Managing Sclerotinia Stem Rot in Canola

Sclerotinia stem rot, also known as white mould, is the greatest disease threat to canola producers in Western Canada. Yield losses can surpass 50 percent in individual fields and 10 to 15 percent across larger areas when conditions are conducive for the disease.^1,2 Managing sclerotinia stem rot can be challenging because of a number of factors. The fungal pathogen that causes the disease (Sclerotinia sclerotiorum (Lib) de Bary) can survive in soil for long periods, making cultural practices like crop rotation and tillage less effective. The pathogen also infects a number of plant hosts living on the Prairies, making it more likely the disease is present and limiting options for rotation crops. The pathogen can be dispersed easily from one field to another by wind-borne spores or by moving soil and crop residue. Most importantly, disease development is highly influenced by environmental conditions during the growing season, particularly moisture, and favored by widespread cropping strategies that promote high yield in dense plant canopies. Finally, newer canola products have demonstrated tolerance to the disease; however, most remain susceptible.^2,3

Developing a management plan including field history and integrated disease management practices can help reduce losses from white mould. An effective, long-term management strategy should include host resistance, cultural practices, chemical fungicides, and biological control. It is also important to consider key aspects of the disease when evaluating different management options.

Key Aspects of the Pathogen and Disease Cycle^2,4

• Survival structures called sclerotia fall to the ground during harvest and allow the fungus to remain viable in the soil for an extended length of time.
• From the soil, sclerotia infect canola plants in the summer by germinating to produce structures called apothecia that extend to the soil surface.
• Apothecia develop and produce millions of ascospores that are windblown or water splashed to infect aerial portions of canola plants.
• Ascospores infect fallen or senescing flower petals on canola plants when water is present on plant surfaces and humidity is 100 percent.

Plant Development and Environmental Conditions for Disease^2,4

• Primary infection occurs during flowering, with the critical stage for damaging disease development from early to full bloom.
• Optimal weather for canola growth that is conducive for disease development:
  • One to two inches of rain or irrigation prior to or within the first two weeks of flowering
  • Prolonged soil moisture (10 days or more above the wilting point)
  • Temperatures of 15 to 25^o
  • Humid conditions and temperatures between 20 to 25^oC after initial infection
• Fields with a history of disease have a higher likelihood of disease development since pathogen inoculum is likely to be present for initial infections. Ascospores can also travel by wind from neighboring fields with a history of the disease.

Symptoms^2,4

Two to three weeks after initial infection on dead flower petals, lesions appear as water-soaked spots on leaves and stems, particularly around leaf axils. As infection develops on the stems, lesions enlarge and turn grey-white in color. Lesions continue to expand and, stems may become girdled, causing plants to wilt and ripen prematurely (Figure 1). As infection further develops, stems will bleach and tend to shred. Premature death may occur, and severely infected plants will often lodge and shatter at swathing. White cottony mycelium of the fungus can appear on the inside and outside of infected stems. Hard, black sclerotia on the outer surface of diseased tissue may also be visible but can be observed inside infected tissue as well (Figure 2).

For more information on the biology, epidemiology, and symptoms of S. Sclerotiorum, please refer to the article Sclerotinia Stem Rot in Canola.

Disease Management

Good Recordkeeping

Keeping good records of disease location and levels can help predict the potential pathogen inoculum load (sclerotia) in the future. It is also important to record yield information relative to disease conditions to help with product selection.⁵

Cultural Control

Crop Rotation: Crop rotation with nonhost crops such as wheat, corn, barley, or grass helps to reduce the number of sclerotia in the soil. Because sclerotia remain viable for long periods of time, a minimum of a three-year rotation with a nonhost is recommended. The pathogen can infect over 400 plant species in addition to canola, including many species in Western Canada like sunflowers, beans, lentils, peas, mustard, and many common broadleaf weeds.^2,4,6,7

