WASHINGTON - A mysterious fungus killing America's bats could spread nationwide within years and represents the most serious threat to wildlife in a century, experts told Congress yesterday.

Displaying pictures of bats speckled with the white fungus that gave the disease its name - white-nose syndrome - experts described to two House subcommittees yesterday the horror of finding caves where bats had been felled by the disease.

As a state wildlife biologist from Vermont put it, one cave there was turned into a morgue, with bats freezing to death outside and so many carcasses littering the cave's floor, the stench was too strong for researchers to enter.

They also warned that if nothing more was done to stop its spread, the fungus could strike caves and mines with some of the largest and most endangered populations of hibernating bats in this country.

At stake is the loss of an insect-eating machine. The six species of bats so far stricken by the fungus can eat up to their body weight in insects each night, reducing insects that destroy crops and forests and that carry disease such as West Nile virus.

"We are witnessing one of the most precipitous declines of wildlife in North America," said Thomas Kunz, director of the Center for Ecology and Conservation Biology at Boston University, who said that $10 million to $17 million was needed to launch a national research program into the fungus.

Merlin Tuttle, a world-renowned bat expert and president of Bat Conservation International in Austin, Texas, said that white-nose syndrome was probably the most serious threat to wildlife in the last century.

"Never in my wildest imagination had I dreamed of anything that could pose this serious a threat to America's bats," Tuttle told the panel. "This is the most alarming event in the lifetime of a person who has devoted his life to recovering these populations."

Since it was first discovered in a cave west of Albany, N.Y., in March 2007, white-nose syndrome has spread to 65 caves in nine states, turning up last winter in West Virginia and Virginia, federal wildlife officials said. Several caves are also thought to harbor the fungus in Canada.

To date it has killed 500,000 to one million bats, mostly common species. But what has wildlife officials concerned is that the fungus looks to be on the verge of entering the Southeast and Midwest, where some of the most endangered and largest populations of bats live.