Skip to content 
Tree diseases can be pretty tricky. Take Armillaria root rot, for example.
This aggressive fungal disease can quietly destroy a tree’s root system and leave it at risk for complete failure while only causing subtle aboveground symptoms. If you don’t happen to notice the honey-colored mushrooms during the fall, you may have little indication that the tree is at risk.
But on the other hand, some diseases cause obvious, terrible-looking symptoms despite not being a serious threat to otherwise healthy trees. Many foliar diseases fall into this category, including tar spot — a common issue for maples across Michigan.
We’ll explain everything you need to know about tar spot on maple trees below, including the symptoms it causes, the way it spreads, and when it may actually put your trees at risk.
Tar Spot Disease of Maples: The Basics
First described in 1794, tar spot has been known to scientists for hundreds of years. Found in both Europe and North America, the disease is caused by a few different, closely related fungi, including:
- Rhytisma acerinum
- Rhytisma punctatum
- Rhytisma americanum
The disease gets its name from the most obvious symptom it causes: dark, tar-like spots and blotches on the leaves of trees. The spots typically become noticeable during the summer, though infection actually occurs earlier in the spring when spores infect newly emerging leaves.
While the disease is quite common, it rarely causes significant decline in trees that are healthy. The infected leaves are eventually shed, and the tree typically produces healthy new foliage the following spring.
However, reinfection can occur in subsequent years. If this occurs repeatedly, or the tree is subjected to additional pests, pathogens, or environmental stress, it may cause the tree to decline.
How Does Tar Spot Spread from One Maple to the Next?
The fungus that causes tar spot is confined to the tree’s leaves; it doesn’t penetrate the vascular system of the tree the way some other fungal pathogens do. So, once the tree sheds its leaves, it is effectively tar-spot-free.
However, those shed leaves will remain colonized by the fungus. The fungus will then overwinter in the leaf litter, giving it the chance to infect trees the next spring, particularly if the temperatures are relatively cool and the humidity high.
Maples Susceptible to Tar Spot Disease
Tar spot affects several maple species commonly planted in Michigan, including:
- Norway maple (Acer platanoides)
- Silver maple (Acer saccharinum)
- Red maple (Acer rubrum)
- Sugar maple (Acer saccharum)
- Black maple (Acer nigrum)
Additionally, though it’s not deliberately planted very often, the boxelder (Acer negundo) is a native maple that is also susceptible to tar spot disease.
It’s not clear whether Japanese maples (Acer palmatum) are vulnerable to tar spot or not. So, homeowners should consult a plant health care professional if similar-looking lesions appear on a Japanese maple tree’s foliage.
Common Symptoms of Tar Spot Disease in Maples
The most obvious symptom of tar spot disease is the namesake dark brown to black spots that appear on the leaves in the summer. However, this is not the only symptom trees can exhibit. A few other clues that may indicate a tar spot infection include:
- Pale green to yellow-green leaf discoloration (chlorosis) that appears early in the season
- Yellow halos or larger yellow areas around dark lesions as they develop
- Small raised black flecks within the yellowing tissue before they merge into dark spots
- Premature leaf drop in severe infections
Treating & Preventing Tar Spot in Maple Trees
As mentioned at the outset, tar spot is usually a relatively mild disease. Most trees will recover on their own and go on to live long, healthy lives.
Accordingly, the best management approach is to support the tree’s overall health through proper watering, applying a layer of mulch over the root zone, and having it pruned regularly by a certified arborist.
You can also help reduce the chances of new infections occurring by practicing proper sanitation. Try to rake up the shed leaves promptly and dispose of them. This will limit the number of fungal spores in the area, thereby reducing the likelihood that your trees will suffer an infection next year.
Some fungicides can be used to treat the disease, but they must typically be applied several times over the course of the season, beginning early in the spring, before symptoms have even appeared. Once infection has occurred, fungicides will not eliminate existing lesions.
So, treatment is rarely advisable because trees generally recover from tar spot, fungicides must be applied preventively, and complete coverage of mature trees can be difficult to achieve.
Tar Spot & Maple Trees: FAQ
Tar spot can certainly grab a homeowner’s attention and spark a number of questions. To help, we’ve answered some of the most common ones our clients have below.
