It's just a matter of time. Asian soybean rust will infect the U.S. soybean crop, says Tadashi Yorinori, a research plant pathologist at Embrapa Soja in Londrina, Brazil, and considered the world's foremost expert on the disease.
Yorinori, who has been working with Brazilian agricultural experts and farmers, was in the United States working with Bayer CropScience crop specialists, training them on disease identification and management. He also addressed a tele-conference of agricultural journalists.
“The disease is unpredictable,” he said. “It has infected areas in the Southern Hemisphere where we did not expect it. It's windborne, so U.S. soybean growers have to be ready.”
Yorinori said the disease was first detected in Paraguay and southern Brazil in 2001. By 2003, 60 percent of the acreage in both countries was infected. “Now, it's all over the Southern Hemisphere, including Brazil, Paraguay, Bolivia and Argentina,” he said.
He classifies Asian soybean rust as “the worst thing that could hit a soybean farmer.” Damage from infection ranges from nothing to 100 percent losses. Average infection ranges from 30 percent to 75 percent loss,” he said.
The problem may be more prevalent in Brazil because of a 12-month growing season. He said growers have soybeans in the ground in some part of the country year-round. The shorter season in the United States may limit infection somewhat, but he's convinced that losses can still be severe.
He said two elements must occur for infection: First, the inoculum must be present. Second, climatic conditions must be conducive. Ideal conditions for severe infection include temperatures less than 86 degrees Fahrenheit and seven to eight hours of leaf wetness, with inoculum present.
A 75-degree to 81-degree range during the day with a 70-degree reading in the evening will create dew, he said. Rainfall also may supply the required humidity for infection.
“But we've also found that the disease adapts to various conditions.”
Scouting will hold the key to management when it hits the United States, Yorinori said. “Farmers likely will need professionally trained scouts to identify the rust,” he said. “It's especially difficult to identify in the early stages. Farmers should collect samples of any suspected infection and test them.”
Early treatment and thorough coverage with an approved fungicide are critical to control, he said.
Jim Bloomberg, manager, fungicide product development for Bayer, said relatively few fungicides are available for effective control in the United States. He said fungicides in the triazole and strobilurin classes may be the only options presently available.
“In the future, USDA is likely to grant a Section 18 to provide more options.”
He said if widespread, severe infections hit now, U.S. growers would not have adequate fungicides to control the disease.
“I think we'll be ready when the rust is identified here,” he said.
Yorinori said varietal resistance may provide one management option but has not been successful in Brazil. “We had one resistant variety but a new race developed and wiped it out.”
He said thorough coverage with a fungicide and proper timing are both essential for control. “Growers may need to change row spacing or reduce the number of plants per foot of row to improve coverage,” he said.
Yorinori also noted that soybean farmers in the southern United States may have another dilemma. Kudzu could be an alternate host.