A tropical storm could have carried the corn disease tar spot into the heart of the U.S. farm belt for the first time, as winds and rain blew in from Latin America, researchers told Reuters.

The U.S. Department of Agriculture has confirmed cases of tar spot in four locations in Indiana and three in Illinois. The fungal disease has been a problem for years in Mexicoand in Central and South America, with farmers fighting infections that can lower yields.

Tropical Storm Bill, which brought rain to the central United States in June after spinning through the Gulf of Mexico, may have transported the disease, said Bill Dolezal, a research fellow for seed company DuPont Pioneer.

"It looks like it came up out of that area," Dolezal said.

The possibility that a storm introduced tar spot illustrates how U.S. efforts to keep out foreign agricultural diseases can be hopeless against the forces of Mother Nature. Plant specialists have asked farmers to look for more cases and report them, and said the disease could appear in other states.

It is relatively rare for crop diseases to enter the United States from overseas, and the USDA is working with experts in Indiana and Illinois to pinpoint how tar spot entered the country.

Kiersten Wise, extension specialist for field crop diseases at Purdue University in Indiana, said she was studying weather records for a link to the U.S. infections.

There is precedent for such an introduction. An active hurricane season in 2004 is thought to have brought a soybean disease called Asian soybean rust into the United States from South America for the first time.

"For a commodity disease, like on corn, boy it's been a while since a new disease has been introduced," said Suzanne Bissonnette, director of the Plant Diagnostic Clinic at the University of Illinois.

Tar spot arrived too late in the growing season to have much affect on crops this year, experts said. Harvests have already begun in Indiana and Illinois.

Next year, the disease will likely need to arrive anew from Latin America to be a problem for farmers, Bissonnette said. It probably will not be able to withstand the cold Midwestern winter because tar spot needs to survive on living tissue, such as corn plants, she said.

A number of different corn varieties have been found to be susceptible to the disease, which appears as black spots, Bissonnette said.

"It really does look like somebody dripped tar on the leaf," she said.

Recently, farm animals have suffered more than crops from new diseases entering the country from abroad. This year, the United States endured its worst animal-disease emergency ever in poultry from a strain of bird flu that originated in Asia.

In 2013, a pig virus never before seen in the United States was found. Previously seen in Europe and Asia, it has since killed millions of baby piglets. It's unclear how the virus arrived.

In May, Kansas - the top wheat-producing state - discovered its first outbreak of a wheat disease called flag smut since the 1930s. The disease had been present in the Pacific Northwest, and it is not known how it arrived in Kansas.

Flag smut can be transmitted by seeds, while tar spot can not, crop specialists said.

Experts hope the U.S. arrival of tar spot does not foretell infections of the wheat disease Ug99, or stem rust. The fungus, which can destroy entire wheat fields, was first found in Uganda in 1999 and then spread to other countries, including South Africa and Iran.

"That's actually the one people are keeping an eye out for," said Judy O'Mara, director of the plant disease diagnostic lab at Kansas State University. "That will blow in."