Rice Farming

Plant disease experts knew it was only a matter of time before a potentially devastating soybean disease arrived in the United States.

Their fears came true when the more aggressive Asian species of soybean rust was discovered in a soybean production field at the Ben Hur Research Farm near Baton Rouge, La., on Nov. 6, 2004. It has since been found in Iberia, St. John, Pointe Coupee, Iberville and St. Mary parishes, La.; a 700-acre soybean field south of Natchez, Miss.; Crittendon County, Ark.; New Madrid and Pemiscott counties in the Missouri “Bootheel;” Seminole County, Ga.; Mobile County, Ala.; and a University of Florida experimental plot near Quincy, Fla. A kudzu sample collected in Florida also tested positive for soybean rust.

Once the initial soybean rust discovery was confirmed, a team of U.S. Department of Agriculture, state and university agricultural officials converged on the 2,500-acre Ben Hur Farm to determine how widespread the infection was. Similar surveys were conducted in nearby states.

Time to plan
The good news is the disease was found after more than 90 percent of the Gulf area’s soybeans had been harvested, giving plant pathologists and other experts time to develop battle plans for the 2005 season.

“It couldn’t have happened at a better time of year. It gives us time to prepare,” says Billy Moore, Mississippi State University Extension plant pathologist emeritus.

Soybean rust is caused by two different fungi. The Asian species, Phakopsora pachyrhizi, which is the more aggressive and the one found in North America, can decrease soybean yields by up to 80 percent if left untreated. The organism interferes with photosynthesis, so the plant can’t grow and reproduce.

Soybean rust spores are easily transported by the wind over long distances. Because kudzu can carry the disease and show symptoms without being killed by the fungus, the team also surveyed dense infestations of the weeds. Altogether, 60 to 90 different plants are soybean rust hosts.

Because the fungus is easily spread through the air, the USDA will not be quarantining fields where the disease is found.

An early warning system
Throughout the 2004 summer, Mississippi State University monitored six sentinel soybean fields planted between Natchez and Mobile Bay. Plant pathologists also conducted biweekly surveys of kudzu as well as checking volunteer soybeans left after harvest in Mississippi and western Alabama.

It wasn’t until November that MSU officials confirmed the first case of soybean rust in that state.

Based on the recent surveys, USDA and state agricultural officials believe the disease’s arrival in the Mississippi Delta was probably related to Hurricane Ivan, which struck the Gulf Coast in late September. Soybean rust was discovered in South America’s soybean producing area in 2001.

To treat or not to treat
What the 2005 season holds in terms of soybean rust pressure is unknown, Moore says.

In Mississippi, MSU plans to plant sentinel soybean fields well ahead of the time growers typically plant. Should soybean rust show up in the sentinel fields, MSU will issue recommendations to treat. Typically that will be at the R1 stage—and the beginning of bloom—and then 20 days later.

In anticipation of soybean rusts’ arrival, several states, including Louisiana, Mississippi, Arkansas, Texas and Missouri, had applied for Section 18 emergency use permits for a handful of fungicides in 2004. The permits would kick in once the disease was discovered, so the products will be available for use during the 2005 season.

Those fungicides include propiconazole (Tilt, Propimax, Bumper), myclobutanil (Laredo) and tebuconazole (Folicur). Registrations for other fungicides to treat soybean rust are currently in the works.

Louisiana State University experts will be developing treatment suggestions during the next month and will present them at grower meetings later this winter, says Clayton Hollier, a LSU AgCenter plant pathology professor in Baton Rouge. Also involved in developing the suggestions are Ray Schneider and Ken Whitham, LSU plant pathology professors in Baton Rouge.

“We’re looking at treating for rust, but also treating to get a benefit from the product or combination of products against other diseases,” Hollier says.

Purdue University officials estimate growers may spend $25 to $35 per acre for fungicides for soybean rust. Although some growers may complain about the added cost to treat soybean rust, Moore says they should also look at the products’ benefits.

In the absence of soybean rust, applying one fungicide application may increase soybean yields by 5 to 8 bushels because it controls other diseases that are below treatment thresholds but still reducing yields, Moore says.

Without fungicide treatments, soybean rust can reduce yields by as much as 80 percent, Moore says.

