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Fungus flattens foliage

Tar spot blunts most common local tree -- Norway maple

Arlington DPW workers get ready to take down a sick tree. The tar spot fungus produces spots on the leaves of the Norway maple.
Arlington DPW workers get ready to take down a sick tree. The tar spot fungus produces spots on the leaves of the Norway maple. (Globe Staff Photo / Joanne Rathe)

ARLINGTON -- A nasty but non lethal fungus that took advantage of the long, wet spring has blunted the typically brilliant autumnal foliage of the Norway maple -- the most common tree across Greater Boston -- and highlighted concern about the increasingly homogenous tree stock in Massachusetts.

"The fungus is the worst I've ever seen it," said Stephen Rae, the forestry supervisor in Arlington. "And it's made the colors everywhere very flat this year."

Tar spot fungus produces black, sooty patches on leaves starting early in the summer. The fungus forces the leaves to brown and curl before their colors peak in October.

This year, the fungus hit the Norway maples particularly hard and, because they constitute such a broad swath of the tree population across the region, it has muted the colors of the leaves this season.

Norway maples were planted en masse across the state 50 years ago when a widespread blight decimated the American elm. The maples were a cheap and a biologically competitive alternative, said H. Dennis P. Ryan III, professor of arboriculture and community forest management at the Stockbridge School at the University of Massachusetts at Amherst.

But, in the ensuing years, the Norway maple has become a little too successful from a forestry management point of view, Ryan said. Arborists and landscapers have relied on the maples so heavily, Rae said, that they constitute about 70 percent of the tree stock in many communities in Greater Boston, including Arlington.

A healthy diversity of trees includes no more than 10 percent of any genus, Ryan said. Otherwise, an area becomes especially vulnerable to a blight.

"If there's one thing we've learned from history, it's that we don't know what's going to come off the next plane at Logan," Ryan said.

At the turn of the 20th century, the American chestnut was destroyed by a blight that was imported from Asia. And, of course, Dutch elm disease destroyed the native American elm population starting around mid century.

Tar spot is not nearly as deleterious as those diseases and is, in fact, usually present in a healthy forest. But, combined with the drought several summers ago and the encroachment of the winter moth north of Boston, arborists from Boston through the Merrimack Valley are worried.

"I'd guess the winter moth is in about 5 percent of the trees in Arlington," Rae said. "And we don't know what that's going to mean."

The moth invaded the South Shore a few years ago and has decimated tens of thousands of acres of trees south to Cape Cod. The long-term consequences are not known but entomologists are introducing predators to fight the moths to avoid worst-case scenarios. Rae said he was positive the infestation will get worse north of Boston, too.

The winter moth has not yet reached into the Merrimack Valley, said John Coppinger, an arborist who works there as well as in Boston.

"But it's just a matter of time," he said. "There's nothing to prevent it from spreading."

Also tempering this year's foliage is a fungus called anthracnose, which causes the veins of a leaf to turn brown before overtaking its edges and then the entire surface.

Like tar spot, that disease lies dormant in the ground in the winter until it's swept up into the lower branches of trees by the wind in the spring. The spores then spread from the lower to the higher branches -- particularly if the spring is very wet.

The combination of the fungi, the drought, and the annual onslaught of road salt has weakened the tree population in Arlington, Rae said. Typically, he takes down between 200 and 400 trees a year. This year, that number is up, though he is not sure by how much. When spring arrives, he expects to have to take down many more than the usual number of trees that fail to survive the winter.

The larger problem, Rae said, is that the trees he removes aren't being replaced with new ones.

"We maybe replace a third of them," he said. "We should be replacing every one we take down with two."

Eric Seaborn, director of the state Urban and Community Forestry Program, said it's the same across the state. Every city and town in the state is mandated to have a tree warden but most programs are poorly funded, typically to the tune of less than $10,000, Seaborn said.

The upshot: Fewer and fewer trees.

On Langley Road, where a four-man bucket crew sawed down its second maple in two months, Robert Morris, 75, watched from his porch with a grimace.

The maple was there 30 years ago when he bought his single-family home overlooking the Mystic Lakes. He watched it get sicker and sicker over the years until it was leaning dangerously over the road. This year, the leaves are sparse and brown.

"It was time for her to come down," he said. "If it was healthy, there would have been a demonstration out there, but it was time."

Douglas Belkin can be reached at dbelkin@globe.com.

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