Rock County’s Tar Spot Problem
While traveling our beautiful county over the last year, doing research for my blog, Rock County Our Story, I’ve noticed that many of our Maple trees have a fungus on them. I did some research on the issue and wrote an article that was later published in the Courier. I thought that as I have two trees on my property with this issue and it’s the perfect time of year to take some action toward clearing the fungus from our trees, I would write another post.
There are several types of this fungus that fall into the group known as Rhytisma. While this is a cosmetic issue and not a condition that is fatal to trees, it is something I have been trying to clear from my trees. This fungus generally affects Maple trees like the Norway Maple, Sugar, and Ash Leaf Maples, but has also been known to infect Willow and Tulip Trees.
The fungus, commonly known as Tar Spot begins in the spring. As the days warm and new leaves open, small needle-shaped fungus spores also begin to emerge from the previous seasons leaves that have overwintered on the ground. The spores are sticky and when they take flight, they attach to and infect budding leaves in the path of the windblown spores. Creating pockets of infected trees across our communities.
If your tree has been infected, you will see it manifest first as small light green or yellow spots on leaves in late spring. As the season warms into summer and fall, the spots become larger and darker with a yellow ring around them. These lesions can reach a diameter of up to 1.5 inches and when looking closely at them, small fingerprint type lines can be seen. By the end of the season trees can look like someone dripped tar all over them. In some cases the leaves will brown up and prematurely fall of the tree.
The Tar Spot fungus was first discovered and studied in Europe in the late 1800’s. Eventually it was introduced to America. It has since spread through the Northeastern US and the Great Lakes region, where Maple trees are quite common.
Clearing the fungus from trees and returning them to their beautiful healthy condition can be done in one of two ways. Fungicides can be used by spraying the infected tree. This can be a difficult process if the tree is mature as its size may prevent one from covering the whole tree, so consulting a professional for what type of fungicide is best and how to safely apply it is recommended.
I contacted Todd Lanigan of the Wisconsin DNR and asked about ways to clear the issue. In his response to my query Mr. Lanigan said “Tar spot is a cosmetic issue that really does no harm to the maple. The best control in a yard situation is to rake up and destroy the infected leaves that fall on the ground.”
The infected leaves can be disposed of by burning, burying, or they can even be placed in a mulch pile. If mulching is the disposal method of choice, the pile should be turned in the spring before any spores can be released. If the homeowner is diligent about keeping leaves off the ground the tar spot problem can be cleared from the tree in just a couple of seasons.
The main difficulty is that when a homeowner works to clear their trees and the neighbors aren’t doing the same we fall into a cycle of reinfection. It really needs to be a neighborhood effort. I spoke to City Hall about sending a flyer with the water bill explaining the Tar Spot issue and how to clear it but there didn’t seem to be any interest as I never saw a flyer.
If you think that you might have a tree with Tar Spot a University Extension article can be found at hort.uwex.edu/articles/tar-spot. I found this to be a helpful resource.
Good luck. If we all work together we can clear our trees.