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 featureĀ 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.

tar spot fungus on norway maple Acer platanoides Rhytisma acerinum 20131103-4So 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.

2 thoughts on “Tar Spot Fungus and The Norway Maple Invasion

  1. The Norway maple leaf was on the $50 and $100 bills when they were released, too. But since no botanist is rich enough to have come in contact with them, the first time it was noticed was on the $20 bill. And then the $10 and the $5. But it won’t be changed until they re-tool the die, which is likely never (or once they turn our plastic money into metal money. Or, even better, stone money).

  2. If you ever look at the major ravines of Etobicoke, Toronto you will find that the Norway maple is the dominant species. You’ll only see Norways as far as the eye can see……………
    What’s even dumber is that the law of Toronto protects all trees in parks, regardless whether they are invasive or not, so it’s pointless if you try to eradicate a tree in a park for the good of the wildlife. XP
    Lets not forget to mention that Norway Maples often drop their branches, thus causing damages to nearby infrastructures ie. your home, even your car and even YOU. That ice storm that occurred here in Toronto back in December 2013 caused many N. maple trees to seriously drop their big major branches, causing many property damages, especially in places like Old Toronto.