White Mold in Soybeans Sporecaster Forecasts and Scouting

Paul Esker & Alyssa Collins, PH.D PennState Extension

Sporecaster app validation to improve PA white mold risk prediction.

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)


Figure 2. Sclerotinia sclerotiorum sclerotia found in the soil at the base of the soybean plants. Sclerotia germinate and produce apothecia (small mushroom-like structures) at soybean flowering, which release ascospores that infect soybean flowers. (Photo: Tyler McFeaters, Penn State

 

 

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.

Background of the Sporecaster App and validation project:

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.

Validation of the Sporecaster 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?

2020 Risk information for the period, July 1 to July 7, 2020

Parameters we use to run the model include:

  • 30-inch (this assumes that the 40% canopy closure requirement is met) or 15-inch row spacing
  • Flowers Present
  • Non-Irrigated
  • 30% Action Threshold

Table 1. Sporecaster predictions for 2020 in various locations throughout PA. This week’s period is from July 1 to July 7, 2020.

County Row Spacing (in) Risk on 7/1 (%) Risk on 7/7(%) Spray recommendation Week trend 30-Day Avg. Temp(°F) Avg. Wind Speed (mph)
Centre 30 74 73 High Constant 80 7
Centre 15 77 69 High Decreasing 80 7
Erie 30 71 65 High Decreasing 77 9
Erie 15 71 71 High Constant 77 9
Indiana 30 79 73 High Decreasing 78 8
Indiana 15 79 77 High Decreasing 78 8
Lancaster 30 30 19 High/Low Decreasing 83 10
Lancaster 15 30 23 High/Medium Decreasing 83 10
Lebanon 30 54 44 High Decreasing 82 8
Lebanon 15 54 55 High Constant 82 8
Lycoming 30 58 47 High Decreasing 84 7
Lycoming 15 59 54 High Decreasing 84 7
Northampton 30 52 50 High Decreasing 82 8
Northampton 15 52 45 High Decreasing 82 8
Somerset 30 73 73 High Constant 76 10
Somerset 15 73 73 High Constant 76 10

Interpretation:

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.

Continued validation and white mold scouting during the summer:

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.

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