The closest confirmed rust on soybeans to North Carolina soybeans in late July was approximately 310 miles from Charlotte, 575 miles from Elizabeth City, 400 miles from Fayetteville, 190 miles from Murphy, 440 miles from Raleigh, 515 miles from Washington, 440 miles from Wilmington, and 380 miles from Winston-Salem, N.C.

“We do not recommend spraying soybeans that have not started blooming with a fungicide to control Asian soybean rust. Such pre-bloom applications have seldom improved yields.  

“Once soybeans start blooming, we would recommend spraying if rust has been confirmed within 100 miles of the field,” Dunphy says.

Of more immediate concern, Koenning says, is frogeye leaf spot, caused by Cercospora sojina

“Growers need to scout their beans for this disease, and if they find it, they need to spray a strobilurin fungicide as timely as they can,” Koenning says.

He adds that long-term management of frogeye leaf spot is difficult because growers can go through several years when the disease isn’t a problem.

Growers may not have a problem with the disease for several years, so new varieties come out and have really good production characteristics, but may lack the same high level of resistance to frogeye leaf spot. “In the interim, growers tend to move toward these newer varieties, and when we get a year with significant pressure from the disease, we have more soybeans that are susceptible, he says.

Despite the changes in soybean varieties, Koenning says, “Most soybean varieties currently grown are resistant to this disease, and the use of resistant varieties is the preferred method of control.”

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Although frogeye leaf spot is seed borne, it tends to be worse in fields of continuous soybeans. Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce. 

Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather.

So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion. This can lead to a situation where frogeye is layered in the canopy at different levels.