Tillage: The effect of tillage on the management of white mould is inconsistent. Sclerotia must be near the soil surface to produce apothecia and ascospores, therefore deep tillage can prevent germination of apothecia. However, because deeply buried sclerotia survive well, they may be returned to the soil surface with any subsequent tillage operation. In addition, any tillage can contribute to the dispersal of sclerotia across fields.^7,8 On the other hand, reduced tillage may help decrease viable sclerotia. According to no-till research in Wisconsin, sclerotia remained on the soil surface and did not survive.⁵

Weed Control: The host range of S. sclerotiorum includes many common broadleaf weeds including shepherd’s purse, thistles, stinkweed, chickweed, hemp-nettle, false ragweed, narrow-leaved hawk’s beard, and others.^2,3 These weeds can serve as sources of pathogen inoculum for canola, therefore, weed control is critical.

Seeding and Row Spacing: Practices that limit high humidity within the plant canopy can lessen white mould incidence and severity. Factors that may impact humidity levels include: site location, seeding rate, row spacing, nitrogen fertilizers, and canola product selection. Well-drained soils with wide row spacing and lower seeding rates can increase air circulation within the crop canopy and reduce the moisture levels necessary for sclerotia to germinate. High nitrogen levels can contribute to dense canopy development and delay crop maturity, increasing incidence of disease. Selecting canola products that mature early and tend to grow more upright in contrast to products that have more of a vining growth habit can help limit disease development.^2,6,8

Controlling Irrigation: When using irrigation, avoid conditions that encourage periods of leaf wetness beyond 12 to 24 hours or during periods of high humidity, especially during flowering. Irrigate early in the morning and stop around noon, making sure plants dry.^4,6,8

Harvest: White mould can develop rapidly in the swath during wet years. Growers can lose up to one third of their crop to sclerotinia stem rot during swathing and also suffer a downgrading of canola due to sclerotia in seed samples.² In addition, the increases in sclerotia development during swathing can allow the pathogen to remain in the field after harvest. Do not to swath canola crops during forecasts for heavy rain if substantial disease is present and clean the combine after harvesting a field with high white mould incidence to prevent movement of sclerotia to fields with low or no white mould.²

Fungicides

Foliar fungicides can help to provide an effective option for the management of white mould on canola. The main considerations with fungicide applications are:

• Timing/Method: Disease incidence and development are dependent on environmental conditions around flowering; therefore, the timing and method of the fungicide application are critical.
• Scouting: Assessment of conditions conducive to disease are key. If the field has a previous history of white mould and weather conditions have been wet for a couple of weeks prior to flowering, a foliar fungicide application between 20 to 50 percent bloom, with optimum timing at 30 percent bloom, is recommended.^2,6 Fungicides are essentially targeting fungal ascospores when flower petals are senescing, so spraying earlier than 20 percent bloom is likely too early and can require additional fungicide sprays later.
• Scouting Techniques⁹
  • Scout the field for small mushrooms at the early blossom stage.
  • After flowering, look for prematurely-ripened plants.
  • Infections begin as soft rot that girdles the stem.
  • Lesions turn white, become brittle, and tend to shred.

For more information and excellent images of canola growth stages, please refer to https://www.canolacouncil.org/canola-encyclopedia/growth-stages/.

• Application: It is critical that the fungicide application cover as many petals as possible and penetrates the canopy to protect leaf axils and bases from possible infection. To cover flower petals adequately, high water volumes and increased pressure should be considered. According to research at Agriculture and Agri-Food Canada, conventional flat fan nozzles, low-drift venturi nozzles, and hollow cone nozzles are effective for fungicide application to help reduce disease severity.^2,3

Proline^® and Proline^® GOLD are Bayer fungicides registered for control of sclerotinia stem rot on canola in Canada. Both products have the proven defense of prothioconazole, with Proline^® GOLD fungicide adding a second mode of action (fluopyram) to improve the level of efficacy against the fungus.

Application of Proline^® fungicide (at 128mL/acre rate) and Proline^® GOLD fungicide (at 253 mL/acre rate) with good coverage at the early to mid-bloom stage of plant growth has demonstrated maximum efficacy and protection from the yield-robbing fungus. Proline^® GOLD fungicide, with its additional mode of action (fluopyram), provides both contact and systemic protection. Proline^® GOLD fungicide is recommended for improved control of sclerotinia in higher disease pressure situations where environmental conditions such as high relative humidity and extended periods of rainfall occur.

