Recent rains and canola entering bloom have prompted questions about white mold and fungicide applications on canola. Below we try to answer some questions about white mold on canola.
Why is canola susceptible at bloom?
Unlike many other pathogens, Sclerotinia (the white mold pathogen) needs flower petals to cause infection. The ascospores can easily digest flower petals, and once the process starts, the disease will quickly spread into branches and stems and can result in yield losses and lodging. Recommendations for fungicide applications begin at early bloom (usually 20-50%) because the earliest infections have the longest time to develop. Consequently, they tend to be lower on the plant and cause greater yield loss.
What conditions are favorable for infection?
Essentially, the answer is cool and wet.
Prior to bloom that pathogen needs water in the soil. Wet soil will allow the sclerotia (the hard black overwintering structure) to germinate, form small mushroom (apothecia) and release ascospores. A minimum of 1-2 inches of rain 1-2 weeks before bloom can initiate germination.
During bloom, anything that keeps the canopy wet for prolonged periods of time will facilitate infection. Multiple days of rainfall, very heavy dews, fog, etc… will provide enough water for infection to start. The wild card here is temperature. We rarely see much white mold when daytime highs are 85 F or higher, but the disease can run rampant when high temperatures are in the 60’s and low 70’s F.
What’s the risk now?
Often, it is a mixed bag across the region. Fields that have had more rainfall, a history of disease, and cooler conditions are at greatest risk. The NDSU Sclerotinia stem rot (SSR) Risk Map and Risk Calculator for Canola, operated by NDSU Canola Pathologist Dr. Luis del Rio and funded by the Northern Canola Growers Association, are useful tools that can be used to help producers determine risk. However, please remember that this map is only relevant when canola is in bloom and that the map is only as good as the weather data used to generate it. NDAWN (North Dakota Agriculture Weather Network) is a fantastic resource, and the envy of most other states, but rainfall can be extremely variable even at a field level.
The most recent risk map (6/26/18) indicates that risk is highly variable throughout the state, and that can change quickly. The risk map and risk calculator can be found online at http://www.ag.ndsu.edu/sclerotinia and through the Northern Canola Growers Association website, http://www.northerncanola.com/. The map and calculator are also available through the Canola Doctor application for Android and iOS-based devices. The application is free and can be downloaded from Google Play Store and the iTunes App Store. The risk maps are updated daily between June 16 and July 31.
Fungicides. If growers determine that a fungicide application is warranted, application timing is very important. Targeting the early bloom stages is ideal (commonly 20%). Pictures and descriptions of bloom stages can be found in the NDSU Extension Publication Sclerotinia of Canola PP-1410 http://www.northerncanola.com/wp-content/uploads/2014/05/Sclerotinia-Control-in-Canola.pdf. Our data demonstrates that multiple fungicides can be effective.