Brazilian soybean farmers have seen firsthand just how devastating an outbreak of Asian soybean rust can be. But that hasn't caused farmers in the world's second largest soybean-producing country to back away from the crop.
To the contrary, Brazilian farmers are believed to have planted a record 23 million hectares (56.8 million acres) of soybeans in the 2004-05 season, a 20 percent increase over 2003-04. All told, Brazil's growers have boosted their plantings by 22 million acres or 65 percent since soybean rust was discovered in the country in 2000-01.
Soybean rust appears to be more prevalent in Brazil this year, according to reports from Brazilian experts. But the disease is not expected to have as much impact on yields this season as the drought in Rio Grande Do Sul, one of Brazil's largest soybean-producing states.
“Asian soybean rust has produced changes,” said Fernando C. Juliatti, a plant pathologist with the Federal University of Uberlandia in the state of Minas Gerais. “But we still expect Brazil to become the No. 1 producer of soybeans by 2008.”
Juliatti, a professor with Uberlandia's Institute of Agrarian Sciences for 20 years, began working with soybean rust four years ago. He accompanied a group of chemical company and farm supply representatives on a recent tour of soybean farms in the state of Mato Grosso.
“When Asian soybean rust was found in Rio Grande do Sul, Parana and Mato Grosso do Sul in 2001-02, it probably only affected about 4,000 hectares (10,000 acres),” said Juliatti. “The next year it had spread to about 400,000 hectares (988,000 acres).
“In 2003-04, we had a large outbreak in many states. We estimate that we lost about 6 million metric tons (236 million bushels) of soybeans last season to Asian soybean rust. Farmers spent about $700 million on fungicide sprays and Brazil's losses to soybean rust totaled about $3 billion.”
Although some farmers reportedly are making three applications of fungicides to control the disease this season, experts like Juliatti say they expect yield losses due to soybean rust won't be nearly as severe this season.
Brazil's farmers have learned they must respond quickly to symptoms of Asian soybean rust.
“Asian soybean rust develops very fast, similar to phytopthora late blight,” said Juliatti. “The disease is very dynamic and, if you don't spray at the R-1 to R-2 stage of development, the losses can be very severe.”
“Timing of applications can be much more important than the products you spray,” said Fabiano V. Siqueri, an agronomist with Foundation Mato Grosso, a Rondonopolis-based organization devoted to improving the quality of life in the state.
Siqueri said that's especially true in the area around Primavera do Leste, a city to the north of Rondonopolis in Mato Grosso, that has the dubious distinction of possessing some of the most severe infestations of Asian soybean rust in Brazil.
“Some farmers around Primavera do Leste have had to make four and five applications of fungicides this season,” he said. Primavera do Leste was the first place where Asian soybean rust was discovered in Brazil, according to some accounts.
Heavy amounts of rain fell in Mato Grosso between September and January, creating ideal conditions for the development of soybean rust. Since the end of January, however, the soybean areas of Mato Grosso and the surrounding states have been dry.
“It's very possible that we have lost more yield because of dryness than we have to soybean rust this year,” said Siqueri.
Soybean rust lesions can appear at almost any stage of development in soybeans, according to Uberlandia's Juliat
“But when we get to flowering, the rust seems to explode if it's not controlled.”
Officially, Brazilian experts recommend that farmers spray a fungicide or mixture of fungicides when 5 percent of the leaves in a sample of 100 plants or more have at least two lesions per square centimeter (about one-sixth of a square inch).
But many growers spray their fields with a preventive fungicide when the soybeans reach the R-1 or R-2 stage and again 14 to 21 days later when soybeans reach the R-5 stage.
“Even if you scout and don't see the disease, with Brazil's climatic conditions, it is very possible for diseases to come,” said Siqueri. “Even if you don't see the disease, if you wait three days, and it is there, your crop may be lost.”
Another consideration is the size of the farms in Mato Grosso, which is now the largest soybean-producing state in Brazil with 5.4 million hectares (13.3 million acres).
“In the case of a grower with 10,000 hectares (25,000 acres) of soybeans, if you start applications at seven days, you can have a lot of damage before you spray the whole farm,” said Siqueri. “And if it's raining every day, you can't treat when needed.
“Because of this inability to cover all the acres or to plant different varieties or to change crops, we don't have the luxury of waiting. Under our conditions, it's almost impossible that soybean rust will not be in the fields, so that's why we spray.”
If it rained in Rondonopolis today, Siqueri was asked, how fast would soybean rust develop?
“More important than the rainfall is leaf wetness,” he said. “If you have good wetness in the field, you can reach a 70 percent infestation in 18 days — if you don't treat for soybean rust. Even without rain, you can still have a lot of dew.”
Siqueri said Brazilian agronomists believe that farmers should protect their fields through the R-7 stage to allow them to complete pod-fill before soybean rust can defoliate the plant.
“The big temptation, especially when it's dry, is to delay the second application of a preventative fungicide in hopes the disease will not reappear,” said Siqueri, “but that can be a mistake. Our recommendation is that growers make the second application at 20 days whether it is wet or not.
“It can be a case of the disease was here, but impossible to see. Another consideration is that when it comes to a curative fungicide, the application may not be as effective.”
He said Brazilian agronomists are making three recommendations to growers:
Plant varieties on the correct date. “The disease appears to spread more in later plantings,” he said.
Monitor the crop carefully. “We used to have a saying in Mato Grosso that farmers planted soybeans and went to the beach,” he said. “We recommend growers scout their fields for soybean rust at least twice a week.”
Apply a fungicide at flowering. “If you get to that stage, and rust is not there, you still must apply.”
For farmers who want to scout their fields, agronomists suggest they pick one point in the field per hectare (2.47 acres) and pull primary leaves from the bottom, middle and top of the surrounding plants. When 5 percent of the leaves are infected with even one or two lesions, a fungicide should be applied.
Members of the group touring the farms in Mato Grosso found out how difficult it can be to find soybean rust lesions in soybeans. The lesions, which resemble those of some other soybean diseases, must be examined with a 20X hand lens to determine if they have the gray pustules in the centers that can produce rust spores.
Siqueri said that, in his opinion, when as little as 0.6 percent of the leaves are damaged on 20 percent of the leaves, the disease is “out of control. That's why we recommend spraying at R-1 or R-2 and again at R-5.”
“You have to realize that in Brazil we have hosts available for soybean rust all of the time,” said Juliatti. “And with the heavy dews or wetness on the leaves for extended periods and temperatures in the 15 to 30 degrees C (60 to 86 degrees F) range we have favorable conditions for soybean rust most of the year.”
Asked if he had any advice for U.S. soybean farmers, Siqueri paused for a moment and said: “Don't underestimate Asian soybean rust.”
Agronomists estimate that Brazil's 23 million hectares of soybeans have been treated with fungicides an average of 1.8 times this season at a cost of about $1 billion.
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