Late Season Soybean Disease Scouting Guides Spray Decisions

By Andi Anderson

Late-season soybean disease management often raises questions about whether fungicides should be applied. According to Dr. Horacio Lopez-Nicora, OSU Extension Soybean Pathologist and Nematologist, the decision depends on the plant’s growth stage, the type of disease present, and whether the disease is likely to impact yields.

“Typically, what is observed in late summer is the above-ground foliar symptoms of the disease. Those symptoms do not always translate to yield losses,” Lopez-Nicora explained. Septoria brown spot is a common foliar disease seen each year, but in most cases, it causes little yield reduction unless a highly susceptible variety is planted.

Some diseases may appear during scouting but have minimal effect. Downy mildew, for example, is caused by an organism related to Oomycetes, not a true fungus. “Spraying fungicides solely to control downy mildew will not be helpful, and we may be spending money for no reason,” Lopez-Nicora cautioned.

Other diseases can become serious depending on environmental conditions. Frogeye leaf spot and cercospora leaf blight are closely monitored each season, though they may or may not appear. White mold is another concern. Infection occurs during the flowering stage (R1 to R2), though symptoms usually appear later in the season between R3 and R5.

Lopez-Nicora emphasized that preventive measures are critical. “Once growers start seeing white mold in the field, there is nothing they can do,” he noted. Preventive fungicide applications during flowering may help, but there are no curative treatments once symptoms appear.

The key to effective disease management lies in the “disease triangle”: a susceptible host, a present pathogen, and favorable environmental conditions. Scouting and correct pathogen identification help growers make informed decisions, ensuring fungicide use is effective and cost-efficient.

By understanding disease lifecycles and growth stages, farmers can avoid unnecessary expenses while protecting yield potential through timely, preventive actions.

Photo Credit: gettyimages-zoran-zeremski