Soybeans face threat from frogeye

By Andi Anderson

Frogeye leaf spot, once primarily a Southern soybean disease, has spread across the U.S. over the past 30 years, reaching as far north as North Dakota. This disease can cause up to 35% yield loss under certain conditions, making it a significant concern for soybean farmers nationwide.

In 2015, Iowa State reported that frogeye leaf spot resulted in the loss of 17.5 million bushels of soybeans across the country. Its growing resistance to popular fungicides compounds the problem.

Alex Schaffer, a research agronomist for the Iowa Soybean Association, emphasized the economic impact of the disease and noted that resistance outbreaks began in the South and have spread across soybean-producing states.

The fungus Cercospora sojina causes frogeye leaf spot. While most seed treatments can kill seed-borne contamination, the fungus can survive on soybean residue and lead to infections following rainfall.

According to Marty Chilvers, a plant pathology professor at Michigan State University, the disease can cycle multiple times in a season, particularly under hot and humid conditions in July and August when soybeans are flowering and developing pods.

Farmers need to be vigilant in scouting for frogeye leaf spot, starting just before flowering around the R1 stage. Lesions appear as irregular gray spots with reddish-brown to purple borders.

Being aware of the soybean variety planted and tracking weather conditions can help guide the intensity of scouting. Fields in continuous soybean production or low-lying areas should be prioritized.

Effective fungicide application is crucial for managing frogeye leaf spot. The optimal time for application is around the R3 stage, which is critical for pod and seed development. However, due to resistance issues, farmers should use a fungicide blend containing at least two effective chemistry classes.

Triazoles and some SDHI fungicides have shown efficacy, but overuse can lead to resistance. Resources like the Crop Protection Network’s fungicide efficacy guide can help farmers make informed decisions.

In areas where frogeye leaf spot has been a problem, seed selection is crucial for managing risk. Using quality, certified seed and considering disease resistance among soybean varieties can help mitigate the impact. In severe cases, crop rotation and tillage may be necessary to reduce soil-resident pathogens.

Regular field visits and careful management practices are essential for protecting soybean yields from frogeye leaf spot. By staying informed and proactive, farmers can better manage this challenging disease and ensure healthier crops.

Photo Credit: michigan-state-university-msu