America's first line of defense against a pathological killer carries no weapons, handcuffs or pepper spray. It relies solely on dark green leaves.

The pathological threat is Asian soybean rust, a foliar disease that has been known to destroy as much as 80 percent of a soybean field's yield potential. The agricultural crime-fighters are hundreds of tiny soybean fields known as sentinel plots.

Sentinels have been planted throughout Indiana and in other farm states, said Greg Shaner, Purdue University Extension plant pathologist. The plots serve as detection sensors for rust infection so that farmers can be alerted in time to protect their crops with fungicide, he said.

"Sentinel plots are intended to be an early warning system," said Shaner, who is supervising the Indiana sentinel plot program. "They are small plots - roughly 50-by-50 feet - that are planted throughout the country in soybean-producing states. We try to get these in the ground a little bit ahead of the normal crop.

"The plots are monitored closely on a weekly basis for the first appearance of rust. Once we know that rust is within a couple hundred miles of where we have sentinel plots, we'll switch to a twice-weekly monitoring."

Sentinel plots have been used to help monitor soybean rust movement in Brazil, a major world soybean producer.

Indiana has about 20 sentinel plots this season. Many have already emerged, Shaner said. Sentinel planting started in early April - weeks ahead of regular soybean planting.

"The sentinel plots are scattered throughout the state," Shaner said. "We have nine Purdue research farms, and we have a sentinel plot at each one of those. The remaining plots are on farms, with a few being maintained by either seed or chemical companies."

Soybean rust is caused by the fungus Phakopsora pachyrhizi. Soybean plants are infected when fungal spores are carried by the wind from one field to another. Once infected, the disease forms tan lesions on the leaves. The leaves eventually die and fall off, damaging the plant's ability to produce soybean seeds.

The rust pathogen reproduces rapidly. A single moderately infected soybean plant can produce about 6 million rust spores per day.

Fungicide is the only control option available for soybean rust.

For rust to infect Indiana soybean fields, the disease must travel up from the southern states where the rust pathogen overwintered. Sentinel plot programs in those states should give Hoosier farmers ample warning if a disease outbreak occurs, Shaner said.

"We think that soybean rust would not appear here until sometime in June or early July," he said. "We will start monitoring our plants once the first leaf is out and try to keep an eye on them each week."

Most Indiana sentinel plots will be treated with fungicide if rust is detected. Fungicide will not be applied to sentinels at Purdue farms, Shaner said.

"We won't spray the plots at the research farms because we want to allow the disease to develop," Shaner said. "The reason is that we desperately need information on the progress of the disease over the season. We want to use that information, in conjunction with weather data, to improve forecasting models that can be used to help farmers decide when to begin using a fungicide."

Application timing is important to ensure that farmers get the most from their fungicide treatments, Shaner said.

"The whole purpose of these sentinel plots is that we don't want farmers to spray too early," he said. "If a farmer sprayed and no rust actually showed up in this area for another three weeks, that spray would be totally wasted and the farmer would have to spray again. Most of these fungicides will provide protection for, roughly, two to three weeks."

For more information about soybean rust and management strategies, log onto the Purdue Plant and Pest Diagnostic Laboratory soybean rust Web site.

Related publication:

Preparing for Asian soybean rust: http://www.ces.purdue.edu/extmedia/ID/ID-324.pdf

Related Web sites:

Purdue Agriculture Soybean Rust Page

Purdue Department of Botany and Plant Pathology