep cool until sent to lab).
  • Check in a ‘W’ shaped pattern in the field stopping at 5-10 locations,
  • Take note of yellowing patches, and problem areas in the field, these are often the source of disease spread and if recurring problems are noted these areas may benefit from a separate management strategy.

Scouting, Government of Alberta

Plant Disease Scouting 101, Government of Saskatchewan Fact Sheet.  

Fungicide Application

Keeping crops healthy and disease free is a priority for producers and one practice that can help achieve these goals is applying fungicide. Producers need to consider both disease risk and economics when deciding whether to apply a fungicide.

Disease risk is influenced by the field history (crop rotation, levels of disease in previous susceptible crops; the environmental conditions such as temperature and precipitation levels that may favour disease development; and the level of variety resistance to the disease). A fungicide should only be applied if the risk of disease is high and an economic return is expected. 

Once a decision has been made to apply fungicide, the two most important considerations are timing and coverage. Plant growth and disease progress can be very rapid so it’s essential to be ready to apply the fungicide when it’s needed. Missing the ideal window is very costly and a fungicide applied at the wrong time can lose much of its effectiveness.

Good coverage is also critical for a fungicide to be effective. To achieve this, target the plant part that needs protection, be it the leaves, stems, or leafstalks, and do so with adequate droplet density. This is because most fungicides do not move from one part of the plant to another very well. That is why it is important to understand the disease, including which plant parts must be covered by spray and where – top, middle, or bottom – they are in the canopy. Then assess how easy it will be for a spray to reach the target zone.

Determine if more than one application should be done by scouting. If applying more than one application, make sure to rotate fungicide groups and do not use a single mode of action more than once.  Consult the product’s label. (Forsythe, Trudy K. “Timing and Coverage are Important Factors in Fungicide Application”)

Management of Fungicide Resistance

Just as with different herbicide groups, fungicide groups also have the potential to develop resistance in the disease population. Any fungal pathogen population may contain some strains naturally insensitive to various fungicides. A gradual or total loss of disease control may occur if these fungicides are used repeatedly in the same fields. Other resistance mechanisms that are not linked to site of action, but are specific for individual chemicals, such as enhanced metabolism, may also exist.

To delay fungicide resistance/insensitivity

  • Use a fungicide rotation – rotate the use of fungicides with others from different groups that control the same pathogens.
  • Tank mix fungicides that have a higher risk of developing insensitivity with other fungicides from a different group.
  • Do not apply more than the maximum number of applications listed on the label. 
  • Avoid consecutive sprays of the same fungicide, or other fungicides in the same group, in a season.
  • Fungicide use should be based on an integrated pest management (IPM) program that includes scouting and accurate record keeping of fungicide use and crop rotation. An IPM program also considers cultural, biological, and other chemical control.
  • Monitor treated fungal populations for signs of fungicide insensitivity. If disease continues to progress after treatment, do not reapply the same product with a higher rate.
  • If you observe fungicide insensitivity, discontinue use of the product and switch to a fungicide with a different mode of action.
  • Contact your local regional crop specialist or certified crop advisor for any additional pesticide management and/or IPM recommendations for specific crops and disease. problems in your area.

Sclerotinia Stem Rot (White Mould)

Sclerotinia Stem Rot (White Mould) in Soybean, Photo Credit Roger Schmidt, University of Wisconsin-Madison, Bugwood.org

CAUSED BY
  • Sclerotinia stem rot is caused by Sclerotinia sclerotiorum.
  • Sclerotinia stem rot is common in many broadleaf crops in Western Canada, and is the most impactful disease with soybeans in Alberta.
  • This disease affects many different broadleaf crops, however, soybeans are more tolerant than canola, dry beans, and sunflowers. If 10% or more plants are affected, yield loss can be expected.
  • If this disease appears late in the season, there will be minimal impact to yield.
  • Overwinters as sclerotia in infected crop debris and soil.
  • Sclerotinia stem rot favours cool, moist conditions such as those found under a thick canopy.

SYMPTOMS
  • White mould is easily recognized by fluffy, white growth on stems. Initial symptoms develop from reproductive (R) 3 to R6 as grey to white lesions at the nodes.
  • Lesions rapidly progress above and below the nodes, sometimes girdling the entire stem. Characteristic black sclerotia will become visible, embedded in the white mycelium on the stem lesions and inside the stem.

PREVENTION AND CONTROL
  • Some varieties are moderately resistant to Sclerotinia, so variety selection can influence disease severity. Open canopies that have increased air flow are also less susceptible to Sclerotinia developing.
  • Crop rotation is difficult, as many other crops are hosts as well, and spores can move long distances in the air to affect nearby crops.
  • Controlling weeds is also important as many broadleaf weeds are hosts as well. Foliar fungicides are available to help manage white mould in soybeans.

IMPACT
  • Sclerotinia stem rot is considered the most impactful disease on soybeans in Alberta.

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Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers.