Purdue News

December 3, 2004

Brazilian researcher: Our soybean rust story a lesson for U.S.

Alvaro Almeida

WEST LAFAYETTE, Ind. - Knowledge is power, especially when it comes to combating a serious new soybean disease, said a Brazilian plant pathologist with ties to Purdue University.

Alvaro Almeida, a lead researcher at Brazil's National Center for Soybean Research, commonly known as Embrapa Soybean, said American farmers should educate themselves on Asian soybean rust before they plant soybeans next spring. Almeida, who earned a doctoral degree in plant pathology from Purdue in 1986, spoke about Brazil's soybean rust experience during a recent visit to the university.

Brazilian soybean growers have battled the fungal disease the past three crop seasons. Farmers in the United States could be joining in that battle as early as next year now that soybean rust has been found in nine southern states.

Asian soybean rust - Phakopsora pachyrhizi - is a fungus spread by wind-blown spores. The disease causes plant defoliation, poor pod set and fill, and, ultimately, reduced yields. Fungicide is the only known control option available to farmers. Left untreated or treated too late with fungicide, a soybean field's yield losses can surpass 80 percent.

American soybean producers and agricultural researchers can learn from Brazil's soybean rust experience, Almeida said.

"It's very important to identify the disease," Almeida said. "This is the major problem in Brazil - identifying when the disease is arriving and when you need to start spraying fungicides."

Almeida said Embrapa Soybean researchers are studying soybean cultivars in hope of identifying genes resistant to soybean rust. One cultivar, called Tiana, appears to show levels of tolerance, but not enough to eliminate fungicide applications, Almeida said.

"Since we couldn't find resistance, the only way to control rust was through fungicides," he said. "To use fungicides, we have to understand very well the epidemiology of this fungus."

Coming up with a fungicide application strategy has been a tale of trial and error in Brazil, Almeida said.

"To avoid this disease, farmers have spent much money on sprays that were unnecessary," he said. "So to use the fungicide at the right time didn't come by chance - we had to work on this subject to advise them. And now we have very good solutions to this problem."

The efficiency of fungicide application is closely related to sprayer technology, Almeida said.

In Brazil, research indicates the most effective rust control is achieved when fungicide is applied in droplets from spray nozzles 11.8 inches above the plant canopy, Almeida said. Farmers typically use between 36 gallons and 47 gallons of fungicide for every 2.5 acres when applying with spray booms and about 8 gallons to 10.4 gallons per 2.5 acres when applying from aerial sprayers, he said.

For most Brazilian soybean fields, two fungicide applications are sufficient to control rust, Almeida said. In some cases, additional applications have been necessary.

Timeliness in fungicide spraying is critical, Almeida said. From the moment a field is blanketed by rust spores to the appearance of spore-filled "pustules" on leaves is about 9-12 days, he said. By day 25, any infected crop untreated with fungicide is beyond help, he said.

Once rust is detected in an area, Brazilian farmers are urged to scout their fields for signs of the fungus. "It is best to scout the bottom leaves initially, then move up the plant," he said.

Other soybean rust observations made by Almeida and his fellow researchers in Brazil include:

• Farmers often misdiagnose the disease when scouting fields. Plants infected with the rust pathogen look similar to those infected with other pathogens that cause brown spot, bacterial blight, downy mildew and bacterial pustule.

• Rust infection previously occurred only after plants reached the flowering stage. Now infection is occurring earlier in plant development.

• "Sentinel" soybean plots planted on the periphery of soybean fields provide good monitoring for early rust detection. The plots must be destroyed as soon as infection sets in to prevent further spread of rust spores, however.

• Kudzu, an important alternate host plant for soybean rust, should not be used in sentinel plots. The reason? Kudzu is an invasive plant that is nearly impossible to destroy.

Like many American farmers who were stunned by the U.S. Department of Agriculture's Nov. 10 announcement that soybean rust had reached the United States, Brazilians were unprepared for the disease when it arrived in that country in 2001, Almeida said.

"Nobody expected to have this disease in Brazil," he said. "When it was reported in Africa we never expected it would cross the Atlantic Ocean. This disease arrived in Paraguay first and Brazil later. It has caused severe losses to farmers. This last growing season losses were estimated at $2 billion, according to researchers from Embrapa Soybean. The disease progressed so quickly that everyone is scared."

Brazilian agricultural officials are working with growers to reduce rust-related yield losses, Almeida said.

"Every week we have a meeting with farmers. Every month we have meetings with more farmers," he said. "This is the only way that we can control the disease, by clarifying how the disease is (developing), what it looks like and, then, management of the disease."

Although Brazil is a major soybean-producing nation and competes with the United States in world markets, Brazilian farmers are not happy to see soybean rust make its way to America, Almeida said.

"We don't wish this disease on anyone," he said. "Perhaps, instead, this can lead to greater international cooperation on finding resistant varieties."

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

Source: Alvaro Almeida, amra@cnpso.embrapa.br

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

Related Web sites:
Purdue Ag Communication Soybean Rust

Purdue Plant & Pest Diagnostic Laboratory Soybean Rust

Embrapa (English version)


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