FROGEYE LEAF SPOT THREATENS SOYBEAN YIELDS

Be prepared to manage disease this summer.

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Once thought to be a Southern soybean disease, frogeye leaf spot has marched across the U.S. over the past 30 years. Now seen reaching as far north as North Dakota, the disease can cause up to 35% yield loss in certain circumstances.

In 2015, Iowa State reported 17.5 million bushels of yield lost nationwide to frogeye leaf spot. Its growing resistance to popular fungicide options makes the disease even more concerning.

“It’s one of the most economically significant diseases of soybeans across the nation,” says Alex Schaffer, a research agronomist for the Iowa Soybean Association. “Resistance outbreaks to fungicide started in the South and have moved across the soybean-producing states.”

Disease life cycle and optimal conditions

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

“Once we get an infection on the leaf, more spores will be produced from that spot,” says Marty Chilvers, a plant pathology professor at Michigan State University. “That can lead to development of the disease, and it will potentially go through multiple cycles in a season.”

Severity depends highly on weather conditions. Heat and humidity, especially in July and August, as soybeans flower and pods develop, can greatly exacerbate the disease.

Extended wetness on leaves opens the door to infection. Dew-prone fields, such as those in valleys or protected by trees, are particularly susceptible.

Infections start as lesions on the soybean canopy. In severe cases, spots can also be found on stems and pods.