Big Huge Icky Giant Tar Spot

Categories: Insects and Diseases, Trees

Giant Tar Spot

The study of plant diseases made me terribly depressed in school, but also made me laugh on occasion. I imagined two plant pathologists walking around, looking at sick leaves. Of course, each of them wouldn’t admit to the other that they didn’t know or recognize some strange disease, so they’re making up names as they go along. Strange fungal spores on a cherry tree? Well, it looks like Black Knot, and the disease is named.

A fall arrival on campus (and occasionally in my inbox as a question) is always the blatetenly named Giant Tar Spot. (“Hey, what’s this?” “You don’t know that? Any idiot knows thats, um, Giant Tar Spot. Yeah, that’s it. Giant Tar Spot”) You guessed it, huge jet black spots on the leaves of primarily Norway Maple.  This, too, is a fungus, Rhytisma acerinum for you latinally inclined. It infected the leaves way back in early summer, but they don’t really show up until now. When it’s too late to treat.

Not that you need to. Late diseases and insects like this look terrible but, let’s face it, the leaves are only around for a couple of more weeks anyway. It’s not a bad year for it this year on campus, we’ve had it much worse. If you’re looking to get rid of it on your tree at home, rake and dispose of the trees at your neighbors…

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