In Toronto most maples you see are not the iconic sugar maple (Acer saccharum; as featured on Canadian flag and the source of maple syrup) but rather an introduced species, norway maple (Acer platanoides). Norway maples have become invasive, spreading into natural habitats and displacing native species. And the invasion goes further, sadly, the new Canadian $20 bills feature norway maple not sugar maple! Blasphemy!
In what seems like a punishment for Canada’s transgressions (well, not really) norway maples are often heavily attacked by tar spot fungus (Rhytisma acerinum) that causes large black lesions on the leaves. I think they look nifty, but people get all upset because their attempts at making nature look exactly like they want are failing. Of course, if they really wanted the true maple experience why didn’t they plant sugar maple (!), but that its own rant altogether.
So I say, spot away Rhytisma acerinum! It is a neat parasitic interaction to observe, but also a good reminder that we are not nearly as good as we think at predicting what will happen when we start moving species around. They may not act like we hope and they could just start spreading in our forests and on our money.
There is a simple lesson: don’t fill cities with foreign species! There are plenty of delightful native ones to chose from.