Take Action Now: White Mold Threat in Soybeans

By Dylan Mangel

As we approach the critical period for managing white mold in soybeans, it's essential to make informed decisions to mitigate potential losses. Sclerotinia stem rot, or white mold, significantly impacted yields in 2023, and proactive management is crucial for this season. The key factors to keep in mind are:

1. Flowering Susceptibility: Soybean plants are most vulnerable to white mold during the flowering stage.
2. Protective Measures: Ensuring flowers are protected is essential for preventing infection and controlling the disease. Late management will not save infected plants.
3. Environmental Conditions: Wet, humid environments facilitate the spread of white mold.

For detailed management strategies, including genetic resistance, cultural practices, and chemical/biological control, please refer to the sections below.

Pathogen Overview

Sclerotinia stem rot, commonly known as white mold, is a fungal disease caused by Sclerotinia sclerotiorum. The fungus survives overwinter in soil as hard, dark structures called sclerotia. In spring, saturated soils and a full canopy favor the emergence of apothecia — mushroom-like structures that release millions of airborne spores during flowering. These spores infect soybeans primarily through senescing blossoms and injured tissue, requiring free moisture for infection. The disease typically prevents successful pod development and produces more sclerotia on and in the stems, which are scattered back onto the soil during harvest and can survive for several years.

Disease Symptoms

Symptoms appear during pod development, starting with wilting and discoloration of leaves. Infected stems and pods show white mycelium and sclerotia, with water-soaked lesions initiating near colonized flowers. Stems become tan, bleached, pithy, and shred easily. Symptoms typically occur six to 14 inches above the soil but can also affect basal parts.

Favorable Environmental Conditions

White mold develops from flowering to pod formation under wet, humid conditions with moderate temperatures (<85°F). It is more prevalent in northern counties and irrigated fields with consistent moisture and cooler canopy temperatures.

Management Strategies

Genetic Resistance: Choose varieties rated against white mold, avoiding highly susceptible ones, and select shorter varieties that do not lodge.

Cultural Practices: Use wider row spacing (30-inch over 15-inch) to delay canopy closure and reduce humidity. Avoid irrigation during flowering. Rotate with non-host crops like corn and wheat, and maintain good weed control, as some weeds and cover crops can host the fungus.

Chemical/Biological Control: Apply foliar fungicides during the susceptibility window at flowering if there is a field history of white mold and favorable weather conditions for disease development. Late applications made after the infection window are not effective. Refer to the Crop Protection Network and the UNL 2024 Guide for Weed, Disease and Insect Management in Nebraska for fungicide efficacy test results to find products that are effective on the pathogen.