Tar spot of maple is ugly and alarming—but not serious
I love fall color this time of year. The reds! The yellows! The oranges! The huge black tar spots on the maple leaves! Unfortunately tar spot disease of maple is one of the colors we're seeing a fair amount of this year. It's ugly and alarming but fortunately not a serious disease.
Tar spot of maple affects most types of maples and also sycamores, willows and tulip trees. It's caused by a fungus called Rhytisma. The fungus enters new leaves in the spring and causes yellow leaf spots. It's not until late summer that the fungus really shows off its stuff by forming dense black mats on the upper side of the leaves (and it really does look like big splats of dripped tar!). These mats are fruiting bodies—just like a mushroom is a fruiting body of an underground fungus—and eventually will produce spores. Rhytisma fruiting bodies overwinter in fallen leaves. Then in early spring the fungus releases its spores—just in time to infect the new leaves forming on the tree. Wind carries the spores from the ground up to the tree and the cycle starts all over again.
The good news is that this disease is mostly cosmetic and won't hurt the tree in the long run, but it does cause early leaf drop which can ruin the beautiful fall color display. The disease is worse in years with a wet spring (like this year). The most important way to prevent tar spot is good sanitation. All the leaves should be removed from the site. They can be burned or even composted, as long as your compost pile isn't near your trees. So if you see tar spot on your maple trees, don't panic, but get busy raking!