March 15, 2005

Keep close eye on far-off soybean rust, pathologist says

WEST LAFAYETTE, Ind. – Asian soybean rust came within a few hundred miles of Indiana this past fall, when the crop-destroying disease made its first thrust into the continental United States.

The near miss prompted many Hoosier farmers to wonder how close soybean rust needs to get before they take action.

"We don't have precise answers at this time," said Greg Shaner, a Purdue University Extension plant pathologist.

Because the fungal disease is new to the U.S. mainland, it is impossible to predict where it will show up and how severe the infection might be in 2005, Shaner said. That doesn't mean farmers are defenseless against rust this year, he said.

Growers should pay close attention to reports of rust in nearby states, weather forecasts and rust updates from Extension and agricultural experts, Shaner said. The information will be critical in helping farmers determine whether they should begin checking their fields for signs of the disease or start applying fungicide to their soybean crops, he said.

"We'll be looking at a lot of different kinds of information to try to alert growers on when they should be scouting their fields carefully or getting the sprayers out," Shaner said.

"One thing we'll look at is how close the rust is. Is it in Tennessee? Is it in Missouri? Is it in Arkansas? Is it in Kentucky? Then we'll get some idea of how much rust is there. In other words, have farmers been able to do a good job of controlling it in those states or is it running rampant in a lot of fields?

"Another thing we'll consider is the movement of air from there to here. That's more than simply looking at wind direction. Our climatologists have come up with sophisticated models that can predict where a volume of air has come from at any given time."

Purdue will plant monitoring – or "sentinel" – soybean plots at several university research farms and other locations across Indiana this spring, Shaner said. Sentinel plots will be inspected two times or more per week for the presence of rust. Farmers will be notified if rust is found in the sentinels.

"There isn't any reason for every farmer to plant a sentinel plot," Shaner said. "There's going to be a lot of work involved in monitoring these plots for the percentage of leaves that are infected. We'll be monitoring disease incidence down to about 1 percent, which means looking at about 150 leaves."

Exactly when soybean rust is most likely to strike the Midwest – if it does at all – is unknown, Shaner said. Some pathologists believe rust won't arrive in major soybean-producing states like Indiana until the crop reaches the R2 growth stage, when plants are in full bloom. Indiana soybean crops usually reach R2 in late June.

Shaner isn't so sure. Farmers should be ready to inspect their fields for rust much earlier if conditions warrant.

"Everybody here in the Midwest seems to think rust probably wouldn't show up until sometime after flowering," Shaner said. "But since we don't really know when it would show up, I think the time to begin scouting would depend, in part, on what we know is going on to the south of us. If we know that there's very little rust developing, even by flowering, then we probably don't need to do a lot of scouting. Conversely, if the rust gets started early, then we'd probably need to start scouting before flowering."

Field scouting could be a challenge, especially in the early stages of infection. Before pustules develop, rust will appear as small light-brown spots. Farmers could mistake rust for several other soybean diseases in their early stages. Pustules most likely will form on the underside of soybean leaves. Farmers will need to inspect leaves with a hand lens to detect the first pustules that develop.

Other than remaining vigilant and spraying fungicide when soybean rust infects their crop, there isn't much else a farmer can do to combat the disease, including early planting, Shaner said.

"At this stage, we don't think there's any reason to make any dramatic shifts in planting date," he said. "One of the arguments for planting early is the thinking that the beans would mature earlier, so you might avoid a second fungicide spray. But planting early doesn't take that much time off of the maturity date because of the day length requirement for soybeans.

"Besides, when you plant early you run other risks, from seedling blights to poor stand establishment. So we recommend that farmers stay with their normal production practices."

This past November, Asian soybean rust, or Phakopsora pachyrhizi, was confirmed in soybean fields in nine southern states. Infection was discovered as far north as Missouri and Tennessee.

The fungal disease, already a problem in South America, Asia, Africa and Australia, produces tan lesions on a soybean plant's leaves. The leaves, which play an important role in the plant's seed-making process, die and fall off.

When a plant is infected, the fungus forms raised areas, called pustules. The leaf surface covering the pustule breaks, releasing thousands of rust spores into the air. Wind can carry spores hundreds of miles before the spores land to infect other fields.

A moderately infected soybean plant can produce about 6 million rust spores per day. In an average acre of 160,000 to 180,000 soybean plants, that adds up to around 1 trillion spores.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Greg Shaner, (765) 494-4651, shanerg@purdue.edu

Purdue Agriculture Soybean Rust Page

Purdue Plant & Pest Diagnostic Laboratory Soybean Rust Page

Ag Communications: (765) 494-2722; Beth Forbes, forbes@purdue.edu
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