Black Spots on Maple Leaves Can Be Treated
Maple tar spot is one of the easiest plant diseases to identify. Most people can spot it even without knowing what it is: if you have seen round black spots on maple leaves, either in a tree or on the ground, you can identify tar spot.
With the number of maple trees in southern Ontario, tar spot is one of the most common plant issues seen in the GTA and a common problem for Toronto maple trees. Tar spot can affect all trees of the Acer, or maple, family. There is some indication the problem is getting worse in Ontario. This includes non-native and invasive maples like Norway maple and Manitoba maple. Maple tar spot is especially common on Norway maple, red maple, silver maple and sugar maple trees.
What Are Tar Spots?
Maple tar spot begins as yellow spots on the surface of the leaf, growing to the more recognizable black spots. The black spots may be ringed by yellow or brown. Yellow spots start appearing in June. The spots grow over time, turning into larger black spots by late August each year.
The spots can be most obvious on Norway maples. These trees are quite common in Toronto, which is one of the reasons this disease is so easy to find. Tar spots on sugar maples and silver maples may be more subtle.
What Causes Maple Tar Spot?
The disease is caused by fungus, specifically the fungi Rhytisma punctatum, Rhytisma americanum, and Rhytisma acerinum.
Is Tar Spot Harmful?
Tar spot only affects the leaves of a maple tree. That means that its worst effect is aesthetic. Few homeowners with maple trees want their beautiful trees marred by black spots on leaves. The spots on the leaves can block sunlight to that area, reducing the tree’s ability to use the sun for energy and nutrients.
Maple tar spot isn’t fatal to your maple tree, but the infection won’t disappear without action. Understandably, many owners will prefer to see their tree with healthy-looking green leaves. The lack of sunlight can also make some leaves drop prematurely.
Maple Tree Tar Spot Treatment
Since the fungus can keep reproducing on damp leaves over the winter, clearing up infected leaves is important to prevent future tar spot outbreaks. Otherwise, the spores will circulate again when the weather warms up next spring.
Cleaning up infected Black Spot Leaves
Composting the leaves, or placing them in your yard waste bag, is effective to stop the fungus from growing. When the leaves aren’t exposed to air, the fungus can’t grow. It’s better to remove fallen leaves in the fall, as manually removing leaves earlier in the year can stress your tree. Removing the leaves in the fall can help make the infection less severe next year.
Pruning to Increase Airflow and Sunlight
Another way to help alleviate the problem is through pruning to increase sunlight penetration and air flow. Fungus spreads through spores in the air so can’t be stopped, but trees with dense canopies are more susceptible.
Supporting the Health of Trees Overall
Trees under stress from drought, girdling roots, pollution and other issues are also more susceptible to all pests and pathogens. This is why Norway Maples are one of the most commonly infected – they have dense canopies and suffer from numerous problems in urban environment.
Mulching or adding soil may also help reduce the recurrence, since moisture can help fungi grow. Vista Tree’s Certified Toronto Arborists can advise you on any issues with soil quality around your maple trees.
Fungicides – though not in Ontario
While fungicide can be applied in certain jurisdictions to treat this disease, Ontario doesn’t allow the application of restricted chemicals for aesthetic treatments. Since Maple Tar Spot is not a disease that will harm the tree on its own, no chemicals can be applied.
In other jurisdictions, an arborist may treat your maple tree with a fungicide to kill the spores and prevent maple tar spot from reappearing. This treatment works best in the springtime. It’s most effective on smaller maples, as it may be difficult to cover enough of the leaves on a large mature maple. The fungicide may be applied twice over a few weeks.
Contact Vista Tree For help with Black Spots on your Maple Tree
If tar spot on your trees is ruining the look of your maples, contact Vista Tree for a consultation! We can help
References
Canadian Tree Fund. (2008). Tar Spot of Maple: Where did it come from and is it getting worse?Canadian Tree Fund – Promoting and Funding Arboricultural Research and Education in Canada. https://canadiantreefund.org/wp-content/uploads/2018/08/2008-Maple-Tar-Spot.pdf
City of Toronto. (2017, November 17). Threats to trees: Diseases. https://toronto.ca/services-payments/water-environment/trees/forest-management/threats-to-trees-diseases/
Cornell University Plant Clinic. (2021). Tar Spot of Maple – Factsheet. Cornell University PDDC. https://plantclinic.cornell.edu/factsheets/tarspotofmaple.pdf
Jack Lakey Contributing Columnist. (2020, November 16). Are your maple leaves dotted with Black spots? They’re called ‘tar spots’ and here’s what they mean. Toronto Star. https://www.thestar.com/news/gta/the-fixer/are-your-maple-leaves-dotted-with-black-spots-they-re-called-tar-spots-and-here/article_aae16ab2-2bbe-57fb-8ead-132880fefe9f.html
Michigan State University. (2014, August 6). Black dots on maple: Tar spot. MSU Extension. https://www.canr.msu.edu/news/black_polka_dots_on_maple_tar_spot
Tar spot of maple | Rhytisma spp. | The Morton arboretum. (2024, March 4). The Morton Arboretum. https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/tar-spot-of-maple-
