Brazil’s Rust Story
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 (Phakopsora pachyrhizi) 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 will be joining them next year, now that soybean rust has been found in nine southern states. The disease causes plant defoliation, poor pod set and fill, and, ultimately, reduced yields. Currently, fungicides are 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. "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 fungicide." Almeida said EMBRAPA Soybean researchers are studying soybean cultivars in hope of identifying genes resistant to soybean rust. One cultivar - ‘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. To use fungicides we have to understand very well the epidemiology of this fungus."
Coming up with a fungicide application strategy has been one of trial and error in Brazil. "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. 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. 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 from the moment a field is blanketed by rust spores to the appearance of spore-filled "pustules" on leaves is about 9-12 days. 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 in that area. "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.
Like many American farmers who were stunned by the U.S. Department of Agriculture's announcement that Asian 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. 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. 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." (Ag Answers, 12/3/04).
