White Mold in Soybeans Sporecaster Forecasts and Scouting

 

White mold is a soybean disease that occurs when spores from the fungus Sclerotinia sclerotiorum infect susceptible flowers, leading to growth throughout the soybean plant (Figure 1). Signs of white mold include white, cottony mycelial growth on the stem and black overwintering structures (Figure 2), called sclerotia, form inside the stem and pods. Plants may also display brown wilting symptoms. However, white mold symptoms will only show up after flowering has ended, and treatment options are no longer available.

 

 Figure 1. White mold wilting symptoms and white fungal growth on the stem. (Photo: Paul Esker, Penn State Extension)

 

 

Preventative fungicide sprays are the only in-season option for white mold control. Early fungicide application during flowering, between R1 and R3, is critical. When determining whether a fungicide application is warranted, it is important to consider weather as a key factor for risk. The risk of white mold infections increases with cool, wet conditions at soybean flowering. A mobile application developed at the University of Wisconsin-Madison, called “Sporecaster,” helps growers predict the risk of infection and if a fungicide application would be warranted.

 

 

The Sporecaster app is based on a model that considers location, weather, and cultural practices to estimate the risk for white mold. The models used by the app make predictions based on the weather factors like daily maximum temperature, wind speed, and daily maximum relative humidity. The app also takes into account row width, canopy closure, and irrigation (if applicable). Growers can set their own action threshold, but typically if risk is above 30-40%, a fungicide spray is recommended. While the app has proven helpful in the Midwest and was validated to be accurate 81% of the time, further validation is being conducted in Northeastern climates like those of Pennsylvania and New York to validate the utility of the app.

 

 

In 2020, we will be monitoring select locations throughout Pennsylvania. We will summarize the risk based on different expected flowering dates, as well as different action thresholds (30% versus 40%). The strength of the Sporecaster app is that we can examine different flowering dates to understand how current weather conditions are influencing the risk model: i.e., is the model trending downward in terms of risk, or is it increasing?

 

 

Parameters we use to run the model include:

 

 

At most locations, the risk remains high for white mold but is starting to trend downwards with the hotter weather. In our monitoring site in Lancaster County, we see that this risk is low to medium (as of 7/7), depending on the row spacing. Dry weather is expected to continue throughout PA, with the 10-day forecast predicting only scattered thunderstorms in most locations. However, white mold development can occur with foggy conditions and mornings that have high dew, as well as cooler temperatures. The 15-inch row spacing is predicted to have slightly higher risk of white mold in most areas.

 

 

To further validate the Sporecaster app, we will scout fields in each of the monitored counties after soybeans reach the R5 growth stage (seed is 1/8 inch long in the pod at one of the four uppermost nodes of the main stem) and symptoms are visible. The disease severity within the field will be compared with the Sporecaster prediction to determine if it was an accurate predictor of white mold incidence.

 

Scouting soybeans for white mold is the first step in protecting future crops. An example scouting protocol includes walking a “W” shape through your field. Make 20 stops throughout the field where you can pull back the canopy and get a closer look for white stems and black sclerotia. Where disease severity is high, in the future, you may consider alternative crop rotations to a non-host, wider row spacing in soybeans, or partially resistant varieties.