PHOTO PROVIDED This picture of a section of a soybean crop shows white mold growing on the stems of the plants.

During the last week of August, the Penn State Extension Agronomy Team conducted the Crop Conditions Tour across Pennsylvania to assess soybean and corn current conditions and potential yields. In Centre County, we noted that the severity of a disease known as white mold or Sclerotinia stem rot of soybean was particularly high. Therefore, let’s explore this disease in more detail.

Initially, white mold will appear sporadic in a field, with small numbers of plants infected. However, in a matter of a few years, inoculum builds up in the soil, and the condition may become a yield-limiting problem for soybean production. We will provide details about the disease to help you identify and understand the factors promoting its development.

What causes

the disease?

Sclerotinia stem rot, or white mold, is caused by the fungus Sclerotinia sclerotiorum. The pathogen survives many years in the soil in the form of hard fungal masses called sclerotia, which resemble black- small rocks or rat droppings with an off-white center. 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.

What are

the symptoms?

Symptoms on infected plants include a light green to gray coloration of the leaves and wilting. Initial symptoms can be observed from beginning pod (R3) to full seed (R6) (1). Infection of the stem also leads to the formation of white to gray lesions around the point of entry of the fungus, usually at the nodes. Lesions on the stem and other plant tissues become covered with a white/cottony fungal growth and newly formed sclerotia that can also be observed within the stem and pods. Expanding lesions girdle the stems, leading to premature death of the plants, seed abortion, and reduced seed size. During harvest, sclerotia will be harvested along with seeds and fall to the ground, remaining until the next growing season.

What factors drive disease development?

The factors required for disease to occur are a susceptible host, the presence of the pathogen (fungus), and an environment conducive to disease. Let’s review each of these factors:

— Susceptible host: No soybean varieties are completely resistant to white mold; however, commercially available varieties vary in their tolerance to the disease. Tolerant varieties should be planted in fields with a history of white mold.

— The pathogen: Sclerotinia sclerotiorum can be introduced into a field as sclerotia when using contaminated seed lots, and sclerotia in soil and infected plant tissues attached to tractor tires and ag implements. Long-distance movement of spores in air currents has the potential to introduce Sclerotinia into new fields (2). One factor that makes disease management more challenging is the longevity of the sclerotia. Based on farmers’ experience, sclerotia can survive in the soil for 4 to 5 years (3). It has been observed that a two-to-three year rotation to nonhost crops (e.g., corn and small grains) can aim at reducing the viability of the sclerotia in the field.

— A conducive environment: The disease is favored by high humidity and cool temperatures (59-68 F). Dense canopies, high plat population, and narrow row spacing create a microenvironment that favors apothecia formation and spore release. When the risk of white mold is high, farmers should consider using the lowest recommended plant population to attain maximum yields and include early maturing soybean varieties. Studies have shown that sclerotia absorb water before forming the apothecia, and a critical moisture content (MC) must be achieved to initiate the process (4). Sclerotia can reach the critical MC even when soils are at 25% saturation, which could explain why apothecia forms under soils with low moisture content (4). When conditions are favorable for infection and fields have a history of white mold, applying a fungicide during reproductive stages can help reduce infection, although it does not eliminate it.

From the road driving at 65 miles per hour, it is difficult to tell if your crop has a problem. We recommend scouting your fields to determine if white mold is present and how severe the disease is. Integrating all cultural and agronomic practices is the best way of managing the disease.

If you have questions about the contents of this article, please get in touch with Adriana Murillo-Williams (axm1119@psu.edu), and Paul Esker (pde6@psu.edu).