Judging from recent posts on social media, many residents in the Kawarthas region have been noticing that some maple trees — especially Norway maples — are already dropping their leaves, which are covered with black spots like somebody dripped tar on them.
No, your maple tree is not dying. It’s a disease called tar spot that’s caused by a fungus.
Although tar spot looks terrible, it’s a cosmetic disease and does not usually affect the long-term health of a tree.
The fungus that causes tar spot survives the winter on fallen leaves. In spring, activated by warmth and moisture, the fungus releases tiny spores that are carried by the wind to healthy new leaves. The spores have a sticky coating that helps them attach to the leaves, where they grow and enter through tiny openings in the leaves, resulting in yellow spots.
During the summer, the centres of these spots turn brown or black, making them look like drops of tar surrounded by yellow borders. Because the leaves are no longer as efficient at photosynthesis, the tree will stop directing water to them and they may turn brown and drop to the ground.
The fungus overwinters again in the fallen leaves and releases its spores in the spring. As moisture facilitates the release of spores, tar spot tends to be worse when there is lots of spring rain.
Scientists estimate that fungi have existed for at least 1.2 billion years, colonizing the land over 500 million years ago, and could have been the dominant form of life around 250 million years ago. There are three related species of the fungus that causes tar spot, and each infects different species of trees.
Rhytisma americanum infects native maples including sugar and red maple. Rhytisma punctatum infects at least seven native maple species as well as sycamore maple. Rhytisma acerinum infects the non-native Norway maple and sycamore maple, as well as the native silver maple.
Both Norway and sycamore maples were imported to North America from Europe, and Rhytisma acerinum was imported along with them. Norway maples are common throughout Ontario, partly because they were extensively planted to replace shade trees lost to Dutch elm disease in the mid-1900s.
Rhytisma acerinum was first described scientifically in 1778 by South African-German mycologist Christiaan Hendrik Persoon, who named it Xyloma acerinum, with the Swedish mycologist Elias Magnus Fries giving the fungus its current name in 1823.
Once healthy leaves have been infected with the tar spot fungus, there’s nothing you can do. However, you can help prevent the fungus from spreading the following spring by raking up diseased leaves and bagging them for curbside green waste collection, if you have it, or taking them to a municipal landfill site.
You’ll want to encourage your neighbours with infected maple trees to do the same. This only works if everyone does it, as the fungal spores can be carried for long distances by the wind.
Composting or mulching the diseased leaves will not prevent the fungus from reproducing. Home composts won’t normally reach the very high temperatures needed to kill the fungal spores, and mulching the leaves won’t destroy all the tar spots, meaning they’ll release their spores in the spring.
