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Black spot disease unlikely to harm trees

By Samantha Craggsstaff writer

As you are wandering through your yard and watching the leaves change colour, do not get nervous if you notice black spots about the size of dimes on your maple leaves.

The black spots, sometimes called "wrinkled scabs," come from a common tree disease called tar spot and are not likely to hurt your tree, says Steve Williams, forester for the Aylmer district of the Ministry of Natural Resources.

"From what I gather, it is a fungus and not caused by an insect," Williams said. "Normally it doesn't affect the leaves at all."

Williams said tar spot commonly appears in mid-summer. Once the leaves fall from the trees, that is the last of it.

"What usually happens is that by fall, the leaves fall off the trees and it's not a problem anymore," he said.

Tar spot is a foliage disease common to red and silver maples, although it has also infected bigleaf, Norway, sugar and sycamore maples. It occurs on trees throughout the eastern and Great Lakes states, Manitoba, Oregon and Washington.

The infection can be detected early through one or more small greenish-yellow spots on the upper leaf surface. The spots can grow to be dark and one-half inch in diameter.

Although Williams said the spots are harmless, to discourage the blight it is wise to rake and destroy leaves beneath infected trees and root-feed trees which have been affected severely for years. Spraying is usually not necessary.

Williams said the largest drawback of tar spot is an unsightly tree but it usually does not influence tree growth.

"In some respects, there is far more to be concerned about in forestry," he said. "It's a little unsightly but nothing to worry about."

Williams said the blight is unpredictable, so it can be widespread one year and not appear the next. He said it also may be more visible some years than others because the number of trees infected varies.

Tar spot affects maples regardless of the tree's age or size. Infections start in early spring, usually from spores released from dead, diseased, over wintered leaves on the ground.

"In most cases, it's not something to worry about," Williams said.