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High Yield Team: The rust report

Soybean rust ran rampant in the Brazilian soybean fields that Bob Rikli and others toured on the 2005 Successful Farming Brazil Crop Tour.

So far, U.S. farmers haven't faced similar rust outbreaks. Yet, the climb of soybean prices into double-digit dollars per bushel makes a potential outbreak much more costly.

"Twelve-dollar beans make a difference," says Rikli, who farms near Murdock, Nebraska. "If we get rust in a big way, we're going to get killed if we aren't prepared."

In 2007, soybean rust showed it could move north. Its most northern inroad occurred in the north-central Iowa county of Hancock. Iowa State University (ISU) rust trackers also detected rust in 13 other Iowa counties. Fortunately, rust's late-September arrival did not slice yields.

For rust to infect Midwest soybean fields this year, spores will need to ride southern wind currents. Even then, dry weather, stifling heat, and the region's intense sunlight can quickly quell potential infections. That's what happened during 2005, 2006, and 2007.

If 2008 weather mimics that of 2004, though, rust outbreaks may occur, says Bob Streit, a Boone, Iowa, crop consultant who also attended the 2005 Successful Farming Brazil Crop Tour.

"Since it came in at the end of 2004, we've had hot and dry years with a high degree of sunshine," he says. "If we have a 2004 type of year with cooler wet weather and more cloud cover, we will be at risk. All the disease looks for is an opportunity with spore load and wind in the right direction after flowering."

Soybean rust has already impacted yields in the Southern U.S. Rust clipped yields up to 25% in several Louisiana fields in 2007, says Daren Mueller, ISU Extension plant pathologist. "Many acres received a second application of fungicides in Louisiana, and some of these were due to rust," says Mueller.

Fortunately, most Southern growers escaped yield damage.

"Louisiana and Mississippi, which are two states on the coast that are the center of the rust-entry area, both set record (per-acre) yields in 2007," says Larry Heatherly, a retired USDA-ARS research agronomist from Seymour, Tennessee, and a High Yield Team (HYT) expert panel member.

"Because of early planting, sentinel plots, and the use of highly effective fungicides, our farmers can manage rust," says Heatherly. Planting before May enables soybeans to dodge the prime rust infection period. Sentinel plots give the first warning that with the right conditions, rust will move north.

"We have a good supply of fungicides and more coming off Section 18," adds Heatherly. (Section 18 federal approval permits unregistered use of a pesticide for a limited time for emergency pest conditions. Pesticides can move from Section 18 to full federal approval).

So what should you do in 2008? Keep triazole fungicides ready, as these mainly curative compounds are the most effective ones on rust. You may also monitor rust's daily movements at www.sbrusa.net.

Until last year, most rust trackers zeroed in on rust traveling along the Eastern and Mississippi River pathways. Spores taking these pathways overwinter on kudzu in Florida and Alabama before arriving in Northern fields via wind currents.

In 2007, though, a record-breaking April Florida frost teamed with a later stifling drought to zap the region's rust spores. "Even in areas where they overwintered, spores did not build up," says Mueller.

Rust that surfaced in Iowa and other Midwestern states arrived via the Western pathway. Rust spores taking this pathway probably originated in Central America and moved northward on the wind.

Early soil-soaking rainfall established rust early in Louisiana and Texas. In mid-July, a rust prediction model by X. B. Yang, an ISU Extension plant pathologist, pegged rust to surface in Iowa sometime in September. Prolific August rainfall in states north of Texas normally would bode well for rust outbreaks.

However, the searing Midwest heat that started in mid-July nixed rust infections during the susceptible R1 (beginning bloom) to R6 (full seed) stage. By the tim rust was first detected in Iowa in late September, all soybeans were out of danger.

The slate is clean for 2008. "The rust pathogen won't survive the winter," says Mueller. For rust to appear in 2008, the establishment cycle will need to repeat itself.

Soybean rust ran rampant in the Brazilian soybean fields that Bob Rikli and others toured on the 2005 Successful Farming Brazil Crop Tour.

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