What’s killing Pennsylvania apple trees?

Mark Boyer describes his home in Schellsburg, Pa., as “in the middle of nowhere, up in Appalachian country” with a commanding view of Chestnut Ridge’s 1,700-foot crest.

But at ground level, he sees something less majestic: Apple trees at his family’s Ridgetop Orchards have been dying for several years, and he has no idea what’s causing it or how to stop it.

"We asked ourselves: ‘Are we going to lose more trees? What’s going on? Is this a by-product of something we’re doing? Chemical? Cultural?’ It was just a big mystery,” Boyer said.

Ridgetop’s 500 acres are perched on an elevation ranging from 1,300 to 1,700 feet, which the family says is ideal for growing cherries, peaches, and apples. But so far, they’ve lost about 17 acres of apple trees — at a cost of about $17,000 an acre — to the mysterious blight.

The disease began attacking apple trees in Pennsylvania and the Northeastern U.S. about six years ago and still baffles those trying to stop it. Pennsylvania produces about 528 million pounds of apples a year, the fourth largest of any state, so any threat to the crop is taken seriously. There is growing concern as a cause remains elusive. No one has a handle on how many trees it has killed or if the blight will end on its own.

Kari Peter, an assistant research professor in tree fruit pathology with Pennsylvania State University, was one of the first to sound an alarm after observing in 2013 what’s now called Rapid Apple Decline.

Peter works out of the university’s fruit research and extension center in Biglerville, Adams County, home of the National Apple Museum. She noticed a three-acre block of apple trees used for research had suddenly started to die without signs of pests or an obvious known disease.

“The trees were dying left and right and it didn’t seem to have any rhyme or reason,” Peter said. “The trees continued to die the following year too. Nothing worked.”

She tried a variety of measures to save the trees, including applications of chemicals. Still, they died. So she reached out to the Pennsylvania Department of Agriculture for help.

“Everyone has theories,” Peter said of the cause.

In 2016, Peter attended a regional meeting of scientists where she learned growers in other states were experiencing the same blight.

All agreed the blight seemed to be attacking a particular root stock of dwarf trees, which are widely grown because they are more compact and yield fruit more quickly. Commonly, problems with the trees seemed to start at the graft. Most apple trees grown to produce marketable fruit start as two pieces: the rootstock, which defines the size of the tree, and the scion, a variety such as Fuji, Honey Crisp, or Gala that gets grafted on.

In Pennsylvania, the issue usually occurs on Malling 9 rootstock, widely known as M.9, Peter said. The problem has been identified on similar rootstock in New York, North Carolina, and New Jersey. The rootstock remains healthy, but the disease attacks the trunk of the tree. Leaves begin to turn yellow, then red. Within weeks, a tree can die and collapse, loaded with fruit.

To address the problem, Penn State joined forces with multiple universities that work with fruit growers in the Northeast. Last month, a Cornell-led research team published a paper in the journal PLOS ONE that cited work by Peter and noted the “unusual decline and collapse of young established trees known as ‘rapid apple decline’ has become a major concern for apple growers, particularly in the northeastern United States.”

The paper dug into speculation about what’s causing the die-off, including wild weather swings, soil composition, pathogens, or insects. The authors said more research was needed to reach a conclusion. Among the most embraced hypothesis: a weather anomaly that began in December 2014 as an exceptionally cold winter, followed by heavy rains, then a severe drought in 2016 during the apple growing season. The belief is that the swings impacted the grafts, which made them more susceptible to cold or pests or disease.

In March, Science Magazine reported that up to 80 percent of orchards in North Carolina “have shown suspicious symptoms.” Trees in Canada have been particularly hard hit after mild winters.

“I don’t believe there is a real straightforward answer,” Peter said. “There could be so many factors: environmental, biological, or a combination of factors that is leading to the decline.”

Peter, who said more than 20 percent of the university’s trees in Biglerville have died, doesn’t rule out climate change as a contributor, but nor does she endorse it as a possibility. Currently, she’s focused on a newly described virus as a potential cause.

Shannon Powers, a spokesperson for the Pennsylvania Department of Agriculture, said a virologist has been assigned to work with Peter.

“It’s a priority for us because of its potential threat,” Powers said. “It’s having a significant impact on orchard owners, but we’ve had a hard time getting our arms around quantifying the loss.”

Boyer, the grower with Ridgetop Orchards, wonders if the problem stems from farmers trying to keep up with the high demand for Fuji, Gala, Honey Crisp and other apple varieties. Where growers once planted 300 trees to an acre, some are now planting 1,200, he said. So, if one rootstock is diseased, it can have an outsize impact.

Boyer also wonders if the M.9 might have developed a weakness. He’s experimenting with a new planting of 30,000 apple trees from a variety of stock and nurseries.

“Instead of using one type of root series and one type of nursery, I’m spreading it all around," Boyer said. “If I only lose 5,000 trees, then it’s a more manageable risk.”