Identifying and Managing Sclerotinia Stem Rot (White Mold) in Canola

As the canola crop progresses, the recent extended rainy period may contribute to Sclerotinia disease issues. Sclerotinia stem rot (also called white mold) is caused by a fungal pathogen called Sclerotinia sclerotiorum. White mold is one of the most destructive diseases in canola. The severity of this disease is extremely variable from year to year, region to region, and field to field.

Factors for Sclerotinia Disease Severity

Symptoms of Sclerotinia white mold do not appear until after the flowering stage; therefore, scouting for symptoms is only possible after the infection occurs. Canola plants are infected with Sclerotinia stem rot when the crop is in bloom. 

There are multiple factors that will affect the amount of disease incidence and severity in canola fields, especially those not treated with a fungicide. These factors include:

  • Crop density
  • Spore quantity
  • Crop height and vigor
  • Rainfall/humidity/leaf wetness
  • Soil moisture
  • Temperature
  • Crop canopy
  • Timing of flower in relation to favorable conditions for sclerotial germination and/or plant infection

What Sclerotinia White Mold Looks Like in Canola

If weather is favorable, small mushroom-like structures called apothecia (Figure 1) will be produced. According to the NDSU Extension Service, as a general rule, 1 to 2 inches of rain are needed within a week or two of flowering to produce the apothecia.

Apothecia resemble miniature golf tees that are usually no bigger than ¼-inch wide and can be difficult to find. However, if they are found around the time of flowering, they are a sign that the pathogen is producing spores and conditions may be favorable for infection. 

Apothecia
Figure 1. Apothecia

Spores can infect floral parts directly, but most infections occur when spores land on dead or dying tissue. Canola petals that die after flowering provide an excellent food source for the spores (Figure 2). Spores that land on these old petals can germinate and the pathogen will begin to grow. Once the pathogen is established, it invades healthy plant tissue and may progress into petioles, branches and the stem. However, if temperatures are too hot during this time (above 86 degrees Fahrenheit), spores may die before they can grow or disease development will not occur until temperatures become cooler.

figure 2
Figure 2
Infected areas become bleached, taking on the appearance of dry bone and frequently become brittle and shredded.
figure 3
Source: www.canolawatch.org, NDSU, and Canola Council of Canada

Yield Loss from Sclerotinia White Mold Infection

Yield losses vary depending on severity and location of infection. A main stem infection could cause the loss of the whole plant, while a branch infection will not cause the whole plant to die. These infections can also lead to other issues that increase the potential for yield losses:

  • Lodging
  • Smaller and fewer seeds
  • Premature ripening
  • Shattered pods

Managing Sclerotinia in Canola with Crop Rotation

There are multiple ways growers can manage Sclerotinia stem rot in canola. First is through crop rotations. Growers should try to avoid more than one highly susceptible crop (sunflowers, canola, dry beans and soybeans) in a rotation. You can also utilize crop rotations by widening out the rotation on your farm by planting two years of non-susceptible crops before going back to canola.

Managing Sclerotinia with Proline® Fungicide

The primary option most growers have chosen in recent years is the use of foliar fungicides.  Proline fungicide from Bayer is available for use on flowering canola fields. Here are tips for spraying Proline:

  • Use rate is 4.3 oz./A to 5.7 oz./A. Use the 5.7 oz./A rate when the risk for Sclerotinia infection is higher.
  • Apply by ground at 15 to 20 gallons per acre or by air at 5 gallons per acre.
  • A Non-Ionic Surfactant (NIS) is recommended with Proline at 1 pint per 100 gallons of water.
  • Proline is rainfast in 2 hours.
  • Proline has a 48-hour restricted-entry interval.
  • Apply between 20% to 50% bloom.

The pictures below help illustrate the correct application timing for Bayer’s Proline fungicide:

application timing for Bayer’s Proline fungicide

The Sclerotinia Risk Map created by NDSU is a tool growers can use to help determine their risk for Sclerotinia stem rot development in North Dakota and some counties in northwestern Minnesota. This map was generated by using environmental information from the North Dakota Agricultural Weather Network (NDAWN) weather stations to determine if conditions are favorable for the germination of sclerotia, development of apothecia and spore release. 

estimated risk of sclerotinia stem rot development for canola

Protecting your canola crop against major yield losses from Sclerotinia stem rot is possible with Proline fungicide. Talk to your local retailer for more information. Be safe this spraying season.

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