Did We Underestimate White Mold in 2023?

During the recent crop tours and other farm visits, we have noted an increase in the severity of white mold of soybean. White mold is a common disease of soybean in Pennsylvania, yet frequently this disease is overlooked, as we observed during recent farm visits. As you head into harvest, this is a great time to estimate the potential for yield loss due to this disease.

Why can white mold catch us off-guard?

Initially, white mold will appear sporadic in a field, with small numbers of plants infected. However, inoculum builds up in the soil in a few years, and the condition may become a yield-limiting problem for soybean production. 

What causes the disease?

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 small, black 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 (Figure 1). 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 (Figure 2) 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 (Figure 3). Expanding lesions girdle the stems, leading to premature death of the plants, seed abortion, and reduced seed size (Figure 4). 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 the disease 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 (Figure 5) 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 plant 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 form even in 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. Different methods are used to assess the incidence (how many plants show symptoms of white mold) and severity (the degree to which the symptoms occur on the plant). While in many breeding and screening programs, a 1-9 scale is used, we typically assess white mold severity using a 0-3 scale (5,6), as follows:

We recommend taking 10 random samples (i.e., locations) across the field examining a minimum of 10 plants per random sample. Incidence would be measured as the number of plants out of the 100 that showed white mold symptoms, while the severity can be quantified as a disease severity index (DSI) calculated as:

The DSI ranges from 0 when there are no plants with white mold to 100 when all plants have white mold and are rated as dead.

For example, let’s say you observed the following number of plants in each category:

The DSI would be (0+30+40+30)/300 = 0.33 x 100 = 33. With the DSI value, you can compare fields and soybean varieties to determine the relative performance and microclimate differences observed during the growing season. Keeping records of this information can help you decide which fields are at highest risk for white mold in the future and what management choices you would like to make for them, including crop rotation, plant spacing, variety selection, and suppressive fungicide applications.

If you have questions about white mold, please get in touch with Adriana Murillo-Williams (axm1119@psu.edu), Paul Esker (pde6@psu.edu), and Alyssa Collins (aac18@psu.edu).

 References:

1. Crop protection network. 2019. White mold of soybean
2. Leyronas C, Morris CE, Choufany M, Soubeyrand S. 2018. Assessing the Aerial Interconnectivity of Distant Reservoirs of Sclerotinia sclerotiorum. Frontiers in Microbiology 9:2257.
3. Adams, PB, Ayers A. 1979. Ecology of Sclerotinia species. Phytopathology 69: 896-899
4. Nepal A., Del Rio-Mendoza LE. 2012. Effect of sclerotial water content on carpogenic germination of Sclerotinia sclerotiorum. Plant Disease 96, 1315-1322.
5. Mueller, DS, Bradley, CA, Grau, CR, Gaska, JM, Kurle, JE, Pedersen, WL. Application of thiophanate-methyl at different host growth stages for management of sclerotinia stem rot in soybean. Crop Protection 23:983-999.
6. Grau, CR. 1988. Sclerotinia stem rot of soybean. pp.56-66. in: TD Wylie, DH Scott (Eds), Soybean Diseases of the North Central Region. St. Paul, MN.