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Corn+Soybean Digest

Scouting, prompt spraying key to managing Asian soybean rust

If they haven't already, Missouri soybean producers should add comfortable boots and a 10- to 20-power magnifying lens to their Christmas lists - they'll need both while scouting their fields for Asian soybean rust in 2005.

"We'll all have to do a little more walking next season," MU Extension plant pathologist Laura Sweets told a group of crop advisors at the MU Crop Management Conference, Dec. 16-17 in Columbia.

Since the announcement in November that the yield-reducing foliar disease had reached the continental United States for the first time - most likely on the winds of Hurricane Ivan - soybean producers across the Southeast and Midwest have scrambled for information.

"Soybean rust isn't new; it's just new to us," Sweets says. "The keys to successfully managing this disease will be correct identification, early detection and prompt action."

Like other foliar soybean diseases found in Missouri, including Septoria brown spot, bacterial blight and frogeye leaf spot, soybean rust begins with small yellow spots on the upper leaf surfaces. "They all start off looking pretty much the same," Sweets says. "Rust usually starts low on the plant, then moves up through the canopy."

What distinguishes soybean rust from other diseases is the spore-producing pustules that develop on lower leaf surfaces. "These pustules are like little volcanoes," she says. "When they erupt, they can produce billions of spores."

Sweets said that while managing other foliage diseases can be accomplished through crop rotation, residue management and variety selection, these management strategies aren't applicable to soybean rust. "There's no resistant variety I could tell you to plant next season, because we don't have a resistant variety," she says. "Managing residues and rotation also have no effect because rust only survives on living host material." Extensive field scouting will be critical to reducing soybean rust's impact on yields should environmental conditions during the growing season be favorable for rust.

"Scout often, especially if rust has already been reported in southern states," Sweets says. "Initially, target your early-planted fields, fields planted to early-maturing varieties, low-lying or protected fields with prolonged moisture in the canopy, and fields with early canopy closure."

When scouting, carefully check leaves low in the canopy. "You need to turn those leaves over and examine the lower leaf surfaces," Sweets says. "Use a hand lens when examining leaves, and if you're uncertain about identifying a disease, send a sample into the diagnostic lab."

If a producer does find soybean rust, several fungicides are labeled for soybeans. Several others have been granted emergency exemption labels for use on soybeans.

"Some products help prevent infection; others are curative when the disease is in its early stages," Sweets says. "The decision on what fungicide to use will be determined by the level of disease in the field, the growth stage of plants in the field, the efficacy of the product and the efficacy of the application."

For fungicides to be effective, they must be applied in a timely fashion, she said. "If the disease is already producing spores, you've missed the boat."

Sweets reminds there are still many unknowns about how soybean rust will develop in the United States. "So much about the severity of the disease will be dependent on weather," she says. "If we have a drought, you could dump a lot of spores and not have a problem. It all depends on the weather during the growing season."

A new MU Extension guide about Asian soybean rust is now available to producers. The guide provides detailed descriptions and photographs of Asian soybean rust, and compares it to other foliar diseases found in the state. It also explains the disease's history, how it develops and outlines management strategies. The guide can be found online at

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