Soybean farmers concerned about frogeye leaf spot need to remember that fungicides work best as a preventive measure for the disease. But what can farmers do if they see the disease? 

David Langston, Virginia Tech Extension plant pathologist, encourages impacted soybean farmers to turn to higher-end fungicides like Lucento, Revylok, and Trivapro for control, since they work best.  

At the 2024 Soybean Field Day at the Eastern Virginia Agricultural Research and Extension Center in Warsaw Sept. 12, Langston explained that these fungicide products are FRAC code 3 fungicides and the FRAC code 3 component in the mixture gives the products their curative activity. 

Warm, humid weather promotes the frogeye leaf spot. High-end fungicides are best for treating this disease and must be sprayed at the R1 to R5 stages if warranted. But Langston stressed that starting with resistant varieties is the best approach.  

Attendees of the 2024 Soybean Field Day at the Eastern Virginia Agricultural Research and Extension Center learn about the efficacy of fungicides against Frogeye leaf spot. (Photo: John Hart)

Other threats to soybeans 

Another disease worry facing Virginia soybean farmers is Sudden Death Syndrome.  

Langston said symptoms of Sudden Death Syndrome include patches of yellow soybeans in an otherwise healthy soybean field. He said tell-tell signs of sudden death syndrome include leaves with a striped appearance and vascular discoloration.  

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“Those sudden death syndrome plants get just as tall as the healthy plants. But then they just start crashing. Pull up the root system of both plants. The root system of a healthy plant is twice as big and twice as developed as the one that has sudden death syndrome. The fungus generally attacks the roots very early in the season. You see more of it in your earlier planted soybeans because it’s a fungus that likes cooler weather,” Langston explained. 

Langston noted that infected plants with the smaller root systems don’t show signs of damage until they reach the pod fill stage, but as the plant reaches the pod fill stage, it crashes due to the infection that occurred back in April or early May which caused that root system not to be developed.  

“When sudden death occurs those plants just die standing up so they will be dead stalks standing up. They don’t fall over. It’s caused by a fungus. It is a fungus that attacks the root system very early in the season,” Langston explained. 

“If you’re going to use a seed treatment, I would use them in those earlier planted soybeans to help combat sudden death syndrome. It’s not going to be great. Don’t expect a miracle, but it does help you get a little more yield out of those bad spots, and you won’t see quite as much sudden death syndrome in your soybean field. You are going to see; it’s just not going to be as bad as it would have been had you not used a seed treatment.” 

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Langston said nematodes, particularly cyst nematodes and root knot nematodes, exasperate sudden death syndrome problems in soybeans. He said the best way to combat nematodes in soybeans is to turn to resistant varieties and a good rotation program, particularly corn. 

“Nematicides and seed treatments with nematicides can give you a little help in low-to-moderate populations. But if you really have a problem, you’re not going to see much benefit with a nematicide, whether it’s an in-furrow nematicide or whether it’s a seed treatment nematicide,” Langston said. 

Langston said most soybean varieties have cyst nematode resistance regardless of maturity group. Most soybean varieties have R3 soybean cyst nematode resistance, but cyst nematodes are overcoming it. In Langston’s research, nearly all of the cyst nematode populations overcome R3 resistance.   

Langston said more soybean lines now have Peking soybean cyst nematode resistance. He said Peking resistance is a more durable type of cyst nematode resistance and works well against most of the races of soybean cyst nematode. However, Peking resistance is only found in Group 5 or higher soybeans. 

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“If you know you have a cyst nematode problem in a field, and you want to combat it with resistant varieties, I recommend rotating varieties that have R3 resistance and Peking resistance from one year to the next. That breaks that cycle a little bit. And certainly, rotations to things like corn are going to help too or any other crop other than soybean is going to help break that cycle some,” Langston told field day attendees.