Contact Vicky Boyd at (209) 571-0414 or vlboyd@sbcglobal.net.

Is it rust or another disease?

Several diseases, such as Septoria brown spot and bacterial pustule, resemble soybean rust in the field to the naked eye. But it takes stronger magnification—either a hand lens or a dissecting scope—to confirm the disease, says Clayton Hollier, a Louisiana State University plant pathology professor in Baton Rouge.

Soybean rust lesions are most common on leaves, but may occur on petioles, stems and pods. Soybean rust produces two types of lesions—tan and reddish brown. Tan lesions, when mature, consist of small pustules surrounded by slightly discolored necrotic area with masses of tan spores on the lower leaf surface.

Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface. As the disease severity increases, premature defoliation and early maturation of plants is common.

Once pod set begins on soybeans, infection can spread rapidly to the middle and upper leaves of the plant. Soybean rust can go from a few pustules in a field to full field defoliation in a matter of about two weeks.

The key to effective control will be catching the disease as early as possible. The longer the disease is allowed to remain untreated in the field, the less effective fungicides are against it.

“It’s going to take time for people to be comfortable [identifying soybean rust] because there are a lot of different spots on the leaves, and it can be very confusing,” Hollier says.

Bacterial pustule, for example, can be easily confused with immature soybean rust lesions. If you put the sample under a dissecting scope, slice through the pustule and see telltale bacterial streaming, then it’s bacterial pustule.

But if you look at the same sample under the scope or strong hand lens and see fungal spores, then you know it’s probably rust.

Shortly after the Louisiana discovery, LSU conducted an intensive training session for its Extension agents on how to recognize soybean rust in the field with hand lenses and in a more clinical setting using dissecting scopes. In February, LSU experts will present a similar identification program at the state consultants’ meeting.

If you suspect you may have soybean rust during the 2005 season, contact your county Extension agent or Extension plant pathologist.

Internet Hotlinks

For more information on soybean rust, visit USDA Soybean Rust Homepage
http://www.aphis.usda.gov/ppq/ep/soybean_rust/index.html

