The American chestnut tree, the nearly extinct leviathan that once dominated the eastern forests and inspired generations of poets and songwriters, may someday make a comeback as a result of discoveries at Michigan State University.

Scientists there have found a naturally occurring virus that can infect the fungus that causes the devastating chestnut blight. The virus renders the fungus almost harmless and allows surviving chestnut roots to send up new healthy shoots that eventually become trees.

Even while the village smithy still kept his stand under the spreading chestnut tree, the mysterious fungal disease began to strike, felling the giants in two or three years, even the biggest ones standing 120 feet tall on trunks up to 13 feet in diameter.

From 1904, when the chestnut blight was first recognized, until the 1950s, when it had largely run its course, the fungus killed an estimated 3.5 billion trees. From the Atlantic to the Appalachians, from Alabama to Maine, vast forests once dominated by chestnuts were taken over by much smaller oaks.

Foresters regard the chestnut blight as causing the worst forest decline in North American history. Carpenters once valued chestnut wood as highly rot-resistant and as beautiful as oak, but easier to work. Woodsmen believed that as the mighty trees declined, the bear population plummeted because the animals could no longer fatten on the nutritious chestnuts before winter hibernation.

Nowadays, the holiday chestnuts roasting by open fires are imported from Italy.

"Most people today have probably never seen a real American chestnut tree, and that's a shame," said Dennis Fulbright, the Michigan State plant pathologist who isolated the helpful virus that promises to bring back the beloved tree. "Italian chestnuts are bigger than American chestnuts but I don't think they're as sweet."

Fulbright's research has focused on a few scattered groves of chestnuts in western Michigan planted long ago by pioneers traveling west. Far from the chestnut's natural range, some of the little stands escaped the blight until recently.

Like many blighted chestnuts elsewhere, some of the Michigan chestnuts have been killed down to, but not including, the roots. The fungus, which spreads under bark, does not kill roots. The chestnut, as Robert Frost once put it, "keeps smouldering at the roots/And sending up new shoots."

But before the shoots grow to more than an inch or so in diameter, the blight strikes them down again.

In some of the Michigan trees, however, the shoots are surviving. They are growing into new, although still small, American chestnut trees. It is in these trees that Fulbright and his colleagues have found the helpful virus.

In recent years, similar viruses have been found in chestnut trees that are sprouting anew in other parts of the country but, until now, no one had succeeded in making the virus spread to other chestnuts still smouldering at the roots.

"We've had some luck at this," Fulbright said. The key experiment was made in a chestnut stand that was just beginning to succumb to the blight. A small laboratory dish of the virus-infected fungus from a recovering grove was tied to the trunk of one dying tree. Within months the less-virulent fungus had not only invaded the tree but spread to other trees in the grove, presumably transported by wind and rain.

Fulbright said the fungus also appeared to have transmitted the virus to the previously harmful fungus already afflicting the trees, making it less virulent and halting the disease.

"I'm optimistic that if further tests show this thing really works, we should be able to cause recovery to take place," Fulbright said.

The origin and nature of the virus remain mysteries. Preliminary tests indicate the virus is quite different from those that appeared in other parts of the country to stop the blight in individual trees but which could not be transmitted.

It differs from ordinary viruses in that it lacks a protein coat. It consists only of a double strand of RNA, or ribonuclease. RNA is a close relative of DNA, the substance making up the genes of animals, plants and many other viruses.

Because protein coats are thought to be essential to a virus' ability to invade a cell, it appears that the mystery virus can exist only inside fungal cells. It is transmitted only when an infected fungal cell fuses naturally with the cells of uninfected fungi, allowing copies of the virus, which the fungal cell makes under command of the virus's genes, to be transferred.

Discovery of the virus, which exists as a kind of parasite on the fungus, promises to fulfill the prophetic conclusion of Frost's poem. "It keeps smouldering at the roots/And sending up new shoots," Frost wrote, "Till another parasite/Shall come to end the blight."