Tuesday, November 17, 2009

Tar Spot on Maple

I love when I find this disease on Red Maple (Acer rubrum).
It doesn't show up until late summer or early fall and it usually only affects a small portion of the leaves. But the spots are large and very distinctive- and they look like tar, hence the common name.

The fungus is Rhytisma acerinum. The specific epithet indicates that the fungus will only infect the leaves of Maple.

When you run your fingers over the leaf, it feels rubbery, as if a dollop of tar dried on the leaf.

Tar Spot can cause premature defoliation, but unless the infection is bad you won't notice.

If your Maple has this disease, the best way to manage it is to rake up the leaves in the fall so they can't cause reinfection the next year. I wouldn't bother unless the trees are being grown for sale.

Wow, right?

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