Tip •  2024-05-23

Five things farmers need to know about white mould

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White mould on soybean stem

Farmers in Eastern Canada are all too familiar with sclerotinia stem rot, or white mould, especially after the high rates of soybean infection during the 2023 growing season.

While the extent of yield loss is largely dependent on the inoculum "load" of a field and the right weather occurring during soybean flowering, white mould can be one of the most devastating soybean diseases farmers face.

Here are five things every soybean farmer needs to know about white mould.

1.    What is white mould?

White mould is caused by the fungus Sclerotinia sclerotiorum, a fungal disease that can cause extensive crop damage and yield loss in soybeans.

The pathogen can survive for many years in the soil in the form of hard fungal masses called sclerotia (dark, irregularly shaped white mould survival structures). The disease is often recognized by fluffy, white growth on soybean stems, but these disease symptoms are often identified in the later stages of disease infection.

White mould is a serious yield robbing disease that can live in the soil, making effective management essential during the current growing season and for the success of future soybean crops.

1.    What are the symptoms?

During the growing season, when cool and humid conditions prevail during flowering and canopy closure, the sclerotia will produce small cup-like structures called apothecia. These structures will release thousands of spores that will infect the stem through decaying flowers.

Sclerotia live in the soil, and within these structures, the disease can survive in soil for up to 10 years. Only sclerotia within two inches of the soil surface can germinate.

Spores are produced under the cap of apothecia. Each apothecium can release more than 10 million microscopic spores over several days, which are carried by the wind to surrounding plants. After release, spores survive for only a few days.

Spores land on senescing soybean blossoms and are able to germinate within soybean plants during a period of prolonged leaf wetness (16 - 48 hours).

Once fungal spores germinate on petals, the infection is able to spread throughout the plant to pods, nodes and stems.

2.    What causes infection?

Humid conditions, during flowering (R1), and especially in lower parts of the plant canopy are ideal for infection, allowing spores to attack the flower petals. Infected flowers fall into the canopy where the disease proceeds to choke the plant of vital water and nutrients.

Infection typically occurs in July across Ontario and Quebec, during the four-week soybean flowering period. Infection signs are not immediately visible. The obvious signs may be apparent within three to four weeks of infection, but generally go unnoticed for another week or more.

Diligent scouting is important, especially if weather conditions favour infection or fields have a history of white mould.

3.    How do I manage infection?

The first step to manage white mould is identifying the disease early. Proactive management strategies, like making a white mould disease management plan and having fungicides on hand can help farmers minimize the spread and reduce the risk of severely spreading the disease in subsequent years. 

When scouting, check stems of potentially infected plants to diagnose white mould. The first signs are gray to white lesions at nodes. Lesions rapidly spread above and below infected nodes and are often covered in fluffy, white growths.

Effective management of white mould requires an integrated cultural, chemical and mechanical approach across several stages of crop development.

Recommended cultural practices include variety selection (including disease tolerant varieties), crop rotation, fungicide application and cropping practices that reduce the intensity and duration of a disease-favourable microclimate within the canopy (i.e., row spacing and orientation, nitrogen fertilizers).

To be effective, fungicides must be applied as protectants before infection. Since soybean flower petals are the sites of primary colonization, fungicide application(s) must be directed at flower petals, especially in the lower portions of the crop canopy (application volume can impact the coverage).

Environmental conditions, length of the crop season and the length of time that flower petals are available for colonization by ascospores could determine the number of applications per season.

4.    What is the best treatment option?

Viatude™ fungicide is the most effective white mould disease control solution for Eastern Canadian soybean farmers.

Complete with Onmira™ active, Viatude provides best-in-class sclerotinia and white mould protection in canola and soybeans. Premium performance is delivered through a unique combination of two of the strongest sclerotinia and white mould actives on the market today, delivering multiple modes of action in an all-in-one premixed formulation for ease of use.

Viatude fungicide delivers a new level of in-season white mould protection for soybeans. For more information, contact your Corteva Agriscience Territory Manager.