Soybean rust can wreak havoc on soybean fields easily causing production losses of 50 percent or more.

The disease has not yet made it into the United States but has advanced into much of the soybean growing areas of South America, and plant pathologists anticipate it is only a matter of time before it makes its way into North America.

“There’s no good news,” Don Hershman, a plant pathologist with the University of Kentucky Cooperative Extension Service, told farmers at the recent Ag Expo. “Last year there was about a $1 billion loss from the disease in Brazil alone because of lost production and chemical costs. Unlike most other rust diseases that commonly occur in corn and wheat here, soybean rust will actually kill the plant. It’s a devastating disease.”

“The time between when you first see symptoms and complete defoliation can be as quick as two weeks and it will occur,” he said. “It is favored by high humidity and moderate temperatures.”

Typically, production loss to disease is between 10 and 30 percent, but with soybean rust the average range is 30 to 50 percent and often even higher.

Unlike most rust diseases already in the United States such as wheat rust, soybean rust has a large number of host plants. It affects many legumes with the big one being kudzu. It is estimated there are 12 million acres of kudzu in the southern United States.

“Rust does not hurt the kudzu and kudzu is a place where many spores can build up,” Hershman said. “Unlike most situations, this organism will build up on kudzu and other legumes so the potential for spore increase and moving into an area is very large and larger than anything we can imagine.”

Hershman said he anticipates soybean rust will be in the United States within five years. It will likely come from Africa or South America, he said, on a weather event. However, there also is concern and some risk that rust could enter the United States on imported grain.  Most scientists believe the airborne route is most likely to occur but anything is possible, Hershman said.

The U.S. soybean belt tends to have ideal weather for an epidemic of rust, he said. While the rust fungus is susceptible to freezing, there are some areas in the United States where it can overwinter every year once it arrives.

Cultural practices are ineffective and effective resistance is not available, Hershman said. Double crop soybeans likely will be more impacted than full season crops because of the delayed planting season.

At present there is only one way to effectively manage soybean rust, and that is to apply a fungicide. Quadris is the only fungicide labeled for soybean in the United States that is highly effective against rust. Kentucky and other states have applied for a section 18 from the U.S. Environmental Protection Agency that would allow additional fungicides to be used to battle rust should it arrive here in 2004. If soybean rust comes into the country, availability and quick access to fungicides could be issues. Once it is discovered, there is not much time to spray. A single application will not be enough and up to three applications may be needed, he said.

 Even then, there will be yield losses.  Farmers are probably looking at upper $20s to $50 per acre as the minimum cost for spraying twice, depending on the chemical and rate applied. That may be sustainable for the short term but farmers may ultimately have to decide if they can continue to bear those expenses with 80 to 90 percent yields, Hershman said.

“Resistant varieties are the greatest future hope, because economically there are problems with that many fungicide applications,” he said.

Resistance does exist but it is short lived and ineffective, he said.

“Novel sources of resistance must be found and they are searching,” Hershman said. “Seed companies and the U.S. Department of Agriculture are putting in a lot of money to find resistance. Working under the current system, they estimate it could be up to 10 years before a resistant variety is commercially available and that’s not good news.”

Biotechnology may provide for more rapid success

For more information on soybean rust visit the UK College of Agriculture Web site .

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