Research Highlights

Combatting Frogeye Leaf Spot in North Dakota

By Carol Brown

As the number of soybean acres increases in North Dakota, farmers are experiencing more disease, weed, and pest issues than before. Weeds such as Palmer amaranth and pests including the soybean cyst nematode and soybean aphid are appearing in North Dakota soybean fields. Recently, a new contender has emerged: frogeye leaf spot.

“We’ve never had frogeye leaf spot in North Dakota, so we’ve not paid too close attention to it,” says Samuel Markell, North Dakota State University Extension plant pathologist. “Then in 2020, it was discovered in a field in the southeast corner of the state.”

Acting proactively, Markell approached the North Dakota Soybean Council to inform them of this new issue. He asked for their help in finding out how widespread the disease was and whether the pathogen was resistant to the strobilurin fungicide group, a resistance that other states are finding. With funding support from the Council, he and his team conducted a survey in 10 southeastern North Dakota counties. They scouted five fields within each county and several areas in each field, collecting more than 300 leaf tissue samples.

“We found it nearly everywhere in the southeast. The counties in that area had frogeye leaf spot in nearly every field we searched,” Markell says. “We went about midway up the state and did not find it. It looks like it is only in the southern half of the state so far, which is where the majority of soybeans are grown in North Dakota.”

Additionally, they found that about one-third of the samples were strobilurin resistant, and the resistance was widespread with no localized hot spots. There were percentages in every county where this fungicide alone wouldn’t work, Markell says.

Frogeye leaf spot is a foliar soybean disease common in southern states and moving to the mid-south and Midwest. The pathogen Cercospora sojina causes the disease, which appears as lesions on the soybean leaf. The disease appears in warm, humid weather and is noticeable when the plants begin to bloom. If not treated, the disease can spread to stems, pods, and seeds.

To learn more about this disease, Markell worked with Carl Bradley, an Extension specialist and plant pathologist at the University of Kentucky, whom Markell refers to as a national authority on this pathogen. Bradley’s lab processed the leaf tissue samples from Markell’s survey to confirm the pathogen presence and its fungicide resistance. 

Funding from the North Dakota Soybean Council helped in part to pay for Markell’s lab manager and a research specialist to visit Bradley’s lab. They received a crash course in how to work with frogeye leaf spot in both the lab and the field, so they can react quickly when this disease returns. 

Since the initial findings in 2020, the weather has not cooperated in North Dakota — for frogeye leaf spot anyway. Markell says that in 2021, during the extreme drought, he saw only one lesion. He’s on the lookout for it in 2022, as the climate has been favorable. August will be the key month for frogeye leaf spot, Markell says.

Markell is also conducting a study to see how fungicides manage the disease, but the drought in 2021 made doing the research in the field nearly impossible as, Markell says, they have to have Frogeye leaf spot in order to study it. 

“We also have great data from Carl Bradley’s work in Kentucky and several other states that battle frogeye leaf spot,” Markell comments. “They’ve got the management recommendations down, but the vast majority of the frogeye leaf spot they see is resistant to strobilurin fungicides. Ours are not there yet, so we may have better luck with pre-mixes that include strobilurin.”

Markell offers some tips to manage frogeye leaf spot for North Dakota farmers that are based on what he has learned from other states:

• Begin scouting at soybean R1 or R2 growth stages. 
• If it is found, a fungicide spray at R3 may be the best timing.
• Use a mix of fungicide modes of action.
• Critically, don’t use strobilurin fungicide alone. If used by itself, it likely won’t manage the disease well because some of the pathogen populations will be resistant. Additionally, this will quickly force a population shift, favoring strobilurin-resistant pathogens. This could result in strobilurins no longer being effective. 

Frogeye Leaf Spot Resources

Soybean Disease Diagnostic Series (PDF file)

Frogeye Leaf Spot in Soybeans (YouTube video):

Published: Oct 17, 2022