Can tar spot kill a maple tree?
Tar spot rarely kills otherwise healthy maple trees. Even when infections are severe and leaves look badly damaged, the disease is generally considered cosmetic. Repeated heavy infections may contribute to stress, but tar spot alone is not usually a serious threat.
Does tar spot affect all maple trees?
No, it does not appear to. Susceptibility varies by maple species and by the particular Rhytisma fungus involved.
Is tar spot contagious from one maple tree to another?
Tar spot can spread between susceptible maples, but it does not move directly from leaf to leaf like some people imagine. The fungi overwinter in infected fallen leaves and release spores the following spring, which can then infect newly emerging maple foliage.
Should I remove leaves infected with tar spot?
Yes. Raking and removing infected leaves in fall can help reduce the amount of fungal material available to produce spores the following spring. It may not eliminate tar spot completely, especially if infected maples are nearby, but good sanitation can reduce disease pressure.
Do fungicides work against tar spot?
Fungicides can help prevent tar spot, but they generally have to be applied early enough to protect newly emerging leaves from infection. Once the black tar-like spots have developed, fungicide applications will not make them disappear.
And because tar spot is usually only a cosmetic concern, fungicide treatment is rarely necessary or appropriate for established landscape trees.
Why does my maple get tar spot every year?
The fungi that cause tar spot survive the winter in fallen infected leaves. If enough infected leaves remain nearby, spores can infect new foliage the following spring when weather conditions are favorable. Nearby maples and leaf litter outside your own property can also serve as sources of spores.
Should I be worried if my maple has tar spot?
Usually, no. Tar spot can make a maple look unhealthy, but the appearance of the leaves is often much worse than the actual effect on the tree.
Give Your Maples the Support Needed to Thrive
Whether your maples are suffering from a mild disease like tar spot, a more serious threat like verticillium wilt, or simply not thriving, the professionals at Longtree Tree Service are here to provide the support and guidance you need.
Contact us today to schedule your free evaluation and give your trees the care they need to thrive.
Sources and References
- MSU Extension – “Tar Spot”
Extension factsheet on tar spot (Rhytisma spp.) of maple, covering the raised black leaf lesions the fungus produces, its disease cycle, and management through sanitation.
https://www.canr.msu.edu/resources/tar_spot - MSU Extension – “When to Control Tar Spots on Maples”
Extension news article addressing the timing question for tar spot control, explaining why the disease is largely cosmetic and fungicide treatment is rarely warranted.
https://www.canr.msu.edu/news/when_to_control_tar_spots_on_maples - Purdue Extension (Plant & Pest Diagnostic Laboratory) – “Tar Spot on Maple”
Plant diagnostic lab report on tar spot (Rhytisma spp.) identification and biology, distinguishing the cosmetic maple leaf disease from unrelated crop diseases sharing the same common name.
https://ag.purdue.edu/department/btny/ppdl/potw-dept-folder/2022/tar-spot-on-maple.html - Cornell Cooperative Extension (Rensselaer County) – “Tar Spot of Maple”
County extension resource on tar spot identification and management, covering the fungus’s leaf-spot symptoms and its generally low impact on tree health.
https://rensselaer.cce.cornell.edu/resources/tar-spot-of-maple - Morton Arboretum – “Tar Spot of Maple (Rhytisma spp.)”
Arboretum plant-care resource describing tar spot symptoms, disease cycle, and the sanitation practices that reduce recurrence, noting the disease rarely threatens tree health.
https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/tar-spot-of-maple-rhytisma-spp/ - City of Waterford, Michigan – “Tar Spot Disease”
Municipal resident-facing page explaining tar spot on maple trees, reassuring homeowners the disease is cosmetic rather than a serious threat. - Molecular Genetics and Metabolism – “Equine Acquired Multiple Acyl-CoA Dehydrogenase Deficiency (MADD) in 14 Horses Associated with Ingestion of Maple Leaves Covered with European Tar Spot (Rhytisma acerinum)”
Peer-reviewed case series by van der Kolk and colleagues linking a metabolic disease in horses (atypical myopathy) to ingestion of sycamore maple leaves infected with tar spot fungus (Rhytisma acerinum).
https://www.sciencedirect.com/science/article/abs/pii/S1096719210002544
