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Corn+Soybean Digest

Rust Ready

When Asian soybean rust attacked fields in Arkansas last summer, Scott Monfort and his team were on it. There was little excuse for growers not being ready for rust.

Outside the $12-plus price plateau being reached by soybean markets in recent months, probably nothing has been a bigger topic for growers than Asian soybean rust — a disease that can defoliate a field in a few weeks and cut production by 80% or more under the right conditions.

Late-summer rains, then the torrential tropical storms that swept through Central America and Mexico had southern and even Midwestern growers concerned over a surge of rust that could have resulted if the storms had turned north last fall.

By last fall, Asian rust had crept into eight states and over 85 counties. By mid-December, it had been reportewd in 324 counties in 19 states, according to USDA. But with an ever-growing network of university plant pathologists and agronomists like Monfort, an Arkansas Extension plant pathologist, as well as private consultants equipped with the ammunition to rescue rust-threatened fields, growers should feel somewhat at ease.

Just don't let your guard down, advises Monfort, who had more than his share of phone calls from worried growers last summer and fall (2007), as rain and more rain pelted parts of that state, bringing rust with it.

“We had reports of rust spreading up through Texas and Oklahoma and monitored it closely as it invaded some of our southwestern and northwestern Arkansas counties,” he says. “But we had plenty of fungicide available to handle any outbreaks.”

Rust is spread by rain and wind in the heart of the growing season. It blows in from the south, usually from Mexico and states bordering the Gulf of Mexico. The southwest Arkansas outbreaks proved an example of how quickly rust can spread in the perfect storm conditions.

After reports from South Texas, Louisiana, Mississippi and other areas, southwest Arkansas started reporting rust in late July. “We found a low incidence of the disease at 1-2%,” says Monfort. “But it went to 30% in three to four days and 100% in a week and a half.

“The good thing was we found it early and got our recommendations out so growers could treat their fields. Most producers took that to heart and sprayed beans that needed to be sprayed,” he says.

Most growers have heard the rust drill. But advice that can prevent a crop from being destroyed should never be ignored, says Monfort.

“Remember, soybeans are susceptible to rust damage if they are in the R1 (beginning flowering) through R6 (full seed) growth stages,” he says. “Beans beyond the R6 stage won't have to be sprayed, but rust could reproduce later on.”

He reminds growers that two groups of fungicides are available to manage rust: triazoles and strobilurin (a list of fungicides available for rust-control use is available at

“The strobilurin group (such as Headline or Quadris) are protectants only,” says Monfort. “You have to have it in the field before the disease comes in.

“The triazoles (such as Laredo, Tilt or Punch) are primarily for control and management of rust and many are available on a Sec. 18 status (for emergency use only; state Sec.18s may vary),” he says. “The time of the application is more important than how it is applied.”

Control measures should be taken when the disease is discovered and not before.

Palle Pedersen, Iowa State University (ISU) Extension soybean agronomist, says that if rust is in a field, fungicide treatment will reduce its presence, which may lessen the disease severity later in the season.

In explaining what growers should watch for in rust, Pedersen says R1 to R5 stages are the most crucial period for seed yield. If rust attacks during early seed filling, leaf loss could significantly reduce yields, so managing rust is critical at this stage, he says.

ISU scientists say some fungicides cannot be sprayed after R5, so be sure to read labels to assure the safety of the fungicide for the plant.

Pedersen says growers should begin to scout for rust at the R1 stage and continue through R5, usually mid- to late August.

He suggests walking through the field in a W-shaped pattern. Growers should periodically examine plants, looking low and deep into the canopy.

CLOSELY EXAMINE THE plants for mottled yellow leaves with pustules (pimple-like structures) on the underside. “Areas in the field with distinct yellowing or browning of the leaves or areas with dense canopy should be targeted in addition to those areas covered by the standard scouting pattern,” says Pedersen. He notes that if pustules are present, it is often too late to minimize a significant yield loss.

A powerful 20X lens magnifying glass may be needed for close examination. Asian rust can resemble other soybean diseases, so growers may need to review university and other soybean disease sources for correct identification. (Close-up photos of rust and various soybean diseases are available at

If rust becomes a threat, one or two applications may be needed. Monfort says the cost of treatments can range from $7/acre up to $20/acre, depending on whether a mix of chemicals is needed to handle the degree of infestation.

“The good prices we're seeing for soybeans means growers will likely take greater measures for control of rust and other problems that may hurt a crop,” he says.

Monfort adds, however, that before committing to a spray program, growers should make sure the soybeans will yield enough to warrant the expense, even if bean prices are high. For example, some soybeans with a 20-bu. yield potential may not be worth spraying.

Sunlight or bright ultraviolet rays will usually help control rust because it kills the spores, says Monfort, adding that rust typically doesn't over-winter in colder climates.

“It would have to be an extremely mild winter where you could have volunteer plants come up and become infected. The farther north the farm, the better,” says Monfort, adding that “crop rotation won't prevent rust from coming back.”

He adds that even though some may argue that the use of some fungicides can improve yields on soybeans even if rust and other diseases are not apparent, he doesn't recommend it. “As a plant pathologist, I only recommend using a fungicide when it's needed,” he says.

So as 2008 planting nears, be ready for rust. Daily rust updates are available through the USDA Soybean Rust Web site at There is also an ASR hotline at 866-641-1847. At press time, rust had been reported as far north as Dallas County, IA.

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