Apply Proline^® fungicide or Proline^® GOLD fungicide when the crop is in the 20 to 50% bloom stage. Best protection will be achieved when the fungicide is applied prior to petals beginning to fall and will allow for the maximum number of petals to be protected. A second application of Proline^® fungicide or Proline^® GOLD fungicide can be applied 10 to 14 days later, up to full bloom, if disease persists or weather conditions are favorable for disease development.

Refer to your Province’s Crop Protect Guides for a complete list of registered fungicides. Consult with crop specialists and always read and follow fungicide label recommendations.

Risk Assessment Tools: Because of the costs associated with fungicide applications and the high variability in the environmental conditions conducive for disease to occur, growers need to determine whether to spray or not. There are several risk assessment tools available, including weather-based forecasting maps and checklists:

• https://www.agr.gc.ca/eng/agriculture-and-the-environment
• https://www.canolacouncil.org/canola-watch/2011/07/07/sclerotinia-checklist-what-does-your-score-mean/
• A newer forecasting tool for sclerotinia risk can be found at www.decisionfarm.ca

Petal testing can also help forecast disease severity by assessing the percentage of canola petals infected by sclerotinia ascospores at the early bloom stage. If greater than 45 percent of petals show infection and environmental conditions are conducive for disease development, a fungicide application is recommended.⁶ One drawback to petal testing is that it requires time from sample collection to results, possibly hampering timely fungicide application.²

Digital tools: Since S. sclerotiorum is highly dependent on environmental factors including relative humidity, rainfall and temperature, field monitoring applications such as Climate FieldView^™ can be used to assess factors relating to increased likelihood of white mould infection. Relative humidity correlated to canopy density (A - field health imagery and accumulated biomass), accumulated rainfall measurements (B) and daily temperature assessments (C) can all be provided through the Climate FieldView^™ application and used by growers as a tool to assess risk of infection and subsequent disease development. The information from Climate FieldView^™ can also be incorporated into an effective scouting program by evaluating the factors leading to increased risk and assessing the need for fungicide applications. Scouting, crop rotation, and application information can be stored in the app as a method of recordkeeping.

Other Controls

Host Resistance: There have been canola products in Canada with sclerotinia tolerance claims, but at this time no products have complete resistance. In trials, tolerant products have shown reduced disease symptoms with the appearance of less lesion development on stems and other plant parts.² Plants with strong stems and a strong branching habit help reduce the likelihood of lodging and disease severity.⁶ When the risk of severe disease infection is present, a fungicide application may still be necessary. For additional information about tolerance and product selection, see the protocol developed by Dr. Lone Buchwaldt and the Pathology sub-committee of the Western Canada Canola/Rapeseed Recommending Committee:

https://www.canolacouncil.org/research/

Biological Control: There currently are two biological products available to Canadian canola growers: Serenade^® OPTI fungicide and Contans^® WG fungicide. Serenade OPTI fungicide is a wettable powder biofungicide produced from the bacterium Bacillus subtilis. Like with fungicides, Serenade OPTI fungicide is recommended for foliar application at the 20 to 30 percent bloom stage. Contans WG fungicide is produced from the fungus, Coniothyrium minitans and is applied pre-emergence to reduce the number of viable sclerotia in an infested field.²

Key Best Management Strategies

• Maintain good records of field history and disease incidence
• Consider hybrid performance, disease tolerance, and a growth structure conducive to air flow
• Follow good cultural practices:
  • 3-year or greater rotation to nonhosts
  • Effective tillage practices
  • Control weeds
  • Reduce plant density and increase row spacing
  • Control Irrigation
  • Do not harvest during heavy rains
  • Clean combine after harvest
• Use fungicides at precise times and proper application methods
• Consider biological controls for long-term strategy to reduce disease in fields