BAR HARBOR — Ah, the lovely maple leaves … with the big, ugly, black blotches.
This is a particularly bad year for tar leaf spot fungus, which mainly affects Norway maples. It is a non-native species, unlike the red maple or sugar maple.
Norway maples are most common in developed areas. Bar Harbor has an especially large number of them, and they are definitely showing signs of the disease.
“The good thing is that it usually comes out later in the summer when the trees have already done most of their growing and photosynthesizing, so it generally doesn’t really harm them, even though it looks horrible,” said Kate Miller, National Park Service ecologist for the Northeast region, who is based in Acadia National Park.
“It’s not something that foresters are concerned about because it’s not really impacting the trees’ overall health.”
There is no known prevention or effective treatment for tar leaf spot fungus.
So, Miller’s advice is, “Don’t worry about it.”
A cool, wet spring seems to be associated with a worse-than-usual outbreak of tar leaf spot disease. Even though early signs of the infection, small black dots on the leaves, can be seen in the spring, infected leaves “retain most of their photosynthetic capabilities throughout the growing season,” according to the Maine Forest Service.
“The buds for growth next year have already been set, and leaf drop is late enough in the season so that refoliation by the tree is not triggered. This means that the energy reserves already stored will be sufficient to allow the tree to develop normally next spring.”