Asian soybean rust was found in fields in the Missouri counties of New Madrid, Pemiscot and Dunklin Sept. 18, said Allen Wrather, University of Missouri Extension plant pathologist at the MU Delta Center, Portageville, Mo.

“These were fresh infections with no visible yellowing of the soybean leaves,” Wrather said. “The infections were fairly extensive, with rust pustules emitting spores.” Earlier in the week, the MU scouts had found no rust.

The infected fields are in the three southernmost counties in Missouri.

“We’ve had ideal weather for soybean rust for the past seven days,” Wrather said. “A slow-moving low-pressure system brought intermittent rain and mild temperatures.”

Last week, rust was found in a growing number of counties in eastern Arkansas, northern Mississippi and Louisiana. South winds probably carried spores from infected fields into Missouri, Wrather said.

“That weather system has moved on east of us,” Wrather added. “Just because we have rust in three southern counties does not mean that rust will be found in counties just to the north.”

With rust arriving late in the season, Missouri producers might not have to spray fungicide.

Most soybean fields in southeast Missouri that emerged in late June are now at the R6 growth stage (filled pods), Wrather said. “Their yield will not likely be reduced by rust.”

However, late-planted soybean plants that emerged in early July are at or below R5 growth stage (pods beginning to fill). “Yields of those plants may be reduced by rust if not treated with a fungicide,” he said.

Weather conditions affect spread of the fungus. Rust spores must have 10 to 14 hours of wet-leaf contact before they germinate and infect the leaves. Warm, dry weather prevents their growth.

“The decision to spray is an economic decision,” Wrather said. “Fields with low yield potential, say 20 bushels, might not pay to spray.” A fungicide application can cost $12 to $15 per acre.

However, late-maturing, high-value fields, such as those growing seed to plant next year, might benefit from a fungicide application.

“Farmers should consider the growth stage and potential yield when making this decision,” Wrather said

As soybean rust infections were discovered closer to Missouri, MU Extension scouts intensified fieldwork in recent weeks. Local scouting should confirm rust in the area before farmers decide to apply a fungicide, Wrather said.

An online map updated daily shows the spread of the soybean rust. See http://www.sbrusa.net for progress reports.

Farmers can get details on rust from regional agronomy specialists through local MU Extension centers.