Norway maple leaves have had an abundance of tar spots this fall.
Fall in Centretown is full of bright red and burnt orange, covered in black tar. Tar spots on maple leaves are common, but it has gotten worse this year.
Botanist Albert Dugal said the Norway maple is particularly hard hit compared to the other species of maple.
“There is very good batch of it this year, maybe because of the extra wet summer that we had,” he said.
“It’s a bumper crop of tar spots for sure.”
Tar spot is a disease distinctive to maple trees, caused by two species of Rhytisma fungus. One strain of the fungus affects native trees and the other is a European strain which affects non-native trees.
Norway Maples, as you can guess from the name, are not native to Ottawa.
Symptoms of the disease begin mid June as small, pale yellow spots which turn into round or irregular black spots on infected leaves.
Tracy Schwets, senior forester for Ottawa’s Forestry Services Division, said last year it was the silver maples that were hard hit.
“Last year the conditions favoured the native strain, and this year the conditions favoured the non-native strain,” Schwets said.
She says this means the native trees will not be affected by this season’s infestation.
Julie Jackson, program co-ordinator of Ottawa Forestry Services, said the disease is common in Ottawa.
“It’s more of a cosmetic thing,” said Dugal, adding it wouldn’t harm the physical health of the tree in any way.
However, heavy infestation can cause early leaf drop, which can be an annoyance to homeowners.
Peter Hall, a member of an environmental advisory committee for the city, said the maple is the fungus’ primary host, and has evolved with the maple.
“It’s not going to kill your tree. It doesn’t look very nice, but that’s the most damage,” Hall says.
The disease tends to go in cycles, though this summer’s unusually warm, damp climate is better growing conditions for the fungus, he said.
New infections start in the spring, when spores are produced within the black stroma, and are carried by air currents to the new, young maple leaves.
A report in Horticulutre Review says some research indicates that tar spots are sensitive to air pollution, and increased levels of infestation are a result of increased pollution.
Old textbooks may suggest some harsh fungicides to deal with the problem, but Hall says they aren’t used anymore.
“One thing people could do in the fall is rake up the infected leaves and burn them – that’s one way to dampen the level of infestation,” Hall says.