Ohio biologist John Pogacnik admits to mixed feelings about having discovered the latest disease imperiling a major American tree.

Pogacnik first noticed American beech trees with striped and shriveled leaves in 2012 during a routine survey of forests owned by his employer, Lake Metroparks. He didn’t think much of it at first: Just a few trees looked sick, and it had been a strange year, with an unusually warm winter and dry spring.

By the next summer, Pogacnik was seeing ailing trees throughout the six-county region in northeast Ohio where his agency manages more than 35 parks. He alerted colleagues at the Ohio Division of Forestry and the U.S. Forest Service.

“I’m glad to have found it, to just put it out there and let people know,” he said. “But it’s still not the greatest feeling in the world.”

Beech leaf disease has now popped up in nine Ohio counties, two other states and Canada, and its spread shows no sign of slowing. The disease has already felled young saplings; mature trees, some hundreds of years old, appear to be on the brink of death. Scientists fear the beech could soon face a plague as serious as those that have devastated chestnut, elm, hemlock and ash trees. “It has all the signs of a significant, emerging pathogen,” said Constance Hausman, a biologist at Cleveland Metroparks.

Scientists are gearing up to fight back, but they face a major challenge: Nobody knows what beech leaf disease is. Searches for a virus, bacteria or fungus — all common tree pathogens — have come up empty. Researchers are facing an arboreal murder mystery.

“At this point I’m not sure anyone is able to rule anything out definitively,” said James Jacobs, a plant pathologist with the Forest Service in Saint Paul, Minn.

The American beech ranges from the Gulf of Mexico to southern Canada, and from the Atlantic Ocean to eastern Texas and Wisconsin. Nature lovers have long admired the tree’s massive trunks and lush, light-green foliage, which turns electric-yellow in the fall. Beech’s smooth gray bark makes an irresistible canvas for carving initials into hearts, many of which long outlast the romances they memorialize.

Though largely shunned by the timber industry, beech is among the most ecologically important trees in the eastern United States. In the north, where oaks are rare, bears, deer and other animals depend on beech nuts for survival. Beech’s almost unmatched ability to grow in deep shade — and the fact that deer don’t prefer its leaves — has made it among the most common trees in the older forests of many eastern states and the District, where it dominates the understory of Rock Creek Park. A beech dieback “would be a huge loss,” Hausman said.

Despite its abundance, the beech has its challenges. For almost a century, a fungal infection carried by an insect has attacked American beech bark and killed many large trees.

The new and mysterious disease is apparently unrelated to the older malady. Infected leaves blacken between their nutrient-carrying veins, then shrivel like bits of paper tossed in a fire. No infected tree has ever been known to recover, Hausman said, though it’s not clear exactly how, or how fast, the disease kills trees.

Scientists and funding agencies, already overwhelmed by tree-killers such as emerald ash borer, responded hesitantly to early reports of the disease.

“To be honest, I initially tried to stay out of it” when Pogacnik first called him, said Enrico Bonello, a plant pathologist at Ohio State University in Columbus. “I said, ‘Let’s wait and see what happens.’ Sometimes you observe things in nature that are very ephemeral; they go away.”

Beech leaf disease didn’t go away. Instead, it spread from Pogacnik’s Ground Zero to nine Ohio counties and parts of New York and Pennsylvania. Hoping to identify a cause, researchers began grinding up leaves from infected and uninfected beech leaves and using a technique that amplifies pieces of DNA unique to fungi, bacteria and viruses, to see whether diseased trees harbor organisms that healthy ones don’t. The studies came up empty.

Bonello and a graduate student are now enhancing the tests using a newer method called next-generation sequencing, which could turn up organisms that the earlier studies missed. “You’re essentially trying to find a needle in a haystack by comparing two haystacks,” he said, “one with a needle, one without.”

If diseased leaves yield DNA not present in healthy ones, the researchers will have a suspect, though they will still need to isolate it and prove that it can infect healthy trees. Bonello expects to have initial results within a year.

The study may turn out to be moot, however, thanks to a possible culprit revealed at a May meeting in Parma, Ohio. Ohio Department of Agriculture plant pathologist David McCann reported that he had found thousands of microscopic worms called nematodes wriggling on infected beech leaves. McCann sent specimens to USDA nematologist Lynn Carta in Beltsville, Md., who discovered that they were related to a bush-dwelling nematode known in New Zealand but never seen in the Americas. Research is now underway to determine whether it is the same species recently found on beech trees in Japan.

McCann doubts that his nematodes will prove to be beech leaf disease’s sole cause. Feeding by those worms tends to create discolored or dead spots on leaves, not the linear bands seen in beech leaf disease.

Others are more optimistic. “Right now it’s probably our best lead,” said Jennifer Koch, a Forest Service biologist based in Delaware, Ohio, who has coordinated much of the research that has been done.

After six years of working on the cheap, beech leaf disease researchers just got some welcome news. In June, the Forest Service released its first dedicated funding — $156,000 — to help track the disease’s spread and accelerate the search for a cause. Hausman is leading the tracking effort; she has helped develop a free smartphone app that will allow foresters and others to quickly enter information about beeches’ locations and conditions.

Koch and scientists at the Holden Arboretum in Kirtland, Ohio, will use part of the funding to infect healthy beeches with McCann’s nematodes and see whether they get sick. If they do, it will be the first break in a six-year-old cold case. The research funding “really changes the landscape in terms of being able to move forward and hopefully find what the problem is,” said David Burke, a biologist at the arboretum.

There’s also the question about how to respond to a mystery pathogen. That decision falls largely to the USDA’s Animal and Plant Health Inspection Service, which can impose quarantines and take other measures to keep pests and disease from spreading. APHIS scientists have participated in some of the research, but a spokeswoman wrote in an email that the agency is not taking other action on the disease.

Faith Campbell, vice president of the nonprofit Center for Invasive Species Prevention in Fairfax, Va., fears that regulators are missing an opportunity to try to limit the damage by restricting the movement of beech trees out of diseased areas. She’s especially concerned that the landscaping industry could inadvertently spread the disease; already, a shipment of infected beech trees from Ohio has shown up at a nursery in Ontario.

“I don’t think we should wait around to see” whether the disease gets worse, Campbell said. “The longer you wait, the more difficult it’s going to be to control.”