Soybean planting is inching along. The April 25 Crop Progress report indicates only 3% of the nation’s soybean crop is in the ground. Tennessee, North Carolina, Nebraska, Missouri, Mississippi, Louisiana, Kentucky, Kansas, Iowa, Illinois and Arkansas have started, but we likely won’t see much movement in the Midwest until corn planting nears completion. However, we are seeing more growers get soybeans in the ground earlier each year.
“We still have some time before common soybean diseases appear,” says Tom Larson, Stine corn technical agronomist. “But as more growers put soybeans in the ground earlier each season, we run the risk of soybean diseases such as sudden death syndrome (SDS) and white mold making an early appearance as they thrive in cooler, wet climates.”
The big four
White mold and SDS are two of the most problematic soybean diseases growers face each year. They’re prolific and overwinter in the soil, so it’s nearly impossible to get rid of the pathogens once they appear in your fields.
“The soybean diseases we see a lot of as agronomists are white mold, SDS and Phytophthora and charcoal rots,” says Todd Schomburg, Stine director of agronomy. “These diseases typically come back year after year, so it’s important to know what to look for, when to look for it and how to manage the problem throughout the growing season.”
White mold, also known as Sclerotinia stem rot, thrives in cool, humid, wet conditions. The disease can be somewhat easy to detect as it has a cotton-like appearance. Look for white, raised spots on plant stems. Symptoms typically appear between the R3 and R6 growth stages. White mold can be detrimental to yields. For growers who experience high white mold pressure every year, there are soybean varieties that offer some resistance to the disease. Crop rotation to small grains and corn can also help mitigate the future spread of white mold.
“White mold can cause significant yield loss,” says Larson. “In some instances, growers may post up to a 10% bushel per acre yield loss. And that number can be a lot higher depending on how early it affects the soybeans. We’ve seen it devastate fields in my region in Iowa.”
SDS prefers cool, wet soils. Symptoms typically appear between R1 through R6 growth stages and can start as random yellow spots between leaf veins. More severe cases evolve to include soft, rotten roots, stem discoloration, complete necrosis of the leaf tissue between the veins, yellowing of leaves and leaf drop. Growers can manage SDS by using tolerant varieties and seed treatments (nematicides). Crop rotation, tillage, planting known SDS fields later and avoiding compaction can also help control the disease, as well as planting into warmer soils. Soybean cyst nematode issues can also exacerbate the disease.
“SDS is one of the major yield-robbing diseases for soybean growers,” says Schomburg. “In fact, experts from Iowa State University Extension and Outreach conducted a study and found that, in areas with severe infestation, SDS can cause up to a 40% yield loss.”
Phytophthora leaf and stem rot is another common soybean disease that occurs in cool, wet conditions. Symptoms appear between VE and R6 growth stages and can include yellowing of leaves, soft/rotting stems and roots and wilted plants. According to Crop Protection Network, “The most characteristic symptom of Phytophthora root rot is a dark brown lesion on the lower stem that extends up from the taproot of the plant.” Tillage, seed treatments and planting resistant varieties are recommended to manage the disease. Some varieties are more susceptible to Phytophthora, so if you have a field history of the disease, it’s important to consult your local Stine agronomist to walk you through the right variety options.
Unlike the other common diseases in our big four list, Charcoal rot thrives in warm, dry conditions. Early symptoms include red to brown lesions on the roots and lower stems. Later on, black specks appear in the same area. When conditions worsen, the disease reaches the canopy and can lead to yellowing of leaves and leaf drop. The best time to scout for charcoal rot is from R5 through R7. A good fertility program mixed with moderate planting populations can help keep mitigate the disease. Selecting the right variety, crop rotation and residue management can also aid in the process.
Other not-to-be-disregarded soybean diseases
Like the corn diseases we discussed last week, there are a number of soybean diseases that, while less common, can’t be overlooked.
Soybean rust is environmental and occurs in hot, wet conditions. While not as prevalent in soybean acres, when conditions encourage growth, it can pose a significant yield risk. According to the University of Minnesota Extension, “Under favorable conditions, the pathogen can cause yield losses greater than 50%.”
Scouting for soybean rust should take place regularly throughout R1 and R6 growth stages. Early symptoms include tan to reddish-brown colored leaf spots on the lower canopy that can blister to release spore masses, which can spread quickly. Fungicide applications are necessary to control the disease.
Frogeye leaf spot — a soil-borne disease — can also lead to significant yield loss. This disease thrives in hot, wet conditions and should be something growers scout for regularly after frequent rains. The best time to scout for frogeye leaf spot is between the R3 and R6 growth stages. Symptoms can include red to red-brown spots on leaves with purple halos surrounding the spots. These spots are often irregular in shape and size. To manage the disease in-season, fungicides can be applied. As the disease survives in crop residue, tillage is recommended.
Brown stem rot occurs in cool, wet conditions and is residue related. Scouting should take place between the R4 and R6 growth stages. Initial symptoms can include browning of the stems and leaf tissue between the veins. Sometimes the stems can look healthy but may be rotting on the inside. After the disease spreads, lodging can occur, which leads to harvest complications.
“Brown stem rot is more prominent in susceptible soybean varieties, so it’s important to work with your Stine agronomist on your best seed options,” says Schomburg. “Crop rotation can also be effective in treating the disease.”
Other soybean diseases such as sunscald, bacterial blight, bacterial pustule, cercospora, downy mildew, stem canker and target spot might also be problematic this year. Learn more about those diseases and tips for scouting here.
For more information on common soybean diseases and how to keep them at bay now and in the future, reach out to your local Stine agronomist or sales rep. University extension services also offer valuable advice on detection and mitigation strategies for these diseases.