Lurking beneath the soil, in residue, and adrift on the wind currents, a band of disease desperados waits to rob your yields. The prevalence of these diseases varies annually, as many factors influence which may have the biggest impact on yield. Staying informed, scouting fields regularly, and adjusting management strategies accordingly can help you stay ahead of these yield-robbing villains.
Let’s look at the most wanted culprits in soybean fields — along with their rap sheets, and how you can bring them to justice. These pathogens lurk in the soil, thrive in humid canopies, and strike when conditions are just right.
Soybean Cyst Nematode
Crime: Soybean cyst nematode (SCN) is soybeans’ most yield-limiting disease. According to the Crop Protection Network, estimated yield losses exceed 125 million bushels annually. Symptoms, including stunted growth and yellowing, often go unnoticed, allowing SCN to spread.
“The numbers can be high enough to cause yield loss, but if the plant isn’t under stress, it may not look any different than a healthy one,” said Matt Montgomery, agronomy education lead at Beck’s Hybrids. “So that’s one of the reasons why we talk so strongly about doing soil sampling and egg counts for SCN, to really find out whether we have a problem or not.”
SCN is visible during the growing season as small, white or yellow lemon-shaped females on roots, which later turn brown and become egg-filled cysts. Cysts persist in the soil, making long-term management essential.
Accomplices: SCN weakens plants, increasing their susceptibility to sudden death syndrome (SDS).
Last Seen: SCN thrives in lighter soils, where reproduction and movement are easier, and is especially problematic in high-pH soils (above 6.5), where egg levels and yield losses are greater. Environmental stressors, such as drought and excessive moisture, further worsen SCN damage.
How to Stop: To combat SCN, rotate to non-host crops; use SCN-resistant soybean varieties; rotate resistance sources to prevent adaptation; and conduct regular soil/SCN tests. The addition of nematicide seed treatments can be used in combination with host plant resistance. Cover crops can also help suppress populations.
Courtesy of SCN Coalition
White Mold
Alias: Sclerotinia stem rot
Crime: White mold spreads through sclerotia — small, black, rat-dropping-like structures that survive in the soil for years. The first visible signs include wilting, and white, cotton-like fungal growth on stems. White mold can eventually lead to plant death.
Last Seen: White mold thrives in cool, humid environments with dense soybean canopies — conditions common in the Upper Midwest.
How to Stop: Fungicide applications at R1 to R3 (flowering stages) can help, but cultural practices play a significant role.
“The goal is to reduce humidity buildup with wider rows and reduce plant population,” Montgomery said. “This allows air through the canopy to reduce the environment that [the] disease really likes.”
Courtesy Provided
Sudden Death Syndrome
Crime: Sudden death syndrome (SDS) is a soil-borne disease. Foliar symptoms typically appear midseason after flowering, showing interveinal chlorosis and necrosis due to a toxin moving from roots to leaves. Leaflets drop, leaving petioles attached, and roots appear rotted, making plants easy to pull. Blue spore masses may form on the main taproot.
Last Seen: SDS thrives in cool, wet soils and often emerges in early-planted soybeans.
SDS is closely linked to SCN, which weakens roots and creates entry points for the Fusarium pathogen.
“The nematode can also serve as a vector, where the fungus is inside the nematode itself,” Montgomery said. “SCN management is a really important part of SDS management.”
However, SDS can occur independently of SCN, making field scouting and soil sampling critical for understanding disease pressure.
How to Stop: Because the pathogen resides in the soil for many years, management requires long-term strategies.
Farmers can mitigate SDS by choosing resistant varieties, using seed treatments, and improving drainage to reduce soil moisture.
Meredith Operations Corporation
Frogeye Leaf Spot
Crime: Frogeye leaf spot (FLS) can cause significant yield losses, up to 35% with early infection and favorable conditions, according to the Crop Protection Network.
The disease first appears as small, circular lesions on soybean leaves, often surrounded by dark-purple to reddish-brown borders, with tan or gray centers. Lesions coalesce into larger dead patches, reducing the plant’s ability to photosynthesize and fill pods. FLS can also infect stems and pods, further threatening yield potential.
Last Seen: FLS is a widespread soybean disease that thrives in warm, humid conditions, especially in fields with frequent rainfall or overhead irrigation. It spreads via wind-borne spores and infected crop residue.
How to Stop: Management strategies include planting resistant soybean varieties, rotating crops, and applying fungicides. Resistance to strobilurin fungicides has been reported. It’s important to use products that contain active ingredients from different fungicide classes for resistance management.
Courtesy Provided
