Every year about this time, I start receiving questions on the ugly black spots that homeowners are seeing on their maple trees. These spots are caused by a fungal disease aptly called Tar Spot. In many cases, these leaves with the black spots are dropping early, which adds to the concern. The black spots that look like big drops of tar look ugly, but the disease doesn’t hurt the tree or affect it’s over all health.
There are three different, but related fungi. All three produce the similar symptoms and can affect almost all species of maples, but mainly occur on Norway, silver and red maples.
How Does Maple Leave Tar Spot Develop?
The disease overwinters on the infected leaves that fall on the ground. As the new leaves begin to open in the spring, the spores on the fallen leaves begin to ripen, split open and are sent by wind currents into the atmosphere. The needle-like spores will land on the new leaves where they may germinate and start thee infection process.
The first symptoms are seen in the early summer as small yellow spots less than 1/8 of an inch in diameter. As the season progresses, a black spot develops within the yellow tissue and enlarges in size and thickness. By the end of August, the spots closely resemble a drop of tar. The spots may also show a pattern of wavy indentations or ripples.
Getting Rid of Maple Leave Tar Spot
Current research has shown that the black tar spot fungus does not cause any long-term damage to the tree. Universities recommend that infected leaves be raked up and removed from the lawn area to lessen the chance of reinfection to the tree.
I am a big advocate of mulching your leaves back into your lawn to add organic matter back into the soil. If Tar Spot is a problem in your trees, you can still mulch the leaves, but not back onto the lawn. They should be collected and place into a mulch pile or bin and then covered so that the leaves don’t produce spores the following spring.
The biggest problem with this recommendation is there are dozens of maples trees in most neighborhoods. Unless everyone is collecting the leaves off their trees, the spores will still be available to spread from lawn to lawn and from maple tree to maple tree. Even if you keep your lawn free of infected leaves, the chance of a tree from “down the block” infecting your tree is always present.
Treatment for Maple Leave Tar Spot
There are fungicides labelled to control Tar Spot on maple trees, but thorough coverage of all leaf surfaces is critical. These may be easy to do with a small tree, but when the tree reaches 20 feet or more, this is almost impossible, even for professional tree care companies.
Any fungicide that is applied only protects a leaf for about 14 to 28 days and then it must be applied again. If the leaves from trees that were infected the previous year are not raked up, the source of infection is still present. It is best to learn to live with a few leaves that have “dollops of tar” on them. There is not much you can do to correct the problem, so learn to live with it.
If you think your tree may have Tar Spot or if any of your landscape plants are showing signs of disease or inset activity, contact your neighborhood lawn care professional at Spring-Green.