Understanding White Mold Disease in Soybeans

Soybean growers in the upper Midwest need to keep watch against white mold outbreaks caused by Sclerotinia sclerotiorum. The disease thrives in cool, wet conditions during the early reproductive stages when flower petals are abundant, and is most often severe in varieties chosen for a high-yield environment – high plant populations with narrow row spacing and a dense, early-closing canopy.

White mold causes yield loss to soybeans by reducing seed number and weight, and can also impact seed quality. Infected fields can have reduced germination. The contamination adds up to problems for many growing seasons to come.

White mold on soybean
Soybean pods infected with Sclerotinia sclerotiorum, otherwise known as white mold. Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org

Signs and Symptoms

Typically, white mold first becomes apparent in July and August, even though most initial infections occur earlier. Single plants within a generally healthy canopy wilt and die rapidly. Leaves remain on the stem but turn brown, and soybean plants may produce no seed. Watch for water-soaked stem lesions that rapidly progress above and below infected nodes, eventually encircling the stem. Over time, infected stems become bleached and stringy. Lesions also can occur on stems, pods and sometimes leaves, resulting in wilting and plant death.

White mold often occurs in patches in a field, and signs of infection may include a white, moldy growth and sclerotia on infected tissues. These fungal growths may be produced inside or outside of stems and pods.

Integrated Management of White Mold

Growers should take precautions, especially those who have a history of white mold in their soybean fields. It’s important to assess the risk as soybeans approach flowering growth stage R1, when plants have at least one open flower at any node, through R3. A dense canopy during flowering may be an ideal environment for white mold.

The best protection against yield loss is understanding how different environmental variables and management practices influence white mold infection.

When white mold infections occur, the following three conditions exist at the same time:

  1. Flowering of susceptible soybean varieties
  2. Shaded, moist and cool soils (40 to 60 degrees Fahrenheit) within the top two inches of the soil profile. This allows sclerotia – which survive in the soil as hard, black structures resembling mouse droppings – to germinate, producing apothecia. This initial phase of the fungus appears as small, tan, cup-shaped mushrooms, which produce millions of ascospores that infect soybean plants through senescing flowers
  3. A cool, wet environment, especially under the soybean canopy

An Integrated Disease Management Approach

Core recommendations for an integrated management approach to white mold in soybeans include:

  • Maintain field records of disease incidence to manage accordingly.
  • Select soybean seed varieties that are pathogen-free and offer the best resistance available.
  • Select the most appropriate maturity group for your growing region.
  • Scout diligently for disease symptoms.

Other cultural practices to help manage white mold include:

  • Reduce plant populations and increase row width.
  • Consider rotating tillage practices.
  • Use fungicides in fields at high risk. Early application at R1 growth stage is most effective.
  • Where irrigation is used, reduce frequency during flowering.

Using Fungicide to Control Disease

Primary treatments for white mold need to be made before the canopy closes, because after canopy closure, it is very difficult to get the active ingredient down into the target zone.

Early fungicide applications on soybeans at the R1 growth state (beginning bloom) provide a greater degree of control than applications occurring at the R3 growth stage (beginning pod set). Fungicide efficacy for white mold declines significantly after symptoms become visible on plants.

Take care to achieve adequate plant coverage deep into the soybean canopy, where infections start. This will increase foliar fungicide effectiveness. Flat fan spray nozzles that produce fine to medium droplets work best. Follow manufacturers’ recommendations for spray volume and pay attention to environmental conditions such as wind speed, which influence spray coverage. Increase spray volume to improve fungicide coverage for soybean fields with a thick canopy.

Bayer recommends to apply 3 fl oz/A of Proline® fungicide at R1 and – if conditions persist – to follow up with a spray of 4 fl oz/A Stratego® YLD at R3.

Dealing with Disease Even After Harvesting

Because white mold occurs under favorable environmental conditions, the sclerotia can remain in the soil for several years, losing viability slowly. The most effective defense against white mold is to keep the fungus out of a field, although this is easier said than done.

Because sclerotia are similar in size and density to soybean seed, they can easily wind up in the grain bin along with infected seed during harvest. Some sclerotia are ejected from the combine in stem tissue, which can further distribute the fungus over the field. Following the harvest of an infested site, the combine can also transport sclerotia into previously uninfected fields.

Avoid harvest of disease-infested fields prior to harvesting healthy fields. If a field with white mold must be harvested first, clean the combine before it is moved to fields that lack a history of the disease. When white mold is present only in a specific area of the field, that area should be harvested last.

For more information about soybean disease control options from Bayer, contact your local Bayer Crop Science US representative or visit the corn section

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