I’ve been noticing big black splotches on maple trees lately, especially on Norway Maples and I wondered what that was. Is it killing the tree? Is it caused by some scary environmental factor?
It turns out it is a common fungus known as Tar Spot, which has been recognized since the 1700s. There are 3 related Ascomycete fungi which cause the splotches. The fungi manifest themselves as splotches end of season and are obvious at that time. It’s considered to be a cosmetic issue which does not affect the health of the tree.
If you get tar spot on your maples and you don’t like it and want to get rid of it, I read the best way to control it is to rake up and remove fallen leaves, because the fungus over-winters in leaf debris.