Peter Blanchard, in a brown work shirt and droopy hat, reaches into a paper bag and pulls out a foot-long section of tree branch. “And here’s the killer,” he says, holding up the branch to a handful of people gathered at the edge of a small woodlot in Rockport called the Harkness Preserve.

Blanchard, a biologist who works with the Nature Conservancy, runs his finger across a reddish-brown patch that has disfigured much of the bark. The affected areas have the melted, shiny-smooth look of burned human skin that has healed over. The scarring was caused by a fungus called “Cryphonectria parasitica,” an aggressive tree blight that Blanchard and a few volunteers have been stalking in this five-acre parcel of woods for the last 10 years.

Blanchard stares closely at the branch and says: “It’s like carrying around Charles Manson in a bag.”

Towering above the group, a few feet away, is one of the killer’s latest victims — a 60-foot-tall American chestnut. The canopy of the tree is thick with large, serrated, lance-shaped leaves that glow under the noon sun. The tree appears to be healthy, except for a a couple of black holes, or cankers, that mar its elegantly furrowed bark. If left untreated, the diseased areas will eventually girdle the tree and kill it, as they have killed nearly every chestnut tree in America over the last century.

Blanchard and the volunteers have been trying to stop the slaughter by injecting a non-fatal blight around the black heart of the killer itself. Their hope is to genetically transform the lethal disease into its weaker cousin, a European chestnut blight that allows its victims to heal their own wounds.

Because new cankers can form while the old ones are being treated, however, Blanchard likens the process to putting out spot fires for years to keep an entire forest from bursting into flames.

“It’s scary, really, to think that we could be losing a lot of these trees,” he said, referring to half of the 40 chestnuts scattered throughout the little preserve. “With a lot of help they could come back, but I don’t think they will ever be dominant in these woods again.”

Up until the turn of the century, the American chestnut filled the forests from Maine to Georgia and west to Illinois. Along with oaks and maples, they were the dominant species of hardwood trees in the country. The grandest of them stood more than 100 feet high, with trunks nearly 12 feet around.

The chestnut trees could live for hundreds of years. The wood was extremely hard and resistant to rot, making it one of the most valuable sources of timber for the early settlers. Its bark was a primary source of tannin, a substance used for dyeing, making inks and medicines, and for converting hides into leather.

The nuts were used to stuff Thanksgiving turkeys from the colonial days to the late 19th century. Roasted, they were a popular food for generations of urban-sidewalk snackers; littering the forest floor, they served as an important supplement to the diets of all manner of woodland animals.

The chestnut’s beautiful canopy once shaded towns and villages across America. In spring, forests blooming with the creamy white blossoms fired the imaginations of poets and artists.

Then in 1904, the people at the New York Zoological Gardens, otherwise known as the Bronx Zoo, began to notice magnificent American chestnuts dying all around the property. Tree experts bombarded the sunken and cracked cankers with every chemical in their arsenals, but the trees kept dying.

According to one theory, the disease arrived in New York on a shipload of Asian chestnut lumber. Borne by rain, winds, insects and birds, the blight soon spread all over the state. It moved through New Jersey and into Pennsylvania, where the alarmed residents, with the help of the federal government, spent great sums of money in an unsuccessful attempt to save their beloved trees.

By 1925, the blight had decimated the chestnut population for more than 1,000 miles. By 1950, more than 9 million acres of trees — essentially the entire native range of the chestnut — was wiped out.

After the disease had moved through the eastern United States, a chestnut blight struck Italy. Everyone thought the disease would ravage the Italian trees as it had those in America. But 15 years later, a forester noticed that the new sprouts were cankered, but not dying as fast as the original trees.

Samples sent in 1972 to the Connecticut Agricultural Research Station near New Haven revealed that when the non-lethal European blight was passed genetically to the killer American disease, the cankers stopped expanding and the tree began healing itself.

For the last few years, repeated applications of the milder, imported fungus has become the only hope of saving the few American chestnuts that remain in Virginia, Connecticut and Maine. The Harkness Preserve, which was deeded to the Nature Conservancy by Mary Cramer of Rockport, contains one of the finest stands of chestnuts left in the state.

One biologist compares the experience of walking into the forest to coming upon a dinosaur, “so unfamiliar are the distinctive, spreading crowns of the chestnut tree.”

In order to save those endangered trees, the volunteers in Rockport rely on a caramel-colored slurry that Blanchard grows in his Mount Desert Island home. The gelatinous goop, which contains a powerful blend of several European chestnut fungi, bubbles up out of petrie dishes scattered all over his kitchen.

“You get attached to fungus after a while,” he says with a smile as the team prepares to inoculate the trees with the life-saving mixture. “When I’m away, I wonder how they’re doing back home. I’ve got a timid French strain and some very aggressive Italians.”

Wells Thurber, a forester from Belfast who has worked on the chestnut project for 10 years, grabs a cordless electric drill and climbs a ladder propped against a diseased tree. After picking away dead wood around a canker, he begins drilling a series of 20 or so holes into the healthy tissue that surrounds it.

On the ground, Kip Hewitt and Kinsley Morse, a husband-and-wife team from Darien, Conn., tear strips of duct tape into small squares and take notes on the day’s operation. With a plastic syringe, Thurber engorges the holes with fungal goop and covers them with the tape to trap moisture.

Jay Cook, a local biologist, and Didier Bonner-Ganter, an undergraduate forestry student from the University of Maine, follow the same procedure on a canker at the base of a nearby tree. This is the third round of inoculations since May, and the talk among the volunteers is friendly, enthusiastic, and hopeful.

Because each of them was born after most of the chestnut trees had disappeared, they have developed a special fondness for the few endangered survivors. Through these trees, and the others being similarly nursed back to health and bred elsewhere, the workers hope the magnificent species might once again take root across the American landscape.

“If we keep up a concentrated effort of this treatment, along with breeding, I’m confident they will occur naturally in the forests,” Thurber says. “Of course, that will be long after I’m gone.”

There is, however, ample evidence that their hard work is paying off even now. Callouses of new, healthy wood have grown around several of the cankers. Although Blanchard admits that is reason for some optimism, he is quick to point out the down side: in a few trees, the good wood has been killed by an overlapping layer of disease, followed by another layer of healthy wood.

Clearly, he says, there are two forces at war in every tree, and no one can be certain of the outcome.

“The trees try to put up walls against the disease every year, but it doesn’t work,” Blanchard says over the whine of the drill. “You have to be careful about being buoyantly optimistic. As Wells says, there’s a bull raging in these woods. We’ve got it by the horns, but will it throw us? Who knows?”