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First spray critical for rust control

Asian soybean rust frequently appears as “hot spots” in soybean fields and can be controlled if found and treated quickly, according to a researcher at a large grower-funded research organization in Parana, Brazil.

But finding such hot spots is not often easy. So the best strategy for the disease for Brazilian growers is to time preventive sprays based on advisories from sentinel programs that track its movement geographically, according to Olavo Correa da Silva, plant pathologist at Fundacao ABC.

Da Silva spoke to a group of U.S. agriculture journalists touring southern Brazil in March. He jokingly welcomed the United States to a dubious international club — countries where Asian soybean rust is active. Rust entered Brazil in 2002 and was discovered in the United States in 2004.

The United States and Brazil also share something else with the disease, according to the researcher. Both have endemic and epidemic outbreaks of the disease.

Endemic areas are where the disease can survive year-round and include northern Brazil and the coastal areas of south Louisiana and Florida, according to da Silva.

Epidemic areas include southern Brazil as well as areas that are north of coastal south Louisiana and Florida. The disease may not always be a problem in epidemic areas. In these areas, environmental conditions may be too harsh for the disease to survive during the winter.

Da Silva said that volunteer soybeans are the primary over-wintering host for Asian rust in Brazil. “We have maps that show which soybean-producing regions of Brazil have environmental conditions in which mature volunteer soybeans can survive.”

The researcher said that many Brazilian soybean producers failed to take the disease seriously when it first entered the country, “because they didn't have many losses.”

However, in 2003-04, most farmers in Brazil “got the fear” of the disease, after widespread heavy rainfall, according to da Silva. “Five million tons of soybeans were lost to soybean rust. Many farmers didn't believe the disease would be that severe. Most farmers are now vigilant about making a first rust application.”

Da Silva said a study conducted by the cooperative indicated the first fungicide application against rust provided about 90 percent of the benefit from using fungicides.

“The earlier you spray against rust, the better. The machinery must be in good shape to deliver the product to the canopy. Rust develops from the bottom to the top. So you have to wet all the leaves, especially down at the bottom.”

Farmers who find hot spots in their fields can control Asian rust, according to da Silva. “Fungicides work very well until there is 5 percent damage in the lower half of the plant. How well it works after that depends on how much latent disease there is in the plant.”

When spraying after rust has entered a field, farmers must have a system for finding hot spots, he said. That is usually a matter of intensive scouting, or luck.

Preventive sprays are the way to go for Albertino Perez, an agronomist and farm manager for Henrique and Anselmo Alberti's farm in Tibagi County.

“In some cases, rust is not that difficult a disease to control. But you have to control it preventively. If not, you can lose money very rapidly. It's better than waiting for rust to appear.”

The family's six farms include 7,090 hectares (17,512 acres) of no-till crops and 7,000 hectares (17,290 acres) of cattle. Summer crops include soybeans, corn and sometimes dry beans. Soybeans comprise around 70 percent of the farm's summer planting, with corn making up the rest.

A country-wide sentinel program — Syngenta's Syntinela — has helped him manage the problem. “They tell me how much to spray and when to spray. I can also estimate how much I will spend.” He also plants small plots of 5 to 10 hectares (12 acres to 24 acres) earlier than his other soybeans as a localized sentinel program to help him track the disease.

One thing helping soybean farmers in southern Brazil is that many already make fungicide applications directed at a number of diseases. “We must not forget about cercospora and septoria,” Perez said. “You can have losses to these diseases, too. Prior to rust, if a farmer did not take care of his crop against the other diseases, losses could be as high as 400 kilograms per hectare (6 bushels per acre). These are heavy losses.”

When soybeans start blooming, Perez's agronomist runs the fields more frequently, even daily, looking for diseases.

Perez has sprayed most of his soybeans once this year for the disease, some twice, using a combination of triazoles and strobilurins.

Another key is to watch the weather. “If you have a drought, you don't have to monitor as closely as you have to during rainy periods.”

Fundacao ABC has eight weather stations that monitor conditions favorable to rust and has developed a model to predict when and where the conditions might occur.

Even non-Portuguese-speaking journalists understood Perez' double-entendre response to the question, “What advice would you give to U.S. farmers for control of soybean rust?” Aware of the fierce competition between the United States and Brazil in soybean trade, he said, “Nao planto soja,” or “Don't plant soybeans.”


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