Frequently Asked Questions About Black Spots on Maple Tree Leaves
Every spring and summer, we get calls from Milwaukee area homeowners concerned about black spots on the leaves of their maple trees. The spots appear on leaves in the spring as small, light green to yellow spots with bumpy, dark discolorations inside them and progress over the spring and summer into larger and thicker black spots about half an inch to an inch in diameter. It looks like the leaves have been splashed with tar. Hence the name: Tar Spot.
Here's what tar spot looks like:
|In spring/early summer, tar spot appears as dark, raised bumps within light green to yellow splotches.||By midsummer, the tar spots have become black splotches 1/2" to 1" in diameter and 1 to 2mm thick.|
What Causes Tar Spot on Maple Tree Leaves?
Tar Spot is a fungal disease caused by fungi in the Rhytisma acerinum genus. It infects silver maple, sugar maple, red maple and Norway maple trees, and the related box elder tree. Trees in damp, sheltered spots tend to be the most prone to tar spotting.
Is Tar Spot Unique to Wisconsin?
No, tar spot is common on maple trees throughout the US.
Is Tar Spot Anything To Be Concerned About?
In most cases, tar spot doesn’t cause any damage to the tree (other than cosmetically), although heavy infections of tar spot can cause leaves to drop well before fall arrives.
Is There Any Treatment for Tar Spot?
Tar spot can be treated with fungicides, but it’s rarely successful—especially if the trees are mature. Usually, we recommend managing the infestation by raking and destroying/disposing of fallen leaves from the tree in the fall. This minimizes the chance of fungi spores that survive on the fallen leaves infecting the tree again in the spring. The problem is that if leaves from other afflicted maples in the neighborhood aren’t also destroyed, the fungus can easily spread to your tree again.
Although some sources say the leaves can be mulched and composted, this requires frequent turning of the leaves to ensure the compost pile stays hot enough to destroy the spores on the leaves. If you are not prepared to do this, you're better off raking them for the seasonal leaf pickup.
Bottom line: Don’t worry if your maples have tar spot. Just make a concerted effort to rake up all the leaves in the fall. Some sources say the spores will die if the leaves are mulched and composted, then turned regularly. If you’re really concerned about tar spot, though, you probably don’t want to take any chances. Your best bet is to get all the fallen leaves to the curb for pickup in the fall.
There’s one other thing you can do: If your neighbor’s trees are also affected with tar spot, let them know what you’ve learned here. If you all clean up the fallen leaves every fall, there’s a chance your trees won’t be reinfected in the spring.