White Mold of Soybean

Sclerotinia sclerotiorum is a fungal pathogen that can infect many crops, including all broadleaf crops such as soybeans, sunflowers, dry beans and canola. This pathogen produces small fruiting bodies known as apothecia (white mold mushrooms), which disperse spores. These spores are dispersed by wind and infect during the flowering stage of soybean production. The pathogen survives as hardened masses of mycelia, or sclerotia, that resemble small rodent droppings. These sclerotia are incredibly hardy and can survive for multiple years in the soil. (Figure 1). This disease can reduce yield by either killing soybean plants prematurely or reducing seed quality.

Sclerotia can potentially survive in the soil for up to 10 years. During the growing season, sclerotia can germinate to form apothecia if there are prolonged soil moisture conditions and cool soil temperatures (around 55-60 degrees Fahrenheit). The formation of these apothecia is also tightly linked to canopy closure within soybeans. Apothecia appear to be small, tan-colored inverted cups that sit close to the soil surface (Figure 2A). Apothecia are commonly confused with bird’s nest fungi, which are also frequently found in soybean fields, especially when heavy crop residue is present from the previous crop (Figure 2B). These bird’s nest fungi do not cause any disease on soybeans and are decomposers of dead plant material.

Once these apothecia form, they release puffs of ascospores into the under canopy of the soybean crop. However, these spores require the presence of flower tissue to infect soybeans. Because of this, soybeans are only susceptible when flowers are present (R1—end of R3 growth stages), when the canopy has closed and when the environment is conducive.

After the initial infection, white mold does not appear for a few weeks, often beginning around the R5 growth stage. Foliar symptoms can be noticed from a distance due to flagging that progresses into wilting or lodging plants (Figure 3). Upon closer examination, the development of white fungal growth or “white mold” can be seen on stems, and typically infected stem tissue will appear bleached or a light tan color when it should still be green. After the fungus has established itself, new sclerotia will begin forming in or on tissue that has been infected. This could occur either on the outside of stems, on the inside of stems or within soybean pods (Figure 4).

1. Resistant varieties – While no varieties are currently available with complete resistance to white mold, some have moderate levels of resistance. Varieties can also have agronomic traits that decrease their susceptibility to white mold. Soybeans with lower maturities typically are considered less susceptible due to the shorter period in which flower tissues are present. Varieties with narrow canopies will have less risk of developing white mold than bushing canopies due to the potential delayed canopy closure during the flowering period. Lastly, soybean varieties that lodge can also be at increased risk of white mold development due to the retention of moisture within the canopy.
2. Crop row spacings – Planting at wider rows (greater than 15 inches) can delay canopy closure and reduce humidity in the canopy during the peak infection period.
3. Planting rates – Soybeans planted at high seeding rates can lead to greater canopy densities, which reduce airflow through the canopy, leading to retention of moisture. It is recommended to reduce planting rates to around 110,000 seeds/ac in areas that have a history of white mold.
4. Crop rotation – Crop rotations may be beneficial if a non-host crop (corn or small-grains) is planted for at least 3 years. However, due to the longevity of sclerotia, this is not often considered a highly effective management strategy.
5. Tillage – Tillage of fields with a history of white mold may influence the increased degradation of the sclerotia. However, the results of tillage have been inconsistent and are most effective if paired with other management practices.
6. Reduce irrigation – As white mold requires consistently high moisture during the infection period, the recommendation is to reduce irrigation during the flowering periods.
7. Cover crops – The use of cereal rye as a winter cover crop ahead of soybeans has been shown to reduce the development of white mold.
8. Biological control – There are some promising biological control products that have the capacity to reduce the risk of white mold development by degrading the sclerotia present in a field.
9. Fungicide applications – Fungicide applications have been a highly effective management strategy for controlling white mold. There are many commercial products with efficacy against white mold. However, these products must be applied at the appropriate time for optimum control. Research from Michael Wunsch at Carrington REC has determined that the optimum applications should be targeted when soybeans have reached the R2 growth stage or canopy closure, whichever occurs first.

For more information on managing white mold in soybeans, visit the Soybean Disease Diagnostic Series on the NDSU Extension website or contact your local Extension agent.