StopSoybeanRust.com
http://www.stopsoybeanrust.com/mc_home.asp

Soybean rust hits the Delta Scout often, treat early to combat new yield-robbing disease threat
By Vicky Boyd Editor
Plant disease experts knew it was only a matter of time before a potentially devastating soybean disease arrived in the United States. Their fears came true when the more aggressive Asian species of soybean rust was discovered in a soybean production field at the Ben Hur Research Farm near Baton Rouge, La., on Nov. 6, 2004. It has since been found in Iberia, St. John, Pointe Coupee, Iberville and St. Mary parishes, La.; a 700-acre soybean field south of Natchez, Miss.; Crittendon County, Ark.; New Madrid and Pemiscott counties in the Missouri “Bootheel;” Seminole County, Ga.; Mobile County, Ala.; and a University of Florida experimental plot near Quincy, Fla. A kudzu sample collected in Florida also tested positive for soybean rust. Once the initial soybean rust discovery was confirmed, a team of U.S. Department of Agriculture, state and university agricultural officials converged on the 2,500-acre Ben Hur Farm to determine how widespread the infection was. Similar surveys were conducted in nearby states. Time to plan The good news is the disease was found after more than 90 percent of the Gulf area’s soybeans had been harvested, giving plant pathologists and other experts time to develop battle plans for the 2005 season. “It couldn’t have happened at a better time of year. It gives us time to prepare,” says Billy Moore, Mississippi State University Extension plant pathologist emeritus. Soybean rust is caused by two different fungi. The Asian species, Phakopsora pachyrhizi, which is the more aggressive and the one found in North America, can decrease soybean yields by up to 80 percent if left untreated. The organism interferes with photosynthesis, so the plant can’t grow and reproduce. Soybean rust spores are easily transported by the wind over long distances. Because kudzu can carry the disease and show symptoms without being killed by the fungus, the team also surveyed dense infestations of the weeds. Altogether, 60 to 90 different plants are soybean rust hosts. Because the fungus is easily spread through the air, the USDA will not be quarantining fields where the disease is found. An early warning system Throughout the 2004 summer, Mississippi State University monitored six sentinel soybean fields planted between Natchez and Mobile Bay. Plant pathologists also conducted biweekly surveys of kudzu as well as checking volunteer soybeans left after harvest in Mississippi and western Alabama. It wasn’t until November that MSU officials confirmed the first case of soybean rust in that state. Based on the recent surveys, USDA and state agricultural officials believe the disease’s arrival in the Mississippi Delta was probably related to Hurricane Ivan, which struck the Gulf Coast in late September. Soybean rust was discovered in South America’s soybean producing area in 2001. To treat or not to treat What the 2005 season holds in terms of soybean rust pressure is unknown, Moore says. In Mississippi, MSU plans to plant sentinel soybean fields well ahead of the time growers typically plant. Should soybean rust show up in the sentinel fields, MSU will issue recommendations to treat. Typically that will be at the R1 stage—and the beginning of bloom—and then 20 days later. In anticipation of soybean rusts’ arrival, several states, including Louisiana, Mississippi, Arkansas, Texas and Missouri, had applied for Section 18 emergency use permits for a handful of fungicides in 2004. The permits would kick in once the disease was discovered, so the products will be available for use during the 2005 season. Those fungicides include propiconazole (Tilt, Propimax, Bumper), myclobutanil (Laredo) and tebuconazole (Folicur). Registrations for other fungicides to treat soybean rust are currently in the works. Louisiana State University experts will be developing treatment suggestions during the next month and will present them at grower meetings later this winter, says Clayton Hollier, a LSU AgCenter plant pathology professor in Baton Rouge. Also involved in developing the suggestions are Ray Schneider and Ken Whitham, LSU plant pathology professors in Baton Rouge. “We’re looking at treating for rust, but also treating to get a benefit from the product or combination of products against other diseases,” Hollier says. Purdue University officials estimate growers may spend $25 to $35 per acre for fungicides for soybean rust. Although some growers may complain about the added cost to treat soybean rust, Moore says they should also look at the products’ benefits. In the absence of soybean rust, applying one fungicide application may increase soybean yields by 5 to 8 bushels because it controls other diseases that are below treatment thresholds but still reducing yields, Moore says. Without fungicide treatments, soybean rust can reduce yields by as much as 80 percent, Moore says. Contact Vicky Boyd at (209) 571-0414 or vlboyd@sbcglobal.net. Is it rust or another disease? Several diseases, such as Septoria brown spot and bacterial pustule, resemble soybean rust in the field to the naked eye. But it takes stronger magnification—either a hand lens or a dissecting scope—to confirm the disease, says Clayton Hollier, a Louisiana State University plant pathology professor in Baton Rouge. Soybean rust lesions are most common on leaves, but may occur on petioles, stems and pods. Soybean rust produces two types of lesions—tan and reddish brown. Tan lesions, when mature, consist of small pustules surrounded by slightly discolored necrotic area with masses of tan spores on the lower leaf surface. Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface. As the disease severity increases, premature defoliation and early maturation of plants is common. Once pod set begins on soybeans, infection can spread rapidly to the middle and upper leaves of the plant. Soybean rust can go from a few pustules in a field to full field defoliation in a matter of about two weeks. The key to effective control will be catching the disease as early as possible. The longer the disease is allowed to remain untreated in the field, the less effective fungicides are against it. “It’s going to take time for people to be comfortable [identifying soybean rust] because there are a lot of different spots on the leaves, and it can be very confusing,” Hollier says. Bacterial pustule, for example, can be easily confused with immature soybean rust lesions. If you put the sample under a dissecting scope, slice through the pustule and see telltale bacterial streaming, then it’s bacterial pustule. But if you look at the same sample under the scope or strong hand lens and see fungal spores, then you know it’s probably rust. Shortly after the Louisiana discovery, LSU conducted an intensive training session for its Extension agents on how to recognize soybean rust in the field with hand lenses and in a more clinical setting using dissecting scopes. In February, LSU experts will present a similar identification program at the state consultants’ meeting. If you suspect you may have soybean rust during the 2005 season, contact your county Extension agent or Extension plant pathologist. Internet Hotlinks For more information on soybean rust, visit USDA Soybean Rust Homepage http://www.aphis.usda.gov/ppq/ep/soybean_rust/index.html StopSoybeanRust.com http://www.stopsoybeanrust.com/mc_home.asp
Return To Top