Corn+Soybean Digest

Vigilance Pays

By all accounts, the U.S. sentinel plot program was a success this past year. We knew where rust was (and wasn't), and by effectively tracking the disease, U.S. soybean growers were able to save an estimated $100 million on fungicide treatments, says David Wright, director of research for the North Central Soybean Research Program.

USDA and the soybean checkoff, through the United Soybean Board and the North Central Soybean Research Program, have again partnered to fund the sentinel plot program, but given what was learned about soybean rust in 2005, the U.S. sentinel plot system will undergo some changes.

There will be fewer sentinel plots this year, according to Don Hershman, Extension plant pathologist at the University of Kentucky. Hershman coordinates the southern sentinel plot network. “We've determined that it's better to be efficient and scout fewer strategically placed plots and do a very good job than to spread ourselves too thin,” he says.

The types of soybeans planted in the plots will change, too. “Last year there was a focus throughout the entire network based on the assumption that we wanted to get to the canopy and flowering stage as early as possible. As a result, we had a lot of very early planted, early variety sentinel plots that were maturing at about the same time rust began to appear,” says Hershman. “This year everyone will plant at least two varieties to stretch maturity across a broader range.”

Because “mobile” or randomly scouted farmers' fields were also an important part of the system last year — many finds were actually in those fields — every state will include some element of mobile plots in their plan.

Based on data from Brazilian sentinel plots, scouting methods will also change. “We know that the potential to find soybean rust increases exponentially once the crop begins to flower, so we'll scout no more than once a week before flowering occurs,” Hershman says. “After flowering, we'll scout once weekly. We've found the disease just doesn't develop that quickly to justify scouting more often than once a week.”

More samples will also go to the lab, according to Hershman. “Last year a lot of states relied on field scouting to identify the disease. There was a big range in how we were assessing for rust,” he says. “One of the things we discovered is that it's unlikely to pick up low levels of rust in a plot unless samples are sent to a lab.”

Hershman notes that each state will determine how many samples are incubated and subsequently observed under a microscope. This is usually done in a lab, but the University of Kentucky will send all its samples to the lab for analysis.

These differences will result in a much more intensive focus on discovering the disease as early as possible.

One thing Hershman points out is that the southern sentinel plot system is very active and key to predicting soybean rust activity throughout the U.S. “Most states in the South don't really have much of a soybean crop, but all of a sudden these folks are being drawn into the soybean rust arena,” he says. “They're glad to do it, but we have to make sure that we support the people doing this work in the South in every way possible. There is a potential weakness in the system if we forget that.”

Looking toward this season, Hershman says many farmers have still not tapped into how to follow the sentinel plot results. All the information is available at He says those who don't have high-speed Internet or aren't comfortable with using a computer should partner with their local ag dealer or Extension agent/educator to regularly track the disease. “We have the tools needed to manage soybean rust, but the only thing standing in the way is paying close attention to what's happening,” he says. “Everyone needs to be diligent.”

Loren Giesler, an Extension plant pathologist at the University of Nebraska-Lincoln coordinates the northern sentinel plot program. He says that most practices in the North will mirror those in the South, with a few minor differences.

Giesler points out that planting early maturing soybean varieties is important when detecting the arrival of the disease, but when it comes to actually tracking disease activity, it will be more useful to scout soybeans that are most representative of what's being planted in that state. Mobile plots won't be as critical in the north early in the scouting process, but will be monitored closely if rust is a threat.

“The sentinel plots are an important ground truth tracking system for the disease,” says Giesler. “There are a lot of models that can predict the movement of the disease, but we really haven't had enough observations to know if they're accurate. If we didn't have the sentinel plot system as a method of tracking the disease we wouldn't know when to pull the trigger to treat it.”

Giesler also points out that researchers don't yet have a good sense for how the disease will move in the U.S. “If inoculum can jump from Columbia as a result of a hurricane, there's the potential for a significant jump within the U.S. as well, so every state needs to remain vigilant, even if the threat of rust doesn't seem imminent.”

Kudzu observations are also important in tracking rust since spores are produced on the prolific weed. Giesler says there are kudzu observation sites in several northern states, including Nebraska.

Giesler encourages farmers to use the system that their checkoff and tax dollars have funded, as well as working with their land-grant universities. He says it's been gratifying to see the collaboration of all the land-grant universities and the scientific community working through the Extension system to make the program work.

“The sentinel plot system is our best tool to monitor the progression of the disease,” says David Wright. “It is from this system that we will base all our recommendations for when to initiate treatment of soybean rust during the upcoming growing season.”

He says researchers are looking at ways to use the sentinel plot system structure to map other pests of economic importance, like soybean aphids.

This year sentinel plots will also be monitored for aphids and the information gathered will be entered in the same database used for rust. “We're in the preliminary stages of determining if we can do as good a job monitoring for other major pests,” says Wright. However, he says understanding the information on aphids will be more challenging because sometimes there is no logic to how aphids behave — they can be found in one field at threshold levels, but there's not a pest to be found in the field across the road. “In general, the idea behind this experiment is to see if we can adapt the system to manage other pests in crops of high value,” he says.

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