Agronomy •  2023-05-10

How to overcome white mould in soybeans

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white mould on soybean stem

Predicting white mould in soybeans isn’t easy. It’s helpful to understand the disease requires three factors to establish: a host plant (soybeans and other susceptible broadleaf weeds), pathogens that release spores to spread infection, and the right environmental conditions (cool moist conditions during flower or near harvest). These three factors are referred to as the disease triangle and can predict the occurrence and severity of field crop diseases, like white mould.

Known as sclerotinia stem rot in canola and white mould in soybeans, both diseases are caused by the fungus Sclerotinia sclerotiorum. Weather and field history are two significant factors that put your soybeans at risk of developing the disease, which shows symptoms late in the season as grey to white lesions on the stems. Unfortunately, if symptoms are identified too late, the disease has already set in, and little can be done to treat soybeans. That’s why a proactive approach to scouting and preventative fungicide treatments are your best option for white mould control.

How to identify

Scouting for white mould is difficult, and if you see symptoms, it’s often too late. Start by assessing the health of your soybean crop and target fields proactively based on those that have the most potential for disease. Any field with previous white mould infection should be considered high risk. Watch weather forecasts for favourable disease conditions, like cool, damp weather or humidity. Dense plant canopies can also be more disease-prone, creating their own cool and moist environments, compared to those that are not as tightly spaced.

Spores released into the air target soybeans at flowering, infecting the flower first, then moving down the plant to transmit the disease throughout the plant. Early signs of infection can be identified by water-soaked lesions at infection sites, followed by cottony white mouldy masses that form on stems.

White mould plant lesions affect plant development, ultimately leading to yield loss. Unfortunately, once lesions are identified, a curative fungicide application isn’t as effective as an earlier preventative application. As the growing season progresses, infected soybean plants are generally killed in patches late in the growing season, and the black bodies (sclerotia) of white mould are sometimes found in the seed at harvest.

Management strategies

Scouting for white mould in June to mid-July is crucial to properly time preventative fungicide applications. Since the disease starts by infecting flower petals, fungicide applications should be timed beginning at the R1 to R1.5 stage. The application decision should be made based on crop risk (previous field infections or infections in close proximity fields) and current or forecasted weather conditions. Heavy dews under a dense canopy can also provide ideal conditions for white mould, so monitor canopies closely when scouting. If in doubt about disease risk or conditions, consult your trusted agronomist, retailer, or local Corteva Agriscience™ Territory Manager for advice. A disease sporecaster app has been developed by the University of Wisconsin, and is available as an additional tool to assess the risk of white mould in your fields. Watch a short video to learn more about this new tool here.

The most effective fungicide available to combat white mould in soybeans is Viatude™, a new multiple effective mode of action solution for resistance management. Viatude contains a unique combination of two highly effective active ingredients, including Onmira™ active, which provides holistic and long-lasting coverage. Acapela™ fungicide is another option, often used as the first application in a two-pass fungicide management strategy for white mould.

Looking for more white mould management strategies? Visit the Corteva Agriscience Agronomy Hub.