INSIGHTS
- White mold is a prominent and potentially devastating soybean disease in certain parts of the growing region.
- Variety selection is the first step in effective white mold management.
White mold is a soybean disease that kills stems from the point of infection up, impacting yield. It is caused by the soilborne fungal pathogen Sclerotinia sclerotiorum, which can survive in soil for years. Because white mold symptoms do not appear until late in the season, it is important to know the factors that encourage growth so the disease can be managed.
- The fungus overwinters as sclerotia
- Sclerotia germinate to produce trumpet-shaped apothecia
- Apothecia contain numerous asci containing ascospores
- Ascospores are forcibly discharged and travel to young susceptible flowers
- Flower infections allow the fungus to enter the stem and kill plant tissues above
- More sclerotia develop (white are young sclerotia and black mature) to allow the fungus to survive over the winter
Figure 1.White Mold Development
The fungus overwinters as thick, walled structures known as sclerotia (A) either in or on the soil or in infected plant tissue (F). Sclerotia that are within the top 5 centimeters of the soil surface can germinate to produce trumpet-shaped apothecia (B), or the fruiting bodies that contain asci and ascospores (Figure 1).1
Asci are filled with ascospores (C), which are forcibly released into the air. Some airborne spores land on susceptible soybean flowers, germinate and infect the plant (D). Flower infections extend into the stem and kill the tissues above the infections (E). Typical symptoms of white mold are flagging or dead plant tops. The fungus will grow on and/or in the plant and develop more sclerotia for survival over the winter (F).
White Mold Identification
White mold first appears on soybean stems as lesions, gray to white in color, at the nodes. It then develops into fluffy or cottony, white growth on the stems and eventually dark black sclerotia along the stem or bean pods (Figure 2).
Figure 2. Cottony, white growth on soybean from white mold.
Figure 3. Areas with environments conducive to white mold (yellow, green, blue) due to frequency of cool, wet weather and corresponding trial locationsGolden Harvest White Mold Screening
- Seven evaluation sites are annually used to evaluate pre-commercial varieties for white mold tolerance.
- Research sites are strategically positioned in areas with conducive environmental conditions.
- Trials are inoculated with Sclerotinia sclerotiorum and irrigated throughout flowering to intensify disease occurrence.
- All Golden Harvest® seed varieties are well characterized for white mold tolerance. Talk to local seed advisors for more advice on locally adapted varieties
Foliar symptoms (yellow or brown leaves) appear later after the fungus has progressed enough to kill the plant. As soybeans become dry or die, the stems will seem bleached, or light in color.
Favorable Conditions for White Mold Development
The following conditions are favorable for white mold growth and development in soybeans:
- Rain during soybean bloom, along with cool temperatures (less than 86°F)
- High relative humidity and moist soil
- Prolonged periods of low soil temperatures (41-59°F)
- Moderate air temperatures and frequent rain just prior to flowering
- To help determine if conditions are favorable for development, consider downloading the University of Wisconsin Sporecaster app: ipcm.wisc.edu/apps/sporecaster/.
White Mold and the Disease Triangle
Best Practices for White Mold Control
Variety Selection:
- No varieties offer complete resistance, but select Golden Harvest varieties have high levels of tolerance and can be effective for managing white mold.
Cultural practices:
- Crop rotation: A minimum of 2 to 3 years of a non-host crop, such as corn or small grains, can reduce sclerotia in the soil.
- Tillage: Inconclusive
- Canopy management: Early planting, narrow rows, high plant populations and high soil fertility all accelerate canopy closure and conditions that favor disease development.
- Irrigation: Avoid excessive irrigation until after flowering.
Chemical control:
- Weed control: Many common broadleaf weeds, such as henbit, velvetleaf and common lambsquarters are also hosts of S. sclerotiorum,1 making weed control an equally important component of disease control.
- Fungicides: Some can help suppress white mold with proper application timing.
Manage white mold with a fungicide when disease is present and conditions are favorable for disease development. Apply Miravis® Neo fungicide at early bloom (R1) to full bloom (R2). If favorable conditions for white mold development continue, apply a second application of Miravis Neo 10 to 14 days after first application. Adjust the rate based on severity of the disease pressure and conditions. An adjuvant may be added at recommended rates. To obtain thorough coverage, apply in sufficient volume.
Effective white mold management begins with variety selection and detailed record keeping of fields for future planning. Other management considerations help keep the disease levels lower to protect yield potential.