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White Mold: Summary of Risk Factors

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This page was adapted from the article, "White Mold: Summary of Risk Factors," which appeared in Crop & Pest Report on July 28, 2022.

In some areas of the state the risk for white mold has increased. Depending on growth stage(s) of the crop(s), proactive management of white mold could be considered if your risk is high. Below is a summary of ten factors you may consider.  

Growth Stages:

  • Broadleaf plants become susceptible to white mold only once they begin blooming (sunflowers are an exception).  This is because the pathogen needs to utilize the flowers as a food source to cause infection.
  • In general, the optimal time to manage white mold with a fungicide application will be somewhere in the early bloom stages of the crop. Applications made in later growth stages miss the critical window to prevent economically important infections.

Weather and Microclimate:

  • Soil moisture. Soils must have some water to begin the disease cycle (when sclerotia germinate 🡪 produce apothecia 🡪 produce ascospores). Historically, only 1-2 inches of rain falling in a 1-2 week period before plants enter bloom is considered the minimum needed for sclerotia to germinate, produce apothecia, and release ascospores.
  • Temperatures during bloom.  Sclerotinia infection and development is optimal when daytime highs are cooler; especially the 60’s and 70’s. White mold can occur at higher temperatures, but it requires longer periods of leaf wetness. 
  • Canopy wetness during bloom.  White mold is favored if the plants are wet for long periods of time. Rain, fog, and heavy dews during bloom are all favorable for disease.  
  • Canopy density and canopy closure. These factors make a BIG difference on the environment in the field, and, we learned recently also impact spray timing and droplet size (see article below).  Once canopy closure occurs, the crop is likely to have a more favorable microclimate for infection and disease development.

Crop and History:

  • Field history.  Fields that have a history of white mold are often more likely to experience epidemics.  Sclerotia (the pathogens survival structure) can survive for many years in a field, so an epidemic a few years earlier may still be influencing this year’s growing season.  Consequently, just because we experienced a drought last year doesn’t mean we won’t have disease.
  • Crop rotation.  A field with a short rotation among susceptible broadleaf crops is more likely to have white mold problems than a field with no white mold history and/or long crop rotations.
  • Crop.  All broadleaf crops can get white mold, but we tend to see sunflowers and dry edible beans be among the most sensitive and flax among the least.
  • Genetics. While most broadleaf crops do not have ‘resistant’ varieties, some varieties will be less sensitive than others. In soybeans, the variability in susceptibility is very pronounced, and soybeans with longer maturity groups are generally more susceptible.