On January 9th and January 21st, winter canola samples from West Tennessee were confirmed to have white mold (Sclerotinia stem rot). These confirmations are a useful reminder that, as we warm up this spring, it is important to scout fields for this disease, especially leading up to bloom to determine the need for fungicide application.
White mold is caused by Sclerotinia sclerotiorum, a fungal pathogen with a wide host range that can survive for years in soil and crop residue using one of its most identifiable features, sclerotia (hard, black structures on the stem surface – see Image 1). In canola, decisions occur around flowering, where fungal diseases can have the biggest impact on yield.
Identification
Typical signs and symptoms consistent with white mold include:
– White, cottony fungal growth on the stem (Image 1)
– Black, irregularly shaped sclerotia forming on stem surface or inside the stem pith (similar in size to sesame seeds or peppercorns) (Image 1, circled)
– Bleached/tan stem lesions (Image 2)
– Petals dropping (during bloom)
Image 1. Sclerotia (circled) on cottony white mold growth on canola stems, (left, mature sclerotium image from BSPP; right, developing sclerotium from field sample received by WTREC).
Image 2. Tan lesions on a canola stem caused by Sclerotinia sclerotiorum, from Canola Council.
Where to scout:
– Lower areas in the field that are more prone to hold water
– Areas with thick canopies that may limit air flow
– Field edges that are near drainage, tree lines, or in shade
– Fields with a history of white mold in canola (or other susceptible broadleaf crops)
Infection and optimal conditions for disease development:
Sclerotia (Image 1) are the primary source of disease outbreaks, surviving for long periods (up to 10 years) in the soil. Sclerotia near the soil surface can germinate directly to produce tufts of fungal strands that can infect nearby stems and leaves in contact with the soil (likely what happened on samples from West TN). Sclerotia can also germinate to produce apothecia – small, cup-like mushroom structures seen in Image 5 (A, B) that release airborne spores called ascospores, which can germinate and colonize flower petals during bloom stages and lower plant parts, causing disease.
Moisture is the main driver of white mold infection and severity, with temperature potentially having a lesser impact. Significant moisture, such as 1 to 2 inches of rain, especially 1- to 2-weeks before flowering, can provide sufficient moisture for sclerotia to germinate and subsequent spore production, as well as moisture in the plant canopy optimizing spore germination and infection on flowers and leaves. While infection and disease development can occur within 55 to 80 F, if temperatures exceed 85 F or are below 50 F disease development can be inhibited.
University of Kentucky and Washington State University notes that white mold risk increases for canola when flowering overlaps with wet weather. Because winter canola typically blooms in early to mid-April in Tennessee, rainy spring conditions can increase risk of infection.
Sclerotinia stem rot look-a-likes:
Blackleg (stem canker)
Often shows darker, sunken lesions rather than bright bleached stems, and typically lacks cottony mycelium and sclerotia (Image 3).
Image 3. Black leg stem canker lesion on a canola stem, from Canola Council.
Grey stem (Pseudocercosporella capsellae)
Appears as silvery patches on the stem (Image 4). Lacks sclerotia and typically occurs too late in the growing season to affect yield.
Image 4. Grey stem lesion on canola, from Canola Council.
Bird’s nest fungi (Nidulariaceae)
Bird’s nest fungi (Image 5, C and D) are harmless decomposers of plant tissue. These fungi can look similar to the apothecia of Sclerotinia sclerotiorum (Image 5, A and B), but are shaped more like a bowl than a martini glass (can remember as “Sclero-martini-a”) and the fruiting bodies of bird’s nest fungi mature to develop “eggs” inside.
Image 5. (A) Apothecia of Sclerotinia sclerotiorum; (B) Closer view of the “martini glass” shape of S. sclerotiorum apothecium emerging from a sclerotium; (C) Developing bird’s nest fungi; (D) Mature bird’s nest fungi with visible “eggs”.
Photo credits: (A) Craig Grau, bugwood.org; (B) sclerotia.org; (C) Brandon Kleinke, Iowa State University; (D) Claire Cooke, University of Tennessee.
Abiotic injury (freeze, wind rub, mechanical damage)
Environmental factors can also cause damage to plants, which can sometimes look similar to a fungal infection (Image 6). Especially with the winter storms this year, freeze and wind injuries can cause bleaching and/or cracking.
Image 6. Abiotic injury on canola leaves, from Canola Council.
