Fungi infecting maple trees in Maine

Posted Aug. 10, 2009, at 9:43 p.m.

ELLSWORTH, Maine — There’s a fungus among us and it’s eating the maples of Maine.

In the past week or so, leaves on maple trees around the state have begun to curl up, turn brown and fall off the trees.

The bad news: The problem is caused by one of a couple of fungal infections that have gotten a boost from the wet weather this year. It is widespread, and at this point, nothing can be done about it.

The good news: Even though severely infected trees look terrible, the infection does not cause any long-term harm to the trees, according to state forestry experts.

“The samples I’ve been seeing have been from Bangor and Brewer especially,” said Bruce Watt, a plant disease diagnostician with the University of Maine Cooperative Extension in Orono. “But it’s widespread.”

The samples Watt has seen show a tar spot fungus that starts out on the leaves as a pale yellow spot that later turns into a black spot.

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Two leaf diseases are prevalent in the state right now, according to Bill Ostrofsky, forest pathologist with the Maine Forest Service — tar leaf spot and anthracnose. Each is a fungus that causes spots on the leaves of maple trees. They are not a new fungus, but the infections become worse in years like this one when the weather is extremely wet in the spring and summer, he said.

The leaves become infected in early May and June when the buds are coming out.

“The fungus grows on the leaf, but the leave can continue to do its work, until about a week ago when they started to brown up and drop off,” Ostrofsky said Monday.

Although both fungi affect all maples to some extent, they have a more severe affect on the Norway maple, a non-native species with reddish purple foliage that has been planted as an ornamental tree for years.

“The seeds of the Norway maple are very prolific and grow readily,” Watt said. “That gives them an advantage over our native maples.”

Watt said the Norway maple is considered an invasive species.

Although the fungus, if severe enough, could stress younger trees and cause some growth loss in older trees, it will not result in any long-term damage, Ostrofsky said.

“The buds are already set for next year,” he said. “They’ll certainly refoliate next year.”

Nothing can be done to help the trees this year, Ostrofsky said.

The fungus can be controlled with a fungicide, but that needs to be applied in the spring, Watt said. It would cost thousands of dollars to spray the larger trees.

The fungal spores winter on the dead leaves and can reinfect the trees in the spring.

Sanitation may be the most cost-effective means of controlling the fungus, according to Ostrofsky. That means raking the leaves from around the tree and disposing of them by burning them, not in a compost.

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