High moisture levels cause disease concerns in Alabama

target spot
Target spot of soybean. Note the concentric rings within the lesion.

Rain, heat and humidity are a recipe for disease in the field.

Edward Sikora, an Alabama Cooperative Extension System plant pathologist, said foliar diseases have become more prevalent on soybeans during the past several weeks.

“We have a number of soybean sentinel plots at different research stations to monitor for soybean rust,” Sikora said. “However, I also use those plots to track other diseases that might pop up during the season.”

Unusual weather

Sikora said the weather is mostly responsible for the higher levels of foliar disease pressure.

“In a general plant pathology course, on the first day they show you the disease triangle,” he said. “One corner will be the host. Another corner will be the pathogen. The other corner is weather.”

He said approximately 80% of all plant diseases are caused by fungi. Approximately 99 percent of the common fungal diseases thrive in warm, wet weather.

“Unless we run into a dry patch in late August or September, these diseases will keep going,” Sikora said.

Soybean rust

Sikora said he expected soybean rust to come back like a lion because last year was the worst year he remembers with the disease.

“Fortunately, soybean rust will probably not be a major concern in 2021, but it is one to keep an eye on for the next month or so,” Sikora said. “We saw significant yield loss last year from soybean rust, the worst in the past 20 years. However, this year the risk of rust is relatively low, but it is a disease we will continue to monitor into September.”

Frogeye leaf spot

Sikora said he detected frogeye leaf spot on soybeans in Escambia County in early August.

“Frogeye leaf spot could be a significant problem,” he said. “It is a fungal foliar disease that can cause up to a 40% yield loss on a susceptible variety. Growers need to be aware of it, especially with our prevailing wet weather conditions.”

Target spot

Target spot is another fungal disease that is well known in cotton. Sikora said there are some soybean varieties that are quite susceptible to the disease.

Sudden death syndrome

Some soybean plants in north Alabama were showing symptoms of Sudden Death Syndrome. This is a disease Alabama farmers may only see every three or four years.

“Symptoms of SDS consist of interveinal yellowing and necrosis, followed by a sudden wilt and death of infected plants,” he said. “SDS is caused by s by a soil-borne fungal disease and is more common in the north-central region of the United States.”

frogeye leaf spot
Frogeye leaf spot typically attacks foliage. in severe cases, it can cause premature defolliation, significantly reducing photosynthesis and yields — photo courtesy Mississippi State University

Although it sounds ominous, this disease is a sporadic disease in Alabama and is not a major concern.

“In years like this, where field soil is saturated, it will only kill plants in small patches in a field” Sikora said. “But it is usually a good sign for the grower in our state because it means there is a lot of moisture in the soil, which is typically good for the crop.”

Soybean disease control

As far as root and stem diseases are concerned, Sikora said there is not much producers can do at this time.

“Realize you have the problem and avoid the areas if you can,” he said. “Consider rotating to a non-host crop like corn, if possible.”

Sikora said with soybeans, frogeye leaf spot and target spot are going to be diseases to focus on this August—and possibly soybean rust later in the season. Producers can usually control these foliar diseases with a single, well-timed fungicide application at the R3 or R4 growth stage.

“Try to get that fungicide application out before the disease shows up. That should give you approximately three weeks of protection,” he said. “Go with a mixture of active ingredients because we have detected frogeye leaf spot populations resistant to strobilurin-type fungicides in Alabama.”

He said this is a good year for producers to protect their crop and their yields because of the high prices for most commodities.

Alabama Cooperative Extension contributed this article.

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