Black Spots on Bigleaf Maple Leaves

Bigleaf Maple | Acer macrophyllum
Bigleaf Maple | Acer macrophyllum

It’s that time of year when the Bigleaf Maple (Acer macrophyllum) leaves are changing colour and falling to the ground. I’ve always noticed the small, circular imperfections on maple leaves at this time of year and have wondered what they were. On the weekend I took a walk in Miracle Beach Provincial Park with my son Alden and took a closer look and photographed several bigleaf maple leaves.

Speckled Tar Spot (Rhytisma punctatum) on Bigleaf Maple (Acer macrophyllum)
Speckled Tar Spot (Rhytisma punctatum) on Bigleaf Maple (Acer macrophyllum)

At home, a little internet research revealed that the black patterns on the maple leaves were actually a type of fungus called Speckled Tar Spot (Rhytisma punctatum). Appropriately named, it does have a speckled appearance when examined closely and looks like a collection of punctuation marks. The plant tissue surrounding the fungus retains chlorophyll longer than the rest of the leaf creating an outer rim of green around the darker centre.

Speckled Tar Spot | Rhytisma punctatum
Speckled Tar Spot | Rhytisma punctatum

This fungus has an interesting life history. It overwinters on the decaying fallen leaves and in the spring it shoots off long filamentous ascospores which are carried by wind to new maple leaves. The fungus establishes on the new leaves and grows throughout the summer until fall when it becomes more noticeable.

Speckled tar spot is just one of many plants, lichen, ferns, mosses, and other fungi that grow on bigleaf maple. To learn more about this fantastic tree read Jocie’s article Meet the Bigleaf Maple Tree. More information about speckled tar spot can be found here.

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