Management
Fungicide sprays
While applying one protective fungicide application at early bloom may be sufficient, if needed, a second application may be warranted two to three weeks after the first application if conditions stay conducive for white mold. More research on fungicide efficacy in our specific region is needed to accurately gauge efficacy of fungicide timing. indicates application timing is likely best between 20-50% bloom depending on forecast (high humidity/wetness), canopy density, and known field history. See Image 7, an illustration of reproductive stages, to better understand bloom development (link to the full Canola Growth and Development illustration by Kansas State University provided below the Image 7). In general, 20% bloom is when 14 to 16 flowers are open and 30% bloom is when at least 20 flowers are open. It is important to note, once symptoms of white mold are apparent fungicides will not be effective/beneficial to those plants. Fungicides need to be applied prior to and during infection stages.
Image 7. Stages of reproductive growth to assist with targeted applications, from Kansas State University. See the full illustration including full vegetative and reproductive development stages here: https://bookstore.ksre.ksu.edu/pubs/canola-growth-and-development_MF3236.pdf.
In a 2024 field trial in North Dakota, fungicide treatments were found to significantly reduce white mold incidence and severity compared to the non-treated check (Table 1). Proline showed the strongest control (0% incidence and 0% average disease severity), followed by Miravis Neo and Propulse when applications were made at ~20% bloom (Chapara et al., 2024). A full list of fungicides labeled for use in canola to manage sclerotinia stem rot in Tennessee can be viewed on Table 2.
Table 1. Efficacy of commercially available fungicides in managing white mold and their influence on yield and test weight (Chapara et al. 2024).
Note: *Performance may differ under Tennessee winter canola conditions. **Topsin is not labeled for use in winter canola.
Table 2. Fungicides labeled for use in canola to manage sclerotinia stem rot in Tennessee (from agrian.com label center).
Rotation
While the fungus can survive up to ten years in the soil, rotating crops with non-susceptible hosts can delay the buildup of S. sclerotiorum inoculum in the field. Non-hosts to rotate with include grass crops (such as corn, wheat, and sorghum) and cotton. While soybean is a host, due to the warmer temperatures, white mold in Tennessee soybean production is not an issue.
Cultural practices
Using recommended seeding rates and fertilizer applications and wider row spacing can help farmers avoid overly dense canopies, that would create more ideal conditions for white mold. Field equipment should also be washed after being in infected fields to prevent contamination to other fields and white mold infested fields should be harvest last, to avoid spreading the disease.
Useful links
Read more about:
Canola Disease Guide – Bayer Crop Science
Great Plains Canola Production Handbook – Kansas State University
Three types of canola grown in the United States – corn-states.com
Harvest Management of Canola – Kansas State University
National Winter Canola Variety Trial results – Kansas State University
Sources used for this article can be found below:
Andreason, S., Damicone, J., & Lofton, J. (2016). Sclerotinia Stem Rot of Canola (EPP-7095). Oklahoma State University Extension.
Website: https://extension.okstate.edu/fact-sheets/print-publications/epp-entomology-and-plant-pathologhy/sclerotinia-stem-rot-of-canola-epp-7095.pdf
Bradley, Carl A. (2025, April 1). Consider a Foliar Fungicide Application on Canola for Management of White Mold. Kentucky Pest News.
Website: https://kentuckypestnews.wordpress.com/2025/04/01/consider-a-foliar-fungicide-application-on-canola-for-management-of-white-mold/
Canola Council of Canada. (2020, July 2). Sclerotinia fungicide: Timing and performance tips.
Website: https://www.canolacouncil.org/canola-watch/2020/07/02/fungicide-for-sclerotinia-timing-and-performance-tips/
Chapara, V., Arens, A., & Jennings, L. (2024). Evaluation of Fungicides to Manage White Mold in Canola. North Dakota State University.
Website: https://www.ndsu.edu/agriculture/ag-hub/publications/2024-evaluation-fungicides-manage-white-mold-canola
de Koff, J. P., & Robbins, C. (2016). Growing Winter Canola for Biodiesel Production. Tennessee State University Extension.
Website: https://digitalscholarship.tnstate.edu/cgi/viewcontent.cgi?article=1053&context=extension
Paulitz, T., Schroeder, K. L., & Beard, T. L. (2015). Sclerotinia Stem Rot or White Mold of Canola (FS188E). Washington State University Extension.
Website: https://wpcdn.web.wsu.edu/wp-ecommerce/uploads/sites/2/product-4128-sku-FS188E.pdf
