The first case of soybean rust has been found on the mainland United States and could affect U.S. crops for the near future, costing farmers millions of dollars, the Agriculture Department (search) said on Wednesday.

A strain of Asian soybean rust (search) was found in two test plots on a research farm near Baton Rouge, La. USDA officials blamed the disease on recent hurricanes that apparently carried the fungus from South America.

The United States is the world's biggest producer of soybeans and is forecast to harvest a record 84.6 million tons in the 2004/05 marketing year. One-third of the U.S. soybean crop is exported for use as livestock feed, vegetable oils and food ingredients.

"We have confirmed a detection of soybean rust," Richard Dunkle, deputy administrator of USDA's Animal and Plant Health Inspection Service (search), told reporters in a news conference. "The chances for wider spread are very high."

The fungus does not pose a threat to humans.

Soybean futures at the Chicago Board of Trade (search) rose 25 to 30 cents a bushel after the news. Wheat and corn futures also rose.

Soybean rust, which seriously erodes crop yields, was never reported before in the continental United States. Hawaii, where many experimental crops are grown, was infected in 1994.

The disease has slowly marched north through South America in recent years. Farmers in Brazil, the world's No. 2 soybean producer, have lost billions of dollars in soybean production. Paraguay, Bolivia and Argentina have also had to deal with the disease.

"We believe the occurrence of this pathogen is related to the path of the hurricane season," Dunkle said.

The United States was hit by four hurricanes this year, which caused billions of dollars in damage.

The USDA previously warned farmers that the wind-borne fungus would eventually spread to U.S. cropland. It has estimated an outbreak could cost U.S. farmers between $630 million and $1.3 billion in its first year.

"The cost is very much going to depend on the nature of the outbreak and how it unfolds over time," said Keith Collins, USDA's chief economist.

Fungicides are the only short-term way to protect U.S. soybean fields. The chemicals will add about $25 an acre, or 15 percent to 20 percent, to the cost of growing soybeans, the USDA said.

U.S. soybean exports are not likely to be banned because of the disease since other rust-infected countries continued to trade, Collins said.

U.S. soybean exports were valued at $8 billion last year.

The disease will have little impact on American soybean production this year because most of the crop has already been harvested, the USDA said.

Bob Odom, commissioner of the Louisiana Department of Agriculture, said soybean rust was found at two research plots owned by Louisiana State University. Louisiana is not a major soy-producing state. "A USDA team is on the way here now to look and see if any more (soyrust) is here or coming," Odom said.

Coastal fields in Texas, Alabama, Mississippi and Florida will be checked for the disease. "We feel that the entire Gulf Coast needs to be surveyed," Dunkle said.

Much of North America, including the major soy-producing states in the Midwest, would not be hospitable to the fungus because of cold winter weather, the USDA said. But there was no clear cut-off point for the disease's northward spread.

USDA told soybean farmers to keep a close watch on their crops. Early symptoms of the disease include small, brown lesions on the soybean plant's leaves.